Hypercursorial Killer Crocs


There are quite a few “croc-grade” archosaurs out there that are peculiar, virtually most branches up until we get to “modern crocs” (i.e., gavials, crocs “proper” and gators, including caimans) seem fairly “croc-like” but differ in some peculiarities. We’ll discuss one of these in a bit. You won’t confuse giant Deinosuchus with any other archosaur other than a croc-grade one, and if you did (like, calling it a dinosaur) then you should be forcibly re-educated. Most croc-grade archosaurs “seem” like crocs, and so we get the phytosaurs, so-called parasuchians because it was once thought they weren’t really crocs, but then later included in the croc-grade, before eventually (as in this month) being made into an archosaur sister taxon; the aetosaurs, armored like typical crocs but with small and strange heads; the notosuchians, a somewhat lumped group that includes the “leaf-toothed” comahuesuchids and the “theropod-headed” sebecosuchids, with special note to the small, probably burrowing “hedgehog-croc” Simosuchus and the leggier “dog-croc” Araripesuchus. This is especially “doggish” when you consider the dental reduction and extreme “canine” development (enlarged caniniform lateral dentition) in Pissarrachampsa sera (Montefeltro et al., 2011).

LPRP/URP 0019, holotype skull of Pissarrachampsa sera Montefeltro et al., 2011. Skull in left lateral and ventral views, modified after the authors.

And it’s that final note, “dog-croc,” that seems most out of keeping. This name is attributed because of the array of large caniniform teeth these crocs have (mostly peirosaurids, sebecosuchids and araripesuchids) along with relatively short snouts. This is because there is a group of crocs that are immensely more dog-like than those baurusuchid, and those are the “sphenosuchians.” Below is a typical example of the leggy, “dog-croc”:

The "true" dog croc, Terrestrisuchus gracilis.

Will the True “Dog-Croc” Please Stand Up?

Legginess is the key here, but it goes beyond just having elongated limbs. “Sphenosuchians” in general, basal crocodylomoprhans in specific, have extremely elongated limbs, and there are several adaptations that compare well with this. First, the femoral caput is turned inwardly and is supported by the iliac blade with a horizontal buttress. Such a buttress is very distinct from that in dinosauromorphans, in which the femur is supported vertically in a parasagittal stance by a lateral process of the ilium, the supracetabular crest; in pseudosuchian archosaurs, and in fact all non-ornithodiran archosaurs, the femur is supported by the entire vertical extent of the ilium, and the expansion of the ilium laterally is matched by one medially, so that as far as it extends to one side, it does so as well to the other.

Pelvic cross-sections at the middle of the ilium. A, psuedosuchian-line archosaurs; B, dinosauromorph-like archosaurs. ILI, ilium; ISC, ischium; SAC, sacrum. Grey arrows point to the suprafemoral butress of the ilium, black arrow in "B" to the medial pelvic foramen.

Not all “sphenosuchians” are like this. Sphenosuchus acutus, the type species, is much shorter-legged and bore a more robust skull, and very similar to the form of Araripesuchus gomesii, the t ype species named by Llewelyn Price in 1959. Hesperosuchus agilis, typically lumped in with these guys, appears to be a lot more basal, and in fact the entire array of “sphenosuchians” find themselves in a recent phylogeny (Nesbitt et al., 2011) to be part of an early step-wise radiation of basal crocodylomorphans.

Crocodylomorpha, based on a strict concensus of 360 MPTs (77tax:412cha), having a length of 1285 steps. (cropped from Nesbitt et al., 2011).

Hesperosuchus agilis is also, like Terrestrisuchus gracilis, very “leggy.” As shown above, this gives these crocs a very strange appearance, and they were also very close to the modern crocs in phylogeny. Much of those crocs would grow, but take to the water, and as a consequence develop a sprawling posture as such a limb set up allowed them easier maneuverability. No longer did they need the elegant, antelope- or dog-like running limbs of their forebears.

But aside from this, these crocs were fairly uniform, with extremely slender limbs, a large posteriorly directed calcaneal heel (which is normally positioned about 45 degrees to the side), and reduced inner- and outermost metatarsals and metacarpals, and even digit loss associated with these shrunken bones. In some cases, the metacarpals and metatarsals are bundled together, forming “cannon” bones, as in mammals. The distal limb segments (pes and manus) are elongated, although the proximal limb segments (femora and humeri) are relatively elongated as well; the former is indicative of increased mechanical advantage in the distal limb, which is useful. These animals have entered cursorial territory.

Sprawling vs. Upright limb designs in two mammals (left, brown rat and domestic dog) and two pseudosuchians ("gator" and "running croc").

These crocs are, for want of a better term, hypercursorial “dog crocs.” More dog-like than the baurusuchids, they developed less dog-like skulls and more dog-like limbs and likely predatory behavior. Smaller, generally, they likely chased smaller prey (much like foxes do) while baurusuchids almost certainly favored and could handle much larger prey.

More importantly, compared to their larger and more robust (and later lived) cousins the baurusuchids, the “sphenosuchians” developed a series of adaptations which is unmatched in any other four-legged group of terrestrial vertebrates to the exception of mammals: like some groups of dinosaurs, they developed elongated cranial and caudal wings of the ilium, increasing the position of the origins of femoral rotator muscles, while also developing a combined forelimb and hindlimb gait and thus various features of the shoulder girdle emphasizing a pectoral/sternal system, elongated and slender scapula, ventrally facing humeral glenoid on the scapulocoracoid, while also retaining a joint between the scapula and coracoid, and canalization and narrowing of the distal limb segments. Among other archosaurs, these features are unknown in quadrupeds, while they are present in mammals; birds and many ornithopodan and theropodan dinosaurs have similar hindlimb features (but lack the elongated calcaneal tuber). Most interestingly, the calcaneal tuber is aligned medially, so that is was practically parallel to the line of action of the ankle joint, while in sprawl-limbed crocs and virtually all other crocs, the tuber is more laterally positioned and aids in pronation of the pes during the step cycle. Loss of the pronation element of the step minimized energy loss during the cycle, and increases parasagittal thrust, more useful for a pursuit predator than a stalker or ambusher. These features place this grade of crocs into a unique position ecologically, and much of this has been little explored.

Fox-Crocs?

While I think it fitting to call Araripesuchus and it’s relatives “dog-crocs,” as Paul Sereno has done in extension to recent discoveries (Sereno & Larsson, 2009), it may be more fitting instead to call them “bear-dog” crocs, as they were stout, short of limb, and likely more suited to grappling and fighting prey, quintessential ambushers, rather than pursuit predators, which evokes the sense of wolves and coyotes rather than the large borhyenoid “bear-dogs.” Thus, if I can’t use “dog-crocs” for “sphenosuchians, I’d like to try using “fox-crocs” for them, instead.

Dogs vs Foxes. The "Dog Croc" Araripesuchus wegeneri, African representative of the otherwise South American taxon, is on the left, from Todd Marshall and via the Project Exploration and National Geographic, in conjunction with Sereno & Larsson (2009); I've added a watermark to reduce copyright theft. On right, muscular study of Terrestrisuchus gracilis, after Crush, 1964 (from me, but also with watermark). Compare the legginess.

Crush, P. J. 1984. A late Triassic sphenosuchid crocodilian from Wales. Palaeontology 27:131-157.
Montefeltro, F. C., Larsson, H. C. E. & Langer, M. C. 2011. A new baurusuchid (Crocodyliformes, Mesoeucrocodylia) from the Late Cretaceous of Brazil and the phylogeny of Baurusuchidae. PLoS ONE 6(7):e21916. doi:10.1371/journal.pone.0021916
Nesbitt, S. J. 2011. The early evolution of archosaurs: Relationships and the origin of major clades. Bulletin of the American Museum of Natural History 292:1-292.
Sereno, P. C. & Larsson, H. C. E. 2009. Cretaceous crocodyliforms from the Sahara. ZooKeys 28:1-143.

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2 Responses to Hypercursorial Killer Crocs

  1. Ranjit says:

    I have confess no matter how many times I see reconstructions of prehistoric crocs like “sphenosuchians” running about gracefully like mammals I can’t quite believe it.

    Sure, modern crocodylians may have reverted to a sprawled stance as they became obligate semi-aquatic species, but would they really have gone back to such a torpid metabolism too? Isn’t it parsimonious to assume that their ancestors were also pretty languid poikilotherms (maybe as energetic as say varanids today) and that they didn’t prance about like canids?

    • Ancestors, yes. Outlying taxa, and the post shows, are notosuchians and baurusuchids, and “rauisuchians” and such. These taxa shift between sprawlers like Simosuchus to relatively high-walking, upright-legged Postosuchus. The energetics of some of these taxa have hardly been questionable, but for some “rauisuchians,” it might be interesting to examine them in more detail, look at their histology.

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