Dinosaurs And Their

The diapsids take over_

The diapsids (see p. 447) were initially small to medium-sized carnivores that never matched the abundance of the synapsids in the Car boniferous or Permian. Things began to change during the Triassic, perhaps as a result of the end-Permian extinction event, which had such a devastating effect on therapsid communities. Small and large meat eaters such as Erythrosuchus (Fig. 17.1a) appeared, one of the first of the archosaurs, a group that was later to include the dinosaurs, pterosaurs, crocodilians and birds. Archosaurs are characterized by an additional skull opening between the orbit and the naris, termed the antorbital fenestra, whose function is unclear.

During the Triassic, diapsids diversified widely, some on land and some in the sea. Some archosaurs became large carnivores, others became specialized fish eaters, others adopted a specialized grubbing herbivorous lifestyle, yet others were small, two-limbed, fast-moving insectivores (crocodilians and dinosaurs), and some became proficient flyers (pterosaurs). It took another extinction event, near the beginning of the Late Triassic (about 220 Ma) to set the new age of diapsids fully in motion. Most of the synapsids died out then, as did various basal archosaur groups. Many new kinds of land tetrapods then radiated: the dinosaurs, pterosaurs, crocodilians and lizard ancestors, as well as the turtles, modern amphibians and true mammals.

The pterosaurs were proficient flapping flyers (Fig. 17.1b), with a lightweight body, narrow hatchet-shaped skull and a long narrow wing supported on a spectacularly elongated fourth finger of the hand. The bones of the arm and finger supported a tough flexible membrane that could fold away when the animal was at rest, and stretch out for flight. Pterosaurs were covered with hair, and were almost certainly endothermic. Some later pterosaurs were much larger than any known bird, such as Pteranodon with a wingspan of 5-8 m, and Quetzalcoatlus with a wingspan of 11-15 m. Most pterosaurs fed on fishes caught in coastal seas, but others were insectivorous.

Early crocodilians were largely terrestrial in habits, walked on all fours and had an extensive armor of bony plates (Fig. 17.1c). Crocodilians were more diverse and abundant during the Jurassic and Cretaceous than they are now. Some even became fully marine in adaptations, to the extent of having paddles instead of hands and feet, and a deep tail fin

Figure 17.1 Archosaurs: (a) skull of the Early Triassic archosaur Erythrosuchus (x0.1); (b) the Late Jurassic pterosaur Rhamphorhynchus, showing the elongated wing finger on each side, and the long tail with its terminal "sail" made from skin (x0.3); and (c) the Late Jurassic crocodilian Crocodilemus, showing the skeleton and armor covering (x0.2). (Courtesy of David Unwin and Danny Grange.)

Figure 17.1 Archosaurs: (a) skull of the Early Triassic archosaur Erythrosuchus (x0.1); (b) the Late Jurassic pterosaur Rhamphorhynchus, showing the elongated wing finger on each side, and the long tail with its terminal "sail" made from skin (x0.3); and (c) the Late Jurassic crocodilian Crocodilemus, showing the skeleton and armor covering (x0.2). (Courtesy of David Unwin and Danny Grange.)

Evrak Par Alama Makineleri Aksam
streptostylic quadrate
Planocephalosaurus

Figure 17.2 Lepidosaurs: (a) the Late Triassic sphenodontid Planocephalosaurus; (b) the Late Jurassic lizard Ardeosaurus; and (c, d) skulls of a modern lizard (c) and snake (d), showing the points of mobility that permit wide jaw opening. (a, based on Fraser & Walkden 1984; b, based on Estes 1983.)

Figure 17.2 Lepidosaurs: (a) the Late Triassic sphenodontid Planocephalosaurus; (b) the Late Jurassic lizard Ardeosaurus; and (c, d) skulls of a modern lizard (c) and snake (d), showing the points of mobility that permit wide jaw opening. (a, based on Fraser & Walkden 1984; b, based on Estes 1983.)

to speed their swimming. The modern croco-dilians - crocodiles, alligators and gavials - all arose in the Late Cretaceous.

The second major diapsid clade, the lepidosaurs, represented today by lizards and snakes, diversified in the Late Triassic. The key forms then were sphenodontids - snub-nosed, lizard-sized animals (Fig. 17.2a) that fed on plants and insects. The group dwindled after the Jurassic, except for a single living representative, Sphenodon, the tuatara of New Zealand, a famous "living fossil". The first true lizards are known from the Mid and Late Jurassic (Fig. 17.2b), and they show characteristic mobility of the skull: the bar beneath the lower temporal opening is broken, the quadrate is mobile, and the snout portion of the skull can tilt up and down (Fig. 17.2c). This process of loosening of the skull was taken even further in the snakes, a group known first in the Early Cretaceous. Snakes have such mobile skulls that they can open their jaws to swallow prey animals that are several times the diameter of the head (Fig. 17.2d).

The age of dinosaurs_

Dinosaurs were the most important of the new diapsid groups of the Triassic, both in terms of their abundance and diversity, and in terms of the vast size reached by some of them. The first dinosaurs were modest-sized bipedal carnivores. After the Late Triassic extinction, a new group of herbivorous dinosaurs, the sauropodomorphs, radiated dramatically, some like Plateosaurus (Fig. 17.3a) reaching a length of 5-10 m during the Late Triassic. Later sauropodomorphs were mainly large and very large animals; some of them, such as Brachiosaurus (Fig. 17.3b) reaching lengths of 23 m or more and heights of 12 m. These giant dinosaurs pose fascinating biological problems (Box 17.1).

The theropods include all the carnivorous dinosaurs, and in the Jurassic and Cretaceous the group diversified to include many specialized small and large forms. Deinonychus (Fig. 17.5a) was human-sized, but immensely agile and intelligent (it had a bird-sized brain). Its key feature was a huge claw on its hindfoot, which it almost certainly used to slash at prey animals. Tyrannosaurus (Fig. 17.5b) is famous as probably the largest land predator of all time, reaching a body length of 14 m, and having a gape of nearly 1 m. The theropods and sauropodomorphs share the primitive reptilian hip pattern, in which the two lower elements point in opposite directions, the pubis forwards and the ischium backwards (Fig. 17.5b). They also share derived charac-

Figure 17.3 Sauropodomorph dinosaurs: (a) the Late Triassic prosauropod Plateosaurus; and (b) the Late Jurassic sauropod Brachiosaurus. (Courtesy of David Weishampel.)

ters of the skull and limbs that show they form a clade, the Saurischia.

All other dinosaurs share a unique hip pattern in which the pubis has swung back and runs parallel to the ischium (Fig. 17.6a), and these are termed the Ornithischia, all of which were herbivores. Two groups of armored ornithischians are the stegosaurs and the ankylosaurs. Stegosaurus (Fig. 17.6a) has a row of bony plates along the middle of its back that may have had a temperature control or display function. Euoplocephalus (Fig. 17.6b) is a massive, tank-like animal with a solid armor of small plates of bone set in the skin over its back, tail, neck and skull: it even had a bony eyelid. The tail club was a useful defensive weapon that it used to whack threatening predators such as Tyrannosaurus.

Most ornithischians were ornithopods, bipedal forms, initially small, but later often large. In the Late Cretaceous, the hadrosaurs were successful fast-moving plant eaters. Many of them have bizarre crests on top of their heads that may have been used for species-specific signaling, and their duckbilled jaws are lined by multiple rows of grinding teeth (Fig. 17.7). Close relatives of the ornithopods were the ceratopsians ("horn-faces"), like Centrosaurus (Fig. 17.6c), which had a single, long nose-horn and a great bony frill over the neck.

There has been a continuing debate about whether the dinosaurs were warm-blooded (endothermic) or not. Evidence for warm-bloodedness is strongest for the small active predators like Deinonychus that might have required the added stamina and speed. However, endothermy is costly in terms of the extra food required as fuel, and it is not clear whether the larger dinosaurs could have eaten fast enough. Indeed, larger dinosaurs would have maintained a fairly constant core body temperature simply because of their size, whether they were endothermic or not.

Dinosaur reproductive habits have also come under scrutiny recently. Discoveries of eggs and nests in North America and Mongolia have shown that many dinosaurs practiced parental care. They laid their eggs in earth nests scooped in the soil, and returned to feed the young when they hatched out. Some of the most spectacular finds are unhatched eggs with the tiny bones of the dinosaur embryos still inside (Box 17.2).

Dragons of the deeps_

During the Mesozoic, several reptile groups became key marine predators. The

Box 17.1 Paleobiology of the largest animals ever

When the monster sauropods of the Late Jurassic were first discovered in the 19th century, many paleontologists thought that they were too big to have lived fully on land. It was assumed that the sauropods lived in lakes, supporting their bulk in the water, and feeding on waterside plants. New evidence shows, however, that life on land was quite possible, and sauropods, like elephants today, could move freely over vast plains, and in and out of the water at times as well.

Sauropods may have divided their feeding preferences by height. Modern herbivores show such niche partitioning, where each animal has its preferred food and feeding mode. Some sauropods such as Brachiosaurus (Fig. 17.3b) may have been like super giraffes, feeding on leaves from very tall trees. Most other sauropods, however, were designed for lower-level browsing, and were probably not able to raise their necks much above horizontal. This allowed several species of sauropods to live side by side, some feeding on low plants, others on mid-height shrubs, and yet others on the leaves of trees.

But how did sauropods get to be so big? Brachiosaurus and relatives reached lengths of 20 m or more, and some weighed as much as 50 tonnes. Did they take 70 or 100 years to reach sexual maturity, growing at the same rate as a modern crocodile, as some paleobiologists have suggested? Or did they grow fast, like modern mammals? An elephant reaches sexual maturity at about 15 years old, while a blue whale grows even faster, reaching sexual maturity at 5-10 years. Blue whales can put on as much as 90 kg per day (equivalent to 30 tonnes per year) during their fastest juvenile growth. How can you tell how fast a dinosaur grew?

The secret is in the bones. Modern reptiles grow in fits and starts - bursts of fast growth when food is plentiful, and very slow growth when they are starved. Typically, there is one good season and one poor season each year, and this is shown in growth rings in the bone. Greg Erickson, a paleontologist at Florida State University in Tallahassee, and Martin Sander, a paleontologist from the University of Bonn in Germany, have sectioned the ribs and leg bones of many dinosaurs, and they have counted the growth lines or "lines of arrested growth" (LAGs). The bones are cut through, and thin sections are glued on to large glass slides, and then ground down to a uniform thickness, so light can pass through. Erickson and Sander have examined these thin sections under a polarizing light microscope that highlights the crystalline apatite components of the fossil bone and allows them to count the LAGs easily (Fig. 17.4a).

Bone histologists, people who study the microscopic structure of bone, argue that each LAG represents a year because this is the case in living reptiles and other animals that have them. Paleontologists can count the LAGs to give the age when a dinosaur died, and then compare these ages with estimates of body masses assessed from the size of the bones. Erickson has looked at a growth series of bones, from tiny (well actually quite big) baby Apatosaurus through juveniles to fully adult specimens. These suggest that Apatosaurus juveniles stayed pretty small for the first 5 years of their life, and then there was a burst of growth from age 5 to 12, when they put on up to 5 tonnes a year, to reach a young adult body mass of 26 tonnes, presumably the age of sexual maturity (Fig. 17.4b). So the LAGs indicate stop-start seasonal patterns of growth, just like a modern reptile, but the rate of juvenile growth is much more like a bird or mammal. Sauropods did not have to wait until they were 100 years old before they could have sex.

Read more about sauropod bone histology in Erickson et al. (2001) and Sander and Klein (2005) and Sander et al. (2006) and find a selection of the best dinosaur web sites at http://www. blackwellpublishing.com/paleobiology/.

(a)
Sauropod Growth Rate
(b) Age (years)

Figure 17.4 Measuring the growth rate of a sauropod dinosaur. (a) Cross-section through the bone wall of the femur of the sauropod Janenschia from the Late Jurassic of Tanzania; the animal was full grown and the femur was 1.27 m long. The section was made by drilling into the bone and extracting a core that was then cut through; the center of the bone is to the left, the outside to the right. Lines of arrested growth are the darker bands, where the bone structure is tighter, indicating a slow-down in growth. These are marked off with tick marks on the side of the slide. (b) Growth curve for the sauropod Apatosaurus based on sections from the limb bones and ribs of several individuals, juveniles and adults, showing how the animal reached adult size with a spurt of growth from years 5 to 12. (Courtesy of Martin Sander and Greg Erickson.)

Sauropod Growth Curve
Figure 17.5 Cretaceous theropod dinosaurs: (a) Deinonychus, and (b) Tyrannosaurus. (a, based on Ostrom 1969; b, based on Newman 1970.)

ichthyosaurs (Fig. 17.9a) were fish-shaped animals, entirely adapted to life in the sea, and they evolved from land-living diapsids. Ichthyosaurs had long, thin, snouts lined with sharp teeth, and they fed on ammonites, bel-emnites and fishes. Exquisite preservation of many specimens shows the tail fin, dorsal fin and the paddle outlines. Ichthyosaurs swam by beating the body and tail from side to side, and they used the front paddles for steering. There are even some remarkable specimens of mothers with developing embryos inside their bellies: like whales and dolphins, ichthyosaurs could not flop up on to land to lay eggs, and they gave birth to live young while at sea.

The second major marine reptile group was the plesiosaurs. Most plesiosaurs had long necks and small heads (Fig. 17.9b), but the pliosaurs were larger and had short necks and large heads. Plesiosaurs fed mainly on fishes, using their long neck like a snake to dart after

0.5 m tall neural spines

ossified tendons

0.5 m tall neural spines

ossified tendons

Figure 17.6 Armored ornithischian dinosaurs from the Jurassic (a) and Cretaceous (b, c): (a) Stegosaurus, (b) Euoplocephalus, and (c) Centrosaurus. (a, c, based on Gregory 1951; b, based on Carpenter 1982.)

Figure 17.6 Armored ornithischian dinosaurs from the Jurassic (a) and Cretaceous (b, c): (a) Stegosaurus, (b) Euoplocephalus, and (c) Centrosaurus. (a, c, based on Gregory 1951; b, based on Carpenter 1982.)

fast-moving prey, and they swam by beating their paddles in a kind of "flying" motion. The extraordinary diversity of tetrapod predators in the sea came to an end 65 Ma during the great Cretaceous-Tertiary mass extinction (see pp. 174-7) that saw the end of the dinosaurs and pterosaurs too.

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