Crocodylian Snout Shape, Mammalian Carnivora, and Hominins

Crocodylians have a long and complex evolutionary history. More importantly, we have a large number of crocodylian fossils. I bring this up because of a number of interesting papers I have read recently that all have a common theme. In this post I will take a look at a paper by Christopher Brochu published in 2001 in the American Zoologist (Crocodylian Snouts in Space and Time: Phylogenetic Approaches Toward Adaptive Radiation, Amer. Zool., 41:564-585 [2001]).

Today there are 23 species spread out across the globe, and where they co-occur they appear to sort themselves ecologically and when they do, there are morphological differences. One of the areas where this is particularly noticeable is in the snout.

FIG. 1. Representative examples of the snout shape categories used in this paper. 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.

Generally speaking crocodylian snouts can be sorted into five basic shapes, which are illustrated in the picture above. 1a is a generalized crocodylian – modern generalized forms eat pretty much whatever they can sink their teeth into. 1b is a slender-snouted form generally associated with a piscivorous diet. 1c is a blunt snouted form that doesn’t seem to have any dietary or ecological associations (although they do tend to occur in forest bound rivers). 1d is a duck-faced form that has no modern analogue. 1e is a ziphodont form that may be a terrestrial carnivore, but also doesn’t have any modern representatives so it is hard to say.
There are a couple of questions that Brochu looks at in this paper. The first has to do with the geographic spread of snout shapes, and the second has to do with whether or not these different snout shapes can be united in clades. Brochu sums it up this way:

Are the crocodiles found in a single unit or region close relatives, with similar skull shapes arising independently in different places? Or did clades of crocodiles sharing a similar skull shape disperse widely, such that individual crocodile species are distantly related to geographic neighbors but closely related to species living elsewhere? In other words, have crocodylian lineages formed geographically-restricted adaptive radiations in the strictest sense of that term as first coined by Osborn (1902)?

And says:

As the Tertiary progresses, we seem to see the origination of endemic crocodylian radiations that are craniologically diverse, whereas clades earlier in the Tertiary are more morphologically uniform and geographically widespread.

The differences in these two patterns are explained by climate change and plate tectonics. Each pattern is looked at through the interpretive lens of the concept of adaptive radiation – something that I will focus on more in the next two posts in this series.