Synapsids and sauropsids are two evolutionary lineages of amniotes, which include all non-amphibian tetrapods and their descendants. Synapsids have an extra hole in their skulls, while sauropsids have a pair of holes behind their eyes. Synapsids diversified faster than sauropsids and gave rise to most of the large Permian animals, including pelycosaurs. The sauropsids took over after the largest synapsids died out, giving rise to dinosaurs, which were wiped out by an asteroid. Synapsids then ruled the world again, giving rise to mammals and eventually humans.
Synapsids include mammals and our distant ancestors, including pelycosaurs and therapsids, while sauropsids is another word for reptiles. Synapside means “fused bow”, a reference to the structure of the skull. Another name for a synapsid is theropsid, meaning “beast face,” as opposed to sauropsid, meaning “lizard face.” Synapsids are sauropsids are the two evolutionary lineages of amniotes, which includes all non-amphibian tetrapods and their descendants (such as whales, which are descended from tetrapods but lost their legs when they became exclusively marine). Early synapsids were called “mammal-like reptiles,” but that’s a misnomer, since they weren’t reptiles at all.
Synapsids and sauropsids split from each other about 320 million years ago, during the late Carboniferous period. Both looked like small lizards. At the time, tetrapods had existed in water for about 45 million years and on land for at least 20 million years. Both are amniotes, i.e. animals with complex eggs that can be laid on land, unlike amphibians, which must lay their eggs in water. Before synapsids and sauropsids split, there were a few stem-group amniotes that fell into neither group. Amniotes were destined to inherit Earth because they are the only land vertebrates that can venture significant distances from water and survive.
The difference between sauropsids and synapsids is defined in terms of the openings in their skulls. Synapsids have an extra hole, used to reduce the weight of the skull and provide an attachment point for the jaw muscles. Sauropsids started out with no holes in the skull, then developed a pair, with each hole behind the eyes. Initially, both groups were “cold-blooded” (ectothermic).
Since the late Carboniferous, the colonization of land by large creatures has been an evolutionary arms race between synapsids and sauropsids. Synapsids got off to a flying start, diversifying faster than sauropsids and giving rise to most of the large Permian animals, including the lucky pelycosaurs, some of which were as big as trucks and had the only apex predators at the time.
At the end of the Permian, the largest synapsids died out, leaving many niches open for exploitation. The sauropsids took advantage of this, giving rise to the dinosaurs, which dominated the Earth throughout the Mesozoic. About 65 million years ago, things turned around again, when an asteroid wiped out all non-avian dinosaurs. Synapsids have once again ruled the world, in the form of mammals. Eventually, synapsids gave rise to humans, arguably the most evolutionarily successful land vertebrate in the history of life on Earth.
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