Review of the Pelycosauria Revisited: Pelycosaurian-Grade Synapsida, Their Role in Updated Interpretations of Early Amniotes, and Their Importance in Understanding the Origin of Modern Terrestrial Food Webs and Ecosystems

Though previously considered an amniote order of their own, pelycosaurs are now recognized as a basalmost paraphyletic grouping within the Synapsida—the lineage that ultimately led to modern-day mammals. Although they are no longer considered a monophyletic taxon, pelycosaurian-grade synapsids still provide critical insight into the basal amniote condition, including our understanding of the systematics, biology, and ecology of those vertebrates that ultimately completed the water-to-land transition. Whereas the familial designations made almost a century ago have not changed dramatically, interfamilial relationships among pelycosaurs have changed significantly. Recent descriptions of new pelycosaurian taxa demonstrate greater similarity in skull morphology between the most basal members of each family. Paleohistology and craniodental analyses suggest that many features previously thought to be those of the more derived synapsids, the therapsids and mammals, began their development much earlier in pelycosaurs. Furthermore, pelycosaurs have also played key roles in determining that the earliest examples of modern vertebrate terrestrial food webs and ecosystems originated in the early Permian, as long 290 million years ago, and well before the previously suggested middle to late Permian and Mesozoic.