A skeleton of the synapsid Dimetrodon on display at the Field Museum. Photo by Ken Angielczyk.
Do you recognize the animal in the picture above? Its name is Dimetrodon and it is a frequent inhabitant of museum exhibits on paleontology and books about ancient animals, so it may look familiar. The elaborate sail on its back is formed by parts of its backbones, or vertebrae; if you run your finger down your back, the bumps you feel are the equivalent parts of your vertebrae. Dimetrodon lived between about 280 and 265 million years ago, and fossils of it are most frequently found in rocks from the Permian Period of Earth history (299 to 251 million years ago) in northeastern Texas. What most people don't realize is that despite its rather lizard-like appearance, Dimetrodon is more closely related to mammals like you and I than it is to any living reptile. The round opening on the side of its skull, just behind its eye socket, is one of the characteristics of its skeleton that informs us about the relationship we share with Dimetrodon.
All living mammals, as well as extinct mammals like woolly mammoths or giant ground sloths, are members of a larger group called Synapsida. In addition to mammals, Synapsida includes a number of other, now extinct subgroups, and Dimetrodon is part of one of these extinct lineages. The oldest synapsid fossils are about 315 million years old, and the synapsid fossil record provides insight into the changes these animals underwent between their origin and the evolution of mammals (about 205 million years ago), as well the evolution of many distinctive mammalian features like our three tiny middle ear bones. During the 110 million years between their origin and the origin of mammals, synapsids came in many shapes and sizes, including carnivores, herbivores with turtle-like beaks, carnivores and herbivores with tall sails on their backs, specialized burrowers, small weasel-like carnivores, hippo-sized herbivores with thickened skulls that may have been used for head-butting, and even a carnivorous species that might have been venomous. They were also the most common animals in terrestrial communities for much of this time, and some synapsid lineages survived the largest mass extinction in Earth history (the end-Permian mass extinction). Therefore studying synapsids is important not only for what it can tell us about our own history, but also for understanding how those ancient communities functioned ecologically and the causes and effects of the mass extinction.
A family tree, or phylogeny, showing relationships among major groups of terrestrial vertebrates. Branches that are connected at their base are descendants of a common ancestor. All synapsids share a more recent common ancestor with each other than they do with any reptile. Synapsida includes eothyridids and all groups to the right of them on the tree; Therapsida includes biarmosuchians and all groups to the right of them on the tree, and Mammalia includes Sinoconodon and all groups to the right of it on the tree. Dimetrodon is a sphenacodontid.
Over the next three weeks, I'll be part of an international team of paleontologists conducting fieldwork in Permian-age rocks in northeastern Brazil. During the course of this trip we hope to find new synapsid fossils in a part of the world where they are currently unknown, and to learn more about other animals that were living alongside them. I'll explain more about the questions underlying this research in upcoming posts.