The shape of tetrapod diversity

For more than 20 years, the University ofBristol's M. J. Benton has been compiling a comprehensive list of the fates of tetrapod families through time. We see several interesting features of the curve that results from this compilation. Note the drop in families during Middle Jurassic time (Figure B13.1.1). This, as we have seen, is an artifact; that is, a specious result. This particular one comes from the lack of find localities more than from a true lack of families during the Middle Jurassic. Then, notice the huge rise in families during the Tertiary. Some of this may be real, and perhaps attributable in part to Tertiary birds and mammals (both of whom are very diverse groups), but some ofit might be another artifact, due to what is called the "pull of the Recent." The pull of the Recent is the inescapable fact that, as we get closer and closer to the Recent, fossil biotas become better and better known. This is because more sediments are preserved as we get closer and closer to the Recent, and a greater amount of sedimentary rocks preserved means more fossils. The big spike at the end of the Jurassic is the Morrison Formation ofthe U.S. Western Interior, a unit that preserved an extraordinary wealth of fossils.

So a curve like Benton's requires skill to understand and to factor out the artifacts. Nonetheless, we can see that, generally, dinosaurian diversity increased throughout their stay on Earth and, as they progressed through the Cretaceous,

Consider this example from the modern world: the large herbivore fauna of Africa is rather different from that of North America. And both differ from that of India. There are no physical connections among these continents that would allow the fauna of one to spread to the other. Each of these faunas - in fact, the ecosystems of which they are a part - has developed in relative isolation, and is therefore distinct. This type of distinctness is called endemism. A region that is populated by distinct faunas unique to it is said to show high endemism. High endemism is caused by evolution on widely separated continents, because there is no opportunity for faunal interchange.

Aternatively, if faunas of two continents appear very similar to each other, then it is likely that some land connection was present to allow the fauna of one continent to disperse to the other continent. Thus we can imagine a region characterized by low endemism; because the continents are closely allied with each other, and there are extensive opportunities for faunal interchange.

It is now clear that, during the Triassic and Early Jurassic, global terrestrial vertebrate faunas were characterized by unusually low endemism. The supercontinent of Pangaea still existed during this time, and land connections were more or less continuous among all of today's continents (see Figure 2.5). The interesting mixtures of faunas outlined here look similar on a global scale during these time intervals. While Pangaea remained united, land connections existed and endemism was low. Here, then, is an excellent example of large-scale, non-biological events driving and modifying large-scale patterns of biological evolution.

Time (millions of years before present)

Time (millions of years before present)

Figure B13.1.1. M. J. Benton's estimate of vertebrate diversity through time. On the x-axis is time; on the y-axis is diversity as measured in numbers of tetrapod families.

dinosaurs continued to diversify. The increase in diversity shown in Benton's diagram may reflect the increasing global endemism of the terrestrial biota, itself driven by the increasing isolation ofthe continental plates.

Jurassic (200-146 Ma)

The Early Jurassic (200-176 Ma)was the first time on Earth when dinosaurs truly began to dominate terrestrial vertebrate faunas. Many of the players in the terrestrial game were now dinosaurs, although they continued to share the limelight with some relict non-amniotes (see Chapter 4), a few of the very highly derived, mammal-like therapsids (including some puny mammals), turtles, pterosaurs, and the newly evolved crocodilians. Interestingly, the Early Jurassic faunas retained some of the low endemism that characterized the Late Triassic world. The unzipping of Pangaea was in its very earliest stages, and it had not gone on so long that the fragmentation of the continents was yet reflected through increased global ende-mism. That was to await the Late Jurassic.

The Middle Jurassic (176-161 Ma) has historically been an enigmatic time in the history of terrestrial vertebrates. As noted in Chapter 2, Middle Jurassic terrestrial sediments are quite uncommon. When we look at the total diversity of tetrapods through time (Box 13.1), the curve all but bottoms out during the Middle Jurassic. Did vertebrates undergo massive extinctions at the end of the Early Jurassic? Probably not. More likely the curve is simply reflecting the serendipitous absence of terrestrial Middle Jurassic sediments on Earth. Without a good sedimentary record to preserve them, we can have little knowledge of the faunas that came and went during that time interval.

Regardless, the Middle Jurassic must have been an important time in the history of dinosaurs. With the dismemberment of Pangaea well underway by this time, dinosaurs had diversified, and endemism was on the rise. Many of the non-dinosaurian tetrapods that characterized earlier faunas - advanced therapsids, for example, were largely out of the picture. The insignificant exception to this, of course, were mammals, hanging on by the skin of their multi-cusped, tightly occluding teeth. The Middle Jurassic must have been a kind of pivot point in the history of dinosaurs, because it was then that most of the major dinosaur groups - sauropods, large theropods, thyreophorans, and ornithopods - assumed their familiar forms and consolidated their hold on terrestrial ecosystems. It's a shame that we cannot know more of this crucial time.

By the Late Jurassic (161-146 Ma; see Figure 2.6), global climates had stabilized and were generally warmer and more equable (less seasonal) than they presently are (see Chapter 2). Polar ice, if present, was reduced. Sea levels were higher than today. Dinosaur faunas were more endemic than ever before.

The Late Jurassic has been called the Golden Age of Dinosaurs.1 Many of the dinosaurs that we know and love were Late Jurassic in age. That special Late Jurassic blend of supposedly equable climates, small brains, and massive size epitomized early dinosaur stereotypes and exerted a fascination on nineteenth- and early twentieth-century dinosaur lovers. Many were large - gigantic sauropods (Brachiosaurus, Diplodocus, Camarasaurus, among others) as well as theropods that reached upward of 16 m - but many were not (for example, Compsognathus). It was during the Late Jurassic that the first known "bird" (Archaeopteryx) appeared. Moreover, this was the time of stegosaurs, ornithopods, and even a few ankylosaurs. By Late Jurassic time, dinosaurs had consolidated their dominance of terrestrial vertebrate faunas.

Cretaceous (146-65.5 Ma)

The Early Cretaceous (146-100 Ma) was a time of enhanced global tectonic activity. With this came increased continental separation, as well as greater amounts of CO2 in the atmosphere,

1. If only because Late Cretaceous dinosaurs weren't fully appreciated in the late 1800s when the expression "dinosaur"

was coined!

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