Synapsid

[[File:Dimetrodon_incisivus_Exhibit_Museum_of_Natural_History.JPG|thumb|165px|A fossil of one of the more well-known synapsids, ''Dimetrodon'']]{{evolution}}
'''Synapsids''' (Greek: "fused-arch") are a [[clade]] of animals that includes [[mammal]]s. Non-mammalian synapsids were once called "mammal-like reptiles", though they were neither; today, the preferred scientific term is "stem mammal".

Synapsids were the largest land vertebrates in the [[Permian]] period. By the end of the [[Triassic]], the group Probainognathia (which includes what would become modern mammals) were the only surviving synapsids.<ref>[http://www.nasmus.co.za/museum/news/greatest-mass-extinction-responsible-making-modern-mammals Greatest mass extinction responsible for the making of modern mammals]</ref> After the [[K-Pg extinction event]] 66 million years ago, synapsids again became the largest land animals and eventually the largest sea animals as well.

==Characteristics==
Synapsid skulls contain a hole behind the [[eyes]] called the temporal fenestrae, which made their skulls lighter and less energy-intensive to grow and may have provided new attachment sites for jaw muscles. They also have differentiated teeth (canines, molars, and incisors)<ref> Angielczch, Kennenth; Kammer, Christian F.; and Frobisch, Jorg. (2013). Early Evolutionary History of Synapsida.</ref> and evolved a different palate shape from their reptilian ancestors.<ref> Hopson, James A. (1987). "The Mammal-Like Reptiles: A Study of Transitional Fossils". The American Biology Teacher. 49 (1): 16–26. doi:10.2307/4448410. JSTOR 4448410</ref>

As soft tissue only preserves in rare cases (even less the further back in the record you go), it is unknown whether early synapsids had hair or mammary glands, though they are occasionally depicted with hair. Certain species of ''Dimetrodon'' have been shown to have tough scales on their undersides that extended to the tail, while ''Estemmenosuchus'' likely had no hair but had smooth skin with glandular depressions. Permian coprolites show that at least some synapsids back then already had hair.<ref>Bajdek, Piotr; Qvarnström, Martin; Owocki, Krzysztof; Sulej, Tomasz; Sennikov, Andrey G.; Golubev, Valeriy K.; Niedźwiedzki, Grzegorz (2016). "Microbiota and food residues including possible evidence of pre-mammalian hair in Upper Permian coprolites from Russia". Lethaia. 49 (4): 455–477. doi:10.1111/let.12156.</ref>

The [[fossils]] of ''Lystrosaurus'', as well as other extinct reptiles and land plants that lived from the Permian to the Cretaceous period, were used as evidence of [[continental drift]] due to fossils showing a clear correlation across the land, even though they were found across entire oceans.

Synapsids are significant in an [[Theory of Evolution|evolutionary]] sense because of their perfect illustration of the process of [[transitional forms|transition]], in this case from reptile-like amniotes to egg-laying mammals (Monotremes). They are no doubt one of the most frightening things to [[creationist]] nutters, who probably try to convince themselves 
they were eaten by [[Dinosaur|velociraptors]] on the [[ark]]
 they lived peacefully with those loving, plant eating tyrannosaurs.<ref>http://http://creation.com/vegetarian-theropod-dinosaurs</ref>

==Classification==
Synapsids are one clade of the amniotes (land vertebrates that lay eggs with shells). The other branch is Sauropsida, reptiles, which include [[birds]] (which are [[dinosaurs]]!). Synapsids are divided into two main clades. The extinct Caseasauria which resembled couch potato lizards<ref>https://en.wikipedia.org/wiki/Cotylorhynchus#/media/File:Cotylorhynchus_NT_small.jpg</ref> and Eupelycosauria which included us and the sailed back herbivore ''Edaphosaurus''. Going further down the line you have Sphenacodontia which includes the famous ''Dimetrodon'' and our ancestors.

Caseasaurs, edaphosaurs, and sphenocodonts were gradually replaced in the mid Permian by a new group of synapsids, the therapsids<ref>https://www.britannica.com/animal/therapsid</ref> which experienced their own flowering. One group of therapsids, called dicyonodonts, were large pig-like browsers that evolved in the Permian and were very successful in the Triassic. They may have even survived until the early Cretaceous in [[Australia]],<ref>https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1691326/</ref> proving that even back then Australia was strange.The other group were the cynodonts, dog-like animals that gave rise to true mammals.<ref>http://www.palaeos.com/Vertebrates/Units/410Cynodontia/410.000.html</ref>

==See also==
*[[Dinosaur]]s, which belong to the sister group, sauropsids
*[[Extinction event]]

==External links==
*[http://www.ucmp.berkeley.edu/synapsids/synapsida.html Berkeley.edu - Introduction to the Synapsida]
* [http://www.talkorigins.org/faqs/faq-transitional/part1b.html Transitional Vertebrate Fossils]

==References==
{{reflist|2}}

[[Category:Animals]]
[[Category:Evolution]]
[[Category:Transitional fossils]] 
[[Category:Paleontology]]

Categories: Animals, Evolution, Transitional fossils, Paleontology