The rise and fall of Dinosauria

Figure 14.11. Willi Hennig (1913-1976) of the Deutsches Entomologisches Institut, the German entomologist who was the father of cladistic analysis (phylogenetic systematics).

Ideas about the rise of dinosaurs underwent considerable rethinking during the second half of the twentieth century. But before turning to these, let's first set the stage for the emergence of dinosaurs in the Triassic. From its outset some 251 million years ago, the Triassic was dominated on land by therapsids. Among these, the sleek, dog-like cynodonts were the chief predators, while the rotund, beaked and tusked dicynodonts were the most abundant and diverse of herbivores (see Figure 13.3a). From the middle and toward the end of the Triassic, these therapsids shared the scene with squat, plant-eating, and swine-like archosauromorphs called aetosaurs and a few carnivorous crocodile-like archosaurs (see Figure 13.4).

Archosaurs began as sprawlers and ended up with either semi-erect (crocodilians and pterosaurs) or fully erect stance (dinosaurs and near dinosaurs (including birds)). Thus changes at the hip, knee, and ankle enabled a fully erect, parasagittal posture in which the legs acted not only as support pillars when standing but also provided for longer strides and more effective walking and running ability (see Chapter 12).

Toward the end of the Triassic, approximately 225 Ma, there was a great change of fortunes for amniotes. The majority of therapsids went extinct (one highly evolved group of therapsids, the mammals, of course survived). And it was dinosaurs that somehow rose to become the dominant terrestrial vertebrates, by which it Figure 14.12. Jacques A. Gauthier, then atthe California Academy ofSciences, who is meant that they became the most abundant, carried out the seminal cladistic studies of bird origins and dinosaur monophyly.

diverse, and probably visible group of tetrapods. The pattern of waxing and waning in dominance (as one group supercedes another in evolutionary time) is called the wedge (Figure 14.13).

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to.00

Synapsid carnivores _____

Diapsid carnivores (including dinosaurs)

Synapsid herbivores

\ Diapsid herbivores \ (including dinosaurs)

Figure 14.13. Two views of the origin of dinosaurs during the Late Trias-sic. (a) Wedge with the edge: Gradual competitive replacement of synapsids, primitive archosaurs, and rhynchosaurs (both herbivores and carnivores) by herbivorous and carnivorous dinosaurs. (b) Wedge without the edge: Rapid opportunistic replacement mediated by extinction.

Synapsid carnivores

Diapsid carnivores (including dinosaurs)

Synapsid herbivores

Diapsid herbivores (including dinosaurs)

Early Triassic

Middle Triassic

Late Triassic

More puzzling, though, was why dinosaurs prevailed. Ideas boiled down to two basic concepts:

1. Dinosaurs out-competed their contemporaries, earning the right, as it were, to be the dominant terrestrial vertebrates.

2. Dinosaurs somehow survived, becau se their non-dinosaurian contemporaries went extinct, leaving the planet to dinosaurs.

Out-competition. In the late 1960s and early 1970s, Alan Charig, Curator of Lower Vertebrates at what was then called the British Museum of Natural History, argued that those archosaurs that had the new, "improved" erect stance were then able to out-compete contemporary predatory, semi-sprawling therapsids for their food sources. The immediate descendants of the flashy new archosaurs were the dinosaurs. The inevitable consequence of such progressive improvements in limb posture, Charig argued, was the gradually changing pattern of faunal succession at the end of the Triassic. We can call this and any other evolutionary advantage a competitive edge; dinosaurs prevailed, according to Charig, by virtue of having better-designed limbs and thereby more efficient terrestrial locomotion.

At nearly the same time, Bakker was making similar arguments about the competitive superiority of endothermy in dinosaurs (see Chapter 12). He believed that, instead of limbs, it was the achievement of internally produced heat that gave dinosaurs (or their immediate ancestors) a competitive edge over contemporary and supposedly cold-blooded therapsids and rhynchosaurs. The same conclusions applied: dinosaurs won, therapsids lost, and the truth of the competitive superiority of endotherms over ectotherms could be read directly from the pattern of faunal succession at the end of the Triassic: the competitive edge produced the wedge.

Wedge without edge? Michael J. Benton of the University of Bristol is not convinced that the edge produced the wedge in the Middle to Late Triassic fossil record of the earliest dinosaurs and their predecessors. In order for edges to lead to wedges, all of the players in the game have to be present to interact with each other. And, according to Benton, they were not (note Figure 14.13). Instead, he suggests that the fossil record of the last part of the Triassic is marked by not one but two mass extinctions. The first appears to have been the more extreme and ultimately most relevant to the rise of dinosaurs. This earlier Late Triassic extinction completely decimated rhynchosaurs and nearly obliterated dicynodont and cynodont therap-sids, as well as several major groups of predatory archosaurs.

Likewise, there is a major extinction in the plant realm. The important seed-fern floras (the "Dicroidium flora," which contained not only seed-ferns, but also horsetails, ferns, cyca-dophytes, gingkoes, and conifers; see Chapter 13) all but went extinct as well, to be replaced by other conifers and bennettitaleans (see Figures 13.8 and 13.9). Dinosaurs appeared as the dominant land vertebrates only after this great disappearance of therapsids, archosaurs, and rhynchosaurs. Thus the initial radiation of dinosaurs, according to Benton, occurred in an ecological near-vacuum, with the rapid loss of the dominant land-dwelling vertebrates setting the stage for the opportunistic evolution of dinosaurs. No competitive edge, because there was no competition.

That there was at least one, and more than likely two, mass extinctions at the end of the Triassic Period is uncontroversial. Naturally, one of the key questions is what might have caused these extinctions. Benton has suggested that the Late Triassic extinctions may be linked with climatic changes - the regions first inhabited by dinosaurs appear to have been hotter and more arid, a change from the more moist and equable - and thence to alterations in terrestrial floras and faunas. The abrupt extinction of the Dicroidium flora may have caused the extinction of herbivores specialized to them and hence the predators feeding on the herbivores.

So, if Benton is right, then perhaps the archosaurian predecessors of dinosaurs may have just squeaked by, survivors not because they were somehow superior to the presumed competition, but because they happened to inherit a deserted Earth. Instead of survival having been something intrinsic to dinosaur superiority, it may have been that they simply had better luck.

Ironically (as we shall see in Chapter 15), 163 million years later the tables again turned, and mammals inherited an Earth this time deserted by the very dinosaurs who, by one means or another, had taken it from them millions of years earlier. The wedge in this case was produced without any edge.

Extraterrestrial extinction. Our trip through the most recent ideas about dinosaur paleontology would not be complete without perhaps the most radical idea of all: extinction by asteroid impact (see Chapter 15). Since H. F. Osborn's time, the extinction of the dinosaurs was viewed by paleontologists as a gradual process of dwindling diversity, beginning well before the end of the Cretaceous. The prevailing view before 1980 was cleverly and succinctly summarized by University of California (Berkeley) paleontologist W. A. Clemens and colleagues who wrote, with apologies to T. S. Eliot:

This is the way Cretaceous life ended

This is the way Cretaceous life ended

This is the way Cretaceous life ended

Not abruptly but extended.6

Clemens and colleagues' article was aptly entitled "Out with a whimper, not a bang."

The revolution, of course, came in the form of the 1980 hypothesis that an asteroid came from outer space, smashed into the planet, and ultimately reset global ecosystems for all time. What a strange, wonderful, and terrifying idea - that extra-terrestrial events are important forces shaping the history of life on Earth. The conceptual revolution provoked by this vision extended far beyond the deaths of a few dinosaurs, and reverberated throughout the geosciences7. The asteroid and its aftermath are the subjects of our last chapter.

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