Although no definitive fossils have been found of the amphibian ancestors from which the reptiles evolved, they are usually considered to have been an early egg-laying offshoot of an amphibian group (Batrachosauria) of which Seymouria (Fig. 4) is an example. Seymouria, Solenodonsaurus and the larger Diadectes (Fig. 4) have sometimes been classified as basal reptiles - but the point at which reptilian characters dominated over those of amphibians is unclear. The remnants of lateral lines on the skull of adult Seymouria suggest that this genus had an aquatic development,while Solenodonsaurus had a dentition intermediate between that of an amphibian and a reptile. (Lateral lines contain sense organs that respond to pressure and thus enable their owners to detect nearby moving objects.)
Increase in size followed quickly after the reptiles began to radiate on land. For these reasons, and perhaps somewhat arbitrarily, I have included Diadectes among the basal reptiles although it is often regarded as a batrachosaur. By the end of the Carboniferous period there were many small captorhino-morph (anapsid) reptiles; while Pelycosauria (Synapsida) measuring more than 1 m in length had also evolved. Captorhinus and Labidosaurus are among the better known parareptiles from the lowest strata of the Permian. They had high, narrow skulls and pointed noses, which are characteristic of reptiles rather than of amphibians.
The basic physiological character that separates reptiles from amphibians is, unfortunately, not revealed in the fossil record. This is the possession of 'cleidoic' or enclosed eggs. Amphibian eggs develop and hatch in water, but those of reptiles are provided with a semi-impervious shell or membrane, and
the presence of water is not necessary for their survival. Ammonia, the chief excretory compound of Amphibia, is extremely soluble in water, but it is toxic when concentrated and would be unsuitable as the nitrogenous waste product of an enclosed embryo. Like insects and birds whose eggs are also laid on land, reptiles eliminate nitrogenous waste in the form of insoluble uric acid. Urea, the principal excretory compound of mammals, would not be suitable as the excretory product of an embryo in an enclosed egg because, when concentrated, it would upset the osmotic relations of the developing embryo. (The urea produced by a mammalian embryo passes through the placenta and is excreted by the mother.) Uricotelic metabolism originally evolved in relation to the possession of cleidoic eggs. It is retained in the developmental and adult stages of reptiles, as it is in insects and birds, because it has the advantage that nitrogenous wastes can be excreted with minimal loss of water.
The early reptiles increased and diversified throughout the Permian period, during which time the world's main reptilian types appeared. Nearly all the individual orders became established during the Triassic,but most of them only reached their maximum development in the Jurassic and Cretaceous periods. We shall now consider some of the earliest forms in greater detail.
Many of the earliest Anapsida, such as Hylonomus (Captorhinidae) from the Middle Carboniferous of Canada,were insectivorous. The remains of these animals are well preserved in the ancient tree stumps into which they crawled in pursuit of insects and worms. Although it closely resembled some of its am-
phibian contemporaries, Hylonomus had a high skull - a typical reptilian feature associated with additional jaw muscles - and almost certainly would have laid cleidoic eggs, although none of these has yet been found (Benton 1996). Other early reptiles had several parallel rows of teeth on both the upper and lower jaws, a character taken to excess by the Upper Permian Moradisaurus which had more than ten rows. This may have been an adaptation to a diet of tough vegetable matter.
Most Palaeozoic anapsids were parareptiles. As already mentioned, Diadectes (Fig. 4) was one of them. Later forms included Pareiasauridae such as the Late Permian Bradysaurus (Fig. 5) a large plant-eater about 2.5 m in length, and the massive Upper Permian Scutosaurus (Fig. 6), which was also herbivorous.
Despite the fact that the Testudines or Chelonia had evolved during the Permian, they did not begin to radiate until the Triassic period and will therefore not be discussed until Chapter 4. Another marine order was the shortlived Mesosauria of the Lower Permian period. Mesosaurus (Fig. 7), the only known genus, was a highly specialised, long-tailed aquatic form, 75 cm-1 m in length with broad hands and feet. The slender jaws were lined with needle-sharp interlocking teeth well adapted either for filter-feeding or for catching fishes (Sect 4.1).
Primitive synapsids were already present in the Late Pennsylvanian and the group took a leading role in the reptilian radiation of the Permian period. The order Pelycosauria is represented most abundantly in the Upper Carboniferous and Lower Permian strata of North America. Most of the pelycosaurs were small- to medium-sized insectivores and carnivores with strong skulls and sharp teeth. One of these was Varanosaurus (Fig. 8), a typical carnivore, about 90 cm in length with a lizard-like appearance little different from that of the anapsids and the primitive diapsids. Others, however, became adapted to a
■ Fig. 10. Dimetrodon (Pelycosauria; Lower Permian; length ca. 1.8 m). (Cloudsley-Thompson 1999)
■ Fig. 11. Cotylorhynchus (Pelycosauria; Middle Permian; length ca. 3 m)
vegetarian diet and were among the first terrestrial vertebrates. A few of the later pelycosaurs, such as Edaphosaurus (Fig. 9) and Dimetrodon grandis (Fig. 10) had large 'sails' supported on vertical spines of the vertebrae. These were probably thermoregulatory devices and acted as radiators (Tracy et al. 1986). In the morning, D. grandis would turn sideways to the sun. It would thus absorb heat with its sail, whilst during the hot part of the day it might have faced the sun or found a shaded retreat and radiated excess heat from the sail (see Sect. 7.5.2). Haack (1986) calculated that it would have taken a large Dimetrodon (weighing some 250 kg) about 12 h basking in sunshine to increase its body temperature from 25 to 30 °C without a sail, but with its sail it would have taken only 3 h. The ability to warm up comparatively quickly would greatly have benefited it in catching its prey. The weakness of this argument, however,lies in the fact that most pelycosaurs and their other contemporaries lacked sails yet seem to have survived quite well (Benton 2004). A very large pelycosaur, Cotylorhynchus (Fig. 11), from the Middle Permian of North America was the ecological equivalent of the gigantic therapsids that lived in South Africa and Russia about the same time.
The second order of Synapida, the Therapsida (Sect. 8.2), encompassed the mammal-like reptiles. The rocks of the Karoo basin of southern Africa are littered with the fossilised bones of these synapsid reptiles. They include both herbivorous and carnivorous Dinocephalia ('giant heads'). Some of the former were quite large. One genus, Moschops (Fig. 12), had massive shoulders and neck. Although the limbs and head were comparatively small, the skull was considerably thickened, as in other dinocephalians, and may have been adapted in this manner for head-butting contests at the time of mating. These thick-skinned reptiles formed the prey of carnivorous forms such as the small Titanosuchus (Dinocephalia; Fig. 13) and Sauroctonus (Gorgonopsia; Fig. 14) a large sabre-toothed therapsid. Although the former was a slender hunter that probably fed mainly on smaller herbivores and juveniles, many of the other titanosuchids were large, heavily-built predators and could not have been very agile or fast. The most common of the herbivorous therapsids were the Dicy-
nodontia ('two dog teeth'). These had large, sausage-shaped bodies, short legs and tails. They either possessed no teeth at all, or had just a pair of tusks; and they sliced the vegetation with sharp, horny beaks. A complex chewing cycle allowed them to exploit a wide variety of plants. When feeding, the lower jaw was first thrust forward, slid back forcibly cutting the food, and then opened from a backwards position. They were preyed on by gorgonopsians along with the carnivorous and other plant-eating dinocephalians. Neither dinocephalians nor
gorgonopsians survived after the Permian, but the dicynodonts flourished for a second time during the Triassic period.
The mass extinction at the end of the Permian period (ca. 250 mya) was the largest of all time (Table 3; Sect. 12.1.2). It has been estimated that considerably more than half the families of animals that had existed during the Late Permian abruptly disappeared. These included numerous groups of marine invertebrates such as the trilobites, many kinds of coral, and most of the brachiopods. Molluscs and fishes also suffered severely. The effects were equally devastating on land. Most of the primitive amphibians and of the mammal-like reptiles vanished (Jablonski 1989; Hallam and Wignall 1997; Benton 2003). Although surviving groups of the latter were eventually to give rise to the mammals (which almost certainly evolved from dicynodonts) the predominant reptiles of the Mesozoic Era were Diapsida. The former included the marine ichthyo-saurs and plesiosaurs: we shall deal with them before considering aerial and terrestrial reptiles. Benton (1996) has given a very clear account of continental drift and of conditions during the Carboniferous and Permian periods.
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