Wednesday, July 18, 2012

Paleo-Myth Number 8: Crocodilians Are An Ancient, Unchanged Family

Archosaur on Archosaur violence

Few archosaurs invoke the title "ruling reptiles" more so than crocodiles. Popular and iconic crocodiles never seem completely satisfied as mere aquatic predators. Never content playing second fiddle to anything in their respective environment- these macropredators challenge, and often win, the dominant mammals around them. And the extent to which crocodiles venture onto land in order to hunt or procure carrion is little appreciated. This reptilian insubordinance hints at a rich and varied ecological history for the group.

While many appreciate that the crocodile lineage is old- it most certainly was not ecologically conservative nor are the extant living species particularly old or "living fossils".  A commonly heard mantra on pop-nature programs is that such and such species of crocodile is hundreds of millions of years old or a "living dinosaur". But in reality modern crocodile species arose in the Late Miocene (Brochu, 2001) and are thus at most 5-7 million years ago. The Nile Crocodile for instance is only 2.5-3.5 million years old (Trutnau & Summerlad 2006). The reason for this persistent belief is both cultural and anatomical. From Brochu, 2001:

Crocodylian skulls have been phylogenetically plastic. But the morphospatial region within which they have varied is rather narrow; a limited number of anatomical solutions may exist for a given ecological problem, and because crocodylians tend to interact with their surroundings with their snouts, similar snout morphologies seem to have arisen multiple times in disparate lineages. This may be partially responsible for the widespread view that crocodylians (and crocodyliforms generally) are “living fossils” that have changed little since the Mesozoic. When the group as a whole (living and extinct) is viewed in a phylogenetic context, they no longer look so static.

Five basic Croc snouts in Tertiary. A. Leidyosuchus canadensis, a “generalized” alligatoroid from the Late Cretaceous of North America. B. Thoracosaurus macrorhynchus, a slender-snouted gavialoid from the Paleocene of Europe. C.Alligator mcgrewi, a blunt-snouted alligatorid from the Miocene of North America (drawing adapted from Schmidt, 1941). D. Mourasuchus, a duck-faced caiman from the Miocene and Pliocene of South America (drawing adapted from Price, 1964 and Langston, 1965). E. Pristichampsus vorax, a ziphodont crocodylian from the Eocene of North America (drawing adapted from Langston, 1975). Drawings not to scale; A through D in dorsal view, E in right lateral view
So the crux of the matter is that differing crocodile lineages, constrained by ecological design, come up with remarkably consistent solutions to fit their given morpho-eco space. In this manner crocodiles appear relatively little unchanged since the Mesozoic but in truth we have multiple lineages adapting to the unchanging constraints of living as an amphibious predator repeatedly throughout their geological history. 

One of the revelations in recent years has been the unexpected diversity of crocs in the Mesozoic.

L-R. Stomatosuchid, Sebecosuchian, Metriorynchid, Spenosuchian. from Tetrapod Zoology
Indeed the variety of crocodiles in this time period borders on the comical- and ultimately suggests that they were aggressive competitors with dinosaurs, marine reptiles, and mammals for an astonishing variety of niches.

Giving theropods a run for their money and perhaps grabbing the top terrestrial predator throne in parts of Cretaceous Gondwanaland were the Sebecosuchids.

Growing up to 4 meters long, these were not insignificant predators and if alive today would no doubt be considered large predators. How and to what extent they interacted with theropods has been of great interest and conjecture. They were most likely ambush rather than pursuit predators and possibly tried to give a devastating first blow to the prey to mortally wound it.

Sebecosuchian attacking an indeterminate ornithopod. Illustration by Author.

But there were also smaller more gracile guys like the Spenosuchians.

Illustration by Author. Spenosuchian.

Indeed this veritable racehorse of a croc so characterized early crocodilians as terrestrial runners that the four chambered heart of modern crocodiles is most parsimoniously explained as an anatomical anachronism leftover from when crocodiles were actually endothermic (warm blooded). As crocodiles expanded into freshwater niches as ambush predators a slower metabolism was evolved that more suited that lifestyle.

There were also burrowing herbivorous crocs, duck billed crocs, and armadillo like crocs- but you can go interwebbing for those if you choose to...

Well, this post is not meant to be exhaustive- I just wanted to dispel the myth of modern crocs as "evolutionary holdovers from the distant past". This topic has been blogged about extensively by Darren Naish, check out the links below.



Brochu, 2001. Crocodylian snouts in space and time: phylogenetic approaches to adaptive radiation
Integrative and Comparative Biology. 

Move Over Theropoda, Sebecosuchia Rules. Tetrapod Zoology.

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