Old Earth Ministries Online Dinosaur Curriculum

Plateosaurus

Quick Facts

Length:  up to 33 feet

Weight:  up to 8,800 lbs

Date Range:   216 - 199 Ma, Late Triassic Period

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 today. It is now among the dinosaurs best known to science, with over 100 skeletons found, some of them nearly complete. German palaeontologist Friedrich August von Quenstedt nicknamed it the Schwäbischer Lindwurm (Swabian dragon) because it was so common a fossil in southwestern Germany.

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 The group name is obsolete, as "Prosauropoda" is not a monophyletic group (thus given in quotation marks), and most researchers prefer the term basal sauropodomorph.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 ten 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", but strongly built, with hands adapted to powerful grasping. The shoulder girdle was narrow (often misaligned in skeletal mounts and drawings), with the clavicles (collar bones) touching at the body midline, as is the case in other basal sauropodomorphs. The hindlimbs were held under the body, with slightly flexed knees and ankles, and the foot digitigrade – the animal walked on its toes. The proportionally long lower leg and metatarsus show that Plateosaurus was adapted to rapid bipedal locomotion. The tail of Plateosaurus was typically dinosaurian, muscular and with high mobility.

The ribs were connected to the dorsal (trunk) vertebrae with two joints, together acting as a simple hinge joint, which allowed reconstructing the inhaled and exhaled positions of the ribcage. The difference in volume between both positions determines the air exchange volume (the amount of air moved with each breath), determined to be ~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 no indicators for postcranial pneumaticity (air sacs of the lung invading the bones to reduce weight) can be found on the bones. Combined with evidence from bone histology this indicates that Plateosaurus was an endotherm.

The type species of Plateosaurus is P. engelhardti. Adult individuals of this species reached 4.8 to 10 metres (16 to 33 ft) in length. Average individuals had a mass of around 600 to 4,000 kilograms (1,300 to 8,800 lb). The older species, P. gracilis (formerly named Sellosaurus gracilis), was somewhat smaller, with a total length of 4 to 5 metres (13 to 16 ft).

Discovery and history

Material assigned to Plateosaurus has been found at over 50 localities in Germany (mainly along the Neckar and Pegnitz river valleys), Switzerland (Frick) and France.

In 1997, workers of an oil platform of the Snorre oilfield located at the northern end of the North Sea were drilling through sandstone for oil exploration when in a drill core extracted from 2,256 meters below the seafloor they stumbled upon a fossil they believed to be plant material. In 2003, the specimen was sent to Jørn Harald Hurum, palaeontologist at the University of Oslo for study. Martin Sander and Nicole Klein, palaeontologists of the University of Bonn, analyzed the bone microstructure and concluded that the rock preserved fibrous bone tissue from a fragment of a limb bone belonging to Plateosaurus, making it the first dinosaur found in Norway. Plateosaurus material has also been found in Greenland.

Taphonomy

The taphonomy (burial and fossilization process) of the three main Plateosaurus sites Trossingen, Halberstadt (both in Germany) and Frick (Switzerland) is unusual in several ways. All three sites are nearly monospecific assemblages, meaning that they contain practically only one species, which requires very special circumstances. However, shed teeth of theropods have been found at all three sites, as well as remains of Proganochelys, an early turtle. Additionally, a partial "prosauropod" skeleton was found in Halberstadt that does not belong to Plateosaurus, but is preserved in a similar position. All sites yielded almost complete and partial skeletons of Plateosaurus, as well as isolated bones. The partial skeletons tend to include the hind limbs and hips, while parts of the anterior body and neck are rarely found in isolation. The animals were all adults or sub-adults; no juveniles or hatchlings are known. Complete skeletons and large skeleton parts that include the hind limbs all rest right side up, as do the turtles. Also, they are mostly well articulated, and the hind limbs are three-dimensionally preserved in a zig-zag posture, with the feet often much deeper in the sediment than the hips.

Earlier interpretations

In the first published discussion of the Trossingen Plateosaurus finds, Fraas suggested that only miring in mud allowed the preservation of the single complete skeleton then known. Similarly, Jaekel interpreted the Halberstadt finds as animals that waded too deep into swamps, became mired and drowned. He interpreted partial remains as having been transported into the deposit by water, and strongly refuted a catastrophic accumulation. In contrast, von Huene interpreted the sediment as aeolian deposits, with the weakest animals, mostly subadults, succumbing to the harsh conditions in the desert and sinking into the mud of ephemeral water holes. He argued that the completeness of many finds indicated that transport did not happen, and saw partial individuals and isolated bones as results of weathering and trampling. Seemann developed a different scenario, in which Plateosaurus herds congregated on large water holes, and some herd members got pushed in. Light animals managed to get free, heavy ones got stuck and died.

A different school of thought developed almost half a century later, with palaeontologist David Weishampel suggesting that the skeletons from the lower layers stemmed from a herd that died catastrophically in a mudflow, while those in the upper layers accumulated over time. Weishampel explained the curious monospecific assemblage by theorizing that Plateosaurus were common during this period. This theory was erroneously attributed to Seemann in a popular account of the plateosaurs in the collection of the Institute and Museum for Geology and Paleontology, University of Tübingen, and has since become the standard explanation on most internet sites and in popular books on dinosaurs. Rieber proposed a more elaborate scenario, which included the animals dying of thirst or starvation, and being concentrated by mudflows.

Current interpretation

A detailed re-assessment of the taphonomy by palaeontologist Martin Sander of the University of Bonn, Germany, found that the mud-miring hypothesis first suggested by Fraas is true: animals above a certain body weight sank into the mud, which was further liquefied by their attempts to free themselves. Sander's scenario, similar to that proposed for the famous Rancho La Brea Tar Pits, is the only one explaining all taphonomic data. The degree of completeness of the carcasses was not influenced by transport, which is obvious from the lack of indications for transport before burial, but rather by how much the dead animals were scavenged. Juveniles of Plateosaurus and other taxa of herbivores were too light to sink into the mud, or managed to extract themselves, and were thus not preserved. Similarly, the scavenging theropods were not trapped due to their lower body weights, combined with a proportionally larger footprint. There is no indication of herding, nor of catastrophic burial of such a herd, or catastrophic accumulation of animals that previously died isolated elsewhere.

Palaeobiology

Posture and gait

This consensus was changed by a detailed study of the forelimbs of Plateosaurus by Bonnan and Senter (2007), which clearly showed that Plateosaurus was incapable of pronating its hands. The pronated position in some museum mounts had been achieved by exchanging the position of radius and ulna in the elbow. This meant that Plateosaurus was an obligate digitigrade biped. Further indicators for a purely bipedal mode of locomotion are the great difference in limb length (the hind limb is roughly twice as long as the forelimb), the very limited motion range of the forelimb, and the fact that the center of mass rests squarely over the hind limbs.

Feeding and diet

Important cranial characteristics (such as jaw articulation) of most "prosauropods" are closer to those of herbivorous reptiles than those of carnivorous ones, and the shape of the tooth crown is similar to those of modern herbivorous or omnivorous iguanas. The maximum width of the crown was greater than that of the root, resulting in a cutting edge similar to those of extant herbivorous or omnivorous reptiles. This is also true in Plateosaurus. Paul Barrett proposed that prosauropods supplemented their herbivorous diets with small prey or carrion.

So far, no fossil of Plateosaurus has been found with gastroliths (gizzard stones) in the stomach area. The old, widely cited idea that all large dinosaurs, implicitly also Plateosaurus, swallowed gastroliths to digest food because of their relatively limited ability to deal with food orally has been refuted by a study on gastrolith abundance, weight, and surface structure in fossils compared to alligators and ostriches by Oliver Wings. The use of gastroliths for digestion seems to have developed on the line from basal theropods to birds, with a parallel development in Psittacosaurus.

Growth, metabolism and life span

Similar to all non-avian dinosaurs studied to date, Plateosaurus grew in a pattern that is dissimilar to both extant mammals and avian dinosaurs. In the closely related sauropods with their typical dinosaurian physiology, growth was initially rapid, continued somewhat slower well beyond sexual maturity, but was determinate, i.e. the animals stopped growing at a maximum size. Mammals grow rapidly, but sexual maturity falls typically at the end of the rapid growth phase. In both groups, the final size is relatively constant, with humans atypically variable. Extant reptiles show a sauropod-like growth pattern, initially rapid, then slowing after sexual maturity, and almost, but not fully, stopping in old age. However, their initial growth rate is much lower than in mammals, birds and dinosaurs. The reptilian growth rate is also very variable, so that individuals of the same age may have very different sizes, and final size also varies significantly. In extant animals, this growth pattern is linked to behavioural thermoregulation and a low metabolic rate (i.e. ectothermy), and is called "developmental plasticity". (Note that is not the same as neural developmental plasticity.)

Plateosaurus followed a trajectory similar to sauropods, but with a varied growth rate and final size as seen in extant reptiles, probably in response to environmental factors such as food availability. Some individuals were fully grown at only 4.8 metres (16 ft) total length, while others reached 10 metres (33 ft). However, the bone microstructure indicates rapid growth, as in sauropods and extant mammals, which suggests endothermy. Plateosaurus apparently represents an early stage in the development of endothermy, in which endothermy was decoupled from developmental plasticity. This hypothesis is based on a detailed study of Plateosaurus long-bone histology conducted by Martin Sander and Nicole Klein of the University of Bonn, Germany. A further indication for endothermy is the avian-style lung of Plateosaurus.

Long-bone histology also allows estimating the age a specific individual reached. Sander and Klein found that some individuals were fully grown at 12 years of age, others were still slowly growing at 20 years, and one individual was still rapidly growing at 18 years. The oldest individual found was 27 years and still growing; most individuals were between 12 and 20 years old. However, some individually may well have lived much longer, because the fossils from Frick and Trossingen are all animals that died in accidents, and not from old age. Due to the absence of individuals smaller than 4.8 metres (16 ft) long, it is not possible to deduce a complete ontogenetic series for Plateosaurus or determine the growth rate of animals under 10 years of age.

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