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Vertebrata

Figure 4.2. Cladogram of the Chordata. Because this is a book about dinosaurs (and not all chordates), we have provided diagnoses for only some of the groups on the cladogram. Bars denote the shared, derived characters of the groups.The characters are the following: I pharyngeal gill slits, a notochord, and a nerve cord running above the notochord along its length; 2 segmentation of the muscles of the body wall, separation of upper and lower nerve and blood vessel branches, and new hormone and enzyme systems; 3 bone organized into elements, neural crest cells, the differentiation of the cranial nerves, the development of eyes, the presence of kidneys, new hormonal systems, and mouthparts; 4 true jaws; 5 bone in the endochondral skeleton; 6 ray fins;

7 distinctive arrangement of bones in fleshy pectoral and pelvic fins (see Figure 4.4);

8 skeletal features relating to mobility on land - in particular four limbs. Consistent with a cladistic approach, only monophyletic groups are presented on the cladogram. Some of the groups may not be familiar; for example, Cephalaspis and Eusthenopteron are not discussed in the text. Cephalaspis was a primitive, jawless, bottom-dwelling, swimming vertebrate, and Eusthenopteron was a predatory lobe-fin, bearing many characters present in the earliest tetrapods. Cephalaspis and Eusthenopteron are included here to complete the cladogram as monophyletic representatives of jawless vertebrates (the non-monophyletic "Agnatha") and lobe-finned fishes, respectively.

Cephalochordates, the closest relative to the vertebrates, are best known to us in the form of the small, water-dwelling lancet (Amphioxus or Branchiostoma; Figure 4.3). Again, known primarily from present-day forms (one specimen was recovered from the middle of the Paleozoic Era), these creatures share with the vertebrates a host of derived features, including segmentation of the muscles of the body wall, separation of

Figure 4.3. Two primitive chordates. (a) Amphioxus (Branchiostoma); (b) an adult sea squirt (Ciona intestinalis); and (c) the larval form of the sea squirt. Note that the juvenile form is free-swimming and has a notochord running down its tail. It metamorphoses into into the stationary adult by planting itself on its nose and rearranging its internal and external structures.

Figure 4.3. Two primitive chordates. (a) Amphioxus (Branchiostoma); (b) an adult sea squirt (Ciona intestinalis); and (c) the larval form of the sea squirt. Note that the juvenile form is free-swimming and has a notochord running down its tail. It metamorphoses into into the stationary adult by planting itself on its nose and rearranging its internal and external structures.

upper and lower nerve and blood vessel branches, and many newly evolved hormone and enzyme systems. As our first cousins among chordates, cephalochordates themselves are united by important derived characters of the head region. Figure 4.2 takes us through the major groups of chordates to Tetrapoda.

Vertebrata We have now reached Vertebrata. The features that unite vertebrates, with one exception, include calcified skeletal tissue (i.e., bone) divided into discrete parts called elements, and a variety of other characters (see Figure 4.2).

Gnathostomata What we might unthinkingly regard as the body plan for all vertebrates is really the plan for a subset of the vertebrates called Gnathostomata (gnathos- jaw; stome - mouth). Gnathostomes are vertebrates with true jaws and a variety of other features (see Figure 4.2). In fact, the absence of jaws is primitive for vertebrates, and the evolution of jaws was an important innovation. So, who are these gnathostomes? Well, we certainly are, as are sharks (starring in the movie Jaws), ray-finned fishes (familiar in our aquaria and filleted with lemon and french fries on our dinner plates), and lobe-finned fish (fleshy-finned creatures that tinkered with muscular fins and, possibly, air breathing). Other, less familiar, extinct gnathostome fishes that do not concern us here also cruised the ancient seas.

Sharks and their relatives belong to a clade within Gnathostomata called Chondrichthyes (chondros - cartilage; ichthys - fish). The remaining fishes are grouped together within Osteichthyes (os - bone), bony fishes that include the ray-finned and lobe-finned gnathostome groups. The ray-finned branch of osteichthyans is called Actinopterygii (actino - ray; ptero -wing) and is one of the most diverse groups (more than 20,000 living species) ever to have evolved among vertebrates (see Figure 4.2). The other

Biological classification: what's In a name?

Organisms are commonly classified according to the biological classification system, the codified series of categories that have been dutifully memorized by generations of nascent biologists.The classification was first developed by the Swedish naturalist Carolus Linnaeus (1707-1778) who, a full 100 years before Darwin first published The Origin of Species, created a formalized hierarchical nomenclature to be applied to the biota. His system of increasingly small categories has been used ever since.The categories that he established - now a memorized mantra - are (in order of decreasing size), kingdom, phylum, class, order; family, genus, species.

Thus a cat would be classified in the following way:

Kingdom: Animalia (a cat is an animal, and not a i plant, or a fungus, or some kind of bacterium)

Phylum: Chordata (the embryo has a notochord);

Class: Mammalia (it has fur and mammary glands)

Order: Carnivora (it bears a distinctive pair of shearing teeth, its carnassials)

Family: Felidae (it has certain features in the ear)

Genus: Felis (it has distinctive soft anatomy in its nose)

Species: domesticus (it's a house cat)

Individuals are generally referred to by italicized generic (genus) and specific (species) names: in the case of a cat, Felis domesticus. Each of these names -indeed, any name in the hierarchy - is considered to be ataxon (plural: taxa).

Organisms have names applied to them by the scientists who first recognize them as new or distinct. Hence paleontologists have created names ranging from Deinonychus ("terrible claw," to acknowledge the formidable claws on the feet of this carnivorous dinosaur) to Cuttysarchus, a small lizard whose name celebrates a well-known scotch whiskey.

All classifications have a purpose, and the biological classification is no exception. We classify by many things; for example, our movies are classified both by subject (Drama, Horror; Comedy, etc.) as well as by suitability for viewing (PG I 3; R, etc.). In the case of the biota, implicit in the classification is the degree of relatedness.Thus all members of a single species are said to be more closely related to each other than anyone is to anything else. At any level in the classification, it is assumed that all members of a group are more closely related to each other than they are to members of an equivalent, but different, group. In the case of the order Carnivora, therefore, it is assumed that all carnivores are more closely related to each other than they are to taxa in any other order In this chapter; we see how this gives a new meaning to the word "Reptilia."

Here we will avoid using most Linnaean terms except for generic and specific ones.The reason for this is that the level in the hierarchy that is designated by any particular Linnaean term is arbitrary: there is no absolute quantity of morphological disparity that is equated with a particular Linnaean term. For example, some people might designate the dinosaur group Ornithischia an order; but some might consider it a suborder Since there is no quantified disparity embodied in the level "order" neither viewpoint is right and, what's worse, we have learned nothing more about Ornithischia, itself. For us, the important facts about Ornithischia are its shared, derived characters and where (relative to other groups) it is situated in the hierarchy. Moreover if a formal name were introduced to designate every rank or monophyletic group revealed by the cladogram, this book, already filled with a formidable nomenclature, would be nothing but an endless list of formal names.The cladogram plainly indicates hierarchical relationships without resorting to formalized Linnaean designations.

Figure 4.4. Some homologous features between lobe-finned fishes and tetrapods. (a) the shoulder girdle of Eusthenopteron, an extinct sarcopterygian; (b) the shoulder girdle in early tetrapods. Because aspects of the foreiimb in different early tetrapods are incomplete, the foreiimb shown here is a composite prepared from two early tetrapods, Acanthostega and Ichthyostega. Key homologous bones are labeled in both drawings.

Figure 4.4. Some homologous features between lobe-finned fishes and tetrapods. (a) the shoulder girdle of Eusthenopteron, an extinct sarcopterygian; (b) the shoulder girdle in early tetrapods. Because aspects of the foreiimb in different early tetrapods are incomplete, the foreiimb shown here is a composite prepared from two early tetrapods, Acanthostega and Ichthyostega. Key homologous bones are labeled in both drawings.

osteichthyan branch, the lobe-fins or Sarcopterygii (sarco - flesh), includes lungfish (of which only three types are alive today); coelocanths (of which only one type is alive today); extinct barracuda-like carnivorous forms; and, surprisingly, tetrapods (which share the derived characters of the group; see Figure 4.2 and Box 4.2). Indeed, bones homologous with those of the limbs, pelvis, vertebral column, and skull of tetrapods are all found within non-tetrapod members of the lobe-finned clade, strongly uniting the tetrapods to other members of this group (Figure 4.4). Those homologies - and many others - indicate that it is here, among the lobe-fins, that the ancestry of Dinosauria - as well as our own ancestry - is to be found.

Tetrapoda Those gnathostomes central to our story are tetrapods. Tetrapoda (tetra - four; pod - foot) connotes the appearance of limbs, an adaptation that is strongly associated with land. According to the conventional Linnaean classification (Box 4.2), there are four classes of tetrapods:

Tetrapoda

1 Amphibia,

2 Reptilia,

3 Aves, and

4 Mammalia

The living amphibians are frogs, salamanders (and newts), and a group of rare tropical, limbless creatures called caeclians. The living reptiles are crocodiles, turtles, snakes, and lizards, and the tuatara, an unusual

Easy Drawings Tuatara

Figure 4.5. Cladogram ofTetrapoda. Diagnostic characters include: at I the tetrapod skeleton (see Figures 4.7 and 4.8); at 2 a lower temporal fenestra (see Figure 4.10); at 3 presence of an amnion (see Figure 4.9); at 4 lower and upper temporal fenestrae (see Figure 4.10); at 5 an antorbital fenestra (see Figure 4.13). Lepidosauromorpha is a monophyletyic group, the living members of which are snakes, lizards, and the tuatara. Chelonia - turtles - are reptiles whose primitive, completely roofed skulls place them near the base of Reptilia.

lizard-like creature that lives only in New Zealand. Mammals and living birds (Aves) are common forms familiar to all of us.

The traditional classification is actually a very inadequate way to reflect the interrelationships of the tetrapods (Box 4.3). Figure 4.5 is a cladogram showing the major groups of tetrapods. It shows their phylo-genetic relations and leads to a very different understanding of vertebra te interrelationships from that implied by the traditional classification. Even the apparently monophyletic Aves (for who cannot diagnose a bird by its feathers?) is most accurately viewed as an artifact of our post-Mesozoic perspective. As this chapter unfolds, we will address many issues relating to tetrapod relationships.

The tetrapod skeleton made easy

Backbone

The tetrapod skeleton is a modification of the skeletal component of the fundamental vertebrate body plan. We shall see in succeeding chapters how, through evolution, dinosaurs have modified this basic skeleton in a variety of ways. The backbone is composed of distinct, repeated structures (the vertebrae), which consist of a lower spool (the j- BOX 4.3

Fish and chips

As 1978 turned to 1979, a provocative and entertaining letter and reply were published in the scientific journal Nature, discussing the relationships of three gnathostomes: the salmon, the cow, and the lungfish.1 English paleontologist L. B. Halstead argued that, obviously, the two fish must be more closely related to each other than either is to a cow. After all, he pointed out, they're both fish! A coalition of European cladists disagreed, pointing out that, in an evolutionary sense, a lungfish is more closely related to a cow than to a salmon. In their view, if the lungfish and the salmon are both to be called "fish," then the cow must also be a fish. Can a cow be a fish?

The vast majority of vertebrates are what we call "fishes.'They all make a living in either salt or fresh water and, consequently, have many features in common that relate to the business of getting around, feeding, and reproducing in a fluid environment more viscous than air But as it turns out, even if "fishes" describes creatures with gills and scales that swim, "fishes" is not an evolutionarily meaningful term because there are no shared, derived characters that unite all fishes that cannot also be applied to all non-fish gnathostomes.The characters that pertain to fishes are either characters present in all gnathostomes (i.e., primitive in gnathostomes) or characters that evolved independently.

The cladogram in Figure B4.3.1 is universally regarded as correct for the salmon, the cow, and the lungfish. In light of what we have discussed, this cladogram might look more familiar using groups to which these creatures belong: salmon are ray-finned fishes, cows are tetrapods, and lungfishes are lobe-finned fishes. Clearly, lobe-finned fishes share more derived characters in common with tetrapods than they do with ray-finned fishes.Thus there are two clades on the cladogram:

1 lobe-finned fishes + tetrapods; and

2 lobe-finned fishes + tetrapods + ray-finned fishes,

Clade I is familiar as Sarcopterygii. Clade 2 occurs at the level of Osteichthyes and looks like part of

Salmon Cladogram
Figure B4.3.1 .The cladistic relationships of a salmon, a cow, and a lungfish.The lungfish and the cow are more closely related to each other than either Is to the salmon.

the cladogram presented in Figure 4.2 for gnathostome relationships. If only the organisms in question are considered, the only two monophyletic groups on the cladogram must be I lungfish + cow; and 2 lungfish + cow + salmon (i.e., representatives of the sarcopterygians and Osteichthyes, respectively).

Which are the "fishes?" Clearly the lungfish and the salmon. But the lungfish and the salmon do not in themselves form a monophyletic group unless the cow is also included.The cladogram is telling us that the term "fishes" has phylogenetic significance only at the level of Osteichthyes (or even below). But we can and do use the term "fishes" informally. "Fish and chips" will never be a "burger and fries."

I See Gardiner B. G„ Janvier; R, Patterson, C., Fortey, R L, Greenwood, R H„ Mills, R. S. and Jeffries, R. R S. 1979.The Salmon, the cow, and the lungfish: a reply. Nature, 277, 175-176. Halstead, L. B. 1978.The cladistic revolution - can it make the grade? Nature, 276,759-760.

Spinal Nerve Cow
Figure 4.6. A vertebra from Apatosaurus, a sauropod dinosaur The nerve cord (indicated by arrow) lies in a groove at the top of the centrum and is straddled by the neural arch.

centrum), above which, in a groove, lies the spinal cord (Figure 4.6). This relationship was first developed in gnathostomes and modified in tetrapods. Planted on the centrum and straddling the spinal cord is a vertically oriented splint of bone called the neural arch. Various processes, parts of bone that are commonly ridge, knob, or blade shaped, may stick out from each vertebra. These can be for muscle and/or ligament attachment, or they can be sites against which the ends of ribs can abut. The repetition of vertebral structures, a relic of the segmented condition that is primitive for vertebrates, allows flexibility in the backbone. In general, however, the tetrapod backbone is considerably more complex than the backbones that preceded it, because in tetrapods the backbone acts not only to facilitate locomotion, but also to support the body out of the water.

Pelvic and shoulder girdles.

Sandwiching the backbone are the pelvic and pectoral girdles (Figure 4.7). These are each sheets of bone (or bones) against which the limbs attach for the support of the body. The pelvic girdle - which includes a block of vertebrae called the sacrum - is the attachment site of the hindlimbs; the pectoral girdle is the attachment site of the forelimbs. Each side of the pelvic girdle is made up of three bones: a flat sheet of bone, called the ilium, that is fused onto processes from the sacrum; a piece that points forward and down, called the pubis; and a piece that points backward and down, called the ischium. Primitively, the three bones come together in a depressed area of the pelvis called the hip socket. The pectoral girdle consists of a flat sheet of bone, the scapula (shoulder blade), on each side of the body, attached to the outside of the ribs by ligaments and muscles.

Chest

A couple of other important elements deserve mention. The breastbone (sternum) is generally a flat or nearly flat sheet of bone or cartilage that is locked into its position on the chest by the tips of the thoracic (or chest) ribs. The rib cage is supported at its front edge by bones in the shoulder (the coracoid bones).

Legs and arms

Limbs in tetrapods show a consistency of form, an arrangement that was pioneered in their sarcopterygian ancestors (see Figure 4.4). All limbs, whether fore or hind, have a single upper bone, a joint, and then a pair of lower bones. In a foreiimb, the upper arm bone is the humerus, and the paired lower bones (forearms) are the radius and ulna. The joint in between is the elbow. In a hindlimb, the upper bone (thigh bone) is the femur, the joint is the knee, and the paired lower bones (shins) are the tibia and fibula.

Beyond the paired lower bones of the limbs are the wrist and ankle bones, termed carpals and tarsals, respectively. The bones in the palm of the hand are called metacarpals, the corresponding bones in the foot are called metatarsals, and collectively they are termed metapodials. Finally, the small bones that allow flexibility in the digits of both the hands (fingers) and the feet (toes) are called phalanges (singular - phalanx). At the tip of each digit, beyond the last joint, are the ungual phalanges. Until very recently, the primitive condition in tetrapods was considered to be the possession of five digits on each limb. Hence, in terms of the numbers of digits they possess, humans, for example, were thought to retain the primitive condition. Now it is known that tetrapods primitively had as many as eight digits on each limb. Early on in the evolutionary history of tetrapods, this number rapidly reduced to, and stabilized at, five digits on each limb, although many groups of tetrapods subsequently reduced that number even further (Figure 4.7).

Head

At the front end of the vertebral column of chordates are the bones of the head, composed, as we have seen, of the skull and mandible (lower jaws). Primitively, the skull has a distinctive arrangement: central and toward the back of the skull is a bone-covered box containing the brain, the braincase. At the back of the braincase is the occipital condyle, the knob of bone that connects the braincase (and hence the skull) to the vertebral column (mammals, unique among vertebrates, have two occipital condyles). The opening in the braincase that allows

Figure 4.7. Exploded view of atetrapod skeleton exemplified by the saurischian dinosaur Plateosaurus.

Figure 4.7. Exploded view of atetrapod skeleton exemplified by the saurischian dinosaur Plateosaurus.

Quadrate Bone DinosaursExploded View Skull Bones

Quadrate

Foramen magnum

Paraoccipital process

Quadratojugal

Quadrate

Occipital condyle

Skull roof Frontals Parietal

Orbit

; Braincase

Squamosal

Wgnum

Cross-section through braincase p| Occipital 1 condyle

Maxilla

Nasals

Maxilla

Premaxilla

Quadratojugal

Quadrate

Foramen magnum

Paraoccipital process

Dentary

Frontals Squamosal

Postorbital

Angular

Articular

Quadrate

Occipital condyle

Lacramal

Parietal

Paraoccipital process

Figure 4.8. Skull and mandible of Plateosaurus.

the spinal cord to attach to the brain is called the foramen magnum (foramen - opening; magnum - big). Located on each side of the braincase are openings for the stapes, the bone that transmits sound from the tympanic membrane (ear drum) to the brain. Covering the braincase and forming much of the upper part of the skull is a curved sheet of inter-locking bones, the skull roof. Among tetrapods, the bones comprising the skull roof have a distinctive pattern with clearly recognizable positions (Figure 4.8).

Primitively, the skull roof has several important openings. Located midway along each side of the skull is a large, round opening - the eye socket, or orbit. At the anterior tip of the skull is another pair of openings - the nares, or nostril openings. Finally, located dorsally and centrally in the skull is a small opening called the pineal, or "third eye." This is a light-sensitive window to the braincase that has been lost in most living tetrapods and thus is not terribly familiar to us.

Flooring the skull, above the mandible, is a paired series of bones, organized in a flat sheet, which forms the palate.2 Tetrapods share a

Figure 4.9. Amniote egg.

variety of derived features. We have seen many of these in tetrapod skeletons: the distinctive morphologies of the girdles and limbs, as the fixed patterns of skull roofing bones. The likelihood of all of these shared similarities evolving convergently is remote; for this reason, these characters establish Tetrapoda as a monophyletic group.

Amniota Amniotes are fully terrestrial, a step which required various means of retaining moisture. Only the embryos of snakes, lizards, birds, and primitive mammals possess in their eggs a membrane, the amnion, that retains moisture and allows the embryo to be continuously bathed in liquid (Figure 4.9). The amnion occurs in conjunction with several other features: a 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 allows those creatures possessing them to sever all ties with bodies of water. This was a key step in the evolution of terres-triality, and, with the advent of these innovations, three great groups of reptiles evolved - Anapsida, Synapsida, and Diapsida (Figure 4.10).

In the mean time, what of the non-amniotes? The living non-amniotic tetrapods are the modern Amphibia (or Lissamphibia), but in the past a bewildering variety of different non-amniotic tetrapods existed. With the exception of the living amphibians, most non-amniotes were extinct by the middle of the Jurassic, not long after the first true dinosaurs appeared on earth.3

2 In mammals, a passage forms between the floor of the nasal cavity and the roof of the oral cavity (mouth) so that air breathed in through the nostrils is guided to the back of the throat, bypassing the mouth. As a result, it's possible for chewing and breathing to occur at the same time. Similar kinds of palates (called secondary palates) are known in other tetrapods besides mammals, but primitively the nostrils lead directly to the oral cavity through tubes called choanae. So, if food were to be extensively chewed in the mouth, it would quickly get mixed up with the air that is breathed in. Obviously, extensive chewing is not a behavior of primitive tetrapods.

3 Although most of these archaic non-amniotes were extinct by the Middle Jurassic, recent discoveries in Australia suggest that a relict few lingered on there until the Early Cretaceous.

Synapsld

Braincase

Diapsid

Figure 4.10. Three major skull types found in amniotes.

Fully roofed temporal region jnr' Upper temporal

Anapsid opening

Fully roofed temporal region

Synapsld

Braincase

Diapsid

Braincase

Figure 4.10. Three major skull types found in amniotes.

Synapsida Synapsids are one of the two great lineages of amniote tetrapods; it is the other that includes the dinosaurs. All mammals (including ourselves) are synapsids, as are a host of extinct forms, traditionally (and, as we shall see, misleadingly) called "mammal-like reptiles." The split between the earliest synapsids and the earliest representatives of the other great lineage, the reptiles (including dinosaurs), 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 their common possession of a distinctive skull type that is a departure from the primitive tetrapod skull type. As we noted earlier, the primitive condition in tetrapods consists of a sheet of interlocking bone covering the braincase. In synapsids, however, the skull roof has developed a low opening behind the eye - the lower temporal fenestra (fenestra - window) - whose position is such that, near its base, the skull roof seems to form an arch over each side of the braincase region (Figure 4.10). From this comes the name synapsid (syn -with; apsid - arch). Jaw muscles pass through this opening and attach to the upper part of the skull roof. Many other features that we won't discuss here unite Synapsida as well.

Synapsids are an important group, and a book of equal size and interest to this one could be written about them. The most famous of the early known synapsids with elongate neural spines is the late Paleozoic Dimetrodon, from the southwestern USA (Figure 4.11). Dimetrodon was a 2 m long, powerful quadruped with a deep skull full of nasty, carnivorous teeth and, assuredly, a malevolent personality to match. Although passed off on the cereal-box circuit as a dinosaur, Dimetrodon is a much closer relative of humans than it is of any dinosaur that ever lived.

Synapsids radiated during the late Paleozoic and, by the Middle Triassic, were the dominant terrestrial vertebrates. In the early Mesozoic, they developed a worldwide distribution and had diversified into a variety of herbivorous and carnivorous habits. By the Late Jurassic, all that was left was a clade of tiny, mangy night-dwellers: mammals. What

Quadruped Dinosaur With Fin
Figure 4.1 I. Dimetrodon grandis, a fin-backed synapsid from the late Paleozoic of eastern Texas, USA. (From Römer; A. S. and Price, L. I. 1940. Review of the Pelycosauria. Geological Society of America Special Paper no. 28.)

happened to synapsids in the Late Triassic and Early Jurassic remains a mystery (see Chapters 5 and 15).

Repti I ¡a The other great clade of amniotes is Reptilia (reptere - to creep; see Figure 4.5). The living exemplars include about 15,000 total species comprising turtles, snakes, lizards, crocodiles, the tuatara, and birds, but - and this knowledge is a prerequisite for admission to Kindergarten - Reptilia also includes dinosaurs (as well as their close relatives, pterosaurs, as well as many other forms). With so many extinct vertebrates, nobody really knows how many members of this clade have come and gone.

Reptilia is diagnosed by a braincase and skull roof that are uniquely constructed and by distinctive features of the neck vertebrae. Figure 4.8 shows the typical reptilian arrangement of bones in the skull roof and braincase.

The inclusion (above) of birds among the living members of Reptilia is counter to the conventional way of classifying birds, but more accurately reflects their phylogenetic relationships. Clearly we have a decidedly different Reptilia from the 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 mono-phyletic group that includes snakes, lizards, and crocodiles must also include birds. The implication of calling a bird a reptile is that birds share the derived characters of Reptilia, as well as having unique characters of their own. These arguments are developed in Chapter 13.

Within Reptilia are two equally important clades: Anapsida and Diapsida. The first, Anapsida (a - without), consists of Chelonia (turtles) and some extinct, bulky quadrupeds that do not concern us here.

Anapsida

Legendary stalwarts of the world, turtles are unique: these venerable creatures with their portable houses, in existence since the Late Triassic (210 Ma), will surely survive another 200 million years if we let them.

Diapsida Diapsida (di - two) is united by a suite of shared, derived features including having two temporal openings in the skull roof, an upper (or supra) and a lower (or infra) temporal fenestra. The upper and lower temporal openings are thought to have provided space for the bulging of contracted jaw muscles, as well as increased the surface area for the attachment of these muscles. There are two major clades of diapsid reptiles. The first, Lepidosauromorpha (lepido - scaly; sauros - lizard; morpho - shape; note that the suffix sauros means "lizard" but is commonly used to denote anything "reptilian") is composed of snakes and lizards and of the tuatara (among the living), as well as a number of extinct lizard-like diapsids.4

Archosauromorpha Finally we come to the other clade of diapsids, the archosauromorphs (archo - ruling). Archosauromorpha is supported by many important, shared, derived characters that are included on the cladogram in Figure 4.12. Within archosauromorphs are a series of basal members that are known mostly from the Triassic. Some bear a superficial resemblance to large lizards (remember, however, that they cannot be true lizards, which are lepidosauromorphs), whereas others look like reptilian pigs (see Figure 16.5).

The last of the aforementioned - prolacertiforms - possess a number of significant evolutionary innovations (Figure 4.12), most notably an opening on the side of the snout, just ahead of the eye, called the ant-orbital fenestra (Figure 4.13). These are the characters that unite Archosauria, the group that contains crocodilians, birds, and dinosaurs. Crocodilians and their close relatives belong to a clade called Crurotarsi5 (cruro - shank; tarsus - anlde); birds and their close relatives constitute a clade called Ornithodira (ornith - bird; dira - neck).

Modern crocodiles are but an echo of what preceded them and in the past there have been sea-going crocodiles with flippers instead of legs, crocodiles with teeth that look more mammalian than crocodilian, and crocodiles that appear to have been well adapted for running on land. Other crurotarsans included a variety of carnivorous (Figure 4.14), piscivorous (fish-eating), and herbivorous forms.6

Ornithodira brings us quite close to the ancestry of dinosaurs. This group is composed of two major clades, Dinosauria (deinos - terrible) and

4 Two marine groups, ichthyosaurs and plesiosaurs (see Figure 18.8) have also been placed among the diapsids, but these are not germane to our story.

5 Some paleontologists prefer J. A. Gauthier's (1986) use of the term Pseudosuchia for this group. Membership in Pseudosuchia is similar to but not identical with that of Crurotarsi.

6 Historically, basal archosauromorphs have all been jumbled together under the name Thecodonts (tfjeco - socket; dont - tooth; see Chapter 13) because their teeth are set in sockets (much as our own are).The teeth in sockets applies to all archosauromorphs (Figure 4.12); how,therefore, can this character be used to distinguish one archosauromorph from another?

Saurischia (including Aves)

Saurischia (including Aves)

Cladogram Mamalia Dan Aves

Figure 4.12. Cladogram of Archosauromorpha. Diagnostic characters include: at I teeth in sockets, elongate nostril, high skull, and vertebrae not showing evidence of embryonic notochord; at 2 antorbital fenestra (see Figure 4.13); loss of teeth on palate and new shape of articulating surface of ankle (calcaneum); at 3 a variety of extraordinary specializations for flight, including an elongate digit IV; at 4 erect stance (shaft of femur is perpendicular to head; the ankle has a modified mesotarsal joint) perforate acetabulum (see Chapter 5 for greater detail); at 5 predentary and rearward projection of pubic processes (see introductory text for Part II: Ornithischia); and at 6 asymmetrical hand with distinctive thumb, elongation of neck vertebrae, and changes in chewing musculature (see introductory text for Part III: Saurischia),

Figure 4.12. Cladogram of Archosauromorpha. Diagnostic characters include: at I teeth in sockets, elongate nostril, high skull, and vertebrae not showing evidence of embryonic notochord; at 2 antorbital fenestra (see Figure 4.13); loss of teeth on palate and new shape of articulating surface of ankle (calcaneum); at 3 a variety of extraordinary specializations for flight, including an elongate digit IV; at 4 erect stance (shaft of femur is perpendicular to head; the ankle has a modified mesotarsal joint) perforate acetabulum (see Chapter 5 for greater detail); at 5 predentary and rearward projection of pubic processes (see introductory text for Part II: Ornithischia); and at 6 asymmetrical hand with distinctive thumb, elongation of neck vertebrae, and changes in chewing musculature (see introductory text for Part III: Saurischia),

Pterosauria (ptero - winged). Pterosaurs, the brainy, impressive "flying reptiles" from the Mesozoic, are highly modified ornithodirans, with as many as 40 derived features that unite them as a natural group. Their smallest members were sparrow sized, and their largest members had wingspans as large as 15 m, making them the largest flying organisms that have ever lived (for reference, the wingspan of the two-person Piper Cub (airplane) is about 12 m).7 That they are unapologetically Mesozoic and utterly extinct has led to their being called "dinosaurs," but in fact they are something utterly different from either birds or dinosaurs. They are pterosaurs. Figure 4.13. An archosaur skull _

with the diagnostic antorbital 7 These extraordinary Mesozoic beasts demand a detailed treatment unfortunately not possible here opening. (see Wellenhfen 1996).

Antorbital openings

Braincase

Antorbital openings

Braincase

Figure 4.14. A reconstruction of the carnivorous crurotarsan Euparkeria.

Dinosaurs This leaves us at long last with the subject of our book, Dinosauria.

Dinosaurs can be diagnosed by a host of shared, derived characters, many of which are elaborated in Chapter 5 (Figures 5.4 and 5.5). Most strikingly, dinosaurs are united by the fact that, within archosaurs, they possess an erect, or parasagittal stance; that is, a posture in which the plane of the legs is perpendicular to the plane of the torso and is tucked under the body (Figure 4.15; see also Box 4.4).

Armadillo Lizard Diet
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