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Abstract
An unusually rich fossil record documents in great detail the evolution of synapsids (mammal-like reptiles and their descendants the mammals). This fossil record extends over 320 million years and provides extensive evidence of the transition between primitive amniotes and mammals. The superior quality of the synapsid fossil record has even enabled scientists to use it to illustrate the concept of evolution and to test evolutionary models12. But a major morphological gap still exists in this fossil record, between Lower Permian and Upper Permian synapsids3. Historically, this gap has been used to divide synapsids into pelycosaurs (primitive mammallike reptiles) and the geologically younger, more advanced therapsids. We studied Tetraceratops to document the origin of therapsids; preparation and restudy of Tetraceratops insignis, originally described as a pelycosaur4'5 demonstrate that this Lower Permian genus bridges the gap between pelycosaurs and therapsids.