Crocodylia

Figure 5.3. H. G. Seeley's evolutionary scenario of the origin of dinosaurs.

Seeley was not a natural group (Figure 5.3).

This major theme, that dinosaurs were diphyletic (i.e., having two separate origins), was continued well into the twentieth century, principally by Friedrich von Huene. Throughout his remarkably long career (he published actively from the early 1900s to the 1960s), von Huene's studies came to epitomize independent dinosaur origins. According to him and many who came after, dinosaurs had at least two and more probably three or four, separate origins from different stem archosaurs, generally called "thecodonts" (see Chapter 13). Certainly, saurischians and ornithischians had separate origins; after all, their hip structure was different. And among saurischians, surely sauropods and theropods had separate origins; after all, they look so different. Finally, among ornithischians, ankylosaur ancestry was also often sought separately within some thecodontian group.

Despite a wealth of dinosaur remains, North America never produced workers intent on the Big Picture of dinosaur phylogeny. Most of the ideas that dominated the history of dinosaur studies throughout much of the twentieth century came from studies of European dinosaur material. Following in the tradition of von Huene, British paleontologist A. J. Charig was the chief purveyor of dinosaur systematics in the 1960s and 70s. His work centered on primitive archosaurian taxa (he called them "thecodonts"), among them the semi-aquatic, carnivorous, and crocodile-like proterosuchids and erythrosuchids, as well as a number of early non-dinosaur groups. It was through these studies that Charig developed a detailed scheme of dinosaur phylogeny in which he traced the various dinosaur lineages to their separate origins among the various "thecodont" archosaurs. Dinosaurs to Charig and to many of his coworkers remained an unnatural group.

Dinosaurs united The first inkling that things were changing as regards dinosaur relationships began in 1974 with a short publication in the British science journal Nature. In it, R. T. Bakker and P. M. Galton attempted to resurrect Dinosauria as a monophyletic taxon, using a number of skeletal features and speculations about dinosaurian physiology. Their analyses - which reunited not only saurischians and ornithischians, but also linked Aves within Dinosauria - met a great deal of resistance and, in a few cases, open hostility (see Chapter 13). Similar resistance was met by J. F. Bonaparte, who also speculated in 1976 that Dinosauria is a true clade.

Starting in 1984, however, cladistic analysis exploded onto the dinosaurian systematic scene. The thrust was four-fold: the disbanding of "Thecodontia," the origin of dinosaurs, the internal pattern of relationships within ornithischian and saurischian clades, and the relationships of birds to dinosaurs. In all four aspects, the changes wrought by cladistic analyses in our understanding of archosaurs in general and dinosaurs in particular were nothing short of revolutionary.

Of the four foci above, we discuss birds as dinosaurs in Chapter 13. Dinosaurian interrelationships will be treated within each of the successive taxonomic chapters (see chapters in Parts II and III). As for "Thecodontia," it has been fully dismembered. There are no unifying diagnostic features that are uniquely shared by all members of the group. For this reason, "Thecodontia" is not monophyletic and we have abandoned it.

Without "Thecodontia," what can be said about dinosaur origins? The multiple roots of Dinosauria might still exist and in fact maybe more obvious now that the cover of "Thecodontia" has been blown. Are there particular archosaurian taxa that share a close relationship with one or the other (but not all) dinosaur groups? To cut to the chase, the short answer is "no."

How we know this to be the case is through the elegant 1986 cladistic work of J. A. Gauthier, now at Yale University. His research provided ample corroboration of a monophyletic Dinosauria, identifying upwards of 10 derived features uniting all dinosaurs with each other. Since then, numerous cladistic analyses of both new and old taxa have confirmed that dinosaurs share a single, most recent common ancestor, itself a dinosaur.

Dinosaurian monophyly Dinosauria can be defined as consisting of all of the descendants of the most recent common ancestor of Saurischia and Ornithischia (Figure 5.4). Such a definition of Dinosauria is quite close to that of Owen, even

Ornithodira

Figure 5.4. Cladogram of Ornithodira, showing the monophyly of Dinosauria. Derived characters include: at I loss of postfnontal, elongate deltopectoral crest on humerus, brevis shelf on ventral surface of postacetabular part of ilium, extensively perforated acetabulum, tibia with transversely expanded subrectangular dijtal end, ascending astragalar process on front surface of tibia.

Ornithodira

Figure 5.4. Cladogram of Ornithodira, showing the monophyly of Dinosauria. Derived characters include: at I loss of postfnontal, elongate deltopectoral crest on humerus, brevis shelf on ventral surface of postacetabular part of ilium, extensively perforated acetabulum, tibia with transversely expanded subrectangular dijtal end, ascending astragalar process on front surface of tibia.

Figure 5.5. Some of the derived characters uniting Dinosauria. (A) Elongate deltopectoral crest on humerus; (B) brevis shelf on ventral surface of postacetabular part of ilium; (C) extensively perforated acetabulum; (D) tibia with transversely expanded subrectangular distal end; and (E) ascending astragalar process on front surface of tibia.

Figure 5.5. Some of the derived characters uniting Dinosauria. (A) Elongate deltopectoral crest on humerus; (B) brevis shelf on ventral surface of postacetabular part of ilium; (C) extensively perforated acetabulum; (D) tibia with transversely expanded subrectangular distal end; and (E) ascending astragalar process on front surface of tibia.

if Sir Richard was not necessarily thinking along such lines. As it turns out, dinosaurian monophyly is upheld by a host of derived features (Figure 5.5), so many that it is now hard to think of some members of the group as having origins elsewhere within Archosauria. These now include (among others) loss of a skull roofing bone - the postfrontal -that lies on the top of the head along the front margin of the upper temporal opening, an elongate deltopectoral crest on the humerus, an extensively perforated acetabulum, a tibia with a transversely expanded subrectangular lower end, and an ascending process of the astragalus on the front surface of the tibia.

The constituent members of Dinosauria - Ornithischia and Saurischia - each have a monophyletic origin (see introductory text to Parts II: Ornithischia and III: Saurischia) and the pattern of internal relationships of taxa within these clades is also reasonably well understood. Still, there are places, as we shall see, where controversy - and therefore intense research activity - still reigns.

Origins How does one find the ancestor of a clade? Simply put, the hierarchy of characters in the cladogram specifies for us what features ought to be present in an ancestor. It is then simply a question of finding an organism that most closely matches the expected combinations of characters and character states. As we have seen, the likelihood of the very progenitor of a lineage being fossilized is nil; however, we can commonly find representatives of closely related lineages that embody most of the features of the hypothetical ancestor.

So far, we haven't yet identified who within Archosauria might have the closest relationship to Dinosauria, in part because the answer is not yet clear. According to Gauthier and K. Padian, pterosaurs - otherwise

Figure S.6. A candidate for closest relative to Dinosauria: F*terosauria as represented by Dimorphodon.

highly modified for flight (Figure 5.6) - may be the closest archosaurian relatives to dinosaurs, together sharing four derived features (see Chapter 4). The clade of pterosaurs + dinosaurs then shares close relationship with a slender, long-limbed animal from the Middle Triassic of Argentina called Lagosuchus (1ago - rabbit; suchus - crocodile) to form what Gauthier termed Ornithosuchia. Lagosuchus (Figure 5.7) was a very small (less than 1 m), long-legged (hence the name) bipedal carnivore or insectivore. The head of this creature is very poorly known. Nevertheless, few paleontologists would disagree that this relatively tiny creature embodies many of the features that were ancestral for all Dinosauria; the diminutive Lagosuchus is probably close to the ancestry of all the spectacular vertebrates encompassed within Dinosauria.

P. C. Sereno, in contrast, places Lagosuchus, as well as several other small, contemporary archosaurs (Lagerpeton (erpet - a creeper), Pseudolagosuchus (pseudo - false)), and Marasuchus (refers to the mara, a rabbit-like rodent that presently lives in Patagonia) as the closest dinosaurian relatives. This clade of dinosaurs and Lagosuchus, called Dinosauromorpha, shares a sigmoidal vertebral column in the neck region, a shortening of the forelimb, and several modifications of the anlde bones and of the metatarsals. On the strength of these features, it appears that Dinosauria shares closest relationship with archosaurs such as Marasuchus, Lagosuchus, Lagerpeton, and Pseudolagosuchus. More far-flung relationships of these dinosauromorphs are with pterosaurs, which, together with a few other taxa, make up Ornithodira - the bird-necks (see Chapter 4). It is only thereafter that these forms join with Crurotarsi to form Archosauria.

There is an interesting and perhaps surprising consequence of this phylogeny. With archosaurs like Lagosuchus closest to dinosaurian ancestry, apparently dinosaurs were primitively obligate bipeds. This

Origins I 93

Figure 5.7. Another candidate for closest relative to Dinosauria: Lagosuchus.

means that the earliest dinosaurs were creatures that were completely and irrevocably bipedal. Because the primitive stance for archosaurs is quadrupedal, and because Dinosauria is monophyletic, it follows that creatures such as Triceratops, Ankylosaurus, and Stegosaurus - in fact, all quadrupedal dinosaurs - must have evolved secondarily (or re-evolved) their quadrupedal stance. They must have (phylogenetically) got back down on four legs, as it were, after having been up on two. In fact, you can see the remnant of bipedal ancestry when you look at a stegosaur or a ceratopsian, in which the back legs are quite a bit longer than those at the front.

The rise of We have now gone through the host of features providing the evidence dinosaurs' t*ie phylogenetic relationships of dinosaurs to other archosaurs closest to them. What, if anything, does this tell us about the success of Superiority dinosaurs? That is, did these features confer any advantage to Or luck? dinosaurs, making them superior to their less well-equipped contemporaries?

Before turning to how features might affect evolutionary success, let's first set the stage for the emergence of dinosaurs in the Triassic. From its outset, some 245 Ma, 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. From the middle and toward the end of the Triassic, these therapsids shared the scene with squat, plant-eating, and swine-like archosauromorphs called rhynchosaurs and a few carnivorous crocodile-like archosaurs. Yet toward the tail end of the Triassic, approximately 225 Ma, there was a great change in the fortunes of these animals. The majority of therapsids went extinct (one highly evolved group of therapsids, the mammals, of course survived), as did the rhynchosaurs, while only the dinosaurs and a few other taxa among archosaurs survived. And it was the dinosaurs that somehow rose to become the dominant terrestrial vertebrates, by which it is meant that they became the most abundant, diverse, and probably visible group of tetrapods.

Many of the character changes found higher and higher in the archosaurian evolutionary tree are those related to limb posture and locomotion. It is generally said that archosaurs began as sprawlers and ended up with either semi-erect stance (crocodilians) or fully erect posture (pterosaurs and dinosaurs (including birds)). A. J. Charig envisioned much of this change in limb posture happening along the lineages that produced the dinosaurs.2 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 15). The archosaurs that had these new, "improved" features were then able, he supposed, to outcompete contemporary predatory therapsids for their food sources, the herbivorous therapsids and rhynchosaurs, both groups that lacked such parasagittal limb posture. The immediate descendants of these flashy new archosaurs -indeed some of the parasagittal-limbed forms themselves - 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. Therapsids lose, dinosaurs win - all by virtue of having better designed limbs and there by more efficient terrestrial locomotion. We can call this and any other evolutionary advantage a competitive edge. The pattern of waxing and waning dominance (as one group supersedes another in evolutionary time) is called the wedge (Figure 5.8).

At nearly the same time, R. T. Bakker was making similar arguments about the competitive superiority of warm-bloodedness - endothermy -in dinosaurs (see Chapter 15 for an elaboration of Bakker's views on this subject). As we shall see, 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 apply: dinosaurs win, therapsids lose. And the truth of the competitive superiority of endotherms over ectotherms can be read directly from the pattern of faunal succession at the end of the Triassic. Again, we have a hypothesis that comes down to the competitive edge producing the wedge. But must the wedge be produced by the edge?

"No," says M.J. Benton (University of Bristol, England). Not that he's adverse to competitive edges and wedges, when and if they can be documented. It's just that they don't appear in the particular fossil record in question; that is, the Middle to Late Triassic fossil record of the earliest dinosaurs and their predecessors. In order for edges to lead to wedges, all

2 Things are complicated by the fact that Charig also viewed dinosaurs as being at least diphyletic; nevertheless the scenario we present is reasonably close to his general views on the evolution of dinosaur locomotion.

Figure 5.8. Two views of the rise of dinosaurs during the LateTriassic (Tr). (a) Gradual competitive replacement of synapsids, primitive archosaurs, and rhynchosaurs (both herbivores and carnivores) by herbivorous and carnivorous dinosaurs, (b) Rapid opportunistic replacement mediated by extinction.

of the players in the game have to be present to interact with each other. And according to Benton, they were not (note Figure 5.8). 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 therapsids, 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 so-called Dicroidium flora, which contained not only seed-ferns, but also horsetails, ferns, cycadophytes, ginlcgoes, and conifers; see Chapter 16) all but went extinct as well, to be replaced by other conifers and bennettitaleans (large cycad-like plants). 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, was done in an ecological near-vacuum, with mass extinction followed by opportunistic replacement. 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; most researchers working on this part of earth history are now providing us with a better, more detailed picture of these extinctions. 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 on them and thereby the predators feeding on the herbivores. According to Benton, far from being a long-term competitive takeover, this rapid loss of the dominant land-living vertebrates set the stage for the opportunistic evolution of dinosaurs.

The end-Triassic extinctions may have been driven by climatic shifts, but Columbia University's P. E. Olsen and colleagues believe that they have identified another, more dramatic forcing factor: asteroid impact. Like the end of the Cretaceous (see Chapter 18), which was marked by severe and abrupt extinctions of the earth's biota, the extinctions at the end of the Triassic also rank in the Extinction Hall of Fame. And like the mass extinction at the end of the Cretaceous, those at the end of the Triassic may also be allied with a "smoking gun": an impact crater close in age to the first of the Triassic extinctions. This impact structure, the Manicouagan crater in northern Quebec, Canada, is 70 km in diameter, large enough, Olsen believes, to have accommodated an asteroid with enough force to have done the job.

Changing climates may produce extinctions, but for catastrophic events they've got nothing on asteroid impacts. Geologists are coming to believe that these must be among the worst and most wideranging disasters that can be suffered by global ecosystems. If it is true that an asteroid contributed to the first of the double extinctions that devastated the earth's biota at the end of the Triassic, 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 18), 160 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 160 million years earlier.

Important readings Bakker, R. T. 1975. Dinosaur renaissance. Scientific American, 232, 58-78.

Bakker, R. T. and Galton, P. M. 1974. Dinosaur monophyly and a new class of vertebrates. Nature, 248,168-172.

Benton, M. J. 1983. Dinosaur success in the Triassic: a noncompetitive ecological model. Quarterly Review of Biology, 58, 29-55.

Benton, M.J. 1984. Dinosaurs' lucky break. Natural History, 6 (84), 54-59.

Benton, M. J. 2004. Origin and relationships of Dinosauria. In Weishampel, D. B„ Dodson, P. and Osmölska, H. (eds.), The Dinosauria, 2nd edn. University of California Press, Berkeley, pp. 7-20.

Charig, A.J. 1972. The evolution of the archosaur pelvis and hindlimb: an explanation in functional terms. In Joysey, K. A. and Kemp, T. S. (eds.), Studies in Vertebrate Evolution. Winchester, New York, pp. 121-155.

Charig, A.J. 1976. "Dinosaur monophyly and a new class of vertebrates": a critical review. In Bellairs, A. d'A. and Cox, C. B. (eds.), Morphology and Biology of the Reptiles. Academic Press, New York, pp. 65-104.

Gauthier, J. A. 1986. Saurischian monophyly and the origin of birds. Memoirs of the Californian Academy of Science, 8,1-55.

Olsen, P. E„ Shubin, N. H. and Anders, M. H. 1987. New Early Jurassic tetrapod assemblages constrain Triassic-Jurassic tetrapod extinction event. Science, 237,1025-1029.

Sereno, P. C. 1991. Basal archosaurs: phylogenetic relationships and functional implications. Journal of Vertebrate Paleontology, 11 (suppl.), 1-53.

Sereno, P. C„ Forster, C. A., Rogers, R. R. and Monetta, A. F. 1993. Primitive dinosaur skeleton from Argentina and the early evolution of Dinosauria. Nature, 361,64-66.

0 0

Post a comment