Published on April 15th, 2021 | by David Marshall Episode 124/125: Crocodylomorph Disparity
Crocodiles are often referred to as “living fossils”, but if we compare modern and ancient species, does that label hold up? What different kinds of morphologies (shapes) did past crocs have and how did they live? How quickly did this past diversity arise and why are we left with so few species today? What’s to stop them from diversifying again?
In this episode, we speak to Dr Tom Stubbs, University of Bristol, about his recent work analysing changes in crocodylomorph disparity through time. We look at some of the weird and wonderful crocs of the past and work through his methods for calculating their rates of evolutionary innovation.
You can read the open access publication here.
There’s a lot of confusing terminology to get through when looking at the different groupings of crocodylomorphs. Let’s start at the bottom with the nile crocodile Crocodylus niloticus. C. niloticus belongs to the sub family Crocodylinae which we call the crocodiles. Crocodylinae is part of the larger order, the Crocodilia which also contains alligators and gharials. Crocodilia is just one order of the crocodylomophs which includes several other order which are now extinct.
The broad-snouted caiman is a crocodylian, but not a crocodile. It is closer related to the alligators and so together form a family called the Alligatoridae. There are many differences between crocodiles and alligatorids, but an easy difference is the shape of the snout. In alligatoroids, they are wider, more U-shaped, whereas in crocodiles, they are more V-shaped. Caiman latirostris Credit: TimSagorski CC BY-SA 4.0.
The gharial belongs to another family of crocodilians called the Gavialidae and is famed for its long thin snout. This is best adapted for catching fish and would not be suited for taking down larger prey like other crocodilians. Across all crocodylomorphs, the shape of the snout is important as it can give us great insights into their ecology (how they lived).
When we look at the fossil record, we can see times when there were many more species of crocodylomorphs (i.e. they had greater diversity) as well as different anatomies (i.e. morphological disparity). Image: Diversity of body plans and jaw shapes in Crocodylomorpha. Credit: Tom Stubbs
Fossil skull and jaws of land-dwelling crocodylomorphs from the Cretaceous. Notosuchians had diverse diets, including insects and plants. Tom’s new research shows they were very fast evolving. Credit: Daniel Martins dos Santos.
Simosuchus, an extinct notosuchian crocodylomorph from the latest Cretaceous. It looked like a “scaly badger” with a short ‘pug-faced’ snout adapted for eating plants. Credit: D. Gordon E. Robertson CC BY-SA 3.0
The thalattosuchians are extinct group ocean-going crocodylmorphs from the Jurassic. They were very diverse and had a range of ecologies including species that lived in a similar way to dolphins and killer whales. Credit: Ghedoghedo.
It can be very hard to quantify a complex shape such as a skull or jaw, but thankfully by using geometric morphometrics, you can get a computer to do it for you! You first have to map out anatomically analogous landmarks, points and curves. With this information, specialist software can give you an idea of how much variation there is within a range of shapes and where most of that variation lies. Credit: Tom Stubbs.
Dr Stubbs was able add that information to a family tree, plot that tree across time and finally determine which groups were innovating, at which times, and how fast. Rapid innovation in skull and jaw shape correlates to groups with new habitats and diets. Very fast rates are found consistently throughout the terrestrial notosuchians, and also in the marine and semi-aquatic thalattosuchians and tethysuchians. These three groups, with very different ways of life, show much faster evolutionary rates through time than average. Credit: Tom Stubbs.