Synapsids are a group of amniotes that includes mammals and their extinct relatives, while sauropsids include reptiles, birds, and extinct relatives. The only living synapsids are mammals, and early synapsids were sometimes called “naked lizards.” Synapsids evolved differentiated teeth and became more mammalian over time. The first synapsid, Archaeothyris, was slightly larger than the earliest sauropsid, Hylonomus. Lystrosaurus was one of the few land animals to survive the Permian-Tertiary extinction event.
Synapsids (“fused bow”) are one of two groups of amniotes, or non-amphibian land animals, which includes mammals and their extinct relatives, the therapsids. The other group, the sauropsids, includes reptiles, birds, and extinct relatives. Together, synapsids, sauropsids, and amphibians make up all tetrapods, both extant and extinct, with the exception of some stem-group tetrapods that existed before these groups branched off from each other about 320 million years ago, in late Carboniferous.
The only living synapsids are mammals. Since the word “mammal” is widely understood and “synapsid” is not, sometimes the word synapsid is used to refer specifically to the extinct group, which was formerly called mammal-like reptiles. More recently, it was realized that these animals did indeed lack some characteristics that are universal among reptiles, and so they were given a group. They are sometimes called “stem mammals.”
A common feature among synapsids is the temporal fenestra, a hole in the skull behind the eyes, intended to reduce the weight of the skull. Modern synapsids are all warm-blooded, but many early synapsids were cold-blooded, making this characteristic not definitive. Like today’s mammals, ancient synapsids had glandular, scaleless skin. It is not known exactly at what point synapsids evolved body hair and mammary glands. Early synapsids are sometimes called “naked lizards,” because they resembled lizards in appearance, only without the scales. Another evolutionary innovation of synapsids was the first differentiated teeth. As synapsids continued to evolve, they became more mammalian and less reptilian.
Both early synapsids and sauropsids looked like small lizards. Interestingly, the earliest known synapsid, Archaeothyris, which lived 320 million years ago, was slightly larger than the earliest known sauropsid, Hylonomus, which lived 315 million years ago, and may have even preyed on it. This role was reversed during the 155 million year age of the dinosaurs, only to reverse again in recent times, when dinosaurs became extinct and mammalian carnivores began to prey frequently on lizards and snakes.
The first synapsid Lystrosaurus was one of the few land animals to survive the Permian-Tertiary extinction event, the “mother of all mass extinctions” in which approximately 99.5% of all individuals and 70% of all species perished. terrestrial vertebrates. For a few million years, Lystrosaurus was one of the few tetrapods to roam the continents, a level of species uniformity not seen in any other geological era.
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