|Erpetosuchus sp. from North America|
Little Erpetosuchus grandi was named and described in 1893 by E. T. Newton and redescribed in 2002 by Benton & Walker. Newton recognized that Erpetosuchus was related to crocodilians but considered it closer to phytosaurs or aetosaurs. While the original specimen was found in northeastern Scotland, a partial skull has turned up in Connecticut (Oslen et al. 2000), which may represent a different species. Newton’s fossil represented essentially the front third of the animal, including a complete skull. The skull and mandible, however, was a natural mold in the rock and Newton used gutta percha material (which is what golf balls are made of) to produce casts from these molds. The results are extraordinarily detailed, even capturing fine details of the outer surface of the skull bones.
|The skull of Erpetosuchus from Benton & Walker|
Erpetosuchus has a particularly large fossa surrounding the antorbital fenestrae. Both the maxilla and the jugal have distinct, large ventral portions that are flattened or at the very least diagonally-facing towards the outside edge of the skull. As a result of this, the palatine bones are narrow—packed in between the flattened ventral surfaces of the maxilla and anterior jugal. The ectopterygoids are quite deep, again due in part to the weirdly diagonally-directed ventral surface of the jugal. In lateral view, the orbits look a bit like an oval that’s in the process of tipping over, but in dorsal view they are more or essentially circles. If you squint, the back of the skull (in lateral view) looks a bit like that of my alligator skull in that the quadrate is strongly diagonally-directed, the squamosal overhangs it, and there’s a small gap between the quadrate and the quadratojugal. Unlike my alligator, however, the lower temporal fenestra of Erpetosuchus is enormous and deeply recessed.
|Skeletal restoration from Benton & Walker|
Erpetosuchus probably has seven cervical vertebrate—the transition between neck and torso is not clear. In any case, it had a very short neck. Its neck and back, however, were covered by two parallel rows of rectangular scutes. Two additional rows may have begun behind the pectoral girdle. Given the proportions of the pectoral elements, Benton & Walker suspect that Erpetosuchus was a bipedal cursor, and its dentition suggests an insectivorous diet. Their phylogenetic analysis (keep in mind this is 2002), Erpetosuchus was found to be a sister group of Crocodylomorpha among then-known croc-line archosaurs. The authors suggest that fragmentary African taxon Parringtonia may represent another erpetosuchid, but more material will be needed to make that determination. They also note that Germany’s Dyoplax was initially referred to the Erpetosuchidae but it has since been reassigned as a genuine basal crocodylomorph. Well, we’ll quickly be revisiting both of these genera.
|Looks like a mummy, right? More like a natural mold.|
|A "best guess" Dyoplax skull from Maisch et al.|
More interestingly, Maisch et al. consider the possibility that Dyoplax is actually a German specimen of Erpetosuchus. If they are the same, then Dyoplax has priority, as it was described almost thirty years before Erpetosuchus.
“Both taxa are from at least roughly coeval beds, and they are also strikingly similar in size. This is particularly important because it excludes the possibility that observed differences may be attributable to ontogeny.”
However, after considering that differences in armor preservation may be taphonomic rather than taxonomic, the authors note that many of the differences in the two animals’ skulls—especially regarding the size and shape of the antorbital fenestra—appear to be genuine. Although it remains an intriguing possibility, Maisch et al. conclude that without better specimens of Dyoplax, synonymizing the two is premature.
|Fragment of the skull table of Tarjadia|
First, let’s take a hard left to the Doswelliidae, a group of basal archosauriforms that also had distinctive armor. Desojo et al. (2011) described Archeopelta arborensis, a Brazilian taxon known from a braincase, a long string of dorsal vertebrae, a sacrum with attached pelvis (and right femur), a right humerus and several osteoderms. Those authors found that Archeopelta, Doswellia, and Arcurri & Marsicano’s Tarjadia share two features related to the structure of the osteoderms. Admitting that the holotype of Tarjadia hardly provides a robust set of characters, Desojo et al. removed it from their analysis and found eight non-osteoderm characters that united Doswellia and Archeopelta.
|Desojo et al. used a Doswellia skeletal in their Archeopelta description.|
In his massive 2016 phylogeny of basal archosaurs, Ezcurra found that the two really do differ in important ways, resurrecting Archeopelta, and placing them all, once again, in a monophyletic Doswelliidae.
|The maxilla of Parringtonia|
|Tarjadia revealed! Yellow bones remain unknown.|
It turns out that Huene’s Parringtonia is in the same camp as Tarjadia: Nesbitt et al. (2018) report that new specimens of Parringtonia have revealed virtually its entire skeleton and guess what? It’s very clearly an erpetosuchid.
|Parringtonia revealed! Look at how big its head is compared to the body.|
Offhand, one wonders if Pagosvenator is potentially a junior synonym of Archeopelta, as both are Brazilian and the latter is so poorly known. Desojo et al. (2011) write that Archeopelta is Landian or Carnian--and so is Pagosvenator. Lacerda et al. (2018) compare the osteoderms of Archeopelta and Pagosvenator but make no other osteological comparisons or even bring Archeopelta up, content to leave it as an unrelated doswellid. However, I think it's possible that the two are synonymous now that Ezcurra et al. (2017) have made a good case for Archeopelta being an erpetosuchid. As I always wind up saying in these essays, more material will be necessary for a more conclusive, uh, conclusion.
Now, what kinds of animals are these cute little crocs? While Benton & Walker believed (based on the proportions of the forelimbs) that Erpetosuchus could have run around on its hind limbs if it wanted to, Parringtonia and Tarjadia present much more traditionally-proportioned quadrupeds. They are also clearly impressively armored. In addition to the osteoderms running down the back, Tarjadia at least has osteoderms on its hind limbs and possibly its forelimbs. In Argentina, Tarjadia shared its habitat with rhynchosaurs, small mammals, and mid-to-large-sized carnivorous pseudosuchians. These were dog-sized cursors going after small game.
There are several features of the skull that I find interesting: the enormous fossa surrounding the anteorbital fenestrae; the enlarged, posteriorly-directed jugal (which gives the lateral temporal fenestra an odd shape); and the fact that the dentary has significantly more teeth than the maxilla. It's also strange that the group is so homogeneous--everyone is roughly the same size, with roughly the same skull shape and dentition, and differ mainly in terms of osteoderm compliment (assuming the differences between Tarjadia and Parringtonia are genuine and not the result of missing data).
This group did not last terribly long: Parringtonia is the earliest member (Ansian) and most of them were extinct by the beginning of the Norian (although North America’s Erpetosuchus appears to be from the late Norian). Prior to the five-minutes-ago publication of Lacerda et al. (2018), a large ghost lineage existed between Parringtonia and everyone else, who turn up in the late Carnian. However, Pagosvenator is from the late Landian/early Carnian, which shortens the temporal gap somewhat. At any rate, erpetosuchids were certainly extinct by the Rhaetian, and thus were spared from whatever traumatic event caused the Triassic/Jurassic extinction.
*Funnily enough, in his original description of Erpetosuchus, Newton also described Ornithosuchus woodwardi. However, he thinks that the latter is a very primitive dinosaur or, at least, an immediate ancestor of dinosaurs.