Description
Plateosaurus is a member of a group of early herbivores known as "prosauropods". The group name is obsolete, as "Prosauropoda" is not a monophyletic group (thus given in quotation marks), and most researchers prefer the term basal sauropodomorph. Plateosaurus had the typical body shape of a herbivorous bipedal dinosaur: a small skull, a long and flexible neck composed of 10 cervical (neck) vertebrae, a stocky body and a long, mobile tail composed of at least 40 caudal (tail) vertebrae. The arms of Plateosaurus were very short, even compared to most other "prosauropods". However, they were strongly built, with hands adapted for powerful grasping. The shoulder girdle was narrow (often misaligned in skeletal mounts and drawings), with the clavicles (collar bones) touching at the body's midline, as in other basal sauropodomorphs. The hind limbs were held under the body, with slightly flexed knees and ankles, and the foot was digitigrade, meaning the animal walked on its toes. The proportionally long lower leg and metatarsus show that Plateosaurus could run quickly on its hind limbs. The tail of Plateosaurus was typically dinosaurian, muscular and with high mobility.
The skull of Plateosaurus is small and narrow, rectangular in side view, and nearly three times as long as it is high. There is an almost rectangular lateral temporal foramen at the back. The large, round orbit (eye socket), the sub-triangular antorbital fenestra and the oval naris (nostril) are of almost equal size. The jaws carried many small, leaf-shaped, socketed teeth: 5 to 6 per premaxilla, 24 to 30 per maxilla, and 21 to 28 per dentary (lower jaw). The thick, leaf-shaped, bluntly serrated tooth crowns were suitable for crushing plant material. The low position of the jaw joint gave the chewing muscles great leverage, so that Plateosaurus could deliver a powerful bite. These features suggest that it fed primarily to exclusively on plants. Its eyes were directed to the sides, rather than the front, providing all-round vision to watch for predators. Some fossil skeletons have preserved sclerotic rings (rings of bone plates that protect the eye).
The ribs were connected to the dorsal (trunk) vertebrae with two joints, acting together as a simple hinge joint, which has allowed researchers to reconstruct the inhaled and exhaled positions of the ribcage. The difference in volume between both positions define the air exchange volume (the amount of air moved with each breath), determined to be approximately 20 l for a P. engelhardti individual estimated to have weighed 690 kg, or 29 ml/kg bodyweight. This is a typical value for birds, but not for mammals, and indicates that Plateosaurus probably had an avian-style flow-through lung, although indicators for postcranial pneumaticity (air sacs of the lung invading the bones to reduce weight) can be found on the bones of only few individuals, and were only recognized in 2010. Combined with evidence from bone histology this indicates that Plateosaurus was endothermic.
The type species of Plateosaurus is P. engelhardti. Adults of this species reached 4.8 to 10 metres (16 to 33 ft) in length, and ranged in mass from 600 to 4,000 kilograms (1,300 to 8,800 lb). The geologically older species, P. gracilis (formerly named Sellosaurus gracilis), was somewhat smaller, with a total length of 4 to 5 metres (13 to 16 ft).
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