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From top to bottom (A) a skull of an Anapsid, (B) a Synapsid (stem-mammal) skull, and (C) a Diapsid skull. [a] Temporal fenestrae are openings in the temporal region of the skull of some amniotes, behind the orbit (eye socket). These openings have historically been used to track the evolution and affinities of reptiles.
Compared to many other ostracoderms, such as the Heterostraci and Osteostraci, anaspids did not possess a bony shield or armor, hence their name.The anaspid head and body are instead covered in an array of small, weakly mineralized scales, with a row of massive scutes running down the back, and, at least confirmed among the birkeniids, the body was covered in rows of tile-like scales made of ...
This placement within the diapsids suggests that the turtle lineage lost diapsid skull characteristics, since turtles possess an anapsid skull. This would make Parareptilia a totally extinct group with skull features that resemble those of turtles through convergent evolution .
An anapsid is an amniote whose skull lacks one or more skull openings (fenestra, or fossae) near the temples. [1] Traditionally, the Anapsida are considered the most primitive subclass of amniotes, the ancestral stock from which Synapsida and Diapsida evolved, making anapsids paraphyletic.
Pantestudines or Pan-Testudines is the proposed group of all reptiles more closely related to turtles than to any other living animal. It includes both modern turtles (crown group turtles, also known as Testudines) and all of their extinct relatives (also known as stem-turtles). [2] Pantestudines with a complete shell are placed in the clade ...
The morphology of the lower temporal fenestra of the skull of Acleistorhinus bears a superficial resemblance to that seen in early synapsids, a result of convergent evolution. Only a single species , A. pteroticus , is known, and it is classified in the Family Acleistorhinidae , along with Colobomycter (also from the Early Permian of Oklahoma).
Although some diapsids have lost either one hole (lizards), or both holes (snakes and turtles), or have a heavily restructured skull (modern birds), they are still classified as diapsids based on their ancestry. At least 17,084 species of diapsid animals are extant: 9,159 birds, [3] and 7,925 snakes, lizards, tuatara, turtles, and crocodiles. [4]
Euryapsids are usually considered entirely extinct, although turtles might be part of the sauropterygian clade [1] while other authors disagree. [2] Euryapsida may also be a synonym of Sauropterygia sensu lato. [3] The ichthyosaurian skull is sometimes described as having a metapsid (or parapsid) condition instead of a truly euryapsid one.