What if anything is a reptile

Organisms are commonly classified according to the biological classification system, first developed by the Swedish naturalist Carolus Linnaeus (1707-1778). His hierarchical system is the very famous (or infamous!) ranking of groups of organisms in groups of decreasing size: kingdom, phylum, class, order, family, genus, species.

Individuals are generally referred to by italicized generic (genus) and specific (species) names, for example in the case of a famous large dinosaur Tyrannosaurus rex. Any name in the hierarchy - representing a group of organisms - is considered a taxon (plural taxa).

All classifications have a purpose, and the biological classification is no exception. We classify for many purposes; for example our movies are classified both by subject (Drama, Horror, Comedy, etc.) as well as by suitability for viewing (PG-13, R, etc.). In the case of the biota, implicit in the classification is the degree of relatedness. Thus all members of a taxon - at any level in the hierarchy - are said to be more closely related to each other than any one is to anything else. And that's where the term "Reptilia," as it's conventionally understood, gets into all kinds oftrouble.

Linnaeus developed his classification long before evolution was proposed, so it really wasn't all about degree of relatedness; it was about grouping similar-looking things. His Reptilia (reptere - to crawl) denoted a group ofscaly, four-legged creatures crawling around on their bellies. And a look at the living representatives of "Reptilia" suggests a certain superficial similarity among the living reptiles: snakes, lizards, crocodilians, and the tuatara.

But are snakes, lizards, crocodilians, and the tuatara really more closely related to each other than they are to anything else? The cladogram in Figure 4.11 and those in Chapter 10 demonstrate that, on the basis of shared derived characters, birds are more closely related to crocodiles than crocodiles are to lizards. And that's just not possible unless a bird is a reptile. But how can a bird be a reptile?

The simplest answer is that clearly we have a decidedly different Reptilia from your parents' (and Linnaeus's) traditional motley crew of crawling, scaly, non-mammalian, non-bird, non-amphibian creatures that were once tossed together as reptiles. If it is true that crocodiles and birds are more closely related to each other than either is to snakes and lizards, a monophyletic group that includes snakes, lizards, and crocodiles must also include birds.

Birds are reptiles because birds share the derived characters of Reptilia, as well as having unique characters of their own (see also Chapter 10). The inclusion (above) of birds among the living members of Reptilia is contrary to the conventional way of classifying birds, but more accurately reflects who they are (and where they come from).

So what, finally, is a reptile? The living reptiles = turtles + diapsids (including birds). Figure 4.7 shows the position of Reptilia on the cladogram and includes some diagnostic characters for the group.

and a rare, limbless, tropical amphibian known as a caecilian. If the living amphibians are any guide, the life cycles of anamniotes are intimately associated with water, as the eggs require, and likely required, an external source of moisture.

Amniotes, by contrast, are fully terrestrial, and need a means of retaining moisture within the egg. The semi-permeable amnion allows gas exchange but retains water, which permits the embryo to be continuously bathed in liquid. The evolutionary appearance of the amnion occurred in conjunction with several other features including a calcified shell, a large yolk for the nutrition of the developing embryo, and a special bladder for the management of embryonic waste. Amniotic eggs can thus be laid on land without drying out, which allowed amniotes to sever all ties with water (other than for drinking). This was a key step in the evolution of a completely terrestrial lifestyle, and is commonly associated with the advent of reptiles (Box 4.2).

There are three great groups of amniotes - primitive amniotes, sometimes termed anap-sids (a - without; apsid- arch), Synapsida (syn - with), and Diapsida (di - two). They're most easily distinguished by the number and position of the openings in the skull roof behind the eyes, called temporal fenestrae (fenestra - window; Figure 4.9). Our main interest is in diap-sids, but we'll detour briefly to look at some basal amniotes and Synapsida.

Fully roofed temporal region

Anapsid

Figure 4.9. Three major skull types found in amniotes.

Fully roofed temporal region

Anapsid

Upper temporal opening

Temporal Opening

Braincase Diapsid (visible through lower temporal opening)

Upper temporal opening

Lower temporal opening

Lower temporal opening

Synapsid

Braincase (visible through lower temporal opening)

Lower temporal opening

Braincase Diapsid (visible through lower temporal opening)

Synapsid

Anapsids and Synapsida

The anapsid condition represents what is thought to have been the original morphology of the skull roof in amniotes. In these amniotes, the skull behind the eyes is completely roofed; they therefore have no temporal fenestrae. The anapsid condition is seen in some long-extinct, bulky quadrupeds that do not concern us here, and persists today only in turtles (Chelonia). Legendary stalwarts of the world, turtles are unique: these venerable creatures with their portable houses, in existence since the Late Triassic (210 million years ago), will surely survive another 200 million years at least if we let them.

Synapsida is one of two great lineages of amniotic tetrapods. All mammals (including ourselves) are synapsids, as are a host of extinct forms, traditionally called "mammal-like reptiles" (Figure 4.10). The split between the earliest synapsids and the earliest representatives of the other great lineage, Diapsida (including dinosaurs), likely occurred between 310 and 320 Ma. Since then, therefore, the synapsid lineage has been evolving independently, genetically unconnected to any other group.

Synapsids are united by a skull roof that is a departure from primitive tetrapods: the skull roof has developed a low opening behind the eye - the lower temporal fenestra (see Figure 4.9). Jaw muscles pass through this opening and attach to the upper part of the skull roof. Synapsids are a remarkable and diverse group of amniotes, and could easily fill a book just like this; but because we're interested in dinosaurs, we'll regretfully move right on past them.

The other great clade of amniotes is Diapsida (see Figures 4.7 and 4.9). The living diapsids include about 15,000 total species including snakes, lizards, crocodiles, the tuatara (a reptile

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