Dinosaurs in the Mesozoic

Throughout much of this book, we have considered dinosaurs as individuals; who they were, what they did, and how they did it. Now we'll step back and take a look at the large-scale ebb and flow of dinosaur evolution. Before we can do that, though, we need to think about what might be missing.

Preservation

Table 13.1 shows the distribution of dinosaurs among the continents through time. The paucity of dinosaur remains from Australia and Antarctica, however, is surely more a question of local preservation and inhospitable conditions today for finding and collecting fossils, than defining where dinosaurs actually lived. Into this mix must be factored geological preservation; some time intervals simply contain more rocks than others. For example, the terrestrial Middle Jurassic is not well represented by rocks, with the result that it artificially appears to have been a time of very low tetrapod diversity (Box 13.1, p. 278). The Late Cretaceous is rather the opposite, with the happy result that we have a rich record of Late Cretaceous dinosaurs. Several methods of estimating the completeness of fossil preservation have been developed to mitigate these problems (Box 13.2, page 280).

Table 13.1. Distribution ofdinosaurs on continents during the Mesozoic Era. Solid blue areas indicate dinosaurs known

Asia

Africa

South America

North America

Europe

Australia

Antarctica

Late Triassic

■n

■n

■nnnn

■nan

■■

■■■

Early Jurassic

■n

■n

■man

■mm

■HI

warn

Middle Jurassic

■n

■n

■mum

■nan

■HI

■■■

Late Jurassic

■n

■n

■nnnn

■mm

Cretaceous

■H

■H

-mm

■mm

Cretaceous

■H

■H

-mm

■mm

■■

Dinosaurs through time

We can think of these geographical and temporal distributions as pages in a notebook, in which each succeeding page represents a new time interval, with new continental arrangements, and new and different assemblages of dinosaurs populating the continents. Considered in this way, the sequence of dinosaurs through time is like a grand pageant through Earth history, in which each interval of time has a characteristic fauna that gives that time a characteristic quality (Figure 13.1).

Now let's look at this information in a more quantitative way: the y, that is the number of different types, of dinosaur genera over time (Figure 13.2). This tracks large-scale, global fluxes in dinosaurs through the approximately 163 million years that they were on Earth.

Figure 13.2. Changes in dinosaur diversity by continent measured through the Late Triassic—Late Cretaceous time interval. Each vertical bar shows the total number of different genera known from that particular time interval. Viewed from this perspective, dinosaurs appear to have steadily increased in diversity as the Mesozoic progressed. (Data from Fastovsky et al., 2004.)

South America North America Europe

Asia

Australia and Antartica Africa

In the beginning . . . the Late Triassic (228-200 Ma)

Figure 13.2 shows that dinosaurs radiated quickly in the Late Triassic. Exactly how dinosaurs came to be the dominant terrestrial vertebrates in the Late Triassic remains tantaliz-ingly shrouded in the mists of antiquity. However, our best data suggest that dinosaurs likely moved quickly into a world abandoned by other vertebrates rather than possessing superior adaptations that somehow allowed them to outcompete pre-existing tetrapods (mainly ther-apsids and primitive archosaurs; see Figures 13.3-13.7 and pp. 309-311) and take charge.

These potentially dramatic ecological steps are not so easily revisited, because the diversity of early dinosaurs is small, and the times of their appearances are not particularly well known. The earliest dinosaurs known are Herrerasaurus and Eoraptor from the Ischigualasto Formation of Argentina, reliably dated to 228 Ma. Indeed, phylogenetic and biogeographical perspectives point to South America as the cradle of Dinosauria.

What we can be sure of is that Late Triassic terrestrial vertebrate faunas were not dinosaur dominated; rather they were an eclectic mixture, including therapsids (advanced synapsids; Figure 13.3), Earth-bound archosaurs (Figure 13.4), primitive turtles (Figure 13.5), some crocodile-like amphibians called temnospondyls (Figure 13.6), and pterosaurs (Figure 13.7). Oh yes, and the very earliest mammals, tiny, shrew-sized, insectivorous creatures, were present (Figure 13.3d). As it turned out, their appearance on Earth was approximately coincident with - or even slightly preceded - that of dinosaurs.

Continental distributions and the Late Triassic fauna. What kinds of evolutionary forces might have been driving the distinctive Late Triassic faunas? The very distributions of the continents likely played a role in the composition of global faunas.

Mesozoic Dinosaurs
Figure 13.3. Late Triassic therapsids ("mammal-like reptiles" and a very early mammal. (a) A large, 2.5 m herbivore (the dicynodont Kannemeyeria); (b, c) two carnivorous cynodontians (Cynognathus); and (d) an early mammal, the tiny (approximately 5 cm) Eozostrodon.
Triassic Mammals
Figure 13.4. Assorted primitive archosaurs. (a) A phytosaur (Rutiodon); (b) an aetosaur (Stagonolepis); (c) a rauisuchian (Posto-suchus); and (d) a primitive crocodile (Protosuchus).
Kannemeyeria

Figure13.6. A temnospondyl (an archaic amphibian) grabbing a snack.

Figure 13.5. A tale of two turtles. (a) The skeleton of a modern specimen "turned turtle" with the shell bones removed; (b) the primitive Triassic turtle Proga-nochelys.

Figure 13.5. A tale of two turtles. (a) The skeleton of a modern specimen "turned turtle" with the shell bones removed; (b) the primitive Triassic turtle Proga-nochelys.

Triassic Temnospondyls

Figure13.6. A temnospondyl (an archaic amphibian) grabbing a snack.

Mesozoic Era Amphibians
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