Skulls

The number and position of temporal openings have been used to classify reptiles into taxonomic groups, and the highlights of this classification system are reviewed here. Reptile skulls lacking temporal vacuities are said to be anapsid (without openings). This group includes the fossil order Coty-losauria, also called stem reptiles because of their ancestral position to all higher reptiles and hence to birds and mammals. The turtles, order Testudines, also are anapsid. Synap-sid skulls have a single temporal opening on each side. The opening is positioned relatively low along the lateral surface of the skull, within the squamosal and postorbital bones. All synapsid reptiles (orders Pelycosauria, Therapsida, and Mesosauria) are extinct, but they are of great interest because of their ancestral position relative to the mammals. The para-psid condition also has a single vacuity on each side, but it is located rather high on the dorsolateral surface of the skull, within the supratemporal and postfrontal bones. Extinct, fishlike members of the order Ichthyosauria constitute the single order of parapsid reptiles, but these animals were probably closely related to euryapsid reptiles that had a single vacuity in much the same position except that it also invaded the dorsal aspects of the squamosal and postorbital bones. Orders of euryapsids were Placodontia and Sauropterygia, both marine and extinct, in the Triassic and Cretaceous periods, respectively. The diapsid condition is characterized by two temporal vacuities on each side of the skull. Major orders include Thecodontia (small crocodilian-like reptiles ancestral to birds and to all of the archosaurs), Crocodylia, Saurischia (dinosaurs with ordinary reptile-type hips), Or-nithischia (dinosaurs with bird-type hips), Pterosauria (flying reptiles), Squamata (lizards, snakes, and several extinct groups), Eosuchia (extinct transitional forms that led to squamates), and Rhynchocephalia (mostly extinct, lizard-like diapsids with one surviving lineage, the tuatara [Sphenodon punctatus] on islands associated with New Zealand; S. punc-tatus may be a superspecies containing two or more separable species).

The order Testudines, which contains all living and extinct turtles, has traditionally been grouped with the primitive cotylosaurs because of common possession of the anapsid condition. Most herpetologists and paleontologists have agreed on this matter for many years. Molecular geneticists, however, have found evidence that turtles may actually be closely related to diapsid reptiles. This finding suggests that the anapsid condition of turtles may be secondary. That is, turtles may have evolved from ancestors that possessed two temporal vacuities on each side of their skulls, but in the course of evolution, turtles lost these openings. Essentially the same idea was proposed early in the twentieth century, not on the basis of genetic evidence but on the basis of a paleon-tological scenario involving a series of extinct but turtle-like diapsid fossils. Few at that time could accept the possibility that temporal vacuities once evolved would ever be abandoned, so this notion was dismissed and has resided in scientific limbo ever since. It has been revived on the strength of genetic data, and this much derided "preposterous idea" may become accepted.

It appears as if there is a contradiction associated with the anapsid status of turtles. Whereas some species are suction feeders with relatively weak mouths, others, such as snapping turtles, have profound bite strength. How is this strength produced, given the absence of temporal openings that would allow large jaw-suspending muscles to anchor (originate) on the dorsal surface of the skull? It turns out that many species of turtles have an analogous adaptation in which sections of dermal bone on the side and back of the skull have become emar-ginated or notched. Temporal openings are holes surrounded by bone. Emarginations are missing sections of the edges of the flat bones that form the ventral or pleural borders of the skull. With substantial sections of these bones missing, jaw-

Parapsid Euryapsid
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Structural types of the reptilian skull: A. Anapsid (no temporal openings); B. Synapsid (lower temporal opening); C. Parapsid (highest temporal opening); D. Euryapsid (upper temporal opening); E. Diapsid (two temporal openings). Bones shown: 1. Postorbital bone; 2. Squamosal bone; 3. Postfrontal bone; 4. Supratemporal bone. (Illustration by Gillian Harris)

suspending muscles have the same opportunity to escape from the dermal-chondral fossae as is made possible by vacuities. Although turtles are, strictly speaking, anapsid, some have taken an alternative pathway that leads to the bite strength necessary for effective jaw prehension of substantial prey or for tearing vegetation. If the anapsid condition is secondary, turtles have substituted an analogous trait that accomplished much the same biophysical effect as did the former temporal vacuities.

Chelydra Egg
Snapping turtle (Chelydra serpentina) embryo in egg. Most reptiles hatch from eggs, although some snakes and lizards are live-bearers. (Photo by Gary Meszaros/Photo Researchers, Inc. Reproduced by permission.)
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