Wednesday Wonders: Erpetosuchus

13 08 2009

Good tidings and well-wishes!

My fellow Triassic aficionados have doubtlessly realized that their beloved period has been unjustly neglected in recent months by my humble blog in favor of perissodactyls and eurypterids among other things. To remedy this situation, I’ve decided to post a bit of coverage on one of the most interesting and controversial archosaurs of the age, Erpetosuchus granti.

Erpetosuchus (partial) skeletal reconstruction.

E. granti skeletal reconstruction.

The type specimen of this critter hails from the Elgin region of modern Scotland, home of Ornithosuchus, Stagonolepis, and the ever-problematic Scleromochlus among many others. Specimens have been discovered in the local sandstone since the mid-1830’s and have received attention from several of the world’s greatest early paleontologists, geologists, and comparative anatomists, including Louis Agassiz, Charles Lyell, Gideon Mantell, Sir Richard Owen, and “Darwin’s bulldog” himself, Thomas Huxley.

Erpetosuchus skull

Erpetosuchus skull reconstruction based on the Connecticut specimen (see below).

In his excellent review of Triassic vertebrate assemblages from around the world entitled “Dawn Of The Dinosaurs: Life In The Triassic”, Nicholas Fraser writes:

“Erpetosuchus is another poorly known small archosaur that is difficult to pigeonhole taxonomically. It was a small animal (skull under 9 centimeters long) with gracile front limbs (the posterior half of the animal is unknown). Superficially, the skull bears a resemblance to sphenosuchian crocodiles, with the quadrate angled forward and a deeply emarginated otic region. However, a closer examination of the sutures (although poorly defined in the sandstone molds) reveals that the quadrate does not hook back under thesquamosal, and it is unlikely that there was a quadrate-prootic contact (a character widey distributed among crocodylians). Neither is the quadratojugal positioned anterior rather than lateralto the quadrate. Furthermore, the radiale and ulnare are not elongated, and the coracoid lacks and attenuated [(“gradually thinning”)] posterior process.”

A rather incomplete specimen of Erpetosuchus (or possibly a closely-related animal) was found in the Newark Supergroup near Cheshire, Connecticut in 1995. In their description of the specimen, Paul Olsen, Hans-Dieter Sues, and Mark Norell (of Columbia University, the University of Toronto, and the AMNH respectively) note that

“There are several minor differences between the two skulls [(theirs and that of the Scottish holotype)], such as the relatively smaller antorbital fenestra, the more gently rounded ventral margin of the orbit, and the size of the maxilliary teeth in AMNH 29300 [(their specimen)]. They probably reflect individual variation and preservational features rather than taxonomically significant characters, and we identify AMNH 29300 as Erpetosuchus sp.”

Erpetosuchus cranial reconstruction.

Erpetosuchus cranial reconstruction.

On the subject of the animal’s maxilliary teeth, many paleontologists consider Erpetosuchus‘ arrangement of teeth to be its most bizarre characteristic. Fraser concurs with this notion, saying

“The most unusual feature of Erpetosuchus is the distribution of the marginal teeth. In the upper jaw, they are restricted to the premaxilla and the very anterior portion of the maxilla. In the lower jaw, there are at least two or three acutely conical teeth situated more posteriorly. [The Newark Supergroup specimen] was first recognized on the basis of this peculiar dentition. Just how this dentition functioned is unclear, but it seems plausible that the acutely conical teeth in the front of the jaw captured large insects such as cockroaches, piercing the exoskeleton and preventing them from escaping. Thus immobilized, the tough exoskeleton could be pierced by the teeth in the lower jaw and crushed against the  broad edentulous [(toothless)] region of the maxillia and palate.”

A galloping Erpetosuchus reconstruction.

A "galloping" Erpetosuchus reconstruction.

(For an incredibly detailed description of Erpetosuchus‘ known anatomy, do go here)

As I’ve said earlier in this post, the taxonomy of this creature is a matter of controversy among Triassic paleontologists. Olsen, Sues, and Norell have offered their own interpretation in their aforementioned paper:

“The phylogenetic relationships of Erpetosuchus have remained unresolved since the original description of E. granti… Most authors… referred Erpetosuchus to the Thecodontia. Since the pioneering phylogenetic analysis by Gauthier (1984), most authors have considered “Thecodontia” a paraphyleticassemblage of only distantly related basal archosaurian taxa…Walker…considered Erpetosuchus ‘a pseudosuchian at best only distantly related to crocodiles’ and interpreted its skull as displaying ‘a remarkable example of convergence towards the crocodilian condition in the attitude of the quadrate and the formation of an otic notch…

In our analysis, Erpetosuchusis the proximate sister taxon of Crocodylomorpha…Unambiguous synapomorphies linking Erpetosuchus and Crocodylomorpha are medial contact of the maxillae to form a secondary bony palate, absence of a postfrontal [(a bone found behind the frontal in many vertebrates)], and parietals fused without trace of an interparietal structure.”

Hopefully, some of the mysteries surrounding this little-known animal will be solved by the discovery of future Erpetosuchus specimens on either side of the Atlantic.

May the fossil record continue to enchant us all!




2 responses

14 09 2009
Tim Morris

My favorite paleo-reverie is of the Elgin Formation. Oases where Lagosuchids(?) Erpetosucus, and tiny Scleromochlus come to drink. Theoretically they could find food in the surrounding desert, as birds and lizards do today. But the oases would probably be an easier chance especially during hard times. Of course, predation would occur between these little critters, with poor scleromochlus getting the raw end of it, except for it possibly being very fast, or refuging in shrubbery or rocks. Of course, the larger Staganolepis, Scaphonyx, and Ornithosuchus would best be avoided as amniotes will willingly snap up small-fry. But their dung and foraging would attract small prey, like insects and smaller tetrapods. Of course, any water might have amphibian larvae and water insects. It must have been a very tranquil place, save for the heat.

14 09 2009

What imagery, sir!

It must have been quite a spectacular time and place indeed.

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