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Synapsids include mammals and our distant ancestors, including pelycosaurs and therapsids, while sauropsid is another word for reptiles. Synapsid means "fused arch," a reference to skull structure. Another name for a synapsid is theropsid, which means "beast face," in contrast to sauropsid, which means "lizard face." Synapsids are sauropsids are the two evolutionary lineages of amniotes, which includes all non-amphibians tetrapods and their descendants (such as whales, which descended from tetrapods but lost their legs when they became exclusively marine). Early synapsids used to be called "mammal-like reptiles," but this is a misnomer, as they were not reptiles at all.
Synapsids and sauropsids split off from each other approximately 320 million years ago, during the late Carboniferous period. Both looked like small lizards. At the time, tetrapods had existed in the water for about 45 million years, and on land for at least 20 million years. Both are amniotes, that is animals with complex eggs that can be laid on land, in contrast to amphibians, which must lay their eggs in water. Before synapsids and sauropsids split, there were some stem-group amniotes that didn't fit into either group. Amniotes were destined to inherit the Earth because they are the only land vertebrates that can venture significant distances from water and still survive.
The difference between sauropsids and synapsids is defined in terms of the openings in their skull. Synapsids have an extra hole, used to reduce skull weight and provide an attachment point for jaw muscles. Sauropsids began with no holes in their skull, then developed one pair, with each hole behind the eyes. Initially, both groups were "cold-blooded" (ectothermic).
Since the late Carboniferous, the colonization of the land by large creatures has been an evolutionary arms race between synapsids and sauropsids. Synapsids got off to a great start, diversifying more rapidly than sauropsids and giving rise to most of the large animals of the Permian, including the successful pelycosaurs, some of which were as big as trucks, and had the only apex predators of the time.
At the end of the Permian, the largest synapsids went extinct, leaving many niches open for exploitation. The sauropsids took advantage, eventually giving rise to dinosaurs, which dominated the Earth throughout the Mesozoic. About 65 million years ago, the tables turned again, when an asteroid wiped out all non-avian dinosaurs. Synapsids ruled the world again, in the form of mammals. Eventually, synapsids gave rise to humans, arguably the most evolutionarily successful terrestrial vertebrate in the history of life on Earth.
To say that "Early synapsids used to be called "mammal-like reptiles," is a misnomer, as they were not reptiles at all." It is a truism, and it is only true because certain phylogeneticists decided that, in order to fit "the reptiles" into a tidy box that fitted their self-imposed rules of cladistics, they would have to redefine what they meant by "reptile". Before this, everyone knew what a reptile was -- scientists and laymen alike.
To force them to fit into a bonafide Clade, they had to decide what to do about the birds and the mammals. For the birds, they had a problem since they are more closely related to crocodiles than the crocs are to other reptiles. The only
option open to them was to classify all birds as reptiles (which to me seems obvious nonsense).
Since mammals are so clearly not mammals, they had to exclude all of the ancestors of the mammals, hence the pelycosaurs and therapsids were reclassified and no longer considered reptiles.
I'll bet if you met Dimetrodon today there would be little doubt in your mind that you had just seen a reptile! I have no problem at all with phylogenetics and cladistics; I just wish they had confined themselves to using the perfectly reasonable term 'sauropsida' and left the already well understood term 'reptile' alone.
@Mor - You are forgetting that birds are also sauropsids. Considering the vast range of birds, I would hesitate to even say that synapsids are the dominant group at the moment. Sure, humans seem powerful, but for sheer numbers, chickens or sparrows or pigeons or seagulls would seem to be doing all right.
Put all those groups together and you've got something. I'm not sure how we are measuring success, but I wouldn't discount birds if it came to taking over the world.
@Mor - Well, if the sauropsids did manage to take over the earth again, it would be with species we hadn't seen before, most likely.
I mean, the dinosaurs weren't waiting in the wings to take over from the synapsids that were there before them.
After the extinction event, they gradually evolved to take their places, and they evolved to fill particular niches as well.
That's one of the reasons I don't think it's going to be all that strange to meet alien life forms that look familiar to us, if they evolved on a similar planet. There are always going to be small flying things, big flying things to eat the small flying things, swimming things, crawling things, running things
, and so forth.
It's the same reason a shark and a dolphin look so much alike, even though they both evolved independently.
And the current day sauropsids would do it again, if they managed to get a foothold in time to beat out other species.
It's interesting to see that pattern, with the rises and falls of synapsids and sauropsids.
Since synapsids are the dominant group at the moment, it makes you wonder whether the sauropsids would have a chance if there was another extinction event?
Because at the moment, they don't really have any contenders. I might believe that crocodiles or maybe Komodo dragons might "conquer" the earth, but aside from that, I'm not sure who I would put my money on.
Too bad, I suppose, that all the dinosaurs died out.
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