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The first turtles already
existed in the era of the dinosaurs, some 200 million years ago.
Turtles are the only surviving branch of the even more ancient
clade Anapsida, which includes groups such as the procolophonoids,
millerettids and pareiasaurs. All anapsid skulls lack a temporal
opening. All other extant amniotes have temporal openings (although
in mammals the hole has become the zygoid arch). Most of the anapsids
became extinct in the late Permian period, with the exception
of the procolophonoids and the precursors of the testudines (turtles).
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However, it has
recently been suggested that the anapsid condition of the turtle
skull may not be a primitive character reflecting anapsid descent,
but rather a case of convergent evolution. More recent phylogenetic
studies with this in mind have placed turtles firmly within diapsids,
slightly closer to Squamata than to Archosauria. All molecular
studies have strongly upheld this new phylogeny, though some place
turtles closer to Archosauria. Re-analysis of prior phylogenies
that affirmed an anapsid ancestry suggests that their inclusion
of turtles within Anapsida was due to both the starting assumption
that they were anapsid (most prior phylogenies concerned what
sort of anapsid they were) and also due to insufficiently broad
sampling of fossil and extant taxa for construction of the cladogram.
While the issue is far from resolved, most scientists now lean
towards a Diapsid origin for turtles.
Above Images Are From The N.O.A.A.
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