We’ve all been there, right? You find an awesome crocodilian fossil, and you want to know how big an animal it came from, but you only have the skull. What a bummer! I guess there’s no way to find out the body size of your rad new crocodilian species after all. Womp womp.

But wait! Thanks to a new paper by Dr. Haley O’Brien and colleagues, recently published in IOB open, there’s an excellent new model you can use to predict fossil crocodilian body size using only the width of an animal’s head!

Image caption: Need to know how big your alligator is? Measure the width of its skull to find out! Figure from O’Brien et al. 2019.

Late-night infomercial pitch aside, this paper is really cool for a couple of reasons. Suchians (aka living crocodiles, alligators, gharials, and caimans plus their extinct relatives) are a very old, very diverse reptile lineage that occupies a range of ecological niches — most today are semi-aquatic, but some extinct species were fully terrestrial, fully aquatic, or even marine dwellers. Body size is an important indicator of how old an animal was, where it lived, and what it ate. Changes in body size between related suchian species are also a clue that rates of evolution may have shifted, which helps us understand how the lineage got so diverse over time.

The size of the femur (upper-hind-limb bone) is usually considered the most accurate predictor of body size in extant crocodilians, but since it’s so rare to find a complete fossil, a lot of times paleontologists are limited to making inferences from just skull material. Suchians especially have fairly flat, robust skulls that are resistant to distortion during the fossilization process, so O’Brien and her colleagues set out to explore how well head width could predict overall body size across the tree.

The authors used a regression model that accounted for phylogeny to assess the correlation between head width and body mass, snout-vent length, and total length in 22 extant crocodilian species. They found that head width is indeed a reliable predictor of these body mass and body length variables, comparable with femur dimensions. They then built a Bayesian predictive model that allowed them to incorporate the uncertainty in the suchian phylogenetic tree (alas, uncertainty is a fact of life in phylogenetics and it’s better to acknowledge it than pretend it doesn’t exist), and used this model to predict body sizes for three species of extinct suchians using only the known head width measurements. If you’re more of a croc fan than a stats fan, just imagine a model where you put your fossil’s head width and estimated phylogenetic position in, and get predicted body mass and length out. Pretty nifty!

These fossil body size estimates predicted by the model are comparable to estimates other researchers have made previously, and are also comparable with estimates made using femur size, so the authors conclude that this model works quite well. In addition to providing and testing a useful model, the authors have made all their code available and encourage others to download it and use the model themselves. Next time you find a fossil crocodilian skull in a cheesy infomercial, you know what to do!

Abby Vander Linden is an evolutionary biologist, functional morphologist, and PhD student at UMass Amherst. You can find her inside a microCT scanner or at www.abbyvanderlinden.com.