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 well 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 (Native American sounding, it was actually the winner of a contest held by mayor Alex Joseph in 1983-84 for a name to replace Glen Canyon City), 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, Merle gave 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.   

Yours truly with Merle and his shirt

• 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.

1 comment:

  1. Great post. I've read this several times now because there's always so much information to absorb in your writings. It always gets me running off and looking things up. I spent about an hour reading about therizinosaurs today.

    That fossiliferous shale and limestone is amazing. Interesting that the Tropic shale is 70+ Ma and yet is so easily weathered and frangible. Also, it makes me wonder why sound wave imaging is not used to locate dinosaur or other fossils in those shale formations.

    Great photos too. I never seem to tire of looking at our incredible western landscape.