Plateosaurus

Plateosaurus ("broadway lizard") is a basal (original) genus of sauropodomorph dinosaur. It is the type and namesake genus of the family Plateosauridae. Finds of these animals date from the Upper Triassic (Middle Norian to Rhaetian), primarily of Central Europe, and are thus about 217 to 201 million years old.

Currently, two species of Plateosaurus are recognized: the type species P. engelhardti from the Late Norian and Rhaetian, and the geologically somewhat older species P. gracilis from the Early Norian. Systematics and taxonomy of plateosaurid dinosaurs are still controversial, however, and a variety of genus and species synonyms exist.

The first find of a Plateosaurus was made in 1834 by Johann Friedrich Engelhardt in the Heroldsberg region not far from Nuremberg. He gave the material to the Frankfurt vertebrate palaeontologist Hermann von Meyer for processing. Meyer described it in 1837 under the name Plateosaurus engelhardti. This made Plateosaurus the fifth scientifically described dinosaur, although the genus was not subsequently used by Richard Owen to define the Dinosauria, since the material known at the time was quite incomplete and difficult to recognize as a dinosaur according to the state of knowledge at the time. Today Plateosaurus is one of the best known dinosaurs, of which well over 100 partly complete skeletons have been found in very good preservation.

Plateosaurus was a bipedal herbivore with a small skull perched on a long, flexible neck, sharp but stout teeth, powerful hind legs, and a strong grasping hand with an enlarged thumb claw that may have been used in foraging or defensive behavior. Unusually for a dinosaur, Plateosaurus had a strong developmental plasticity: adult individuals were not all more or less the same size, but between 4.8 and 10 m long and weighed between 600 kg and 4 tons. The lifespan of most individuals studied so far was between 12 and 20 years.

Despite the rich finds and the good preservation of the material, the appearance of Plateosaurus was controversial for a long time, which led to partly contradictory reconstructions. Some researchers formulated hypotheses on locomotion, diet, palaeobiology and ecology, and taphonomy of the finds that, according to current research, are in clear contradiction with the geological and palaeontological findings inferred from skeletal anatomy and site conditions. Since 1980, the taxonomy and taphonomy, and since 2000, the biomechanics and other aspects of the paleobiology of Plateosaurus have been reexamined, fundamentally changing the overall picture of the genus.

Because of its abundance in southern Germany, the paleontologist Friedrich August Quenstedt gave Plateosaurus the nickname "Swabian Lindwurm".

Description

Plateosaurus belongs to a group of early herbivorous lizard pelvic dinosaurs that were traditionally called prosauropods. Today, instead of "prosauropods", most researchers speak of "basal Sauropodomorpha", i.e. sauropodomorphs that are below the Sauropoda in the phylogenetic tree. Plateosaurus displays a typical bipedal herbivore design: a small skull sits on a long, flexible neck of ten cervical vertebrae, the torso is stocky and merges into a long, flexible tail of over 40 vertebrae. The arms of Plateosaurus are among the shortest found in "prosauropods", but were strong, and the hand adapted to grasping large objects. The shoulder girdle was narrow (often incorrectly reconstructed in skeletal mounts and drawings), with clavicles touching in the middle, as known from other "prosauropods". The legs were long and held vertically under the body, and the foot was digitigrade. Plateosaurus was thus a toe-walker and adapted to brisk bipedal walking.

The skull of Plateosaurus was small and narrow. The snout was occupied by many small, leaf-shaped teeth, five or six of which were in the premaxillary (intermaxillary), 24 to 30 in the maxillary (upper jaw), and between 21 and 28 in the mandible. The serrated, broadly leaf-shaped tooth crowns were capable of crushing plant material. It is likely that Plateosaurus fed exclusively on plants. The eyes were directed laterally, not forward, giving good all-round vision but not stereoscopic vision. Sclerotic eye rings are preserved in some finds. Such rings of ossified plates protect the eyeball from injury.

As is common for dinosaurs, the trunk ribs of Plateosaurus were two-headed, that is, they bifurcated at their upper end into an upper and a lower branch, each of which was provided with an articular head (capitulum and tuberculum, respectively). The articular heads formed with corresponding articular surfaces (diaposphysis and parapophysis, respectively) of the respective dorsal vertebra a hinge joint endowed with moderate play. The interaction of all these joints with the respiratory musculature enabled the ribcage to expand and contract again in the rhythm of breathing. With the aid of a computer-modelled skeleton, taking into account the natural position of each vertebra and the consequent alignment of the axes of rotation of the two respective hinged ribs, it was possible to estimate a respiratory volume of about 20 l for a specimen weighing about 690 kg, which corresponds to a value of 29 ml per kilogram of body weight. This latter value is typical of many birds, for example geese, and is significantly higher than the average value for mammals. This suggests that Plateosaurus probably possessed avian-like lungs with unidirectional airflow and ventilation through air sacs, even though Plateosaurus does not show postcranial pneumatization of the skeleton (openings and cavities indicating the presence of air sacs in the bones). Together with circumstantial evidence from the structure and growth of the bones, this suggests that Plateosaurus was warm-blooded.

The type species of Plateosaurus is P. engelhardti. Adult individuals of this species reached a body length between 4.8 and 10 meters and a body weight of 600 kg up to 4 tons. The second, older species P. gracilis (formerly Sellosaurus gracilis) was slightly smaller and reached body lengths of 4 to 5 meters. There is no definite knowledge about the skull of this species, as the affiliation of a very well preserved skull (specimen number GPIT 18318a) in the collection of the Institute of Geosciences Tübingen to P. gracilis is questionable.

A large specimen of P. engelhardti in size comparison with a human being


Skull cast of P. engelhardti in the Royal Ontario Museum

Discovery and history

The chemistry professor Johann Friedrich Philipp Engelhardt discovered some vertebrae and leg bones near Heroldsberg near Nuremberg in 1834. He left these to Hermann von Meyer for processing, who described them in 1837 as the new genus Plateosaurus with the species name engelhardti. Since then, the remains of well over 100 individuals of Plateosaurus have been found.

Altogether, material of Plateosaurus has been found at more than 50 sites in Germany, mainly along the Neckar and Pegnitz rivers, but also in Switzerland and France. Three sites are of particular importance, because here particularly much and especially well preserved material was found. Between 1910 and 1930, excavations in a clay pit in Halberstadt in Saxony-Anhalt yielded between 39 and 50 skeletons, most of which belong to Plateosaurus, but also to Liliensternus and Halticosaurus. Some of the plateosaur material was placed by Otto Jaekel with P. longiceps, a species now considered a younger synonym of P. engelhardti and thus invalid. Most of the material went to the Museum für Naturkunde in Berlin, where large parts were destroyed during World War II. The site in Halberstadt is now covered with a residential area.

At the second rich site of Plateosaurus in Germany, a quarry in Trossingen, between the Black Forest and the Swabian Alb, bones were discovered by children at the beginning of the 20th century and given to their teacher. The teacher informed Eberhard Fraas, curator at the Stuttgart Natural History Collection (now the Staatliches Museum für Naturkunde Stuttgart). The site was opened up in three excavations between 1911 and 1932. During excavation campaigns led by Fraas (1911-1912), Friedrich von Huene (1921-23), and finally Reinhold Seemann (1932), a total of 35 complete or largely complete skeletons of Plateosaurus were found, as well as parts of another ca. 70 individuals. A new excavation began in 2007 (as of October 2010).

The Plateosaurus fossils in the clay pit of Tonwerke Keller AG in Frick (Switzerland) were first noticed in 1976. The bones from this site were mostly severely deformed during the compaction and consolidation of the clays during diagenesis. Nevertheless, Frick offers skeletons comparable in completeness and posture to those from Trossingen and Halberstadt. In 2015, an excavation team at Frick uncovered an eight-metre-long skeleton of a plateosaur; it is the largest dinosaur skeleton ever found in Switzerland.

In 1997, during an oil exploration well in the Snorre oil field in the northern North Sea, a bone was drilled from a depth of 2651 m below sea level, which workers initially thought was a plant fossil. However, the bone was identified in 2003 as a fragment of a Plateosaurus leg bone. Other finds have also been made on the Greenland mainland.

P. engelhardti specimen number MSF 23 in the Sauriermuseum Frick, the most complete Plateosaurus skeleton from Frick so far.