How Doth the Hypercarnivorous Crocodile…

A Dakosaurus tries to snap up a pterosaur. Artwork by Dmitry Bogdanov, from Wikipedia.

Not long after her trip down the rabbit hole, the reluctant heroine of Charles Lutwidge Dodgson’s Alice’s Adventures in Wonderland is left shaken by the nonsensical strangeness of her surroundings. She tries reciting her school lessons to settle her nerves, but practicing does not provide her with any comfort. Her arithmetic doesn’t add up, geography is lost on her, and when she tries to recite Isaac Watt’s ode to the humble bee “Against Idleness and Mischief”, it comes out as a backwards tribute to a very different sort of creature:

[Alice] crossed her hands on her lap, as if she were saying lessons, and began to repeat [the poem]; but her voice sounded hoarse and strange, and the words did not come the same as they used to do: —

“How doth the little crocodile
Improve his shining tail,
And pour the waters of the Nile
On every golden scale!

How cheerfully he seems to grin,
How neatly spreads his claws,
And welcomes little fishes in
With gently smiling jaws!”

A sly ambush predator, the devious crocodile was the natural antithesis of Watt’s industrious bee. The crocodile did not have to work hard for its meals and did not show any sort of moral compunction; all it had to do was wrap itself in the shimmering light of the river and wait for the unwary.

Dodgson’s doggerel was inspired by the sort of clever crocodiles which have been snagging prey at the water’s edge for tens of millions of years, but today’s assortment of wait-and-strike predators are only a shadow of the diversity that once existed among the crocodylomorpha. Living crocodiles, alligators and gharials are only the still-living stem of this multi-branched limb which split off from similar reptiles during the Late Triassic over 220 million years ago (a time of major faunal change when the first dinosaurs were undergoing their own evolutionary radiation). During the eras that followed, the crocodylomorphs were adapted into a variety of forms – from tusked, armadillolike species to fearsome terrestrial predators which would have been stiff competition for predatory dinosaurs – including one peculiar group of marine predators.

As summarized in a new Journal of Vertebrate Paleontology paper by scientists Marco de Andrade, Mark Young, Julia Desojo and Stephen Brusatte, between 171 and 136 million years ago there lived a variety of ocean-going crocs called metriorhynchids. These creatures looked as if they had been designed by committee. Although possessing the basic crocodile body plan, their limbs were adapted into paddles and their tails were kinked downwards to support wide, shark-like caudal fins. These were not crocodiles which just so happened to go out to sea every now and then, but true pelagic predators well-suited to life in the open ocean, and clues to their hunting habits can be found in their teeth.

If you were to inspect the teeth of a living crocodylian – something best done from a reasonable distance – you would see that their dental equipment consists of a simple array of cones. This type of tooth pierces, grips and crushes, but does not cut or slice – when consuming large prey, crocodylians much thrash their victims about to tear off a piece small enough to swallow. Not so for at least two genera of the metriorhynchids. According to the new research by de Andrade and colleagues, both Geosaurus and Dakosaurus were hypercarnivorous crocs which relied on a more sophisticated set of dental cutlery.

Unlike their modern-day cousins and even many other metriorhynchid crocs, both Geosaurus and Dakosaurus had finely serrated teeth. This is technically known as “ziphodont” dentition, and has often been seen among predatory dinosaurs and prehistoric, terrestrial crocodylomorphs. This tooth type was much rarer among marine reptiles, however, and even though species of Dakosaurus had larger serrations than Geosaurus species, the presence of these cutting teeth in both genera hints that they probably had different dining habits than their close relatives.

Based upon the tooth type of the prehistoric marine crocs and previous studies of how ziphodont dentition relates to diet, the authors of the new study hypothesize that Geosaurus and Dakosaurus did not solely subsist on a diet of fish. Their serrated teeth, short snouts, deep jaws and large size (over 4 meters in length) are indicative of predators which delivered powerful bites to large prey, and biomechanical studies have suggested that these marine crocodiles would have been capable of the “death roll” which their extant relatives use to rip off large chunks of flesh from their victims. Geosaurus and Dakosaurus were active predators capable of chasing and subduing animals larger than they were, and they would have been quite distinct from the little crocodile of Alice’s poem.

Given that both Geosaurus and Dakosaurus probably relied upon large prey, it might be expected that they existed in different places and times to prevent overcompetition. As documented by the fossil record of two approximately 150-million-year-old fossil sites in Germany, however, these predators coexisted with each other. What allowed them to survive side-by-side likely came down to a matter of taste. As interpreted by de Andrade and his co-authors, the differences in tooth anatomy between the crocodiles may indicate that Geosaurus and Dakosaurus – while both hypercarnivores – had been driven to slightly different prey choices through competition in an ecological pattern termed niche partitioning. Whereas Dakosaurus had Tyrannosaurus-type teeth ideal for puncturing through the armor of ammonites and other crocodiles, Geosaurus had teeth similar to that of great white sharks which would have been best utilized on softer prey. If these predaceous crocs were both relatively specialized in the manner the scientists have proposed, they could have occupied the same marine habitats, though, given their fearsome enameled arsenal, it would not be surprising if they sometimes fell prey to each other, too.

References:

DE ANDRADE, M., YOUNG, M., DESOJO, J., & BRUSATTE, S. (2010). The evolution of extreme hypercarnivory in Metriorhynchidae (Mesoeucrocodylia: Thalattosuchia) based on evidence from microscopic denticle morphology Journal of Vertebrate Paleontology, 30 (5), 1451-1465 DOI: 10.1080/02724634.2010.501442

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