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America was part of Laurasia 250 million years ago as Pangaea began to break apart. Reconstruction from Ronald Blakey/Colo- rado Plateau Geosystems, Inc. Used with permission.

evolution among the flora and fauna during the Triassic period. As noted in chapter 1, the Trias- sic was the time of the great reptilian radiation on land and in the sea. The Permo-Triassic extinctions had cleared many of the major ecologic niches in both the terrestrial and marine realm of their pri- mary late Paleozoic occupants. On land the reptiles were the principal beneficiaries of the evolution- ary opportunities created by the extinctions. So many different groups of reptiles developed dur- ing the transition from the Paleozoic to the Meso- zoic that the Triassic assemblage is a confusing and complex array of rapidly evolving lineages. To complicate matters even more, some of the Trias- sic reptiles, such as the mammal-like therapsids, represent “holdovers” from the earlier late Paleo- zoic aggregate. The ancestors of the dinosaurs were included somewhere in that complex hoard of rep- tiles, accompanied by many other specialized rep- tiles representing separate lineages.

While it is impossible to review all the different groups of early Triassic reptiles fully here, it is help- ful to identify two broad categories before we exam- ine the Triassic vertebrate fossils from Utah and adjacent areas. Recall that dinosaurs belong to a large group of reptiles known as the Diapsida in reference to the two temporal openings in the skull (see the appendix for more details). This large group includes dinosaurs, birds (direct dinosaur relatives), croco- diles, snakes and lizards, pterosaurs, and many other extinct groups. Lizards and snakes separated from the other archosaurian diapsids in early Triassic time as a separate group, the Lepidosauria. Our princi- pal interest is in the remaining archosaurs, because the dinosaurs, their ancestors, and their descendants are all found within this group. As part of the great reptilian radiation of the early Triassic period, the archosaurs diverged into two main groups: the Pseu- dosuchia (crocodilians and their relatives) and the Ornithodira (a group that includes the dinosaurs and birds, along with the ancestors of both).

These three groups—the Lepidosauria, the Orni- thodira, and the Pseudosuchia—can be considered

clades, a natural grouping that includes only evolu- tionarily related animals. Clades do not have a tax- onomic rank such as family, order, or phylum but may consist of many such groups. A clade can be considered a discrete branch on the tree of life, sep- arate from all other branches. In addition, the con- cept of a clade can be used to subdivide a large group (like the diapsid reptiles), into smaller units of more closely related creatures, known as sister groups or subclades. A cladogram is a graphic rep- resentation of the relationships between evolution- arily related groups of animals (fig. 2.2). The greater the separation between the groups represented on a cladogram, the more distant is the relation- ship between them. Thus each branch on the tree of life may have numerous smaller branches diverg- ing from it; but any branch can be a clade, regard- less of how many subbranches it contains. One of the things that complicates the evolutionary history of the dinosaurs is that they evidently arose during the time when many different clades were diverg- ing from the reptilian trunk of the tree simultane- ously. The great burst of reptilian evolution created 2.2. Cladogram of the diapsid reptiles. The Ornithodira

includes birds and dinosaurs that are more closely related to the crocodiles than they are to lizards and snakes.

a confusing array of reptiles that paleontologists are still trying to sort out. Be mindful as well that clades are constructed by paleontologists by identifying

skeletal features as either primitive (plesiomorphic) or advanced (apomorphic) and reconstructing the branching pattern accordingly. This requires some judgments that might change in light of new fos- sil discoveries. Finally, a clade can be defined in dif- ferent ways; various scientists may apply different names to a given group. Paleontologists are in gen- eral agreement that the three clades we have iden- tified are valid, but revisions may occur as we learn more about the earliest dinosaurs and their rela- tives. Let’s examine some of the more common rep- tiles known from the early Triassic strata of the Utah region to illustrate the variety of creatures that existed as the first dinosaurs appeared.

The phytosaurs (fig. 2.3B) are one of the most common pseudosuchian reptiles found in the Trias- sic rocks of the Southwest. These semiaquatic rep- tiles were predators that looked much like modern crocodiles and seem to have lived in or near rivers. With long pointed snouts and jaws lined with sharp conical teeth, the phytosaurs were probably vora- cious predators of fish and smaller reptiles living in the rivers that they inhabited. The largest phytosaurs were nearly 20 feet (about 6 meters) long. Aetosaurs (fig. 2.3C) were also crocodilelike in their general body plan, but these animals were herbivorous and possessed heavy plates of bony armor (scutes) along with prominent defensive spikes in the shoulder region and along the flanks. Some of the aetosaurs had blunt snouts, evidently for rooting through plant litter and soil, along with leaf-shaped teeth for shearing vegetation. These features suggest that they ate roots, low-growing shrubs, or aquatic plants growing in or along the banks of Triassic rivers.

Rauisuchians include several types of carniv- orous pseudosuchian reptiles that were quadru- pedal with more or less erect limbs and thus were better adapted for terrestrial locomotion than the phtyosaurs and aetosaurs. The largest rauisu- chian in North America was Postosuchus, a 12-foot- long (4-meter) predator that probably weighed in at some 400 pounds (fig. 2.3G). The rauisuchians looked a bit like massive long-legged alligators. 2.3. Nondinosaur vertebrates of the late Triassic Chinle

Formation of the western United States: A. Metoposau-

rus, a carnivorous amphibian; B. Rutiodon, the most

common of the pseudosuchian phytosaurs; C. Desma-

tosuchus, a herbivorous semiaquatic aetosaur; D. Place- rias, a dicynodont therapsid reptile; E. Paradapedon, a

rhynchosaur with a skull 6 inches long; F. Hesperosu-

chus, a small lizardlike pseudosuchian; G. Postosuchus,

a large rauisuchian with a skull 2 feet long. A–D and F: from Colbert 1972; E: from Carroll 1988 (cited in chapter 1 references); G: after Chatterjee 1986.

Dicynodonts (fig. 2.3D) were large plant-eat- ing mammal-like reptiles (therapsids) that typically had paired tusks for rooting. The dicynodonts were squat and thick-bodied, with large heads tipped with a beaklike apparatus. Canadian paleontolo- gist D. A. Russell refers to the dicynodonts as “cow- turtles” (Russell 1989). The name is fitting, for it describes their general appearance and is also sug- gestive of the combination of reptilian and mamma- lian skeletal features. The therapsids also included some carnivorous forms, known as cynodonts, that had an overall doglike appearance.

Rhynchosaurs were squat, rather piglike herbivo- rous reptiles represented by Paradapedon (fig. 2.3E), one of the most common North American mem- bers of the group. The bones forming the snout of most rhynchosaurs curved over the lower jaw to form a prominent “overbite.” This feature, coupled with small peglike teeth, probably allowed the rhyn- chosaurs to root through soil and plant litter to find their fodder.

In addition to these extinct groups of reptiles, the earliest representatives of more familiar forms also emerged during the Triassic. The remains of prim- itive members of the turtle (anapsid reptiles) and lizard clans (lepidosaurs) can be found in rocks of Triassic age. The earliest ancestors of the archosaurs (true crocodiles, dinosaurs, and birds) were also emerging at about the same time. The landscapes of the world were definitely covered with reptiles dur- ing the Triassic.

Other than the early ornithodirans, none of the groups of reptiles identified above are closely related to the dinosaurs. Sometime in the early Tri- assic, among the ongoing reptilian riot, a group of slender-legged, mostly quadrupedal carnivores emerged. This set of reptiles has traditionally been assigned to the group known as the “thecodonts,” many of which are now thought to be primitive ornithodirans, the group to which the dinosaurs belong. Several Triassic ornithodirans might be con- sidered dinosaur ancestors, because they were evi- dently better adapted for terrestrial locomotion than

their contemporaries from the Triassic. With limbs positioned directly beneath the body and ankles and feet designed for swift walking or running, these creatures became extremely successful and varied, eventually developing carnivorous members that were at least partially bipedal. Were these the ear- liest dinosaurs? The question is difficult to answer, because the evolutionary trend toward increasing land mobility (including bipedal stance) was occur- ring in several different lineages within different archosaur clades. The term “convergent evolution” is used to describe the development of similar fea- tures in separate lineages through natural selection for similar traits. It now appears that the traits that distinguish the dinosaurs were developing to differ- ent degrees in several groups of archosaurs during Triassic time.

No one knows for sure where or exactly when the first dinosaurs originated. South America seems to have many candidates for the “first dino- saur” award, and many paleontologists believed until recently that this continent was the cradle of the Dinosauria. Staurikosaurus, a 6-foot-long (2-meter) bipedal predator from Brazil, seems to be on the threshold of the Dinosauria, because the pelvis has a small opening in it for the end of the femur and the hind limbs were evidently posi- tioned vertically beneath the body. Its bones were recovered from rocks that are not precisely dated but seem to be between 235 and 225 million years old (middle Triassic age). Staurikosaurus is so primitive in other aspects of its skeleton, however, that it cannot be confidently assigned to either the Ornithischia or the Saurischia. Other, perhaps slightly younger, South American reptiles are sim- ilarly dinosaurlike but are comparatively primi- tive. Herrerasaurus and Eoraptor, for example, are both extremely “dinosaurian” bipedal carnivores from the late Triassic of northwestern Argentina. Of the two, Eoraptor is more primitive and lacks some key features of the Dinosauria. Herrerasaurus is more typical of later theropods but is still primi- tive in comparison to the true theropod dinosaurs.

To make matters even more intriguing, a contem- porary dinosaur from the same area, Pisanosaurus, is clearly related to the ornithischian dinosaurs. Pisanosaurus was unquestionably a plant-eater, as evidenced by its well-worn simple teeth, but it did not have the fully developed “dinosaurian” type of ankle. Scientists continue to debate whether or not Pisanosaurus represents the oldest ornithis- chian dinosaur or something close to the ancestor of that group. The absolute age of these specimens is somewhat uncertain; most evidence suggests that they are around 228 million years old (Rogers and others 1993). So we can conclude that the age of dinosaurs was on the threshold of opening some 17 million years after the Triassic began in South America. By that time at least three groups of dino- saurlike creatures existed on that continent.

New evidence, however, suggests that the dino- saurs may not have originated in South America after all. In Africa, for example, Euparkeria pos- sessed reduced forelimbs, suggesting that it was beginning to use its hind limbs as the main method of propulsion. This predator looked for the most part like a small carnivorous dinosaur and was very similar to what many scientists perceived as the archosaur ancestor of the first dinosaurs. Asil- isaurus, a quadrupedal omnivore 5–10 feet (1.5–3.3 meters) long that was recently described from Tri- assic rocks in Tanzania (Nesbitt and others 2010), seems to have a mix of dinosaur and nondinosaur

skeletal features. The remains of a relatively large quadrupedal sauropod dinosaur are known from Triassic rocks in South Africa that are poorly dated but clearly demonstrate the presence of true dino- saurs in the mid-Triassic (Yates and Kitching 2003). On the nearby island of Madagascar vertebrate remains have been uncovered that likely represent at least two species of plant-eating dinosaurs known as prosauropods (Flynn and others 2009) from rocks that are 225–230 million years old. Footprints and body fossils in Poland suggest the presence of dino- saurs in Europe as early as 250 million years ago, very close to the beginning of the Triassic (Brusatte and others 2011).

Thus it appears that the dinosaurs first evolved within the ornithodiran lineage sometime in the early or middle Triassic, 240–250 million years ago, but we can’t be certain where. Perhaps it was in South America, Africa, or Eurasia. But wherever the dinosaurs first emerged from the confusing multi- tude of Triassic reptiles, they were without doubt a minor component of the terrestrial fauna. Several other groups of nondinosaur reptiles, including the pseudosuchians, appear to have been more abun- dant and varied at the time the dinosaurs emerged. It was not until after the end of the Triassic period that the dinosaurs became a prominent element in terrestrial vertebrate faunas. By that time their dis- tribution was nearly worldwide, with several major clades represented on most continents (fig. 2.4).