What Is a Synapsid? Our Own Branch, Which Ruled Before the Dinosaurs

The group is defined by a single opening in the skull behind each eye socket, which is where the name comes from and which distinguishes the lineage from the diapsids, who have two such openings and who include reptiles, birds and dinosaurs. Those openings began as spaces reducing skull weight and providing attachment for jaw muscles, and their arrangement is a reliable marker of ancestry because the pattern is inherited rather than repeatedly invented. The split between the two lineages occurred in the Carboniferous, over three hundred million years ago, which means that the last common ancestor of a human and a lizard lived before the first dinosaur by a very long interval. That is why calling early synapsids mammal-like reptiles is doubly misleading: they are not reptiles, since reptiles are on the other branch, and they are not mammal-like so much as early members of the branch mammals eventually emerged from.

The fossil record documents the transition in unusual detail and the changes accumulate gradually:

• •Pelycosaurs, including the familiar sail-backed forms of the early Permian, sprawling and reptile-like in posture despite their position on our branch
• •Therapsids, appearing in the middle Permian, with limbs drawn further beneath the body, larger jaw muscles and differentiated teeth
• •Cynodonts, a therapsid group appearing in the late Permian, with a secondary palate separating breathing from chewing, and evidence of whiskers implying hair
• •Progressive jaw joint change, in which bones at the rear of the reptilian jaw shrank, moved and became the middle ear bones, a transition documented by a remarkable series of intermediate fossils
• •Teeth differentiated into incisors, canines and molars with precise occlusion, permitting chewing rather than only gripping
• •Mammals proper in the late Triassic, small, probably nocturnal and insectivorous, living alongside the dinosaurs that had by then taken over

The movement of jaw bones into the middle ear is among the best-documented major transitions in the fossil record and is worth understanding because it is so frequently cited. In a reptile the jaw joint is formed between two particular bones and a single bone conducts sound in the ear. In a mammal the jaw joint is formed between two different bones and three bones conduct sound, and the two extra ones correspond precisely to the reptilian jaw joint bones. A long series of fossils shows those bones progressively reduced, with several intermediate forms having both jaw joints operating simultaneously, which answers the obvious objection that an animal cannot afford to lose its jaw joint while acquiring another. Embryology independently confirms the correspondence, since the bones develop from the same structures in a mammalian embryo. The transition improved hearing of high frequencies, which suits a small nocturnal animal locating insects, and it also allowed the jaw joint to be strengthened for chewing.

Synapsids dominated terrestrial vertebrate communities through the Permian, occupying the roles of large herbivores and large predators worldwide, and the end-Permian extinction removed most of them. That event, the most severe in the fossil record, eliminated a large majority of marine and terrestrial species, and the ecosystems that recovered in the Triassic were dominated by the other branch, with archosaurs and then dinosaurs occupying the roles synapsids had held. The surviving synapsid lineages included the cynodonts, from which mammals descended, and those mammals remained small for over a hundred million years while dinosaurs held the large-bodied roles. That is a long period of being ecologically marginal, and it ended only with the end-Cretaceous extinction, which removed the non-avian dinosaurs and permitted the mammalian radiation. The whole history therefore involves one branch being dominant, reduced by catastrophe, marginal for an enormous interval, and dominant again after a second catastrophe, which is not a story about superiority at any point.

One skull opening behind each eye marks the branch mammals belong to, which separated from the reptile line over three hundred million years ago, so the early members are not reptiles and the usual phrase is wrong twice. Jaw bones became middle ear bones through a documented series with both joints operating at once. Synapsids ruled the Permian, were nearly destroyed at its end, and stayed small for a hundred million years.