Paleobiogeography and Evolutionary History of Thyreophora

Stegosaurs are firmly documented only in western North America, eastern Africa, western Europe, eastern Asia, and Australia. Moreover, their temporal distribution is limited to the Early Jurassic (at the earliest) through the Late Cretaceous, although Cretaceous stegosaurs are comparatively rare in relation to their Jurassic predecessors. Ankylosaurs are known from Middle Jurassic through to Late Cretaceous strata, and well-defined specimens have been recovered in North America, Europe, Asia, and Australia. Their remains also have been found in high-latitude localities, such as Alaska and Australia, meaning that a few species may qualify as "polar dinosaurs." Poorly-defined ankylosaur material is reported from South America, but well-documented ankylosaur trackways have been described from Upper Cretaceous strata in Bolivia. The latter discovery is the best evidence for ankylosaurs in South America, and it bodes well for future discoveries of more thyreophoran body and trace fossil finds in this and other southern continents.

Based on paleoenvironmental analyses of facies associated with thyreophoran body fossils, stegosaur remains occur in fluvial, near-shore marine, and lakeshore deposits. Ankylosaurs are in similar facies but specifically those indicating semi-arid conditions. Of course, taphonomic considerations dictate that the burial site of a

Thyreophoran skeletal remains have been found on every continent except Antarctica, but they are most common in the Cretaceous strata of North America and Asia.

i dinosaur is not necessarily representative of where it lived (Chapter 7). Certainly in the cases of stegosaurs and some ankylosaurs, their skeletal material was buried in aquatic environments that an armored animal should have rarely frequented. However, the completeness and orientation of some ankylosaur skeletons in the Cretaceous of Mongolia argue that many of these skeletons underwent little or no transport. In taphonomic studies of these dinosaurs, the environments are hypothesized as semi-arid or arid with sparsely vegetated dunes. These dunes could have collapsed on hapless ankylosaurs; alternatively, they could have been buried by sandstorms (Chapter 7). Some of these ankylosaur skeletons show the bodies positioned horizontally to inclined, with legs underneath and heads pointed up. This evidence collectively suggests animals that were attempting to excavate themselves. An alternative hypothesis, that these 2- to 3-tonne dinosaurs were burrowers, is not taken seriously. In contrast to their Asian relatives, North American ankylosaurs seemed to have preferred moister areas, which correspondingly had more vegetation.

Interestingly, the previously low number of ankylosaur or stegosaur tracks reported from the geologic record was used as an indirect paleoenvironmental indicator. This is because if any of these animals dwelled in uplands well above coastal areas, preservation of their tracks would have been less likely. However, this argument lacks evidence, thus wherever extensive stegosaur or ankylosaur trackways are found, the hypothesis is easily falsified. Thus far, known ankylosaur tracks were made in sediments deposited on lakeshores and other coastal environments, which indicates that these ankylosaurs were not upland species.

As far as evolutionary lineages are concerned, Genasauria is the clade under which both ornithopods and thyreophorans are placed and both of these clades had a common ornithischian ancestor. Ornithischians as a clade began during the Late Triassic, but no thyreophorans are known from that time. Based on current evidence, thyreophorans probably did not develop until the Early Jurassic. The most basal thyreophorans are the Early Jurassic Scelidosaurus of England, Scutellosaurus of the southwestern USA, and Emausaurus of Germany, which are all considered as outgroups from the clades Ankylosauria and Stegosauria. Of these three, Scutellosaurus, a small (1- to 2-meters long), long-tailed ornithischian, is probably the most primitive. Moreover, it bears some resemblance to the primitive ornithischian Lesothosaurus (Chapter 11), which comes from Lower Jurassic strata of South Africa. Cladistic analyses, however, confirm that Lesothosaurus falls outside of Genasauria and is thus not as closely related to Scutellosaurus as the latter is to Emausaurus and Scelidosaurus. One major feature that distinguishes thyreophorans from other primitive ornithischians is abundant osteoderms (scutes) on the dorsal and lateral parts of the body.

Scelidosaurus, first described by Richard Owen in 1863 (Chapter 3), was closer to the ideal of a thyreophoran in that it was a robust (about 4 meters long) dinosaur with limb proportions more apt for a quadruped. It also had numerous osteoderms, which helps to identify it as an undoubted thyreophoran, although it cannot be placed strictly within either Ankylosauria or Stegosauria. Interestingly, nearly 130 years passed until the description of another basal thyreophoran from the Early Jurassic, Emausaurus.

Of the thyreophorans, ankylosaurs were apparently the most conservative in their evolutionary histories. Once they had evolved their extensive body armor and other distinctive ankylosaur features, they stayed within the parameters of this basic body plan throughout the entirety of their geologic range. Nevertheless, they certainly underwent some diversification during the Late Cretaceous and especially trended toward larger sizes. Stegosaurs, which did not have such a long geologic range, probably underwent rapid evolution from the Early to Middle Jurassic before their greatest success during the Late Jurassic. Although still present during the Cretaceous, they were seemingly sparsely distributed and especially rare during the Late Cretaceous. Because so few thyreophoran fossils are known from between the stratigraphic intervals of the stegosaur- and ankylosaur-bearing zones, little is known about their lineages or the probable factors influencing their natural selection.

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