The earliest reptiles are known from the early Pennsyl-vanian (323-317 million years ago, or mya). They were quite small and lizardlike in appearance, and their skulls, jaws, and tooth structures strongly indicate that they were insectivorous. In fact, it is thought that they evolved in tandem with insect groups that were beginning to colonize the land. Some Pennsylvanian (323-290 mya) amphibians of the microsaur group also evolved into insectivores that were so superficially similar to early reptiles that, for a time, they were classified as such.

The amniote egg evolved in the earliest reptiles. This allowed for the first true occupation of the land by tetrapods, for the amniote egg allowed the embryo to develop in an aquatic microcosm until it was ready for terrestrial life; this paved the way for the huge adaptive radiation that eventually took place among the reptiles. Robert Carroll of the Red-path Museum in Montreal, Canada, has pointed out that the earliest reptiles probably occupied the land before the am-niote egg was developed fully. An analogy may be found in a few modern salamanders (small, somewhat lizardlike amphibians) that lay tiny non-amniote eggs in moist terrestrial places, such as under logs or in piles of damp leaves. These eggs hatch into tiny replicas of the adults rather than going through a larval stage, such as occurs in frogs and other salamanders. The evolution of the amniote egg took place when the membranes within the eggs of the earliest reptiles became rearranged in the form of various sacs and linings and the outer membrane incorporated calcium into its structure to form a shell.

The calcareous (limey) shell afforded protection for the developing embryo and was porous enough to allow for the entrance and exit of essential elements, such as oxygen and carbon dioxide. Food for the developing embryo was supplied in the form of an extra-embryonic sac full of yolk and a system of blood vessels that allowed the yolk to be transferred to the embryo. The amnion itself formed a sac that contained a fluid within which the embryo was suspended. This provided a "private pond" (a term used by the late Harvard University scientist Alfred S. Romer, the world leader in vertebrate paleontology from the late 1940s until his death in 1973) for its occupant and kept the fragile embryonic parts from sticking together. A unit composed of a part of the al-lantois and the membranous chorion next to the shell allowed for the absorption of oxygen and the excretion of carbon dioxide. A sac formed by the allantois stored the nitrogenous wastes excreted by the embryo. The origin of the amniote egg was one of the most important evolutionary events that ever occurred.

Turning to other anapsid reptiles, turtles were one of the first reptiles to branch off the amniote stem. Turtles are first known from the late Triassic (227-206 mya) but probably evolved in the Permian (290-248 mya). Aside from protection, the turtle shell (which is essentially a portion of its skeleton turned inside out) has other critical functions. A large percentage of the red blood cells of most land vertebrates are formed in the marrow of the long bones. Turtles, however, need sturdy legs to support their shells; thus, their limb bones are very dense, with little space, if any, for red blood cell-producing marrow. It has been shown that the turtle shell is filled with canals and cavities where red blood cells are produced in quantity. Moreover, rather than being just an inert shield, the turtle shell is the site of calcium metabolism and is important in the process of temperature regulation, by absorbing heat during the basking (sunning) process.

Other attributes of turtles include the ability of some of them to absorb oxygen in the water through patches of thin skin on the body, in the lining of the mouth, or within the cloaca, a terminal extension of the gut wall. Some turtles can freeze solid in the winter and thaw out in the spring with no harmful effects—a process that also takes place in various frog species. The ability of turtles to survive severe injuries is well known, and many species can exist in the absence of oxygen for long periods of time. Leatherback turtles have a current-countercurrent blood flow similar to that of deep-sea mammals and can dive in the sea at great depths and remain active

Phylogenetic relationships of terrestrial vertebrates

(Illustration by Argosy. Courtesy of Gale.)

in very cold temperatures. The incubation temperature of the eggs of many species of turtle determines the sex of the hatch-lings. In some cases "cold nests" produce females and "warm nests" produce males, but the opposite can also occur. Turtle eggs have large amounts of yolk compared with those of many other vertebrates. This "egg food" sustains the young during long incubation periods.

The origin of turtles is somewhat in doubt. An early anap-sid with expanded ribs, Eunotosaurus, once was proposed as the ancestral form, but the skull of Eunotosaurus was not turtlelike. On the other hand, the body skeleton of early reptiles called procolophonids had a shell, and the skull was somewhat turtlelike. Owenetta, a procolophonid found in the Upper Permian (256-248 mya) of South Africa has nine advanced characters in the skull and one in the humerus that are shared with Proganochelys, an unquestionable Triassic (248-206 mya) turtle. Owenetta lacks a shell, however; thus it has been suggested that the skull changes in turtle ancestors preceded those that led to the origin of the shell.

True turtles (order Testudines) are composed of three major suborders: the proganochelydians, the pleurodirans, and the cryptodirans. The proganochelydians are the most primitive and are known from the late Triassic to the early Jurassic (206-180 mya). The shell is similar to that of modern turtles, except that it has extra bones and the head and limbs cannot be retracted effectively into it. The skull lacks teeth except for a few on the palate. The pleurodirans and cryp-todirans have no teeth in the skull and can retract the head, neck, and tail into the shell.

In the pleurodires the neck swings sideways when it is retracted, so that the turtle looks out with only one eye. In the cryptodires the neck folds over itself when it is retracted, so that the turtle gazes with both eyes. Oddly, the neck differentiation in these turtles did not occur until the late Cretaceous (99-65 mya). Reflecting their Gondwana origin, one finds only cryptodires in the Northern Hemisphere, whereas

Hylonomus Skeleton
Skeleton of one of the earliest known amniotes, Hylonomus lyelli, from the early Pennsylvanian of Joggins, Nova Scotia. Remains were found within the upright stump of the giant lycopod Sigillaria. (Illustration by Marguette Dongvillo.)

a significant number of pleurodires occur in the Southern Hemisphere. Seaturtles and soft-shelled turtles, familiar animals throughout much of the world today, appeared in the Upper Jurassic (159-144 mya); by the time the neck specializations came about in the late Cretaceous, all turtles were essentially modern. A few very big seaturtles lived in the Mesozoic (248-65 mya) seas during the time when dinosaurs dominated the land. Archelon of the Cretaceous (144-65 mya) had a shell length of 6.3 ft (1.9 m). But the largest known turtle is a freshwater pleurodire (side-neck group) turtle with a shell length of 7.6 ft (2.3 m). Aptly named Stupendemys, this turtle was collected on a Harvard field trip to Venezuela. The animal came from Pliocene (5.3-1.8 mya) sediments, not long ago at all in terms of geologic time.

Several odd reptile clades branched off the anapsid stem, including the elephantine, terrestrial pareiasaurs, and the slim, marine mesosaurs. The pareiasaurs (among them, the well-known genus Scutosaurus) have been found in the Middle and Upper Permian (269-248 mya) of Africa, western Europe, Russia, and China. They were up to 10 ft (3 m) long and had an upright stance (unlike that of other amniotes of the time) and stout limbs that supported the massive body. The head of pareisaurs was short and thick, with heavily sculptured bones protecting the eyes and tiny brain. The teeth had compressed, leaf-shaped crowns like those of modern leaf-eating herbivorus lizards; thus, pareiasaurs were probably the first large land herbivores.

Mesosaurs, on the other hand, were the earliest truly aquatic reptiles. These gracile animals were close to 1 yd (0.9 m) long, about a third made up of the tail. Fossil mesosaurs are known only from the adjacent coasts of southern Africa and eastern South America, reflecting the fact that Gondwana split into the two continents. In the mesosaurs the snout was very long, and the mouth was filled with long, needlelike teeth. These teeth apparently formed a specialized straining devise that allowed the animals to feed upon small crustaceans, possibly those found in the same fossil beds as the mesosaurs. The long, compressed tail probably was used for propulsion in swimming. The posterior tail vertebrae have fracture plains, indicating that caudal autonomy (voluntary shedding of the end of the tail during stress) could have occurred; this feature may have allowed them to escape from the grasp of predators. The skull of mesosaurs originally was thought to have a temporal opening, but this was later disproved.

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