Tuesday, October 28, 2014

A Visit to the Miocene Sea at Maryland’s Spectacular Calvert Cliffs: A Geologic and Paleontologic Overview

In “101 American Geo-Sites You’ve Gotta See,” author Albert B. Dickas listed Maryland's Calvert Cliffs as number 32. With curiosity piqued and the Cliffs renowned for their diverse and well-preserved fossiliferous assemblages - having attracted tourists and scientists as early as 1770 - I decided to visit the site in July. Only a 90 minute drive southeast of Washington, the Calvert Cliffs is a spectacular and scenic, almost continuous 50 km wavecut bluff along the western side of the Chesapeake Bay.

A geological contradiction to the Eastern Seaboard's otherwise flat Coastal Plain, the Cliffs rise in places to as much as 40 meters. They are cut into actively eroding, unconsolidated sandy, silty and clayey sediments of the lowermost portion of the Chesapeake Group. Throughout much of the Miocene, the region was the site of a shallow, inland arm of the temperate Atlantic Ocean during climatic cooling, uplift of the Atlantic Coastal Plain and eustatic changes in sea level. The stratal package preserves the best available record of middle Miocene marine and, less frequently, terrestrial life along the East Coast of North America.

Looking obliquely at Calvert Cliffs from the cuspate foreland at Flag Ponds, the undeformed beds of the Chesapeake Group can be seen to have a regional dip of less than one degree to the southeast (about 2m/km). That allows the exploration of progressively younger strata in a southward direction and vice versa. The strata is primarily Miocene with a coarse-channel, fluvial and tidal deposited overburden variably from the Pliocene and Pleistocene. Typical of bluffs adjacent to the bay, the beach and cliff-vegetation is small or non-existent. Notice the slump and collapse material at the base. Its presence at the toe is generally short-lived. The beach at Flag Ponds is very popular for sunning, swimming and strolling, but fossil collecting at the cliff-toes, which is a very productive locale, is prohibited and prevented by the presence of a large fence.

The Cliffs of Calvert reside on the broad, flat, seaward-sloping landform of the Coastal Plain of Maryland on the western shore of the Chesapeake Bay in Calvert and southernmost Anne Arundel Counties. Located on finger-like Calvert Peninsula, the Cliffs are on the higher western section of the Plain, while the lower section, known as the Eastern Shore (always capitalized), is a flat-lying tidal stream-dissected plain, generally less than 60 feet above sea level. 

Red arrow indicates the region of Calvert Cliffs on the Atlantic Coastal Plain of the eastern shore of the Calvert Peninsula within the Chesapeake Bay.
Modified from USGS map

Chesapeake Bay is an estuary - the largest of 130 in the United States - with a mix of freshwater from the Appalachians and brackish water from its tidal connection to the Atlantic Ocean. The Bay's central axis formed by the drowning of the ancestral Susquehanna River by the sea that flowed to the Atlantic from the north during the last glacial maximum of the Pleistocene Ice Age some 20,000 years ago - when the planet's water was bound up in ice and global seas were lower.

Multiple advances of the Laurentide Ice Sheet blanketed Canada and a large portion of northern United States during Quaternary glacial epochs. Its advance never reached the region of Chesapeake Bay, having extended to about 38 degrees latitude. Being in a contemporary interglacial period - the Holocene - global seas are higher, but not at the level during the deposition of the Chesapeake Group at Calvert Cliffs during the Miocene.  

During the time of deposition of the Calvert Cliffs, fluctuating seas drowned portions of southern Maryland. A shallow, protected basin of the Atlantic Ocean called the Salisbury Embayment was located in Virginia, Maryland, Delaware and southern New Jersey. It was one of many depocenters along the Eastern Seaboard, separated by adjoining highs or arches. The Salisbury Embayment structurally represents a westward extension of the Baltimore Canyon Trough that extends into the central Atlantic on the continent's shelf and slope.

Major Structural Features of the Atlantic Coastal Plain from New York to Florida
The embayments are depocenters - major sites of sediment accumulation. The Fall Line is the inner limit of deposits on the Coastal Plain at the Appalachian Piedmont Province.
Modified from Ward and Strickland, 1985.

The depositional history and relief of the Cliffs of Calvert are testimony to the elevated level of the seas in the Miocene. In addition, the Embayment resides on the “passive” margin of North America’s east coast, which is typified by subsidence and sedimentation rather than seismic faulting and volcanic activity found on the continent’s “active” west margin. The Embayment was and is in a constant dynamic state - passive but far from inactive. 

How did the passive margin and the Coastal Plain of Maryland? What is the sediment source of the depocenters? What fauna occupied the ecosystem of Calvert Cliffs? These questions can't be adequately addressed without gaining an appreciation for the geological evolution of North America's East Coast. Here's a brief synopsis.

The supercontinent of Rodinia fully assembled with the termination of the Grenville orogeny in the Late Proterozoic and brought the world's landmasses into unification. Rodinia's tectonic disassembly in the latest Proterozoic gave birth to the Panthalassic, Iapetus and Rheic Oceans, amongst minor others. Rodinia's fragmentation was followed by the supercontinent of Pangaea's reassembly from previously drifted continental fragments throughout the Paleozoic. 

A succession of orogenic events...
Using modern co-ordinates, the east coast of Laurentia (the ancestral, cratonic core of North America) experienced a succession of three major tectonic collisions during Pangaea's assembly – the Taconic (Ordovician-Silurian), Acadian (Devonian-Mississippian) and Alleghanian (Mississippian-Permian) orogenies. Each orogen built a chain of mountains that overprinted those of the previous event, closed intervening seas and added crust to the growing mass of the continent of North America. 

The Paleozoic Assembly of Pangaea
In the Early Devonian (400 Ma), following the Taconic orgeny, the Acadian collisional event has initiated mountain building on Laurentia's east coast with the closure of the Iapetus Ocean. The megacontinent of Gondwana, lying across the Rheic Ocean, is converging upon Laurentia towards its eventual subduction zone.
Modified from Ron Blakey and Colorado Plateau Geosystems. Inc.

A final orogeny assembles Pangaea...
The penultimate Alleghanian orogeny between Laurussia (Laurentia, Baltica and Eurasia) and the northwest Africa portion of Gondwana - the two largest megacontinental siblings of Rodinia's break-up - culminated with the late Paleozoic formation of Pangaea. That unification event constructed the Central Pangaean Mountains - a Himalayan-scale, elongate orogenic belt some 6,000 km in length within Pangaea at the site of continental convergence. Calvert Cliffs (red dot) had not yet evolved, but its deep Grenville basement was in place along with the orogen that would eventually blanket its coastal seascape with sediments from the highlands.

The Supercontinent of Pangaea
By the Late Pennsylvanian (300 Ma), the closure of the Rheic Ocean brought various microcontinents, magmatic arcs, and the northwest African and Amazonian aspect of Gondwana into an oblique transpressive collision with Laurussia (summarily Laurentia, Scandinavia and Eurasia). Convergence built the elongate Central Pangaean range composed of segments from South America and Mexico through North America and into Europe and Asia. The Chesapeake Bay, yet to form, is landlocked at the red dot.
Modified from Ron Blakey and Colorado Plateau Geosystems. Inc.

Supercontinental fragmentation...
Beginning in the Late Triassic, Pangaea’s break-up created a new ocean – the Atlantic – and fragmented the Pangaean range. As a result, the Appalachian chain remained along the passive eastern seaboard of the newly formed continent of North America from Newfoundland to Alabama, while severed portions of the range were sent adrift on rifted continental siblings. A supercontinental tectonic cycle is apparent between Grenville formation of Rodinia and its fragmentation and Alleghanian formation of Pangaea and its dissassembly.

The orogen's remnants form the Anti-Atlas Mountains in western Africa, the Caledonides in Greenland and northern Europe, the Variscan-Hercynian system in central Europe and central Asia, the Ouachita-Marathons in south-central North America, the Cordillera Oriental in Mexico, and the Venezuelan Andes in northwestern South America. As a result of Pangaea's fragmentation, the region of the future Calvert Cliffs (red dot) was finally positioned in proximity to the sea, awaiting the orogen to erode and Cenozoic eustasy to flood the landscape and deposit the Chesapeake Group during the Miocene.

The Continent of North America
The Atlantic Ocean began to form with the disassembly of Pangaea in the Late Triassic. In the Late Jurassic (150 Ma), the modern continents are drifting apart. The Appalachian Mountain range has begun to erode and shed its deposits on the developing Atlantic Coastal Plain (light blue). The Chesapeake Bay, yet to form, is located at the red dot.
Modified from Ron Blakey and Colorado Plateau Geosystems, Inc.

North America's new passive margin...
By the Cretaceous, the Appalachian highlands had eroded to a nearly flat peneplain, sending voluminous fluvial sediments to the continental margin and shelf in a seaward-thickening wedge. Under the weight and the effect of lithospheric cooling, the passive marginal shelf began to subside and angle seaward as global warming and rising seas drowned both the coast and cratonic core of North America. The Salisbury Embayment, along with others up and down the coast in a scalloped array, was created by the tilting and reactivation of normal faults that extended westward to the Appalachian foothills. Initially, the regional dip was to the northeast, but Neogene uplift left the Coastal Plain's beds dipping to the southeast.

North America in the Late Cretaceous (89 Ma)
The entire shallow, subsiding passive margin of North America's east coast (light blue), along with the depocenters of the Salisbury Embayment (ellipse), were drowned by global high seas beginning in the Late Cretaceous. Notice that the cratonic core of Laurentia has been submerged by two converging arms of the sea from the north and south that formed the Western Interior Seaway. Also, notice the narrow active margin at North America's west coast, the site of convergent tectonics. This eustatic condition will not prevail with sea level progressively dropping as it fluctuated throughout the Pliocene, the Pleistocene and Holocene.
Modified from Ron Blakey and Colorado Plateau Geosystems. Inc.

Subsidence and sedimentation...
Although classified as a passive continental margin, the "active" shelf received nonmarine 
deposition through most of the Cretaceous. With the exception of the Oligocene, the Tertiary saw wave after wave of marine deposition on Maryland's subsiding shore spurned by rising global seas. During the Late Oligocene, Miocene and early Pliocene, the Chesapeake Group was deposited. The Group's lower three stratal components are exposed as wave-eroded bluffs at Calvert Cliffs. Their ~70 m of strata preserve nearly 10 million years of elapsed time. It forms the most available sequence of exposed Miocene marine sediments along the East Coast of North America.

The depocenter of the Salisbury Embayment is drowned by high seas during the Middle Miocene (15 Ma). This eustatic condition will not prevail with sea level progressively dropping as it fluctuated throughout the Pliocene, into the Pleistocene and present.
Modified from Ron Blakey and Colorado Plateau Geosystems. Inc.

The Chesapeake Group's Cliffs of Calvert...
Vacillating seas consequent to Pleistocene glaciation-deglaciation periodically flooded and exposed the land, backflooding rivers and bays such as the Chesapeake. Our present interglacial epoch - the Holocene - has re-exposed the Chesapeake Group and the Cliffs of Calvert to erosion. In spite of not having experienced a significant phase of tectonic convergence for over 200 million years, the modern Appalachian Mountains have been rejuvenated and river-incised during the late Cenozoic, possibly isostatically in response to ongoing erosion or by mantle forcing.

A glowing Maryland sunrise illuminates the strata of the Calvert Cliffs looking south.
Photo Courtesy of Stephen J. Godfrey, Ph.D., Curator of Paleontology, Calvert Marine Museum

North America's most easterly geomorphologic region is the siliciclastic sedimentary wedge of the Atlantic Coastal Plain that began to form with the breakup of Pangaea. It’s a low relief, gentle sloped, seaward-dipping mass of unconsolidated sediment over 15,000 feet thick, deposited on the margin of North America Appalachian-derived rivers and streams, and spread back and forth by the migrating shorelines of vacillating seas throughout the Cenozoic. In Maryland, progressing to the northwest from the Coastal Plain, one encounters the Piedmont Plateau at the Fall Line, the Blue Ridge, the Valley and Ridge and the Appalachian Plateau Provinces.

From east to west through the Appalachian orogen with an underlying Grenville basement, the physiographic provinces of Maryland are synonymous with those up and down the East Coast of North America. Notice the Fall Line, separating the Piedmont from the Coastal Plain, and the Chesapeake Bay, dividing the Plain into higher western and lower eastern subdivisions.

The five physiographic provinces are a series of belts with a characteristic topography, geomorphology and a specific subsurface structural element. The overall trend is from  southwest to northeast along the eastern margin of North America. Their strike and geologic character have everything to do with the tectonic collisions that built mountains, recorded periods of quiescence, formed and fragmented at least two supercontinents, and opened and closed the oceans caught in between.

Location of Calvert Cliffs on the Chesapeake Bay's western shore of the Atlantic Coastal Plain (light colored)
Modified from USGS map

Calvert Cliffs is on the Chesapeake Bay's western shore of the Atlantic Coastal Plain, as are the major cities on the mid-east coast - Philadelphia, Baltimore, Richmond, Washington, etc. If you connect them on a map, you'll locate the approximate western boundary of the Coastal Plain called the Fall Line or Zone. It's the geomorphic break (and geographic obstacle) where a 900-mile long escarpment of falls and rapids separates the Coastal Plain from hard, metamorphosed crystalline rock of the Piedmont foothills to the west.

The Piedmont and Blue Ridge share similar types of crystalline igneous and metamorphic rocks of the core of the Appalachians. The majority of Blue Ridge rocks are related to events of the Precambrian and Cambrian from Grenville mountain building to the Cambrian rift basins, while most of the Piedmont rocks were transported and accreted to North America. A discussion of the details of tectonic derivation and geologic structure of the remaining westerly provinces is beyond the scope of this post. 

The Calvert Cliffs consist largely of relatively undeformed and unlithified strata of silts, sands and clays of the Calvert, Choptank and St. Mary's formations (Shattuck, 1904) in ascending order of the Chesapeake Group. The formations are interrupted by a series of erosional unconformities and other hiatal intervals and preserve nearly 10 million years of elapsed time.

Flag Ponds Nature Park
The cliffs in the distance are the same as the top of the post. Bluffs directly adjacent to the bay generally have very narrow or no beach material (0-3 m) and little to no vegetation on the face. The erosion rate there is historically uniform (0.3-0.6 meters per year), where they are susceptible to slumping and collapse facilitated by a slope angle of nearly 70 degrees. In this area of Calvert Cliffs and Cove Point to the south, "fossil" bluffs are stabilized and preserved inland of the shoreline as the beach acts as a barrier and protects toe slope erosion from wave action as it migrates to the south along longshore currents. At Cove Point the migrating-landform is a prograding cuspate foreland. Southward bluffs become protected from wave action as new beaches are deposited at the bluff-toes. As the foreland migrates to the south, the beach will recede and active bluff erosion will recommence. The changes induced by the foreland occur on a "decadal rather than centennial scale, which places the rate of slope failure on a human scale." This passive Atlantic margin is anything but!
 (Martha Herzog, USGS).

The Miocene succession was deposited as a complex package representing a first-order transgressive-regressive cycle with numerous superimposed smaller-scale perturbations of sea level. Overall, the record is one of gradual shallowing within the Salisbury Embayment and is reflected in the character of the strata that progresses from inner to middle shelf to tidally-influenced, lower-salinity coastal embayments. Deposition occurred under subtropical and warm temperate conditions in a shallow marine shelf environment at a maximum water depth of more than ~40-50 meters.

The exposures include not only the Calvert Cliffs but the Westmoreland and Nomini Cliffs along the Virginia Shore of the Potomac River. Debated for more than a century, estimates for the basal Calvert range from early Early Miocene to mid-Middle Miocene. 

Miocene Stratigraphy of Calvert Cliffs
The lower Chesapeake Group's Miocene section has been subdivided using a multitude of stratigraphic systems including three formations of 24 stratigraphic beds and molluscan zones, many of which have been renamed as members based on locality. In addition, various depositional sequences and events have been described.  Dates are in millions of years (Ma).
Modified from Ward and Andrews, 2008, and Carnevale et al, 2011.

A general absence of beaches below the cliffs is a characteristic of the region. Direct wave undercutting at the cliff-toe, freeze-thaw erosion, underground seepage at sand-clay interfaces and mass-wasting (the average inclination is 70 degrees) are accompanied by rapid wave removal of colluvium (slope debris). Long-term rates can exceed 1 m/yr. Slumps, rotational slides and fallen trees are constantly being generated and removed. For these reasons, beachcombing for fossils and excavating the cliff-toes is a dangerous and prohibited venture. Collected is allowed in designated parks along the shoreline such as Calvert Cliffs State Park, Flag Ponds Nature Park and Brownies Beach, and private beaches given the owner's permission. 

Two vying factions in regards to the Cliffs are scientists that reap the benefit of ongoing erosion and the homeowners, who seek real estate in close proximity to the cliff edge for the sake of a bay view. For the former, the best way to preserve the Cliffs is to let them erode naturally, while the latter would like to riprap (with stone or concrete), bulkhead, sandbag and groin-field the Cliffs to preserve them and their property. In dealing with Mother Nature, in this regard, shoreline protection reduces the risk of cliff failure, although it doesn't eliminate it. It's only a matter of geologic time. 

Rocky Point just downbay (south) of the Calvert Cliffs Nuclear Power Plant
Exposed are the Choptank and the St. Mary's Formations, the latter often oxidized to an orange color. The boundaries between the members and subdivisions are readily discernible. Zonation of thinly laminated beds of cobbles, mollusk shells, molds and casts can be made out even at this distance. Uppermost strata (upland deposits) consists of undulating Pleistocene alluvium, possibly where Pliocene beds were beveled off during a Pleistocene embayment. Again, notice the slump and collapse material, and destabilized trees sliding down the slope from above. Many homes perched on the cliff-edge have met a similar fate.
Photo Courtesy of Stephen J. Godfrey, Ph.D., Curator of Paleontology, Calvert Marine Museum

To reach otherwise inaccessible sections of the Cliffs and lessen the inherent dangers of collapse and burial, this mode of exploration employs descent from above. Excavation into the unlithified substrate is facilitated by an air-powered drill using a scuba tank's compressed air.

From Google Science Fair 2014. See the video here.

The formations preserve more than 600 largely marine species that include diatoms, dinoflagellates, foraminiferans, sponges, corals, polychaete worms, mollusks, ostracods, decapods, crustaceans, barnacles, brachiopods and echinoderms. The macrobenthic fauna (large organisms living on or in the sea bottom) act as good indicators of salinity. The Calvert and Choptank are dominated by diverse assemblages of stenohaline organisms (tolerating a narrow range of salinity); whereas, the younger St. Mary's Formation exhibits an increasing prevalence of euryhaline molluscan assemblages (tolerating a wide range of salinity). The changes within the assemblages reflect the fluctuating freshwater conditions within the Salisbury Embayment.  

Vertebrate taxa include sharks and rays, actinopterygian fish, turtles, crocodiles, pelagic (open sea) birds, seals, sea cows, odontocetes and mysticetes (whales, porpoises and dolphins).

In addition, the Cliffs preserve isolated and fragmentary remains of large terrestrial mammals (peccaries, rhinos, antelope, camels, horses and an extinct group of elephants called gomphotheres), palynomorphs (pollens and spores), and even land plants (from cypress and pine to oak) that bordered the Miocene Atlantic Coast and were carried to the sea by floodplains, rivers and streams sourced by the Appalachians.

In the Miocene, mammalian diversity was reaching unprecedented levels, increasing in size and filling every conceivable niche on every continent. Grass-grazing, multi-stomached herbivorous artiodactyls (such as giraffes, antelopes, cattle, camel, pigs and hippopotami) were slowly replacing dominant perissodactyls (such as horses, rhinos and tapers) that developed after the end-Cretaceous extinction.  
Modified from Jay Matternes

Miocene River Environment by Karen Carr with permission

Armed with a plastic garden rake in hand from the local hardware store, I collected this fossil potpourri (below) on a 60 minute stroll along the beach at Flag Ponds. Their abundance and availability is a testimony to the richness and diversity of the Miocene marine fauna. The fossils are continually being generated from the eroding cliffs and their underwater extensions. 

I am uncertain as to the origin of the mammalian bony fragments, but I suspect they are largely from marine fauna (i.e. dolphin, whale, seal, etc.) rather than terrestrial, since the former vastly outnumber the latter. Cetaceans such as the whale have repurposed an air-adapted mammalian ear for the differentiation of underwater sounds. The shell-like otic tympanic bulla is a thickened portion of the temporal bone located below the middle ear complex of bones. The large dense bone is well preserved and can easily be mistaken for an eroded fragment of rock.

The crocodile tooth is an indication that rivers and swampy habitats existed in the marginal marine environment of Calvert Cliffs. Shark teeth are the most common vertebrate fossils preserved at the Cliffs. Their numbers are commentary on the favorable paleoenvironmental conditions that existed in the Salisbury Embayment. Of course, sharks continually produce and shed teeth throughout their lives (facilitated by the absence of a long, retentive root structure) and are composed of durable and insoluble biogenic apatite (which favors their preservation). Calvert Museum collections contain as many as 15 genera. The main constituents are Carcharhinus, Hemipristis, Galeocerdo, Isurus and Carchrius. Carcharhiniformes shed about 35,000 teeth in a lifetime!

Appearing in the fossil record about 395 million years ago (middle Devonian), the Class Chondrichthyes (cartilaginous skeletal fish) is divided into two subclasses: Elasmobranchii, which includes sharks, rays and skates, and Holocephali (chimaeras). Elasmobranchii are distinguished by their 5-7 pairs of gill clefts, rigid dorsal fin, presence or absence of an anal fin, placoid dermal scales, teeth arranged in series within the jaws and the upper jaw being not fused to the cranium. Along with a dolphin tooth and a few gastropods, here are a few specimens collected from the surf at Calvert Cliffs State Park, about five miles southeast of Flag Ponds. 

Top Row: Hemipristis (Snaggletooth shark), Isurus (?), Carcharhinus (?)
Middle Row: Carcharhinus (?), Hemipristis (?), Porpoise tooth, Hemispristis (?)
Bottom Row: Ray plate, Turritella shell, ray plate, Scaphopoda mollusc shell.
Any corrections or additional insight regarding these fossils is welcomed.

From these small samples, one can glean the ancient habitat at Calvert Cliffs during the Miocene. The combined study of both fossils and rock layers are essential in reconstructing the paleo-geography of the Cliffs. 

By far, the largest but rarest shark teeth at Calvert Cliffs (but widely distributed within the world's oceans) are those of a "Megalodon," an extinct species of shark that lived from the late Oligocene to early Pleistocene (~28 to 1.5 million years ago). The shark’s distinctive triangular, strongly serrated teeth are morphologically similar to those of a Great White shark (Carcharodon carcharias), a fact that fuels the debate over convergent dental evolution versus an ancestral relationship.

An allometric relationship exists between tooth width and body length in modern sharks. A tooth that is 5.5 inches wide correlates with a body length of 60 feet, making Megalodon the largest shark to have lived, weighing as much as 100 tons. I calculated this Megalodon tooth in my collection to come from a 49 footer. That's ten feet longer than a school bus!

The controversy has resulted in a taxonomic name of either Carcharodon megalodon or Carcharocles megalodon - commonly abbreviated as C. megalodon. The “Meg” is regarded as one of the most powerful apex predators in vertebrate history. On rare occasion, extinct cetacean fossilized vertebrae have been uncovered with bite-marks suggestive of mega-tooth shark predation or scavenging. Megalodon is represented in the fossil record by teeth and vertebral centra, since the majority of its cartilaginous skeleton is poorly preserved.

C. megalodon dining on cetaceans
With permission from artist Karen Carr with permission

Dr. Stephen Godfrey of the Calvert Marine Museum has been conducting paleontological studies in the Calvert Cliffs for some 15 years. After rafting along the shoreline to the excavation site, this paleontologist is taking measurements of a trace fossil interpreted as an infilled tilefish burrow from the Plum Point Member of the Calvert Formation deposited some 16 million years ago. In a 2014 paper in the JVP (see reference below), well preserved,  partially complete, largely cranial remains of tilefish are described. They have been collected over the past three decades from Miocene deposits outcropping in Maryland and Virginia. 

Ruler in hand, paleontologist W. Johns investigates the exposed tilefish burrow.
Photo Courtesy of Stephen J. Godfrey, Ph.D., Curator of Paleontology, Calvert Marine Museum

Lopholatilus ereborensis, a new species of family Malacanthidae and teleost (infraclass of advanced ray-finned fish), inhabited long funnel-shaped vertical burrows that it excavated for refuge within the cohesive bottoms of the outer continental shelf of the Salisbury Embayment that likely inhabited other parts of the warm, oxygenated waters of the western North Atlantic outer shelf and slope. The species name was derived from 'Erebor,' the fictional name for the Lonely Mountain in J.R.R. Tolkien's The Hobbit. Like the mountain-clan of dwarves, the tilefish mined the substrate.

This is a close-up view of an infilled, cylindrical-shaped, tilefish-excavated burrow in erosional cross-section. Investigations of extant tilefish show they were shelter-seeking within horizontal clay substrates. The tilefish used as a head-first entrance and tail-last exit for protection from predators. The burrows were subject to infill and collapse, taphonomously preserving the tilefish if within.
Photo Courtesy of Stephen J. Godfrey, Ph.D., Curator of Paleontology, Calvert Marine Museum

Schematic drawing showing three Miocene tilefish burrows. The fish on the left is actively excavating a new burrow. In the center, the burrow has been infilled, preserving the fish that habitated the burrow. On the right, the fish has taken refuge within its burrow.
Photo Courtesy of Stephen J. Godfrey, Ph.D., Curator of Paleontology, Calvert Marine Museum

Lopholatilus ereboronsis moderately deep skull and short snout in left lateral view with interpretive illustration. For orientation, the large circular structure is the orbit with anterior to the left. Post-cranial structures (vertebral skeleton) are missing. The fossil preserves anatomical detail as if a live dissection. One can clearly differentiate the entire suspensorium (jaw connections) of the maxilla (upper jaw) and a few teeth, the dentary (lower jaw) and the quadrate (ancestral jaw-joint), etc. Five extant genera of tilefish inhabit the waters of the Atlantic, Indian and Pacific Oceans.
Photo Courtesy of Stephen J. Godfrey, Ph.D., Curator of Paleontology, Calvert Marine Museum

In an attempt to see what fossilized remains might have filtered down from overlying Miocene and more recent strata, I walked the beach until finding a break where a wash had carved a trough through the bluffs. I immediately spotted a white object glistening in the sun in a stream bed filled with gravel and tiny sharks teeth that turned out to be the worn remnants of a Colonial-era smoking pipe.

Pipes of clay were first smoked in England after the introduction of tobacco from Virginia in the late 16th century. Sir Walter Raleigh, an English sea captain, was one of the first to promote this novel habit acquired from Native Americans that practiced its ritual use for many centuries. By the mid-17th century clay pipe manufacture was well established with millions produced in England, mainland Europe, and the colonies of Maryland and Virginia. For various reasons, clay pipe demand declined by the 1930’s.

Clay pipes were very fragile and broke easily, and along with their popularity, they are commonly found at Maryland and Virginia colonial home sites. In Colonial-era taverns, clay pipes that were passed around were supposedly broken off at the stem for the next user in the interest of hygiene. Some clay pipes can be dated by the manufacturer's stamp located on the bowl, which was unfortunately missing in this lucky (but well calculated) find.

• Evolution of Equilibrium Slopes at Calvert Cliffs, Maryland by Inga Clark et at, Shore and Beach, 2014.
 • Frequency of Effective Wave Activity and the Recession of Coastal Bluffs: Calvert Cliffs, Maryland by Peter R. Wilcock et al, Journal of Coastal Research, 1998.
• Geologic Evolution of the Eastern United States by Art Schultz and Ellen Compton-Gooding, Virginia Museum of Natural History, 1991.
• Maryland's Cliffs of Calvert: A Fossiliferous Record of Mid-Miocene Inner Shelf and Coastal Environments by Peter R. Vogt and Ralph Eshelman, G.S.A. Field Guide, Northeastern Section, 1987.
• Miocene Cetaceans of the Chesapeake Group by Michael D. Gottfried, Proceedings of the San Diego Society of Natural History, 1994.
• Miocene Rejuvenation of Topographic Relief in the Southern Appalachians by Sean F. Gallen et al, GSA Today, February 2013.
• Molluscan Biostratigraphy of the Miocene, Middle Atlantic Coastal Plain of North America by Lauck W. Ward, Virginia Museum of Natural History, 1992.
• Slope Evolution at Calvert Cliffs, Maryland by Martha Herzog, USGS.
• Stargazer (Teleostei, Uranoscopidae) Cranial Remains from the Miocene Calvert Cliffs, Maryland, U.S.A. by Giorgio Carnevale, Stephen J. Godfrey and Theodore W. Pietsch, Journal of Vertebrate Paleontology, November 2011.
• Stratigraphy of the Calvert, Choptank, and St. Mary's Formations (Miocene) in the Chesapeake Bay Area, Maryland and Virginia by Lauck W. Ward and George W. Andrews, Virginia Museum of Natural History, Memoir Number 9, 2008.
• The Ecphora Newsletter, September 2009.
• Tilefish (Teleostei, Malacanthidae) Remains from the Miocene Calvert Formation, Maryland and Virginia: Taxonomical and Paleoecological Remarks by Giorgio Carnevale and Stephen J. Godfrey, Journal of Vertebrate Paleontology, September 2014.
Variation in Composition and Abundance of Miocene Shark Teeth from Calvert Cliffs, Maryland by Christy C. Visaggi and Stephen J. Godfrey, Journal of Vertebrate Paleontology, January 2010. 

I wish to express my gratitude and thanks to Stephen J. Godfrey, PhD., Curator of Paleontology of the Calvert Marine Museum, for providing valuable support (personal communications, October 2014), documentation and photographs of Calvert Cliffs and his recent excavation and publication.

Calvert Cliffs Marine Museum was founded in 1970 at the mouth of the Patuxent River in Solomons, Maryland. Visit them here, but do go there! You can join the museum here.

The Ecphora is the quarterly newsletter of the Calvert Marine Museum Fossil Club. Ecphora gardnerae gardnerae is an extinct, Oligocene to Pliocene, predatory gastropod and the Maryland State Fossil, whose first description appeared in paleontological writings as early as 1770. Sadly, riprapping (rock used to protect shorelines from erosion) has covered one of only two localities in the State of Maryland where the fossil can be found. The other is on private land and off limits without permission. Ironically, Marylanders can find their state fossil in Miocene strata of Virginia. Download copies of current and past newsletters here or simply subscribe.

Tuesday, September 23, 2014

A “Walk on the Moon of Big Water” with Merle Graffam – Discoverer of the Utah dinosaur Nothronychus graffami

Vox Clamantis In Deserto
"The voice of one crying out in the wilderness"

From the Dartmouth College motto adapted from the Gospel of Mark and
subtitle of Merle Graffam's treatise entitled Fossils from the Tropic Shale

Although some dinosaurs may have spent time feeding in open water and possibly a few may have become strongly amphibious as implied by some trackways, it’s a common misconception that dinosaurs colonized the seas. If so, what were the bones of a terrestrial dinosaur – a new species of therizinosaur - doing amongst the marine fauna of the Late Cretaceous Tropic Shale, at least 60 miles from the nearest dry land at the time?

Artist Victor Leshyk’s portrayal of the proto-feathered, Late Cretaceous dinosaur Nothronychus graffami dining upon mangroves growing marginal to the Western Interior Seaway. The therizinosaur is thought capable of balancing tripodally on its massive pelvis while raking in tree branches with its long slender claws, which it passed to its toothless beak.
From the Museum of Northern Arizona. Visit Victor here. Visit MNA here.

If you’ve ever visited the badlands outside of the tiny southern Utah town of Big Water, you know the meaning of the word “barren”. The landscape consists of a coarse, brownish sandstone bedrock covered by a monotonous repetition of eroding, low-slung, blue-gray mounds of fine mud turned-to-shale against a backdrop of buff-colored, sandstone cliffs littered at the base with dislodged blocks of stone. Little grows and nothing moves, other than the wind and the imperceptible forces of gravity and erosion that are incessantly at work.  

The region is so drab and desolate that locals call it ‘The Moon.’ To geologists and paleontologists – who are of the same ilk - it’s all hauntingly beautiful and exciting beyond anything imaginable, not just for its appearance but for the story of its formation and the bounty of lifeforms that are preserved. Personally, I couldn’t wait to get out there with geologist and acclaimed author Wayne Ranney, and Merle Graffam - namesake of the dinosaur Nothronychus graffami.

Indeed, there’s nary a soul in sight on The Moon unless you stumble upon Merle - retired commercial artist, Big Water resident, Bureau of Land Management Park Ranger at the Big Water Visitor Center, and amateur paleontologist par excellence. Merle takes regular walks on the Moon, combing the ancient seabed for marine fossils with the intuition, trained eye, laser focus and insatiable curiosity of a seasoned field expert. 


Merle knows The Moon and the fossils preserved within in it, all creatures of a long gone sea - megafaunal marine and brackish-water invertebrates such as oysters, gastropods, solitary corals, inoceramid bivalves and ammonites, and marine vertebrates such as fish, rays and sharks, turtles, crocodilians and an occasional short-necked plesiosaur. Above the sea soared pterosaurs and early avians with toothed-beaks. As bleak and depauperate as the landscape looks now, at one time, The Moon was the site of a thriving marine ecosystem. 

Creatures above and within the Late Cretaceous Western Interior Seaway
Adapted from nd.gov

Terrestrial deposits marginal to the sea preserve extensive skeletal remains and trackways that attest to a diverse dinosaur fauna that plied the shoreline's habitats, while diminutive, insectivorous mammals hid in the shadows amongst the gymnosperms and newly-evolved angiosperm plants. It takes considerable imagination to view this ancient land and seascape while standing on the landscape of The Moon.

The lowland-sea interface of The Moon in the Late Cretaceous consisted of many paleoenvironments - habitats that supported a rich array of lifeforms. Seaward, deeper muds led to sandy shores and various sea grasses. Mixed salinity-plants such as mangroves thrived nearshore and onshore, possibly the haunt of therizinosaurs. Further inland, larger trees such as cypress and hardwoods thrived.
Modified from Plateau Magazine, 2007. 

On one of Merle's lunar constitutionals in 2000, he made an unsuspecting discovery that would change his life. What's more, it would rewrite a portion of dinosaur phylogeny, offer a new perception of dietary plasticity amongst theropods and expand our knowledge of biodiversity within the Cretaceous ecosystem.

Merle discovered a small toe-bone - a phalange - in the Tropic Shale that eventually would lead to the remains of a spectacular dinosaur skeleton at the crest of a small hillock of eroding marine sediments of the Tropic Shale. It proved to be the most complete therizinosaurid yet discovered.

Very excited at the excavation site, Merle exclaimed, "Here's the spot!"

With a population barely of 475, the settlement of Big Water is a tiny speck on the map (green laser dot) located in Kane County on Highway 89 in southernmost Utah near the Arizona border. On maps from the late 50’s and early 60’s, it's called Glen Canyon City and housed workers who built the nearby Glen Canyon Dam.

The name Big Water seems a misnomer, since the high desert and badlands are as dry as a bone with an average rainfall of barely six inches a year. The nearest “big water” is a slender arm of Lake Powell called Wahweap Bay about 10 miles down the highway to the southeast, where a trip downstream leads to the dam that impounds the Colorado River. So where’s all the water at Big Water?

Wayne Ranney aims his laser-pointer at Big Water on a topographical relief map at the Big Water Visitor Center. The highway takes you down and across the Glen Canyon Dam that impounds Lake Powell. 

Big Water could have been named for the Navajo Aquifer, an underground formation with an estimated 400 million acre-feet of potable water that spans most of southwestern Utah and some of northern Arizona. But in reality, the name Big Water was simply the winner of a contest held by Mayor Alex Joseph in 1983-84 for a name to replace Glen Canyon City when the town was incorporated. Big Water sounded Native American, and the residents liked it.
Geologically, the name is highly appropriate, since a much earlier actual “big water” submerged the entire region and a wide swath of North America during the Late Cretaceous. That sea was responsible for the layered deposits at the Moon of Big Water – and as we shall see - much more. 

Panorama from Scenic Byway 89 looking northeast from Big Water.
The eroded, gray badlands at the cliff-base are composed of Tropic Shale, while the cliffs are of composed of resistant Straight Cliffs Sandstone. Beneath the Tropic is the Dakota Sandstone. The high desert's sand is a Quaternary mix of unconsolidated surficial deposits. Click on the photo for a larger view.

Big Water is just outside the Kaiparowits Plateau section of the Grand Staircase-Escalante National Monument in south-central Utah, which in turn is on the western margin of the Colorado Plateau - an arid region of high relief centered over the four corners region of Utah, Arizona, Colorado and New Mexico. President Bill Clinton controversially designated the region a national monument in 1996 - an area rich in geology, paleontology and human history.

The three sections of the Monument record sedimentation throughout the Mesozoic. The centrally-located Kaiparowits Plateau section is exemplified by plateaus, buttes and mesas carved in rocks acquired in the Cretaceous when the region was situated along the western shore of an extensive inland body of shallow water called the Western Interior Seaway.


Three sections of the Grand Staircase-Escalante National Monument in southern Utah with Big Water on Highway 89 near the Arizona border.
Modified from Utah Geological Association, Second Edition DVD.

Today, the Monument is elevated two kilometers along with that of the Colorado Plateau, but throughout the Paleozoic much of western Laurentia (the cratonic core of the supercontinent of Pangaea) was situated at sea level. Beginning in the latest Proterozoic and throughout most of the ensuing Paleozoic, an ocean called the Panthalassic (Paleo-Pacific) lapped onto the continent's western margin, leaving limestone deposits now deeply buried below the Monument. In the Cretaceous Period of the Mesozoic, marine waters returned as the Western Interior Seaway - only this time inland and following Pangaea's disassembly in the Mesozoic. Like the Panthalassic (that became the Pacific Ocean), the Seaway left its mark in the form of sedimentary deposits preserved on the Great Plains and the Colorado Plateau. In the Tertiary, the Colorado Plateau and the Rocky Mountains were uplifted, casting the sea from the continent's craton and stripping off much of the sea's depositional legacy from the Plateau. 
By what geological process did the Kaiparowits section and The Moon of Big Water come to be flooded in the Cretaceous by an inland sea?
In the Late Triassic, Pangaea began to break apart in the north-central Atlantic Ocean. As rifting progressed at the mid-ocean ridge, newly-formed North America began to drift westward, while nascent Europe, Africa and South America headed east and south, respectively. Beginning in the latest Jurassic at the west margin, a tectonic plate collision initiated between the overriding continental plate of North America and the east-directed, subducting Farallon plate of the Pacific Ocean.
Plate convergence resulted in a mountain-building deformational event called the Sevier orogeny that extended over 1000 km eastward into the craton. The event had far reaching consequences for deposition across the continent's mid-section, particularly during the Cretaceous - with the most stratigraphically complex sequence of sedimentary rocks on the Colorado Plateau.
North American tectonics during the Early Cretaceous (125 Ma)
Two large arms of the rising sea are about to converge, held up temporarily by tectonic barriers such as the Trans-Continental Arch and the Ouachita Uplift. Ocean basins, inherently deeper, are designated as dark blue: whereas, shallow epicontinental (epeiric) basins and continental shelves are light blue.
Adapted from Ron Blakey and Colorado Plateau Geosystems Inc.

The Sevier front consisted of a fault zone, an active volcanic arc, low-angle thrust slices and a broad foreland basin. The retroarc basin - so called because it was 140-200 km cratonward of the thrust front - was an asymmetric depression created in response to the load superimposed by the east-advancing wedge of thrust sheets that downwarped the lithosphere. I
n response to ongoing Sevier thrusting, the foreland migrated eastward and continued to rapidly subside. The basin received massive amounts of detritus delivered by rivers across alluvial plains from the encroaching front from the west and southwest.

East-directed subduction of the Farallon plate beneath the North American plate initiated loading that drove lithospheric flexure and subsidence. The resulting accommodation space that formed preserved up to 20,000 feet of sediment and received an influx of marine waters from the north and south.
Modified from Plate Tectonics by Frisch et al

In the Early Cretaceous (Aptian to Albian), the basin began to flood with marine waters from the north and south, connecting the Boreal and Tethyan seas. By the Late Cretaceous, long arms of the sea converged forming an inland epicontinental sea (epi is Greek for above). Nearest the front, deep-water sediments pass upward into shallow-water sediments recorded with conglomerates that pass into sandstones and shales, which in turn pass into carbonate marine sediments well to the east.

Late Cretaceous oblique, north view of the asymmetric Western Interior Seaway illustrating the subducting Farallon slab, the Sevier orogen and Western Interior Seaway.
Modified from Wikipedia

The development of the inland sea occurred by active subsidence of the foreland but was assisted at a time of eustasy (global high seas). Sea level changes are affected by the volume of water contained in the ocean basins and the volume of water displaced from the basins. For example, melting polar ice adds to the basins causing glacioeustasy, and shifting plates and shallowing basins removes water called tectonoeustasy. It's a rather simplistic scenario but not far from reality.

Pangaea's aridity during the Triassic and Jurassic - demonstrated by widespread eolian sandstones and evaporites in the west - was replaced by a humid, subtropical climate in the Cretaceous, as North America drifted out of lower, equatorial latitudes. Concurrently, as Atlantic seafloor spreading increased, the ocean basin shallowed, displacing vast quantities of water, while extrusive continental volcanics associated with rifting elevated temperatures 18°F (10°C) higher than average.

Submitting to the global greenhouse, melting polar ice further drove seas higher. Low-lying regions - coasts, interior lowlands and cratonic platforms - drowned worldwide including the subsiding basin of the Sevier foreland. And in its wake, the seas left vast sequences of sedimentary rocks. The great flood is known as the Zuni transgression - the greatest of six major high water events of the Phanerozoic. As an aside, our modern world with rising seas is in a state of Holocene (post-Pleistocene) glacioeustasy. Now back to the Cretaceous Seaway!

The six major transgressions of the Phanerozoic Eon with the Cretaceous Zuni highlighted.
Modified from Earth System History, Second Edition, 2005 and msubillings.edu.

At its zenith in the Late Cretaceous, the Western Interior Seaway in places was almost 300 meters deep. Inland seas are built on buoyant continental platforms and are relatively shallow compared to deeper-denser ocean basins. The sea connected the Arctic Ocean and Hudson Bay with the Gulf of Mexico, and stretched from Utah in the west to the western Appalachians in the east. It split North America into two massive landmasses - eastern Appalachia and western Laramidia - and divided the terrestrial ecosystem forcing it to pursue independent courses of evolution, as did the resident faunal populations riding on Pangaea's drifting continental siblings.

North America in the Late Cretaceous (92 Ma)
The Western Interior Seaway (light blue and white) has flooded the foreland across the continent's mid-section, uniting the waters of the Arctic and Hudson Bay with the Gulf of Mexico, while creating two massive continental islands. Laramidia, in the far north, formed a land bridge through Beringia connecting North America and Asia. Submerged Big Water is located at the red dot. Notice coastal flooding on the subsiding shelf of all newly-formed Atlantic passive margins. Dark blue represents deep ocean basins.
Adapted from Ron Blakey and Colorado Plateau Geosystems Inc.

Also in the Late Cretaceous, Laramidia formed an arctic land-based connection with northeast Asia called Beringia. The loosely defined region in the vicinity of the Bering Strait has intermittently persisted through Recent times. During the Pleistocene, an Ice House climatic condition created regressive global seas exposing the land bridge; whereas during the Greenhouse conditions of the Cretaceous, the land was devoid of polar ice, having formed tectonically from a series of accretionary events. Like the Pleistocene connection that allowed the passage of Paleo-Indians and mammalian megafauna (the Asian saber-toothed cat comes to mind), the Cretaceous bridge (up to a 1,000 miles wide) likely allowed faunal and floral exchange in a similar manner in both directions.

Approximate extent of the Beringia Land Bridge
Adapted from ic.arizona.edu

In the Late Cretaceous, Laramidia experienced a major evolutionary radiation of dinosaurs possibly related to new biomes generated by the Sevier front and foreland, and may have been infused by immigrant fauna that migrated across the bridge from Asia (or vice-versa). The relevance of Cretaceous paleography will become relevant in our forthcoming discussion of therizinosaurs from Laurasia (the combined landmasses of North America and Eurasia that formed Pangaea with Gondwana of the Southern Hemisphere). 

During the Late Cretaceous for nearly 25 million years, the Western Interior Seaway dominated paleography and sedimentation over a vast area of the Southwest. At least two major and numerous minor transgression-regression sequences - called cyclothems - are recorded in the rock record.

Marine waters advanced onto the continent's downwarping interior, rising and falling with starts and stops while the shoreline shifted to and fro from east to west. As the sea advanced onto land, the sandy shore was buried by new, higher shores, while previously deeper muds migrated as well. Terrestrial deposits met marine that vied for space in an overlapping, alternating geometry, all related to the whim of the vacillating sea. When the sea eventually reversed its direction, the opposite layered architecture was deposited as newer shorelines formed on previously deeper muds called a transgressive-regressive sequence - visible stratigraphically.

The west part of the GSENM was elevated by Sevier tectonics before sediments were deposited in coastal areas ahead of the encroaching inland sea from the east. All Upper Jurassic and a good part of Middle Jurassic rocks were removed by erosion before Cretaceous sediments were deposited.

Generalized geologic map of the southernmost Kaiparowits section of the Monument (dotted line). The region represents a structural basin but is topographically high, having achieved its relief along with that of the Colorado Plateau. Big Water is just outside the boundary. Note the Cretaceous deposits of the Dakota, Tropic and Straight Cliffs in the region.
Modified from of Grand Staircase-Escalante National Monument, Utah by Doelling et al, 2000.

Initial alluvial plain and coastal plain deposits were met by the sea's rapidly-rising westward advance called the Greenhorn Cyclothem (late Cenomanian to middle Turonian). Deposited in the sea's first transgression in the early Late Cretaceous about 95 million years ago came coarse, yellow-brown beach sands of the shallow marine Dakota Formation, deposited on either the Morrison Formation (east) or the Entrada Sandstone (west). The Dakota contains a record of shallow brackish and marine water environments, lush coastal swamps and sandy expanses incised by rivers and streams emptying into the sea. 

In deeper waters, the Dakota grades into dark, organic-rich Mancos Shale - called the Tropic Shale regionally - and consists of exceptionally fossiliferous blue-gray silts and muds formed about 93 million years ago. The type section crops out around the town of Tropic, Utah, about 50 miles to the northwest. Elsewhere in Utah, Tropic stratigraphic equivalents have been referred to the Tununk Member of the Mancos Shale, the Tropic equivalent in most of the Southwest.

On top of the sequence with the sea retreated to the east lies the four-membered Straight Cliffs Formation, an overall regressive sequence rich in coal that followed the previous marine incursion about 85 million years ago. The sea returned again bringing with it another sequence of deposits, seen elsewhere on the Kaiparowits Plateau and in the Grays Cliffs of the Grand Staircase.

The aforementioned lithologies are conformable and form a classic transgressive-regressive sequence that documents the greatest widespread rise in sea level of the Cretaceous recognized worldwide. In summary, the foreland basin's sedimentary infill represents a record Sevier orogen tectonics, flexural subsidence, weathering and sedimentation and eustatic sea level change.

The dissected landscape rocks of The Moon of Big Water preserve Upper Cretaceous transgressive Dakota sandstone, shale and some coal buried beneath eroding gray Mancos muds and regressive cliff-forming Straight Cliffs sands and coals.  

In 2000, at the conclusion of a large plesiosaur excavation in the Tropic, Merle turned to Dr. Dave Gillette - Utah's former state paleontologist and current Colbert Curator of Vertebrate Paleontology at the Museum of Northern Arizona in Flagstaff. Pulling a bone from his pocket, Merle uttered the now famous phrase "Hey Dave! What's this?" 

Dave recognized the toe bone, but it was clearly not from a plesiosaur, the large marine reptile found with increasing frequency on the Tropic seabed thanks to Merle's keen eye of discovery. The bone was too small to be from a hadrosaur, a terrestrial, duck-bill dinosaur found in large numbers along the shoreline far to the west. 

Stumped by the implication of a dinosaur bone so far from land, they later returned to the site, found more bones and initiated an excavation. The dinosaur's identity was a mystery well into the dig. According to Dave, “We weren’t thinking ‘therizinosaur’ at first, because at that time they were known only from Asia. From that first toe bone, we thought maybe we had a big ‘raptor’ (an agile, hunting dinosaur). But when we found peculiar bones of the massive hips, we knew we had a sickle-claw dinosaur. They were like nothing we’d ever seen.”

The active therizinosaur excavation site in the Tropic Shale. A project can require the removal of up to 20 tons of overburden and take a thousand hours of field and laboratory time. 
Photo by Dave Gillette

For Merle's discovery and contribution, Graffam became the species namesake. Following the dinosaur's reconstruction, the therizinosaur was featured in an exhibit at the Museum of Northern Arizona from 2007-2009 and at the Carl Hayden Visitor Center at the Glen Canyon Dam in 2012. The actual bones of N. graffami are in storage at the Natural History Museum of Utah, Salt Lake City. Several casts are on display such as the one inside the entrance to the Museum of Northern Arizona in Flagstaff. 

For half a century, therizinosaurs have remained a poorly known and understood group of theropod dinosaurs with an extremely unusual combination of anatomical features. That's changed largely in the last decade with new discoveries in Cretaceous deposits in Mongolia, China and western North America.

Artist Victor Leshyk’s portrayal of the feathered dinosaur Nothronychus graffami beneath Pterandon-filled Late Cretaceous skies. Its sickle-claws are the hallmark of the family Therizinosauridae. The volcano, a ubiquitous cliché in dinosaur art, is a reminder of tectonic activity located further to the west that intermittently showered vast regions of the Southwest with datable volcanic ash.

Unlike earlier theropod dinosaurs that exhibit predatory morphological adaptations and carnivorous inclinations, therizinosaurs exhibit the characteristics inherent of herbivores. These are thought to include: tightly-packed, leaf-shaped cheek teeth (as opposed to elongate, typically Theropod meat-cutting teeth), an inset tooth row (suggesting fleshy cheeks necessary for plant mastication) in tandem with a rostral rhampotheca (keratinous, toothless bird-like beak to facilitate an herbivorous diet), a massive, highly derived pelvis (to accommodate a large gut synonymous with plant digestion), the development of large load-bearing hind limbs (to support a large abdomen), the loss of cursorial hind-limb adaptions (typical of predatory, swift carnivorous theropods) and an increased vertebral count (long neck speculated to increase browsing range similar to sauropods).

Therizinosaurs, especially more derived forms such as Nothronychus graffami, are thought to have been slow, large waddling, pot-bellied creatures rather than the quick, graceful gaited members of related Theropod predators. In spite of their likely herbivory, the group is thought to possess defensive capabilities with its powerful claws.

The specimen of Nothronychus graffami (holotype UMNH VP 16420) was missing the skull, a majority of the cervical vertebral series and a few elements of the distal extremities (grayed-out bones). Missing elements of the skeleton were borrowed from Erlikosaurus andrewsi from Mongolia. The therizinosaur in the middle is a hypothetical feathered reconstruction and below, drawn with traditional scales.
Modified from Zanno, 2009, Gillette, 2009 and Victor Leshyk and Plateau Magazine 2007.

The tail was short and unnecessary for its mode of non-predatory diminished speed and thought to have provided upright, tripodal support for plant consumption. Unlike most theropods, the pes was curiously tetradactyl (four toes, which is a throwback to the ancestral dinosaur condition) with blunt unguals (claws), while its manus was tridactyl (three fingers) with elongated, recurved claws - the distinctive anatomical feature that gives the clade its name. Therizinosaur means "sickle-claw reptile". These Therizinosauroid features became increasing expressed from basal forms through more derived forms.

Evolutionary osteological progression from basal Falcarius to Nothronychus.
Take note of the increase in size, postural change, tail shortening, longer neck, size of the gut, massivity of the hindlimbs and acquisition of forelimb claws.
Used with permission by Scott Hartman of skeletaldrawings.com

What's more, being members of Theropoda, the entire clade was thought to possess rudimentary proto-feathers - integumentary-derived structures such as hair, scales and nails. Please visit my daughter's post on feathers here for the evolution of this dinosaurian structure.

An imaginative interpretation of a proto-feathered adult therizinosaur accompanied by juveniles
Used with permission by artist Damir G. Martin. Visit him here.

Taking their singular, albeit bizarre morphology and fragmentary fossil record into account, it comes as no surprise that therizinosaurs have endured a convoluted taxonomic history within Dinosauria and have been variously assigned to nearly all of its major subclades.

At one time, the family Therizinosauridae was referred to as the now-outmoded, group Segnosauria (segnis means slow in Latin) based on their heavy bodies, short legs, and sloth-like claws with a comparable lifestyle. They have been variously regarded as gigantic turtles, aberrant theropods and sauropodomorphs. Based on their retroverted (opisthopubic) pelvis (posteroventrally-directed pubis bone which was aimed backwards as in ornithischian, bird-hipped dinosaurs), they were considered to be phylogenetic intermediates between herbivorous prosauropods and early ornithischians.

Pelvic girdle of a cast of Nothronychus graffami on display at the Carl Hayden Visitor Center at the Glen Canyon Dam. Although the therizinosaur is a Theropod, whose pubis is typically pointed forward, their pelvis is opisthopubic with the pubis bone retroverted, pointed backwards.

With increasing numbers of discoveries in Asia and North America from the Cretaceous, the diversity of therizinosaurs has begun to exhibit remarkable growth. Yet, a significant impediment to ascertaining phylogenetic relationships has been the paucity of both ancestral and transitional forms. Speculation is gradually being replaced with resolution.

Therizinosaurs are now considered to be unequivocal descendants not only of theropods but of the coelurosaurian clade and maniraptoran subclade with a sister relationship with Oviraptorosauria (see below). Thus, therizinosaurs are Saurischian, Theropodal, Tetanurian, Coelurosaurian, Maniraptoran dinosaurs and members of the family Therizinosauridea. 

Therizinosaurs also share the tree with more highly derived Avialae (birds) and possess avian-associated characters such as a pneumatic-skeleton (hollow light-weight bones that facilitate a high rate of respiration, and later, powered flight), a pygostyle (shortened tail with fused vertebrae), feathers (for thermo-regulation, sexual dimorphism and brooding) and an avian-trending brain (for enhanced sight, sound and mechano-reception).

One of many proposed Theropod phylogenies, the origin of facultative herbivory, that is omnivory, and the point of dietary diversification is posited at the base of Maniraptoriformes within the ellipse.
Adapted from Zanno, 2009 and from Araujo et al, 2013.

Interrelationships between specific therizinosaur taxa remains less clear. Until recently, the fossil record was restricted to Asia. With discoveries in North America such as Nothronychus mckinleyi (the first undisputed North American therizinosaurid from the Upper Cretaceous of New Mexico) and Falcarius utahensis (the third therizinosaur discovered in America, the most morphologically primitive therizinosaur yet discovered and a sister taxon to the clade of Therizinosauroidea from the Lower Cretaceous Cedar Mountain Formation of east-central Utah), we can begin to ask questions about origination, geographic and stratigraphic range, and even potential faunal mixing between immigrant and endemic clades. 

One of many parsimonious phylogenies proposed for therizinosaurs
50% Majority-rule consensus tree
Modified from Pu et al, 2013.

Did basalmost Falcarius utahensis originate in North America and certain populations expand into Asia? With recent finds in China such as Eshanosaurus deguchiianus from the Early Jurassic, the presence of derived coelurosaurian lineages including therizinosaurians is being pushed back earlier. Did therizinosaurs differentiate from coelurosaurian ancestors before the breakup of Pangaea into Eurasia and North America and/or did they migrate across the tectonic Beringia Land Bridge that was established in the Early Cretaceous between northeast Asia and northwest North America? Was there more than one dispersal event? At various times, Asian endemic therizinosaurids show faunal similarities with North American forms. Did endemic forms mix with immigrant forms? Do North American therizinosaur taxa exhibit an Asian affinity or vice versa?   

This imaginative diorama depicts a mix of Cretaceous fauna on the Beringia Land Bridge. A therizinosaur (in the box) has been tentatively identified on trackways in the Yukon.
Illustration by Karen Carr and the Perot Museum of Science and Nature. Used with permission.

Based on therizinosaur's osteological anatomy and soft tissue reconstructions, taking into account the habitats in which they likely thrived, and using animal analogues such as the sloth, certain dietary assumptions have been made about therizinosaurs and the Coelurosaurian clade in which they belong - once thought to have been obligate carnivores. In a little over a decade, doubt has been shed on that notion, raising the possibility or even likelihood that "dietary diversification was more commonplace among 'predatory' dinosaurs than previously appreciated" (Zanno, 2009).

In fact, therizinosaurs are the most widely regarded candidate for herbivory among theropods. Dietary plasticity and facultative (capable of rather than restricted to) herbivory (omnivory) is thought to have afforded the group the potential to invade and exploit ecospaces early in evolution for survival.

With a similar body shape and large claws on their front feet, Nothronychus graffami is shown as a bipedal browser analogous to the Giant Ground Sloth of the Ice Age.
Artist Victor Leshyk

Wayne Ranney and I met Merle bright and early at the Visitor Center in Big Water for a tour of the Moon. Crossing the dry streambed of Wahweap Creek, we travelled on a planar surface of Tropic mudstone, occasionally bouncing around on the hummocky, eroded terrain. The easiest places to build roads on the Colorado Plateau, although the most difficult places to maintain them, are on the Mancos-Tropic Shale. The soft rock weathers readily, forming broad, flat expanses and easy routes to get from here to there.

Our first destination was the very spot of Merle's once-in-a-lifetime discovery. After a surprisingly short drive from the Visitor Center, we left our vehicle and began to ascend a large, loose mound of gray mudstone and claystone to a noticeably beveled off area. 

Standing at the site of the former dig, let's let Merle tell the story. Although he's recounted the details many times in well over a decade since the find, it's clear that his enthusiasm hasn't diminished one iota. I'm filming, while Wayne is interjecting commentary. 


The excavation consumed the better part of two years and included the removal of tons of overburden. Some of the skeletal elements were compressed by compaction, while the skeleton was slightly disarticulated due to settling after coming to rest in the soft marine sediments of the Tropic. The hillocks and empty washes in the Tropic Shale were created in more recent times as the flat-lying muddy sea bottom succumbed to the forces of erosion.

Merle and Wayne debate the mysterious circumstances of the terrestrial therizinosaur's burial at sea.

One important question that has plagued paleontologists is how the dinosaur came to be buried in marine mud 60 miles out to sea with the nearest shore confirmed geologically near present-day Cedar City? To date, no definitive answer exists, although theories include "bloat and float" - having died on or near land and washed out to sea buoyed by decomposing body-gases followed by burial- and "lost at sea" - a less plausible scenario of having been caught by a flood or storm, floated out to sea alive, perhaps attacked by predators, and eventually buried nearly fully articulated. The skeleton of N. graffami was located in a supine position, belly-up, implying that it settled to its final resting place in the Tropic mud while buoyed by gases in a typical death pose.

I've seen the same "bloat and float" taphonymous (mode of fossilization) entity in New Jersey where terrestrial duck-billed dinosaur remains (the state fossil) have been found buried in glauconitic sands of the flooded Late Cretaceous continental shelf on Jersey farmland.

As the Tropic continues to erode, a few small osseous remnants of N. graffami have weathered to the surface since its excavation over a decade ago. It's that fact, among many others, that keeps Merle coming back to The Moon. There's always something new to be found on the ever-changing landscape. Come back the next day, and a new discovery will be awaiting you. What appears to be a static landscape is entirely the opposite!

Leaving the therizinosaur excavation site, we headed southeast on the flats of Wahweap Wash to further explore the Tropic's marine bounty.
Is their any doubt that the Tropic is a marine deposit? This horizon is literally covered with disarticulated bivalves, typically inoceramids (clams), pycnodontids (oysters) and  a few ammonites. I've seen similar marine exposures in lag deposits of the Late Cretaceous marl of the New Jersey coastal plain but not as richly concentrated.
Because of the fast rate of evolution in inoceramids and ammonites, they have become an important biostratigraphic tool for dating and identifying depositional boundaries in the Late Cretaceous of the Seaway - along with datable bentonite ash beds intermittently generated by volcanics to the west and southwest of the subsiding foreland. As a result, the Tropic Shale has been well constrained in the Kaiparowits Basin as upper Cenomanian-lower Turonian with Vasconceras diartianum-Prionocyclus hyatti ammonite biozones. 
Turritella is an extremely common Cretaceous gastropod (snail) fossil in North America, whose descendants are still extant today. The shells are tightly-coiled and spiraled in the shape of elongated cones. In this region, the Tropic's mudstone-siltstone is highly-calcareous and extremely well-lithified. In the Kaiparowits Basin, the lower two-thirds is bluish-gray due to its high carbonate content; whereas, the upper third is darker and noncalcareous. Wayne suggested that it may have been diagenetically-altered.
Notice that fine mudstone has entered and lithified within the Turritella's conical shell. When the shell eventually erodes away, it will leave a perfectly shaped internal cast called a steinkern (German meaning "stone" and "grain or kernal"). I found the identical gastropod in the Mancos Shale of the Seaway - the Tropic equivalent throughout the Southwest - near Ship Rock in northwestern New Mexico about 150 miles to the east.
As sea level rose during the Seaways first transgression onto land, the Dakota Formation was deposited. With the sea's westward advance, deeper Tropic muds covered the Dakota, onlapping and interfingering with it. Walking the contact, we were able to view the bedding planes within the Tropic and the magnitude of layered invertebrate remains.
The region is situated along the northern terminus of the Echo Cliff monocline, seen in the inclination of the landscape. Compressionally-generated monoclines formed across the Colorado Plateau with the ongoing subduction of the Farallon plate at a shallower angle during the Laramide orogeny.  
We're walking on a Late Cretaceous marine and brackish-water oyster bed, where shells accumulated and became disarticulated, smashed by the high energy wave system nearshore. Some areas are depauperate, while others are so rich in bivalves that they formed a shell-pavement. 
For every shark tooth I found, Merle's trained eye found ten - and in half the time. It was obvious that Merle possesses an uncanny ability of finding fossils. I asked him how he goes about it. He answered, "You need to have a second sense when you walk. I simply go where it feels right. That will lead you to bones and teeth."  There's a well-camouflaged Ptychodus tooth below concealed amongst mudstone rubble. 

Of the many shark species that plied the Seaway, Ptychodus in the "Greenhorn Sea" was widespread. Ptychodus was a hybodontiform ("hump-backed" tooth) shark that lived from the Cretaceous to the Paleogene. It grew to 32 feet and was a benthic (bottom-loving) molluscivore (bivalve-loving) predator. The teeth are square or quadrilateral in shape, with broad, low crowns that overhang a blocky, short root. 
Ptychodus teeth were arranged in straight, closely-spaced, parallel teeth rows that formed a bivalve-crushing pavement type of dentition.
In addition to invertebrates, the Tropic Shale also contains an abundant and diverse marine vertebrate fauna including at least five different short-necked plesiosaur genera, two genera of turtles, a normal chondrichthyan-osteichthyan assemblage - and of course a therizinosaur dinosaur, albeit terrestrial. Owing to the poor preservation of the cartilaginous skeletal structures, chondrichthyans are represented largely by teeth and dermal ossicles. Here are some of the interesting remnants that we came across.
This region of The Moon is on Bureau of Land Management land. The official website states  that visitors to BLM lands "are welcome to collect reasonable amounts of common invertebrate, such as ammonites and trilobites, and common plant fossils, such as leaf impressions and cones, without a BLM permit." Casual, hobby collecting is allowed "for non-commercial personal use, either by surface collection or the use of non-powered hand tools resulting in only negligible disturbance to the Earth's surface and other resources.”
By noon, Merle, Wayne and I had spent considerable time baking in the sun with our heads trained downward, walking the Tropic and scouring the seabed for fossils. Notice the vehicle for scale. 
A visit to the Tropic wouldn't be complete within mention of the indigenous vegetation. The Moon is sparsely vegetated, but Opuntia cacti add incredible color to the landscape, especially in Spring. Referred to as Prickly Pear, the brilliant crimson of this cactus is almost painful to the eyes in the bright sun. In the Southwest, there are many varieties all of which are native to the Americas. Many possess alkaloids with biological and pharmacological activities (for diabetes and hypertension). Most are edible, and some are used to make an alcoholic drink, while others have psychoactive properties.
This hardy shrub of Prince's Plume, in the mustard family, is highly recognizable by the bright yellow flowers clustered along the stem. It's native to the western United States and prefers alkaline and gypsum-rich soils, typically found in deserts. The plant is toxic since they concentrate selenium from the soil, necessary for cellular function. Coincidentally, selene means "moon" in Greek.

Desert Globemallow is also native to the American Southwest and grows well in alkaline, sandy soil and clay. The plant was used by Native Americans as a food source and for medicinal purposes. 
Back at the Visitor Center in Big Water, Wayne and I got the grand tour of the facility. Merle is an extremely personable and friendly guy, who is very affable and chock full of stories. In all, it was a fantastic and memorable day walking The Moon of Big Water with Merle's paleontological prowess and Wayne's geological knowledge.   
• A New North American Therizonosaurid and the Role of Herbivory in Predatory Dinosaur Evolution by Lindsay E. Zanno  et al, Proceedings of the Royal Society, 2009.
• Ancient Landscapes of the Colorado Plateau by Ron Blakey and Wayne Ranney, 2008.
An Unusual Basal Therizinosaur Dinosaur with an Ornithischian Dental Arrangement from Northeastern China by Pu et al, 2013. 
• A Taxonomic and Phylogenetic Re-evaluation of Therizinosauria (Dinosauria:
Maniraptora) by Lindsay Zanno, Journal of Systematic Paleontology, 2010.
• At the top of the Grand Staircase by Alan L. Titus and Mark A. Leowen, 2013.
Correlation of Basinal Carbonate Cycles to Nearshore Parasequences in the Late Cretaceous Greenhorn Seaway by William P. Elder et al, 1994.
Discovery and Excavation of a Therizinosaurid Dinosaur from the Upper Cretaceous Tropic Shale (Early Turonian), Kane County, Utah by David D. Gillette et al, 2002.
• First Definitive Therizinosaurid From North America by James I. Kirkland and Douglas G. Wolfe, 2001.
• Fossils from the Tropic Shale by Merle H. Grafam, 2000. Personal copy from the author.
• Geological Evolution of the Colorado Plateau of Eastern Utah and Western Colorado by Robert Fillmore, 2011.
• Geology of the American Southwest by W. Scott Baldridge, 2004.
• Geology of Utah's Parks and Monuments, Utah Geological Association by Douglas A. Sprinkel et al, 2003.
Herbivorous Ecomorphology and Specialization Patterns in Theropod Dinosaur Evolution by Lindsay E. Zanno and Peter J. Makovicky, 2011.
On the Earliest Record of Cretaceous Tyrannosaurids in Western North America: Implications for an Early Cretaceous Laurasian Interchange Event by Lindsay E. Zanno and Peter J. Makovicky, 2010.
• The Geology of the Grand Staircase in Southern Utah by the Geological Society of America, 2002.
• The Pectoral Girdle and Forelimb of the Primitive Therizinosaiuriod Falcarius Utahensis by Lindsay A. Zanno, 2006.
• Therizinosaur -  Mystery of the Sickle-Claw Dinosaur by David D. Gillette, Arizona Geology, Published by the Arizona Geological Survey, 2007.
• Therizinosaur – Mystery of the Sickle-Claw Dinosaur by David D. Gillette, Ph.D., Plateau, Museum of Northern Arizona, 2007.
• Vertebrate Paleontology by Michael J. Benton, 2005.