Skull
From EvoWiki
The skull is a hollow bone shell which surrounds and protects the brain. As such, skulls are found only in organisms which possess skeletons.
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Anatomy
The skull can be divided into two distinct clusters of bones:
Other accessory bones, not always directly articulating with the skull but in the same region, are:
Morphology
Skull morphology is an important basis of classification for major tetrapod groupings. All tetrapods belong to one of three groups: anapsids, diapsids, or synapsids.
Anapsids
The anapsids are a group of amniotes, characterized by skulls without openings (fenestra)near the temples. The only extant members are the Testudines (turtles, tortoises, and terrapins), though other extinct groups are known from Permian and Triassic fossils. The anapsids have traditionally been treated as a subclass of the class Reptilia, but as this group is paraphyletic, they are sometimes placed in a separate class Anapsida.
Most of the anapsid orders, including the millerettids, nyctiphrurets, and pareiasaurs, became extinct in the Permian-Triassic mass extinction. Both the procolophonoids and some ancient ancestors of the testudines managed to survive into the Triassic, and Testidunes are the only surviving order.
Diapsids
Diapsids ("two arches") are a subclass of tetrapods that developed two holes (temporal fenestra) in each side their skulls, about 300 million years ago during the late Carboniferous period. Living diapsids are extremely diverse, and include all birds, crocodiles, lizards, snakes, and the tuatara. While some lost either one hole (lizards), or both holes (snakes), they are still classified as diapsids based on their ancestry. There are 14,600 living species of diapsids, including most flying and poisonous vertebrates.
The ancestral openings are above and below the eye, and allow the jaw to open wider, and the attachment of larger, stronger jaw muscles. Extinct groups include the dinosaurs (except for birds), pterosaurs, plesiosaurs, mosasaurs, and ichthyosaurs.
Synapsids
Synapsids ("fused arch"), formerly known as "mammal-like reptiles", are a class of amniotes that developed one hole in their skull (temporal fenestra) behind each eye, about 320 million years ago (Ma) during the late Carboniferous. The 3,500 species of living synapsids are currently the dominant land animals, and include both aquatic (whales) and avian (bats) species.
The synapsids known as pelycosaurs and caseasaurs were the first successful group of amniotes, spreading and diversifying until they became the dominant large terrestrial animals in the late Carboniferous (>290Mya). They were sprawling, bulky, cold-blooded, had small brains, and were the largest land animals of their time, ranging up to 3 m (10 ft) in length. Many, like Dimetrodon, had large sails that may have helped regulate their body temperature. They lasted through the Permian and became extinct during the Permian-Triassic extinction event about 250 Ma.
The therapsids, a more advanced group of synapsids, appeared during the first half of the Permian, and went on to become the dominant large terrestrial animals during the latter half. Some groups survived the Permian extinction and went on to dominate the early Triassic as well, though the more lightly built diapsid amniotes, such as the thecodonts and archosaurs, started to become more common. The Triassic-Jurassic mass extinction event finally wiped out all large synapsids and left room for the diapsid archosaurs known as dinosaurs to spread, and they dominated the terrestrial ecosystem for the rest of the Mesozoic Era. The remaining Mesozoic synapsids were small, ranging from the size of a shrew, to the badger-like Repenomamus.
While marginalized, synapsids continued to evolve and developed sophisticated teeth, which tend to vary greatly in size and shape; everything from flat, multi-cusped molar teeth to the sharp cone-shaped canines. The first known mammal appeared during the Jurassic. There is no fossil evidence until much later because soft tissues are rarely preserved, but many traditional mammalian features, like bearing live young, milk-producing teats, fur, and warm-bloodedness, probably appeared in the cynodonts, the immediate ancestors of mammals.
The hole in the synapsid head allows the attachment of larger lower jaw muscles. A synapomorphy of synapsids is dentary-squamosal jaw articulation, or a fusing of the lower jawbones into one long piece (the dentary), which hinges at the quadrate of the skull. Smaller jawbones--the incus, malleus, and stapes (also known as the hammer, anvil, and stirrup)--that did not fuse migrated to the inner ear and allow for sophisticated hearing (in reptiles there is only one bone, the stapes).
External links
- Introduction to Anapsida from UCMP
- Diapsida. Michel Lauren and Jacques A. Gauthier. Tree of Life Web Project. June 22, 2000.
- Diapsida Cladogram
- Tree of Life - Synapsida
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This page is part of the EvoWiki Encyclopedia of Anatomy and Physiology. |
