Plateosaurus

***Plateosaurus*** (probably meaning "broad lizard", often mistranslated as "flat lizard") is a genus of plateosaurid dinosaur

that lived during the Late Triassic period, around 214 to 204 million years ago, in what is now Central and Northern Europe and Greenland, North America. *Plateosaurus* is a basal (early) sauropodomorph dinosaur, a so-called "prosauropod". As of 2011, two species are recognised: the type species *P. engelhardti* from the late Norian and Rhaetian, and the slightly earlier *P. gracilis* from the lower Norian. However, others have been assigned in the past, and there is no broad consensus on the species taxonomy of plateosaurid dinosaurs. Similarly, there are a plethora of synonyms (invalid duplicate names) at the genus level.

Discovered in 1834 by Johann Friedrich Engelhardt and described three years later by Hermann von Meyer, *Plateosaurus* was the fifth named dinosaur genus that is still considered valid. Although it had been described before Richard Owen formally named Dinosauria in 1842, it was not one of the three genera used by Owen to define the group, because at the time, it was poorly known and difficult to identify as a dinosaur. It is now among the dinosaurs best known to science: over 100 skeletons have been found, some of them nearly complete. The abundance of its fossils in Swabia, Germany, has led to the nickname *Schwäbischer Lindwurm* (Swabian lindworm).

*Plateosaurus* was a bipedal herbivore with a small skull on a long, flexible neck, sharp but plump plant-crushing teeth, powerful hind limbs, short but muscular arms and grasping hands with large claws on three fingers, possibly used for defence and feeding. Unusually for a dinosaur, *Plateosaurus* showed strong developmental plasticity: instead of having a fairly uniform adult size, fully grown individuals were between 4.8 and 10 metres (16 and 33 ft) long and weighed between 600 and 4,000 kilograms (1,300 and 8,800 lb). Commonly, the animals lived for at least 12 to 20 years, but the maximum life span is not known.

Despite the great quantity and excellent quality of the fossil material, *Plateosaurus* was for a long time one of the most misunderstood dinosaurs. Some researchers proposed theories that were later shown to conflict with geological and palaeontological evidence, but have become the paradigm of public opinion. Since 1980 the taxonomy (relationships), taphonomy (how the animals became embedded and fossilised), biomechanics (how their skeletons worked), and palaeobiology (life circumstances) of *Plateosaurus* have been re-studied in detail, altering the interpretation of the animal's biology, posture and behaviour.

*Plateosaurus* had the typical body shape of a herbivorous bipedal dinosaur: a small skull, a long and flexible neck composed of 10 cervical vertebrae, a stocky body, and a long, mobile tail composed of at least 40 caudal vertebrae.[1][2][3] The arms of *Plateosaurus* were very short, even compared to most other "prosauropods". However, they were strongly built, with hands adapted for powerful grasping.[2][4] The shoulder girdle was narrow (often misaligned in skeletal mounts and drawings),[4] with the clavicles (collar bones) touching at the body's midline,[2] as in other basal sauropodomorphs.[5] 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.[2][6][7] The proportionally long lower leg and metatarsus show that *Plateosaurus* could run quickly on its hind limbs.[2][4][6][7] The tail of *Plateosaurus* was typically dinosaurian, muscular and with high mobility.[4]

*P. engelhardti* skull cast, Royal Ontario Museum

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.[1][2][8] 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).[1][2][8] The thick, leaf-shaped, bluntly serrated tooth crowns were suitable for crushing plant material.[1][2][8] The low position of the jaw joint gave the chewing muscles great leverage, so that *Plateosaurus* could deliver a powerful bite.[8] These features suggest that it fed primarily to exclusively on plants.[8] Its eyes were directed to the sides, rather than the front, providing all-round vision to watch for predators.[1][2][8] Some fossil skeletons have preserved sclerotic rings (rings of bone plates that protect the eye).[1][2][8]

Size comparison between a maximum size individual of *P. engelhardti* and a human

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 these two positions defines 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.[4] This is a typical value for birds, but not for mammals,[9] and indicates that *Plateosaurus* probably had an avian-style flow-through lung,[4] although indicators for postcranial pneumaticity (air sacs of the lung invading the bones to reduce weight) can be found on the bones of only a few individuals, and were only recognised in 2010.[10][11] Combined with evidence from bone histology[12][13] this indicates that *Plateosaurus* was endothermic.[13][14]

The type species of *Plateosaurus* is *P. engelhardti*.[15] Adults of this species reached 4.8 to 10 metres (16 to 33 ft) in length,[13] and ranged in mass from 600 to 4,000 kilograms (1,300 to 8,800 lb).[7] 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)

a common species is a green variant with white stripes along the back