Plateosaurus (meaning 'flat lizard') is a genus of plateosaurid prosauropod dinosaur that lived during the Late Triassic period, around 216 to 199 million years ago in what is now Central and Northern Europe. The latest research recognizes two species: the type species P. engelhardti from the late Norian and Rhaetian, and the slightly earlier P. gracilis from the lower Norian, although 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 at the genus level.
Discovered in 1834 by Johann Friedrich Engelhardt and described three years later by Hermann von Meyer, Plateosaurus was the fifth dinosaur genus to be named that today is still considered valid. However, it was not one of the three genera originally used by Richard Owen in 1842 to define Dinosauria, because at the time it was poorly known and difficult to identify as a dinosaur. Plateosaurus is now among the best known dinosaurs, with over 100 skeletons found, some of them nearly complete. Plateosaurus was a bipedal herbivore with a small skull on a long, mobile neck, sharp but plump plant-crushing teeth, powerful hind limbs, and a grasping hand with a large thumb claw possibly used for defence and feeding. Unusually for a dinosaurs, Plateosaurus showed strong developmental plasticity: fully grown individuals were between 4.8 and 10 metres (16 and 33 ft) long and weighed between 600 to 4,000 kilograms (1,300 to 8,800 lb). Commonly, the animals lived for 10 to 25 years.
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 conflicted with geological and palaeontological evidence, but have become the paradigm of public opinion. Since 1980 the taxonomy and taphonomy, and since 2000 the biomechanics and palaeobiology of Plateosaurus have been re-studied in detail, totally altering the interpretation of the animal's posture and behaviour.
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[edit] Description[]
[1][2]Size comparison between P. engelhardti and a humanPlateosaurus is a member of a group of early herbivores known as prosauropods.[1] Plateosaurus had the typical body shape of a herbivorous bipedal dinosaur: a small skull, a long and flexible neck composed of ten cervical vertebrae, a stocky body and a long, mobile tail composed of at least 40 caudal vertebrae.[2][3][4] The type species of Plateosaurus is P. engelhardti.[5] Adult individuals of this species reached 4.8 to 10 metres (16 to 33 ft) in length.[6] Average individuals had a mass of around 600 to 4,000 kilograms (1,300 to 8,800 lb).[7] The second, older species, P. gracilis (formerly named Sellosaurus gracilis), was somewhat smaller, with a total length of 4 to 5 metres (13 to 16 ft).[8]
[3][4]P. engelhardti skull cast, Royal Ontario MuseumThe skull of Plateosaurus was small and narrow.[2][3][9] The snout carried many small, leaf-shaped, socketed teeth in both the upper and lower jaw, five to six on the premaxilla, 24 to 30 on the maxilla, and 21 to 28 on the dentary.[2][3][9] The teeth had serrated, leaf-shaped crowns suitable for crushing plant material.[2][3][9] The low position of the jaw joint gave the chewing muscles great leverage, so that Plateosaurus could deliver a powerful bite.[9] These features suggest that it fed exclusively on plants.[9] Its eyes were directed to the sides, rather than the front, providing all-round vision to watch for predators.[2][3][9] Some fossil skeletons have preserved sclerotic rings.[2][3][9]
The ribs of Plateosaurus were connected to the dorsal vertebrae with a hinge joint, and its lung volume has been estimated at ~20 l, or 29 ml/kg bodyweight,[10] which is a typical value for birds, but not for mammals.[11] This indicates that Plateosaurus probably had an avian-style flow-through lung,[10] although no indicators for postcranial pneumaticity (air sacs of the lung invading the bones to reduce bone weight) penetrating the bones can be found on the bones.[12] The tail of Plateosaurus was typically dinosaurian, with high mobility.[10]
[edit] Discovery and history[]
[5][6]P. engelhardti, collection number F 33 of the Staatliches Museum für Naturkunde Stuttgart, Germany, in dorsal view. The skeleton was kept in articulation as found at Trossingen (Germany) by Seemann in 1933. It has the typical folded hindlimbs of most Plateosaurus finds. Unusually, the anterior body is not twisted to one side.In 1834, physician Johann Friedrich Engelhardt discovered some vertebrae and leg bones at Heroldsberg near Nuremberg, Germany.[5] Three years later German palaeontologist Hermann von Meyer designated them as the type specimen of a new genus, Plateosaurus.[5] Since then, remains of well over 100 individuals of Plateosaurus have been discovered.[13]
Between the 1910s and 1930s, excavations in a clay pit in Saxony-Anhalt dug up between 39 and 50 skeletons that belonged to Plateosaurus, Liliensternus and Halticosaurus.[14] Some of this material was assigned to P. longiceps, a species today considered a junior synonym of P. engelhardti and described by palaeontologist Otto Jaekel in 1914.[15] Most of the material found its way to the Museum für Naturkunde in Berlin, and part of it was destroyed during World War II.[14] The Halberstadt quarry today is covered by a housing development.[14]
The second major German locality with Plateosaurus finds, a quarry in Trossingen in the Black Forest, was worked repeatedly in the 20th century.[14] Between 1911 and 1932, excavations during six field seasons led by German palaeontologists Eberhard Fraas (1911–1912), Friedrich von Huene (1921–23),[3][16] and finally Reinhold Seemann (1932) revealed a total of 35 complete or partially complete skeletons of Plateosaurus, as well as fragmentary remains of approximately 70 more individuals.[14]
[7][8]P. engelhardti, collection number MSF 23 of the Sauriermuseum Frick, Switzerland, in dorsal view. This is the most complete Plateosaurus skeleton from Frick.The Plateosaurus skeletons in a clay pit of the Tonwerke Keller AG in Frick, Switzerland, were first noticed in 1976.[13] While the bones are often significantly deformed by taphonomic processes, Frick yields skeletons comparable in completeness and position to those of Trossingen and Halberstadt.[13]
Overall, 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.[14] 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.[17] 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,[17] making it the first dinosaur found in Norway. Plateosaurus material has also been found in Greenland.[18]
[edit] Classification and type material[]
Basal sauropodomorph phylogeny simplified after Yates, 2007.[19] This is only one of many proposed cladograms for basal sauropodomorphs. Some researchers do not agree that plateosaurs were the direct ancestors of sauropods |
Plateosaurus was the first prosauropod to be described,[1] and is the type genus of the family Plateosauridae, to which it gives its name. Initially, when the genus was poorly known, it was only included in Sauria, being some kind of reptile, but not in any more narrowly defined taxon.[5] In 1845, von Meyer created the group Pachypodes (now unused) to include Plateosaurus, Iguanodon, Megalosaurus and Hylaeosaurus.[20] Plateosauridae was proposed by Othniel Charles Marsh in 1895 within Theropoda.[21] Years later, it was moved to Prosauropoda by von Huene,[3] a placement that was accepted by most authors.[22][23] For many years the clade only included Plateosaurus and various junior synonyms, but later two more genera were considered to belong to it: Sellosaurus[24] and possibly Unaysaurus.[25] Of these, Sellosaurus is probably a junior synonym of Plateosaurus.[8]
The type series of Plateosaurus engelhardti included "roughly 45 bones".[26] However, of these nearly half are lost today.[4] The remaining material is kept in the Institute for Paleontology of the University of Erlangen-Nuremberg, Germany.[4] From it, Markus Moser in 2003 selected a partial sacrum (series of fused hip vertebrae) as a lectotype.[4] The type locality is not known for certain, but Moser attempted to infer it from previous publications and the colour and preservation of the bones, concluding that the material probably stems from the "Buchenbühl", roughly 2 kilometres (1.2 mi) south of Heroldsberg near Nuremberg, Bavaria, Germany.[4]
The type specimen of Plateosaurus gracilis, an incomplete postcranium, is kept at the Staatliches Museum für Naturkunde Stuttgart, Germany.[8] The type locality is Heslach, a suburb of Stuttgart, Germany.[8]
[edit] Etymology[]
The etymology of the name Plateosaurus is not clear. As pointed out by Markus Moser[4] the original description contains no information, and various authors have offered differing interpretations. Moser details that the first explanation was offered by Agassiz, who listed Greek platy/πλατη (paddle, rudder; Agassiz translates this as Latin pala = spade) and sauros/σαυρος (lizard).[27] Agassiz consequently renamed the genus Platysaurus,[28] probably from Greek platys/πλατυς (broad, flat, broad-shouldered), creating an invalid junior synonym. This derivation was copied by later authors, so that Plateosaurus is often translated as "broad lizard" or "flat lizard". Often, claims were made that platys/πλατυς is supposed to have been intended as a reference to the laterally flattened teeth of Plateosaurus,[29] which is impossible because the teeth were unknown at the time of description. Moser suggests the Ancient Greek plateia/πλατεια (broad way) as an alternative, which corresponds better to the stem of Plateosaurus, which clearly is "plate-", and not "platy-". Thus, the name should be translated as "broadway lizard".[4]
[edit] Taxonomy[]
[edit] Valid species[]
[9][10]Restoration of P. gracilis, formerly known as SellosaurusThe taxonomic history of Plateosaurus is complex and confusing.[4] As of 2009, only two species are accepted as valid,[4][8] the type species P. engelhardti and the older P. gracilis, previously referred to as its own genus Sellosaurus. Galton showed clearly that all cranial material from Trossingen, Halberstadt and Frick pertains to one species.[9] Markus Moser performed the most extensive and detailed investigation of all plateosaurid material from Germany and Switzerland, concluding that all Plateosaurus and most other prosauropod material from the Keuper stems from the same species as the type material of Plateosaurus engelhardti.[4] Moser considers Sellosaurus to be the same genus as Plateosaurus, but does not discuss the species level, that is whether S. gracilis is identical to P. engelhardti.[4] Palaeontologist Adam Yates of the University of the Witwatersrand casted further doubt on the generic separation, and included the type material, but not all assigned finds of Sellosaurus gracilis, in Plateosaurus as P. gracilis.[8] Previously, von Huene had already concluded this in 1926.[3]
[edit] Invalid species[]
[11][12]Plateosaurus engelhardti (previously P. quenstedti) skull and neck at the Museum für Naturkunde, BerlinAll named species of Plateosaurus except P. gracilis have turned out to be junior synonyms of the type species or invalid names.[4][8] Huene[1] practically erected a new species and sometimes a new genus for each relatively complete find from Trossingen (three species of Pachysaurus and seven of Plateosaurus) and Halberstadt (one species of Gresslyosaurus and eight of Plateosaurus).[13] Later, he collapsed several of these species, but remained convinced that more than one genus and more than one species of Plateosaurus was present in both localities. Jaekel also believed that the Halberstadt material included several plateosaurid dinosaurs, as well as non-plateosaurid prosauropods.[15] Systematic research by Peter Galton drastically reduced the number of genera and species. Galton synonomized all cranial material,[9][30][31] and described differences between the syntypes of P. engelhardti and the Trossingen material, which he referred to P. longiceps.[32] Galton recognized P. trossingensis, P. fraasianus and P. integer to be identical to P. longiceps.[33] Markus Moser, however, showed clearly that P. longiceps is itself a junior synonym of P. engelhardti.[4] Furthermore, a variety of species in other genera were created for material belonging to P. engelhardti, including Dimodosaurus poligniensis, Gresslyosaurus robustus, Gresslyosaurus torgeri, Pachysaurus ajax, Pachysaurus giganteus, Pachysaurus magnus and Pachysaurus wetzelianus.[4] However, it must be noted that there is much prosauropod material from the Germany Knollenmergel in museum collections, most of it labeled as Plateosaurus, that does not belong to the type species, and possibly not to Plateosaurus at all.[34] Some of this material is not diagnostic; other material has been recognized to be different, but was never sufficiently described.[15]
[edit] Dubious material[]
Several prosauropod species are considered to be nomina nuda ('naked names') or nomina dubia ('doubtful names') due to their fragmentary nature and poor preservation.[35] For example, Teratosaurus trossingensis von Huene 1908 is known from only a partial right hind limb, and may belong to practically any Triassic prosauropod.[8]
[edit] Taphonomy[]
The taphonomy of the three main Plateosaurus sites Trossingen, Halberstadt (both in Germany) and Frick (Switzerland) is unusual in several ways.[13] All three sites are nearly monospecific assemblages, meaning that they contain only one species.[13] This is highly unusual, and requires very special circumstances to explain.[13] However, shed teeth of theropods have been found at all three sites, as well as remains of Proganochelys, an early turtle.[13] All sites yielded almost complete and partial skeletons of Plateosaurus, as well as isolated bones.[13] The partial skeletons tend to include the hind limbs and hips, while parts of the anterior body and neck are rarely found in isolation.[13] The animals were all adults or sub-adults; no juveniles or hatchlings are known.[13] Complete skeletons and large skeleton parts that include the hind limbs all rest right side up, as do the turtles.[13] 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.[13]
[edit] 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.[36] Similarly, Jaekel interpreted the Halberstadt finds as animals that waded too deep into swamps, became mired and drowned.[15] He interpreted partial remains as having been transported into the deposit by water, and strongly refuted a catastrophic accumulation.[15] 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.[16] 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.[16] Seemann developed a different scenario, in which Plateosaurus herds congregated on large water holes, and some herd members got pushed in.[37] Light animals managed to get free, heavy ones got stuck and died.[37]
A different school of thought developed almost half a century later, with 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.[14] Weishampel explained the curious monospecific assemblage by theorizing that Plateosaurus were common during this period.[14] 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,[38] and has since become the standard explanation on most internet sites and in popular books on dinosaurs.[4] Rieber proposed a more elaborate scenario, which included the animals dying of thirst or starvation, and being concentrated by mudflows.[39]
[edit] 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[36] is true:[13] animals above a certain body weight sank into the mud, which was further liquidified 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.[13]
[edit] Palaeobiology[]
[edit] Posture and gait[]
[13][14]Example of an outdated skeleton mount in quadrupedal posturePractically any imaginable posture has been suggested for Plateosaurus in the scientific literature at some point in time. Von Huene assumed digitigrade bipedality with erect hind limbs for the animals he excavated at Trossingen, with the backbone held at a steep angle at least during rapid locomotion.[3][40] In contrast, the main investigator of the Halberstadt material, Jaekel, initially concluded that the animals walked like lizards, i.e. quadrupedally with a sprawling limb position, plantigrade, and laterally undulating the body.[41] Only a year later, Jaekel instead favoured a clumsy, kangaroo-like hopping,[2] a change of heart for which he was mocked by German zoologist Gustav Tornier,[42] who interpreted the shape of the articulation surfaces in the hip and shoulder as typically reptilian. Fraas, the first excavator of the Trossingen lagerstätte, also favoured a reptilian posture.[36][43] One publication on this topic was written by Müller-Stoll, who listed a number of characters required for an erect limb posture that Plateosaurus supposedly lacked, concluding that the lizard-like reconstructions were correct.[44] However, most of these adaptations are actually present in Plateosaurus.[7][10]
[15][16]Cast of a P. engelhardti skeleton mounted in the correct bipedal fashion, Natural History Museum of MilanFrom 1980 on, an increased understanding of dinosaur biomechanics and studies by palaeontologists Christian and Preuschoft on the resistance to bending of the back of Plateosaurus[45][46] lead to widespread acceptance of an erect, digitigrade limb posture and a roughly horizontal position of the back.[4][35][38][47][48][49] Many researchers were of the opinion that Plateosaurus could use both quadrupedal gaits (for slow speeds) and bipedal gaits (for rapid locomotion),[45][46][47][48] while Wellnhofer insisted that the tail curved strongly downward, making a bipedal posture impossible.[49] However, Moser showed that the tail was in fact straight.[4]
This consensus was changed by a detailed study of the fore limbs of Plateosaurus by Bonnan and Senter (2007), which clearly showed that Plateosaurus was incapable of pronating its hands.[50] 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.[7][10][51]
Plateosaurus shows a number of cursorial adaptations, including an erect hind limb posture, a relatively long lower leg, an elongated metatarsus and a digitigrade foot posture.[7] However, in contrast to mammalian cursors, the moment arms of the limb extending muscles are short, especially in the ankle, where a distinct, moment arm-increasing tuber on the calcaneum is missing.[3] This means that in contrast to running mammals, the Plateosaurus probably did not use gaits with aerial, unsupported phases. Instead, Plateosaurus must have increased speed by using higher stride frequencies, created by rapid and powerful limb retraction. Reliance on limb retraction instead of extension is typical for non-avian dinosaurs.[52]
[edit] Feeding and diet[]
[17][18]P. engelhardti skull in semi profileImportant 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.[35] Paul Barrett proposed that prosauropods supplemented their herbivorous diets with small prey or carrion.[53] The old, widely cited idea that all large dinosaurs, such as Plateosaurus, swallowed gastroliths (gizzard stones) 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.[54][55]
[edit] Growth and metabolism[]
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.[56] 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, with initially faster growth, then a reduction in growth rate after sexual maturity, and almost, but not fully, stop growing at 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.[6]
[19][20]Two P. engelhardti facing offPlateosaurus 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 indicates 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.[6]
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.[6] 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.[6]
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