The many faces of synapsid cranial allometry
Just as humans and squirrels are part of the larger group called mammals, mammals are part of a larger group of animals called synapsids. But mammals are the only synapsids left. All the other synapsid groups –like the beaked, tusked dicynodonts, the fanged gorgonopsians, and the sail-backed, lizard-like Dimetrodon – went extinct long ago. In fact, the heyday of non-mammalian synapsids was the Permian period, 298-252 million years ago, millions of years before the first dinosaur.
For scientists interested in studying big patterns in mammal evolution, the fossils we have of these non-mammalian synapsids are priceless. They can tell us when and how important features of mammals, like their sensitive ears and complex teeth, first evolved. We were interested in understanding the origins of pattern that’s widespread in mammals: larger species having proportionally longer faces than smaller species they’re closely related to.
That change in proportions between animals of different sizes is called allometry, meaning “different measures”. In closely related mammals, say, all the species of squirrels, we find that the bigger species in that group tend to have significantly longer faces than we would expect if their skulls were just scaled-up versions of the skulls of their smaller relatives. We also see allometry within species – younger mammals also tend to have shorter faces than the adults they grow into.
We wanted to know if the same allometric pattern exists in non-mammalian synapsids, and if so, when it first appeared. So we photographed more than 800 fossil non-mammalian synapsid skulls from museums around the world and used a set of techniques called geometric morphometrics to quantify their shape. You can see our results can see our results summarized in the figure below, which shows the largest and smallest skulls from each group and the shapes our model predicted for them.
Of the seven groups we tested, only the predatory gorgonopsians (E), clearly followed the mammal pattern. In another predatory group, the therocephalians (F), we actually found that larger species had slightly shorter faces than smaller species – the opposite of the pattern we expected! In cynodonts (G), the group containing the closest relatives of mammals, we couldn’t detect any allometric pattern at all.
That told us that allometry we see in mammals probably evolved within mammals and not in earlier synapsids. But why does it exist? We suspected allometry between species might have something to do with allometric growth in a species. So we compared the allometry between all dicynodont species (D) to the allometry within a single species, Diictodon feliceps, and found that the patterns paralleled each other.
Why are they the same? Natural selection could favor mutations that stop animals’ regular developmental plan a little bit early or extend it slightly past the norm if it generates a more advantageous body size. Instead of evolving a new way of changing size, evolution co-opts a plan for growth that all animals already have. It’s not yet clear whether this is what’s happening in mammals, but we hope that future studies will improve our understanding of why these allometric patterns between species exist and how they evolve.
“The many faces of synapsid cranial allometry” by Isaac W. Krone, Christian F. Kammerer and Kenneth D. Angielczyk is the featured paper in the September 2019 issue of Paleobiology. The full article, published Open Access, can be found here.