Thursday, May 31, 2012

The Eurypterid "Eurypterus remipes" is the Official Fossil of the State of New York: Part I - Evolution, Phylogeny, Morphology and Tectonics


Based upon the array of appendages that emanate from its peculiar catfish-like carapace, Dr. S.L. Mitchill in 1818 mistakenly thought he had discovered a fossil fish of the genus Silurus. Correctly recognizing the arthropod nature of the fossil, zoologist James DeKay in 1825 assigned its taxonomic classification as Genus Eurypterus and its species designation as remipes, although he thought it was a crustacean.

Eurypterus remipes was eventually designated the official fossil of the State of New York in 1984, an appropriate choice, attributable to its abundance (more than 10,000 specimens have been recovered), and since the majority of productive eurypterid-bearing regions of the world are found within the state’s borders.

This fossil of Eurypterus remipes, over 400 million years old, is entombed within a dolostone slab
of the Phelps Waterlime. It is presumed (notice the telltale bend in the tail or telson, the body out of axial alignment and the damaged carapace) to be a molted exoskeleton (or exuvia) rather than a preserved carcass, as most are. Its segmentation and body divisions are well defined from its trapezoidal carapace to its pointed telson, as are its compound eyes and swimming paddles. Missing from view are the remaining appendages, six pairs in all.
(Photographed at the R.H. Langheinrich Museum of Paleontology)

Looking like a mix between a scorpion and a lobster on steroids, the extinct eurypterid (you-RIP-ter-id)  or “sea scorpion” was the largest known arthropod, an immense and diverse menagerie of both aquatic and terrestrial invertebrate animals that also includes insects, lobsters, crabs, centipedes, scorpions, spiders and mites. Eurypterids were not true scorpions, although they not only share a profound resemblance and evolutionary relationship to scorpions (both are members of Phylum Arthropoda and Subphylum Chelicerata), they also share aquatic paleoecologies having co-existed together. 



SEA SCORPIONS OF THE SILURIAN SEA
Eurypterids were named from the Greek words for “wide wing” or “broad paddle” referring to the pair of large swimming appendages located on either side of their head structures. The species desgination of the state fossil "remipes" is from Latin meaning "oar" and "foot." They are thought to have lived an aquatic existence in warm, shallow, marginal marine environments in the Middle Paleozoic, and mainly lacustrine, fluvial, and estuarine environments later in the Late Paleozoic. Based on the evaporitic content of the waterlimes in which they are more commonly found (in eastern North America), it is also thought that they exhibited a certain salinity tolerance.

Their apparent dual respiratory system (four pairs of book gills housed within branchial chambers) have led some paleontologists to suggest that eurypterids made forays onto land, an evolutionary transition that many arthropods ultimately made. 

Although eurypterids are extinct, they are morphologically well known owing to the large numbers of exquisitely preserved specimens. However, many aspects of their biology and ecology remain somewhat speculative, clouded by a preservational and collection bias, as happens when making assumptions on ancient fossilized remains.

Eurypterid genera of New York State (clockwise from upper left): 
Dolichopterus, Stylonurus, Eurypterus, Hughmilleria, Eusarcus and Pterygotus
 (By the famous paleontological illustrator Charles R. Knight from Clarke and Ruedemann, 1912)


EARLY BEGINNINGS TO END PERMIAN EXTINCTION
Earliest arthropods...
Arthropods, the phylum to which eurypterids belong, are thought to have evolved from annelids (segmented worms) 500 to 600 million years ago, although several members of the Neoproterozoic Ediacara biota may represent the earliest ancestors. Arthropods have remained the dominant component of animal species diversity beginning with their earliest recognition in the Early Cambrian. Facilitated by walking on their jointed appendages, a myriapod arthropod of the centipede-millipede family made the first pioneering steps onto land during the Silurian between 490 and 443 million years ago.

Earliest chelicerates...
The earliest chelicerate, an arthropod subphylum, was possibly Sanctacaris or “saintly crab” after the latinized version of Santa Claws. It was discovered in 1981 in a Burgess Shale-type locality near the famous Walcott Quarry high in the Canadian Rockies. Chelicerates are thought to have given rise to eurypterids, scorpions, horseshoe crabs, mites and spiders. Fossils such as Sanctacaris provided the basis for interpreting the origins of the living arthropods within the Cambrian radiation. Both members of Phylum Arthropoda and Subphylum Chelicerata, the Middle Cambrian Sanctacaris (left) and the Late Silurian eurypterid (right) are separated by about 100 million years of evolution.

Notice the similarities of the two chelicerates. In particular, Santacaris (left) and a eurypterid (right) both possess a head shield with its six pairs of appendages beginning with a distinctive chelicerae.
(Modified from Wikipedia and John Augier)

Sanctacaris exhibits the characteristics that clearly identify it as both an arthropod and a chelicerate particularly the distinctive grasping-appendages near its mouthpart. Called a chelicerate appendage, this structure both defines and names the subphylum as a group. Evolutionary variations of the appendage also morphed it into a specialized claw. More on that later.

Earliest eurypterids...
The earliest known eurypterids were from the Early Ordovician of Wales, and by the by the Late Ordovician, eurypterids were present in shallow marine settings of Laurentia. By the Silurian and Devonian they were mainly from sediments in restricted, near-shore marine environments such as the “Bertie waterlime” in New York State. We’ll visit such a locality later in the following post (Part II). Eurypterids went extinct during the Permian-Triassic extinction event 251 million years ago along with 96% of all known marine life. Represented by perhaps 241 species (possibly a somewhat inflated number), eurypterids are relatively rare fossils worldwide, partly due to their unmineralized exoskeletons.


THE DEFINING STRUCTURES OF ALL ARTHROPODS
Arthropods (Greek meaning “joint” and “foot”) are invertebrates (lacking a backbone) and are characterized by a body plan consisting of repeating segments (grouped into modular body divisions called tagma), bilateral (left-right) symmetry, jointed, tubular appendages (a tremendous competitive innovation), and a rigid-yet-flexible chitinous exoskeleton (external rather than internal and secreted by the cuticle).

This lobster (Subphylum Crustacea) is a typical arthropod.
(From bio1152.nicerweb.com)
Our extinct eurypterid (Subphylum Chelicerata) has all the characteristics of the arthropod that it is.
(A model from the Smithsonian National Museum of Natural History’s Hall of Fossils)



A BODY DIVIDED SHALL NOT FALL
The repeating body segments of arthropods, called metamers, are grouped into modules called tagma (Greek for a military unit) in which segments became fused and specialized in a plethora of ways. Metamerism had its origins not only within Phylum Arthropoda, but with Annelida (segmented worms) and Chordata (animals with a backbone, vertebrates like you and I). It conferred competitive advantages by allowing greater complexity in structure and function such as allowing the independent movement of separate body segments.

Arthropods are highly compartmentalized, divided tagmatically into a front, middle and rear section. For example, insects and crustaceans possess 3-tagma: a head, thorax and abdomen (some with cephalothorax and abdomen). Trilobites (extinct) also possessed 3-tagma: a cephalon, thorax and pygidium (tail). Eurypterids (such as Pterygotus from Lang's Quarry seen below) possessed 2-tagma: a prosoma (fused head and thorax) and opisthosoma (abdomen) with its abdomen in turn divided into mesosoma and metasoma.


This ventral view of the specimen Eurypterus remipes, identified as a member of the Eurypterina Superfamily by the swimming paddles (the sixth appendage), is again a molted exoskeleton. Dolichopterus of the same superfamily and also of the Bertie Waterlime looks similar in appearance but with larger eyes and a somewhat flattened penultimate appendage (the fifth appendage).
(Photographed at the R.A. Langheinrich Museum of Paleontology)

OUT ON A LIMB
The appendages of arthropods became specialized as well. At first, unspecialized segments bore a pair of appendages or “legs”, one on each side of the body and one pair per body segment. Each pair of appendages consisted of two branches, an outer branch (“gill” branch) for respiration in water and an inner branch (“leg” branch) for locomotion. The entire structure is called a biramal limb meaning “two branched.”
This conceptualized biramal limb consists of a gill- and a leg-branch.
(Modified from Wikipedia)
The spider-like diagram is a conceptualized single body segment of a multi-segmented marine arthropod. The biramal appendage attached to the base of the segment has a pair of gill- and leg-branches. As arthropods evolved and body segments became fused into modular tagma, gill- and leg-branches also became modified, specialized or even lost. Leg-branches became specialized for walking and with paddles for swimming. Pincers and stingers evolved for defense, antennae for sensing the environment, and graspers for acquiring and manipulating a meal. Marine arthropods lost their gill-branches (becoming uniramal) when they became terrestrial, losing the need for gills on land.


THE DEFINING STRUCTURES OF THE CHELICERATES AND OUR EURYPTERIDS
Eurypterids, being members of Phylum Arthropoda, are members of the Subphylum Chelicerata. Chelicerates have all the aforementioned arthropod characteristics in addition to a chelicera (Greek for “claw horn”), the only appendage anterior to the mouth. Their preoral appendage evolved into a specialized mouthpart from a leg-branch for cutting, chewing, piercing, sucking, siphoning, grasping and filtering, all aimed at directing a meal to the mouth. In the eurypterid, the chelicera functions in grasping and is the first of six pairs of appendages (designated by Roman numerals and with a pair for each segment) arranged in a radiating-array around the mouth, a highly efficient predatorial morphology.

Chelicerates are defined by their "jaws" called chelicerae, which also give the group its name (red). In spiders, the chelicerae function as fangs. In horseshoe crabs and eurypterids, the chelicerae serve as graspers of food. Chelicerates lack antennae, but insects have them, actually modified from the first appendage that forms the chelicerate's mouthparts. Being arthropods but not chelicerates, therefore, insects feel with the first pair of limbs on their heads, and chelicerates grasp and bite with them. The pedipalps (blue) are the second appendage. (paleontologyonline.com by Jason A. Dunlop)
The second pair of appendages are called pedipalps and are usually sensory in function (homologous with mandibles in crustaceans and insects). The last four pairs of appendages are for purposes of locomotion, also  evolved from leg-branches, but the sixth appendage became specialized as a paddle for swimming. Note that no antennae are present within the group such as developed in crustaceans and insects.

The chelicera of eurypterids is usually small and generally poorly preserved in fossils. But in pterygotids, it is greatly enlarged, having evolved into long, prehensile claws for the capture of prey.
(By Karen Carr)

EURYPTERID ANATOMY 101
Putting it all together, a typical eurypterid possessed a large, flat, semicircular head (prosoma) protected by a dorsal head-shield (carapace), highly reminiscent of horseshoe crabs (also a chelicerate but member of Order Xiphosura rather than Order Eurypterida). On the dorsal surface, the prosoma housed two pairs of eyes, a large compound pair (consisting of many small photoreceptors) and a smaller, light-sensitive, median pair (ocelli) with an uncertain functionality. On the ventral surface, six pairs of jointed appendages (walking and swimming legs), each arising from a body segment surrounded the mouth.

The abdomen (opisthoma) possessed respiratory book gills, reproductive organs (eurypterids were sexually dimorphic but with one sex which is thought to be differentiated by female horn organs for sperm storage and male claspers), and was armored with tergites dorsally and sternites ventrally. The eurypterid’s scorpion-like tail (telson) facilitated in swimming, but it is not inconceivable that it may have borne venomous glands similar to modern terrestrial scorpions. Got predator?  


A eurypterid (dorsal top and ventral bottom)
(From Wikipedia.com)

HOW DO EURYPTERIDS FIT INTO THE ARTHROPOD FAMILY TREE?
Based on morphological structures, ancestry and molecular data, many schemes of arthropod and chelicerate classification exist with a profusion of names (often synonymous and polyphyletic), the subject of ongoing debate, persistent re-analysis, countless revisions and lack of universal agreement. The following arthropod  phylogeny (chart below) was derived by nuclear protein-coding sequences (Napier, 2010).
 
For purposes of our simplistic discussion, let’s divide Phylum Arthropoda into five basic groups or subphyla (one of which is extinct), not being overly concerned with phylogenetic relationships. We have:
     1.) Myriapods (centipedes and millipedes)
     2.) Trilobites (extinct Paleozoic marine bottom-dwellers)
     3.) Chelicerates (spiders, mites, horseshoe crabs, scorpions and extinct eurypterids)
     4.) Crustaceans (lobsters, crabs, shrimp, barnacles, brine shrimp and others)
     5.) Hexapods (insects)

Much can be said, but let it suffice to say that eurypterids and arachnids (scorpions, spiders, mites and ticks) are considered sister taxons, sharing a common chelicerate ancestry with xiphosurans (horseshoe crabs). As we shall see (post Part II), eurypterids and scorpions also share a common ecology amongst specimens found at Lang’s Quarry.


EURYPTERID ORIGINATION AND GLOBAL DISPERSAL
The earliest known eurypterid is from Wales (Late Ordovician ~460 Ma), while those last preserved near the Permian extinction are from Russia (Late Permian ~250 Ma). During their total range of about 210 Ma, eurypterid fossils are found not only in New York, but in countries as diverse as China, Germany, Norway, Spain, Great Britain, Canada, Siberia, Brazil, Argentina, Bolivia, Libya, South Africa, Vietnam and the Baltic region. Where did eurypterids originate? How and when did they become dispersed throughout the globe?

I have indicated (red dots) the countries and approximate locations of eurypterid fossil discoveries on the globe during the Late Cretaceous (94 Ma). With the origination of Eurypterus eurypterids, first perhaps from Baltica, their subsequent invasion of eastern Laurentia, and their ultimate dispersal throughout the globe, one can predict the geographic locality of potential eurypterid future discoveries. For Eurypterus, furthering our knowledge will rely on focusing on the countries derived from the terranes of Laurentia, Avalonia, Baltica and the Rheno-Hercynian (Tetlie, 2007).
(Modified from scotese.com)
It has long been known that almost all eurypterids originated mostly from marginal marine settings of the Paleozoic continent of Laurentia, but also at a time when Laurentia, Avalonia, Baltica and the Rheno-Hercynian terrane were in close proximity tectonically (Tetlie, 2007). Following Pangaea’s dissociation, eurypterids were distributed globally throughout the various countries represented above while being “rafted” on the continents on which they lived. As a result, we find similar Late Silurian ecosystems (and eurypterids) in New York State separated by the Atlantic Ocean from similar ecosystems in Wales, Scotland and Norway.

The ancestors of Eurypterus remipes appear to have first originated in Baltica and later invaded Laurentia (recall that Baltica collided with Laurentia during the Acadian Orogeny). Their origination, invasion of Laurentia and global dispersal lies in a tectonic explanation. Differences in geographic origination exist amongst the various other clades of eurypterids.

This tectonic "big picture" of the Late Silurian (~420 Ma) shows the Taconic island-arc having collided with Laurentia and having formed a mountain chain and foreland basin. Baltica (to the north) and its associated fused terranes of Avalonia are amalgamating with Lauentia. Later, Avalonia will fuse with Laurentia to form Laurussia, and even later Gondwana will have its turn. The ancestors of E. remipes are thought to have invaded Laurentia from Baltica. (Modified from Ron Blakey, NAU Geology) 
In addition to tectonics as an explanation for their global distribution, could eurypterids have navigated the open seas and populated continents afar? Interestingly, a morphological explanation for their distribution may exist (Tetlie, 2007). Eurypterids can be broadly divided into walking and swimming forms based on the morphology of the posteriormost prosomal appendage (revisit my Eurypterid Anatomy 101 section above for a review). One swimming-clade apparently reached its destination on Gondwana before it amalgamated with Pangaea implying that mode of locomotion was a factor in the clade’s dispersal.


EURYPTERID HABITAT DEVELOPMENT IN NEW YORK STATE
Four orogenies, their foreland basins and clastic wedges
How might the aquatic habitat of eurypterids have evolved in New York State? Following the break-up of the supercontinent of Rodinia in the Neoproterozoic, its rifted continental siblings drifted throughout the globe, one of which was equatorially-situated Laurentia. Beginning in late Middle Ordovician time and extending through the Paleozoic, the closing of the Iapetus and Rheic Oceans signaled a nearly continuous succession of compressional tectonic regimes on Laurentia’s eastern margin with four convergence events: the Taconic, Salinic, Acadian and Alleghenian Orogenies. Each collision built mountains, contributed to the growth of Pangaea, and supplied vast quantities of sediment from the eroding mountain belts to the accommodation space of a succession of foreland basins (formed during lithospheric downflexure in response to orogenic crustal thickening).

In this Middle Ordovician paleomap (485 Ma), a Taconic island-arc complex
is on a tectonic collision-course with the passive eastern margin of Laurentia.
(Modified from Ron Blakey, NAU Geology)

The Taconic Orogeny, the first of four
The Taconic Orogeny (late Middle Ordovician), a complex of island-arc terranes that collided with northeast Laurentia, resulted in the formation of a foreland basin referred to as the Queenston Delta in New York. This, first-of-many, deltaic complex marked the initiation of the Appalachian Basin that eventually extended from southern Quebec in Canada to northern Alabama. Sediments debouched into epeiric seas of the forelands forming an expansive clastic wedge that prograded cratonward and resulted in a complex mosaic of open, marginal marine and terrestrial facies. This scenario was basically repeated with each foreland of each successive orogeny to varying extents.

The scenario of orogenic collision, mountain range uplift and foreland basin formation with a prograding clastic wedge repeated through the Paleozoic on the eastern margin of Laurentia. In this diagram, the Taconic Orogeny, its foreland and the Queenston Delta are developing.
Another cross-sectional perspective of the lithospheric downflexure that formed the expansive Queenston Delta's clastic wedge and the mountainous front of the Taconic Highlands. Note the underlying Cambrian and Lower Ordovician basement structures from Laurentia's former passive slope and shelf.
  (Source unknown)

An unconformity at the Ordovician-Silurian boundary and a major extinction event coincided with global glaciation at the end of the Ordovician. Deglaciation resulted in the seas reinvading the land in the Silurian with a marine transgression. In New York, the Late Silurian is represented by the Salina and Bertie Groups that record an arid landscape with basins of intermittent high salinity. Access to marine seas of normal saline content was unavailable. A notable exception to the more sparse fauna were eurypterids that seemed to have led a prolific existence in the briny Late Silurian waters of the “Salina”, and in particular, the “Bertie” seas.

The Bertie waterlimes of the “Hypersaline Seaway”
As barriers developed that prevented open circulation to the sea, salinity increased enabling the precipitation of anhydrite, gypsum and halite at considerable thickness. The land-locked seas of the Late Silurian that prevailed over east Laurentia were reflected in the deposits of the Salina and Bertie Groups. This environment led to the development of a “hypersaline seaway” comparable to the Red Sea.

In New York the Bertie Group of carbonate sediments accumulated during the very late phases of Silurian deposition upon the Salina Group, both at or near sea level. Shared amongst their strata are fine, mud-cracked dolostones and evaporites indicative of restricted tidal flats to shallow lagoonal settings. Both groups, but particularly the Bertie, possessed stratigraphic members called “waterlimes,” a fine-grained, dolomitized mud (the name relates to their properties as natural cements that harden under water) that contain abundant eurypterid-bearing horizons, and associated fauna and flora. 

By their distinctive and abundant preservation within the hypersaline waterlimes, eurypterids are interpreted as being indigenous to these waters. But did they merely aggregate there in large numbers for purposes of molting and mating or were their remains washed into the region after death from nearby brackish or freshwater estuaries? Such are the challenges of reconstructing an ecology and biology from ancient fossiliferous remains. 

Eurypterids were most diverse between the Middle Silurian and Early Devonian with their absolute peak of diversity in the latest Silurian. The Silurian closed with normal marine conditions returning to northeastern Laurentia, and along with it, a normal marine faunal and floral representation.


Silurian age rocks preserved at the surface throughout New York State
(From paleoportal.org)

IN CONCLUSION
In this post (Part I), we investigated the basic morphology, evolution and phylogeny of eurypterids, their origination and dispersal throughout the globe, and the tectonic formation of their habitat in the State of New York. In the following post (Part II), we'll hunt for fossil eurypterids at the Lang Quarry and visit the R.A. Langheinrich Museum of Paleontology.


SUGGESTED READING
Distribution and Dispersal History of Eurypterida (Chelicerata) by O. Erik Tetlie, 2007.
Testing the Mass-Moult-Mate Hypothesis of Eurypterid Paleoecology by Matthew B. Vrazo and Simon Braddy, 2011.
The Eurypterida of New York VI by Clarke and Ruedemann, 1912.
The Rise and Fall of the Taconic Mountains by Donald Fisher, 2006.
Geology of New York by Y.W. Isaachsen et al, 2000.
The Trilobites of New York by Thomas E. Whiteley, 2002.
Eurypterids Illustrated by Samuel J. Ciurca, Jr., 2008-2010.
Fieldtrip Guidebook, NYS Geological Association, Fiftieth Annual Meeting (1978), Fifty-fourth (1982), Sixty-second (1990), Sixty-sixth (1994) for publications by Samuel J. Ciurca, Jr.
Wonderful Life by Stephen Jay Gould, 1989.


SUGGESTED WEBSITES
eurypterid.net and eurypterids.net/EurypteridLinkIndex.html by Samuel J. Ciurca, Jr.
statefossil.org/news.htm by Allan and Iris Lang
cpgeosystems.com/nam.html by Ron Blakey


ACKNOWLEDGEMENTS
I wish to thank Allan and Iris Lang for their time and generosity in making their collection at the museum available for viewing and photography. I also want to thank Allan for his private tour of the quarry. The Lang facility is open to the public by appointment only. Contact information is available on their website.


Many thanks also to paleontologist Samuel J. Ciurca, Jr. of Rochester, New York for his personal communications. Sam has been studying, collecting, meticulously documenting, and publishing on eurypterids (and associated flora and fauna) for over 50 years. He has donated thousands of specimens from his personal collection to institutions such as the Yale Peabody Museum’s Division of Invertebrate Paleontology recognized as the Ciurca Collection, the Smithsonian Institution and the Buffalo Museum of Science.
 


1 comment:

  1. Intimidatingly thorough and well done for someone who's not a career paleontologist!
    The delivery of the subject, fully within context, should serve as an example to all those within scientific communication.

    ReplyDelete