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Prosauropoda

Prosauropoda or prosauropods (/proʊˈsɔroʊpɒd/) were a group of early herbivorous dinosaurs that lived during the Triassic and early Jurassic periods. They were frequently the predominant herbivore in their environment, and quickly reached large size (6 to 10 meters long [20 to 33 ft]). All prosauropods had a long neck and small head, forelimbs shorter than the hindlimbs, and a very large thumb claw (inherited from the thecodontosaurs) for defense. Most were semi-bipedal, although at least one large form (Riojasaurus) was fully quadrupedal. They were originally thought to be the ancestors of the sauropods, but are now considered a parallel lineage.

Changing definitions

The Prosauropoda were originally defined as the early, bipedal, Triassic ancestors of the great sauropod dinosaurs. More recently, cladistic analysis suggests that rather than being ancestral to sauropods, prosauropods were a sister clade. Recent studies of the genus Massospondylus reveal that the Prosauropoda is indeed monophyletic.

The problem however lies in what genera are considered prosauropods. Upchurch (1997) proposes a Node-Based Definition: Blikanasauridae, Thecodontosauridae, Anchisauridae, Plateosauridae, Melanorosauridae, and all sauropodomorphs closer to them than sauropods. More recently, on the basis of studies of early sauropodomorphs Adam Yates proposed a cladogram in which the primitive genera Saturnalia, Thecodontosaurus, and Efraasia (basically, a paraphyletic Thecodontosauridae) represent basal outgroups prior to the prosauropod-sauropod split. Anchisaurus (despite its classic "prosauropod" build) is now recognised as the most primitive sauropod (Yates 2004). The melanorosaurs and blikanasaurs are very early members of the sauropod line.

Technical diagnosis

The Prosauropod skull was approximately half the length of the femur; their jaw articulation was slightly below the level of the maxillary tooth row. Their teeth were small, homodont or weakly homodont, spatulate, with coarse marginal serrations; manual digit I bore a twisted first phalanx and an enormous, trenchant ungual medially directed when hyperextended. Prosauropod digits II and III were of subequal length, with small, slightly recurved ungual phalanges; digits IV and V were reduced, and lacked ungual phalanges. Typical Prosauropod phalangeal formula was 2–3–4–3. The blade-like distal parts of the pubis formed a broad, flat apron. The fifth pedal digit was vestigial; the femur had a longitudinal crest proximal to the lateral condyle. The lesser trochanter was a weak ripple proximodistally lying on the latero-anterior surface, and the main parts of the trochanter were below the level of the femoral head (Gauffre, 1993).

History and general description

Sauropodomorphs first appeared on the supercontinent of Pangaea as small (1.5 to 3 meters long [5 to 10 ft]) forms during the middle or late Carnian age (the earliest part of the late Triassic). They are known from Brazil (Saturnalia and Unaysaurus), Madagascar (recently discovered), and Morocco (Azendohsaurus).

Prosauropods retained the same body plan, but by the later Early or Early Middle Norian age had doubled in linear dimensions, as indicated by the 4 to 6 meter long (13 to 20 ft) Plateosaurus gracilis of the Lower and Middle Stubensandstein of Germany. This animal in turn gave rise to other species of Plateosaurus, and this animal — 8 meters long (26 ft) and around 1,500 kg or more in weight — dominated the Late Norian environment, persisting into the Rhaetian age. Meanwhile in Argentina an even larger prosauropod, Riojasaurus, served a similar role This animal, 10 meters long (33 ft), was so big it had to walk on all fours. Curiously, in southern Africa at this time the megaherbivore niche was taken not by prosauropods but by basal sauropods, as indicated by Euskelosaurus, Melanorosaurus and Blikanasaurus, and Antetonitrus. Interestingly, while sauropodomorphs dominated the Norian and Rhaetian large herbivore niche, the large carnivore niche continued to be ruled by the Crurotarsi (e.g. ornithosuchids and 'rauisuchians').

The end-Triassic extinction killed off the basal sauropodomorphs like Thecodontosaurus, Riojasaurus and species more closely related to sauropods such as Melanorosaurs and Blikanasaurus. However, 'prosauropod' species such as Anchisaurus survived, as did true sauropods. While the first sauropods diversified, the early Jurassic prosauropods radiated out in a number of medium sized (4 to 6 meter long [13 to 20 ft]) megaherbivores like Massospondylus, Lufengosaurus and Yunnanosaurus and were as successful as their late Triassic predecessors.

The prosauropod reign came to an end in the late Early Jurassic. Both prosauropods and anchisaurs died out at the same time, while the basal sauropods survived and continued to radiate.

Classification

After Yates (2003) and Galton (2001)

Suborder Sauropodomorpha

Plateosauria

Family Plateosauridae

Family Massospondylidae

References

Gauffre F.-X. (1993): The Prosauropod Dinosaur Azendohsaurus laaroussii from the Upper Triassic of Morocco. Palaeontology 36(4): 897–908.

Upchurch, P (1998), The phylogenetic relationships of sauropod dinosaurs. Zool. J. Linnean Soc. 124: 43–103.

Yates, A. M. (2004) Anchisaurus polyzelus (Hitchcock): the smallest known sauropod dinosaur and the evolution of gigantism among sauropodomorph dinosaurs: Postilla, n. 230, 58 pp.

Yates, A.M. & Kitching, J. W. (2003) The earliest known sauropod dinosaur and the first steps towards sauropod locomotion. Proc. R. Soc. Lond.: B Biol Sci. 2003 Aug 22; 270(1525): 1753–8.