LE Representative genera of Prosauropoda with approximate geologic age and where they occur

Genus

Age

Geographic Location

Ammosaurus

Early Jurassic

Western and eastern USA

Anchisaurus

Early Jurassic

Eastern USA

Azendohsaurus

Late Triassic

Morocco

Camelotia

Late Triassic

England

Coloradisaurus

Late Triassic

Argentina

Euskelosaurus

Late Triassic

South Africa, Zimbabwe

Jingshanosaurus

Early Jurassic

China

Lessemsaurus

Late Triassic

Argentina

Lufengosaurus

Early Jurassic

China

Massospondylus

Late Triassic

South Africa

Melanorosaurus

Late Triassic

South Africa

Mussasaurus

Late Triassic

Argentina

Plateosaurus

Late Triassic

Germany

Riojasaurus

Late Triassic

Argentina

Ruehleia

Late Triassic

Germany

Saturnalia

Late Triassic

Brazil

Sellosaurus

Late Triassic

Germany

Thecodontosaurus

Late Triassic

England

Yimenosaurus

Early Jurassic

China

Yunnanosaurus

Early Jurassic

China

but they were initially misidentified as human bones. A later diagnosis showed them as rightfully belonging to Anchisaurus of the Early Jurassic. Regardless of any such missteps, paleontologists immediately recognized the kinship of prosauropods and sauropods. Because the former preceded sauropods in the geologic record, they were considered as ancestral to sauropods. For this reason, they were given the name "Prosauropoda," first dubbed in 1920 by Friedrich von Huene (Chapter 3). Although current evidence indicates that prosauropods were not ancestors of sauropods, like many other mislabeled dinosaur taxa the name has stuck and will likely continue to be used indefinitely. Compared to other major clades of dinosaurs, prosauropods were not only early on the scene but relatively short-lived; their range was from the Late Triassic through to the Early Jurassic. Different prosauro-pod species went extinct by the end of the Triassic or the end of the Early Jurassic, but the clade as a whole was relatively diverse throughout its 50-million-year-old history (Table 10.1).

Plateosaurus is perhaps the archetypical prosauropod because it is both the most abundant in the fossil record and best studied, thanks to its numerous remains in Late Triassic rocks of Europe. Its skull and postcranial elements also have some important distinguishing features (Fig. 10.3A), which include:

■ Small but long skull that is relatively compressed laterally.

■ Postorbital part of skull turned downward.

■ Large nares but small, laterally placed orbitals.

■ Relatively large olfactory bulb, as determined from endocranial casts.

FIGURE 10.3 (A) Skull of Late Triassic prosauropod Plateosaurus of Germany. Anatomical features of the skull. (B) Complete specimen; Naturhistoriches Museum Basel, Basel, Switzerland. See Figure 5.3 for overall anatomy.

■ Premaxilla and maxilla together containing as many as 36 teeth, whereas dentary contains as many as 28.

■ Roughly serrated and leaf-like teeth.

The postcranial features (Fig. 5.3) of Plateosaurus include:

■ Long neck in comparison to most theropods, yet shorter than those of most sauropods, with an atlas and a total of 10 cervical vertebrae.

■ 15 dorsal, 3 sacral (attached to the pelvic bones), and nearly 50 caudal vertebrae.

■ Weight seemingly associated more with its posterior.

■ Phalangeal formula on pes of 2-3-4-5-1, with small unguals, and phalanx on digit V seemingly vestigial.

■ Phalangeal formula on manus of 2-3-4-3-2, with unguals present on digits I through to III.

■ Enlarged ungual on digit I that is deviated from the rest of the manus (discussed later).

Using functional morphology as a guide for examining both the skull and postcranial skeleton, the anatomical data suggest the following interpretations:

1 Plateosaurus was a mostly quadrupedal but facultatively bipedal animal;

2 it could raise up on its hind limbs to grasp and tear off plant materials, such as in trees;

3 it could bring plant material closer to its mouth using its clawed hands; and

4 it could shear this plant material with its teeth and jaws, with minimal chewing before swallowing.

The long neck also apparently aided in this high-browsing mode of life, as it could have reached vegetation out of the reach of other large herbivorous contemporaries in the Late Triassic. In this sense, during the Late Triassic and Early Jurassic, Plateosaurus and most other prosauropods might have been the ecological equivalent of modern giraffes.

In most respects, other prosauropods only varied slightly from this basic model, and the few important differences probably reflect adaptations to different environmental conditions and niches. For example, the Late Triassic Riojasaurus of Argentina was considerably larger (11-m long) than Plateosaurus (8-m long). This size difference suggests that Riojasaurus adapted to different plant foods or otherwise gained an advantage from being larger, perhaps as a defensive measure against increasingly larger ceratosaurian predators. The Early Jurassic Yunnanosaurus of China had pencil-like teeth more adapted for raking vegetation, which differed from the probable shearing function of the serrated teeth in Plateosaurus. Anchisaurus, Sellosaurus, and Thecodontosaurus had relatively larger orbits and smaller nares than Plateosaurus. Perhaps most importantly, they were smaller than most other prosauropods and accordingly could probably move faster. This interpretation is augmented by the ratio of metatarsal III (the middle one) to the tibia of each prosauropod species, where higher ratios should reflect abilities for faster movement. For example, Riojasaurus (= slower) has a ratio of about 0.4, whereas Anchisaurus is close to 0.7 (= faster).

Prosauropods as a clade represent a good start for the sauropodomorph lineage and their longevity in the geologic record was certainly an indicator of their success, even though sauropods surpassed them by the end of the Early Jurassic. Whether the Early Jurassic arrival of other large herbivores, such as sauropods and thyreophorans (Chapter 12), was part of an ecological displacement of prosauropods is unknown. Nevertheless, their extinction was the first one for any major dinosaur clade. This circumstance provoked comparisons to the paleoenvir-onmental conditions associated with later dinosaur extinctions and aided in working out any similarities in patterns or trends.

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