The first reptiles - forms like Hylonomus (Sect. 2.4) appeared in the Middle Carboniferous (Benton 1996). These shortly led to the three main divisions of reptiles, the anapsids, diapsids and synapsids, characterised by the temporal openings or fenestrae in their skulls (Fig. 2). Indeed, knowledge of the interrelationships of reptiles depends mainly upon their fossil skeletons, of which skulls are by far the most useful and important. They are of four different types (Fig. 2). In the subclass Anapsida, the dermal bones of the temporal region of

■ Fig. 2. Reptilian skulls showing arrangement of temporal openings. The various bones have not been named. (Cloudsley-Thompson 1999 based on Young 1981)

the skull behind the orbit present an unbroken surface and there are no apertures between them. This character separates the parareptiles ('beside reptiles') and the Testudines or Chelonia (turtles and tortoises) from all other reptiles. In more advanced reptiles, there are one or two fenestrae surrounded by bony arches in the temporal region. These holes allow the jaw muscles to bulge into them, thereby facilitating their actions during feeding. The Lepidosauria and Archosauria, probably the most successful subclasses of reptiles, have two fenestrae and are therefore known as Diapsida. The lower temporal arch is incomplete in the Squamata, while the upper arch, too, has been lost in some extant lizards and in snakes (Table 2). Only a single fossa and arch are present in the remaining subclasses. In parapsid reptiles this is situated high on the skull. This condition is found in the Ichthyosauria and Plesiosauria, both of which probably evolved from diapsid precursors. In the remaining subclass, Synap-sida, there is also only one fenestra but, in the earlier forms at least, this was located lower in the skull (Fig. 2). The term synapsid means fused arch. It was applied to the group by early workers who believed, incorrectly, that the single arch had been derived from fusion of the two arches found in diapsid reptiles (Table 2).

When Table 2 is considered in relation to Figs. 1 and 2, the relationships between the various taxa or groups of reptiles should become clear. The only reptiles found in the world today are Testudines or Chelonia, Lepidosauria, and Crocodylia. The remainder are extinct.

One final point requires clarification. It will be seen that the dinosaurs ('terrible lizards') incorporated two orders, Saurischia and Ornithischia. The Sauri-

■ Table 2. Simplified classification of reptiles Class Reptiles

Subclass Anapsida ('without arches') Order Parareptilia ('beside reptiles') Family Mesosauridae ('middle lizards') Order Testudines or Cheloniaa (turtles and tortoises)

Subclass Sauropterygia ('reptile fins')

Placodonts, nothosaurs, plesiosaurs etc.

Subclass Ichthyopterygia ('fish fins') Ichthyosaurs ('fish lizards')

Subclass Diapsida ('double arches')

Infraclass Lepidosauria ('scaly lizards')

Order Rhynchocephaliaa ('beak headed') Order Squamataa ('scaly ones')

Lizards, snakes, and amphisbaenians

Infraclass Archosauria ('ruling reptiles') Order Thecodontia ('socketed teeth') Order Crocodyliaa (crocodiles) Order Pterosauria ('wing lizards') Pterodactyls, etc

Superorder Dinosauria ('terrible lizards') Order Saurischia ('lizard pelvis')

Theropod and sauropod dinosaurs Order Ornithischia ('bird pelvis')

Ornithopod dinosaurs. Stegosaurs, ankylosaurs, and ceratopsians

Subclass Synapsida ('fused arches')

Order Pelycosauria ('basin lizards') Order Therapsida ('mammal arches') Mammal-like reptiles a Groups containing extant representatives schia comprised two suborders Theropoda and Sauropoda. The earlier thero-spods were small, bipedal dinosaurs but, by the end of the Cretaceous period, they had produced Tyrannosaurus and its relatives, the largest carnivores ever to be seen on land. The Sauropoda, on the other hand, were quadrupedal vegetarians culminating in the immense Jurassic Apatosaurus, Diplodocus, and Seismosaurus. The second order of dinosaurs, the Ornithischia, appeared in the Jurassic and reached a peak in the Cretaceous period, by which time the sauropods had already become less common. The first ornithischians were bipedal, like Iguanodon, but several lines reverted to a quadrupedal habit and some of the latter developed heavy armour and defensive spines.

The main difference between the two orders lies in the shape of the pelvis (Fig. 3). In the Saurischia, this was similar to that of other reptiles, but in the Ornithischia it was bird-like. In both orders, the hip socket was surrounded by

■ Fig. 3. Pelvic bones of dinosaurs. Left A saurischian; centre a primitive ornithischian; right a typical ornithischian. Ilium (white); ischium (stippled); pubis (black). (Cloudsley-Thompson 1999)

three bones: the ilium above, the ischium below and behind, the pubis below and in front. In the Saurischia the pubis pointed forward and downwards as a single prong; while, in the Ornithischia, it usually had two prongs, one projecting forward and upwards, the other backwards so that it lay just below the ischium. In certain primitive ornithischians the forward prong was absent.

The Triassic archosaurs gave rise to the pterosaurs and the dinosaurs: the saurischian dinosaurs gave rise to birds in the Jurassic period. Recent sequencing of whole mitochondrial genomes suggests that turtles may be related to a bird/crocodile clade or linear branch. (The term clade is derived from a Greek word meaning a twig or branch.) Birds are much more closely related to Mesozoic reptiles than mammals are, and the Testudines may be more like diapsids than has previously been realised (see Benton 1990a).

The most bird-like of the dinosaurs were the Cretaceous theropods, whereas birds were well established by the end of the Jurassic period. Birds flourished during the Mesozoic Era and colonised numerous ecological niches. The number of known fossilised bird taxa has increased greatly in recent years, shedding light on the large temporal and evolutionary gap between the Late Jurassic Archaeopteryx lithographica on the one hand, and superficially diver-like hespe-rornithiform and pelican-like ichthyornithiform birds on the other. These findings provide additional evidence for the hypothesis that birds evolved from cursorial, bipedal carnivorous dinosaurs during the Jurassic period.

Although doubts have been expressed regarding the hypothesis that birds evolved from dinosaurs, the discovery that the microscopic Haversian canals (which carry nutrients to bone cells) of coelurosaurian dinosaurs were organized in the same way as they are in birds today, suggests a strong link between the two. Furthermore, the bundles of collagen fibres which bind bone materials have the same irregular structure in both birds and coelurosaurians. These matters will be discussed further in Chapter 11 (Sect. 11.5).

Our knowledge of extinct animals depends almost entirely upon the study of fossils, as already mentioned. Usually only the hardest parts of animals, such as bones and shells, become fossilised. It should be remembered that countless generations of innumerable species have existed in the past without leaving any traces, and that the fossils we do have represent only a minute fraction of the billions of reptiles that populated the Mesozoic Era. Yet,it is with these that palaeontologists work to reconstruct individual skeletons and even entire extinct faunas. The object of the present book is to clothe these skeletons with flesh, and to show something of the ways in which their possessors lived.

Taxonomy is the scientific ordering and labelling of groups of similar organ-isms,while systematics is the study of the diversity of organisms within clades. Clades are groups of organisms that are monophyletic - that is, they have a common ancestry. Such groups are known as taxa and are assigned a Linnaean rank. Standard ranks are phylum, class, order, family, genus and species. Between these are intermediate groups and subgroups at different levels, e.g. superclasses, subfamilies and so on. In addition, divisions, cohorts and other ranks may be added. This system was the one most widely used until the 1970s and has the advantage of showing which taxa are most closely related, and it indicated their hierarchical levels. For convenience it has been generally adopted in the present book.

It must be realised, however, that an order such as Thecodontia (Table 2) includes archosaurs that are more advanced than earlier reptiles, but more primitive than dinosaurs and birds. At the same time, they are not characterised by any unique features and they lack the specialisations of the more advanced forms. Cladistics, or phylogenetic systematics, is therefore now used by most taxonomists in place of the earlier evolutionary systematics, because it shows the closeness of the ancestry between groups that arose when populations split - a matter of little concern here (Holtz and Brett-Surman 1997). For a detailed cladistic classification of Mesozoic reptiles, see Benton (2004) and Palmer (1999): earlier classifications are given by Romer (1966) and Young (1981). See also Blount and Crowley (2001). The classification of dinosaurs has been given extensive treatment by Weishampel et al. (1990), Currie and Padian (1997), and Farlow and Brett-Surman (1997), among others.

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