Thecodontosaurus

*Thecodontosaurus antiquus*

***Thecodontosaurus*** is an extinct genus of herbivorous basal sauropodomorph dinosaur which lived during the late Triassic period (Norian and/or Rhaetian age). Its remains are known mostly from Triassic "fissure fillings" in South England. *Thecodontosaurus* was a small bipedal animal, about 2 meters (6.5 feet) long. It is one of the first dinosaurs that were discovered and one of the oldest that existed. Many species have been named in its genus but only the type species *Thecodontosaurus antiquus* is seen as valid today. It lived 203-201 MYA.

*Thecodontosaurus* had a rather short neck supporting a fairly large skull with quite big eyes. Its jaws contained many small- to medium-sized, serrated, leaf-shaped teeth. This dinosaur's hands and feet each had five digits, and the hands were long and rather narrow with an extended claw on each. This dinosaur's front limbs were much shorter than the legs, and its tail was much longer than the head, neck and body put together.

Although not actually the earliest member of the group, *Thecodontosaurus* is the most primitive well-known representative of the sauropodomorph dinosaurs. Originally it was included under the Prosauropoda (Upchurch 1998) but more recently it has been suggested that *Thecodontosaurus* and its relatives were prior to the prosauropod-sauropod split (Yates & Kitching 2003). New reconstructions show that its neck is proportionally shorter than in more advanced early sauropodomorphs.

The original type specimen of *Thecodontosaurus* was a victim of World War II bombings by the Germans. The remains of this dinosaur and other material related to it were destroyed in 1940. However, more remains have been found at a number of localities, including Bristol. Some of this new material pertains to a juvenile specimen that may belong to a distinct species, *Thecodontosaurus caducus* Yates, 2003. The dinosaur *Agrosaurus macgillivrayi* (Seeley, 1891), once thought to be from Australia, but more likely from England and misidentified, is probably synonymous with *Thecodontosaurus antiquus* (Vickers-Rich *et al.*, 1999).

In 2007, a paper by Yates, Galton, and Kermack put forth the claim that *Thecodontosaurus caducus* belongs to a different genus, which they have named *Pantydraco*.

In January 19, 2022, the fossil bones of *Thecodontosaurus antiquus* reveal that it walked on two legs making it quick and agile dinosaur to *Diplodocus*.

## Discovery and naming

### Thecodontosaurus antiquus

In the autumn of 1834 surgeon Henry Riley and curator of the Bristol Institution Samuel Stutchbury began to excavate "saurian remains" at the quarry of Durdham Down, at Clifton, presently a part of Bristol. They provided their initial description in 1836, naming a new genus: *Thecodontosaurus*. The name is derived from Greek *thekè*, "socket", and *odous*, "tooth", a reference to the fact that the roots of the teeth were not fused with the jaw bone, as in present lizards, but positioned in separate tooth sockets. *Thecodontosaurus* was the fifth dinosaur named, after *Megalosaurus*, *Iguanodon*, *Streptospondylus*, and *Hylaeosaurus*, though Riley and Stutchbury were not aware of this, the very concept Dinosauria only being created in 1842. In 1843 John Morris in his catalogue of British fossils provides a complete species name: *Thecodontosaurus antiquus*. The specific epithet, "*antiquus*", means "ancient" in Latin.

The original type specimen or holotype of *Thecodontosaurus*, BCM 1, a lower jaw, fell victim to heavy World War II bombings by the Germans. Many remains of this dinosaur and other material related to it were destroyed in November 1940 during the Bristol Blitz. However, most bones were salvaged: 184 are today part of the collection of the Bristol City Museum and Art Gallery. Also, later some more remains were found near Bristol at Tytherington. At present in total about 245 fragmentary specimens are known, representing numerous individuals. In 1985 Peter Galton designated another lower jaw, a right dentary, as the neotype, BCM 2. The remains have been found in chalkstone infillings, breccia deposited in fissures in older rocks. The age of these deposits was once estimated as old as the late Carnian but recent studies indicate that they date from the Rhaetian.

### Other Species

Apart from the original type species, *Thecodontosaurus antiquus*, seventeen other species would be named in the genus.

Riley and Stutchbury also found some teeth of carnivorous phytosaurians that they named *Palaeosaurus cylindrodon* and P. platyodon. In the late nineteenth century the theory became popular that such remains would have belonged to carnivorous prosauropods: animals with the body of *Thecodontosaurus* but with heads armed with slicing teeth. In 1890 Arthur Smith Woodward accordingly named a *Thecodontosaurus* platyodon, in 1908 Friedrich von Huene a *Thecodontosaurus cylindrodon*. Though still defended by Michael Cooper in 1981, the hypothesis that such creatures existed has now been totally discredited. On one occasion material of *Thecodontosaurus* was by mistake described as a separate genus. In 1891 Harry Govier Seeley named *Agrosaurus macgillivrayi*, assuming the remains had been collected in 1844 by the crew of HMS Fly on the northeast coast of Australia. It was long considered the first dinosaur found in Australia, but in 1999 it transpired that the bones probably belonged to a lot sent by Riley and Stutchbury to the British Museum of Natural History and then mislabelled. Already in 1906 von Huene had noted the close resemblance and renamed the species *Thecodontosaurus macgillivrayi*. It is thus a junior synonym of Thecodontosaurus antiquus. It also happened that remains of completely different animal groups were referred to Thecodontosaurus. *Thecodontosaurus latespinatus* and *Thecodontosaurus primus*, both named by von Huene in 1905, were based on material from *Tanystropheus*. In 1908 von Huene tentatively renamed *Plateosaurus elizae* Sauvage 1907 into *Thecodontosaurus*? elizae; it is a nomen dubium based on theropod remains. The proterosuchid Ankistrodon Huxley 1865 was renamed Thecodontosaurus indicus. A larger category consists of related basal sauropodomorphs assigned to *Thecodontosaurus*. These include in chronological order: *Thecodontosaurus gibbidens* Cope 1878; *Thecodontosaurus skirtopodus* Seeley 1894; *Thecodontosaurus polyzelus* (Hitchcock 1865) von Huene 1906; *Thecodontosaurus hermannianus* von Huene 1908; *Thecodontosaurus diagnosticus* Fraas 1912; *Thecodontosaurus minor* Haughton 1918 (= "*Thecodontosaurus minimus*"); *Thecodontosaurus dubius* Haughton 1924;[8] *Thecodontosaurus browni* (Seeley 1895) von Huene 1932 and *Thecodontosaurus alophos* Haughton 1932. All these are today referred to other genera or seen as nomina dubia. The last of these is *Thecodontosaurus caducus* named by Adam Yates in 2003 for a juvenile specimen found in Wales; in 2007 this was made the separate genus *Pantydraco*.

Presently, the only valid species is thus *T. antiquus*.

## Description

From the fragmentary remains of *Thecodontosaurus* most of the skeleton can be reconstructed, the main exception being the front of the skull. *Thecodontosaurus* had a rather short neck supporting a fairly large skull with large eyes. Its jaws contained many small- to medium-sized, serrated, leaf-shaped teeth. This dinosaur's hands and feet each had five digits, and the hands were long and rather narrow with an extended claw on each. This dinosaur's front limbs were much shorter than the legs, and its tail was much longer than the head, neck and body put together. On average, it was 1.2 metres (3.9 feet) long, 30 centimetres (12 inches) tall, and weighed 11 kilograms (24 lbs). The largest individuals had an estimated length of 2.5 metres (8.2 feet).

Michael Benton in 2000 noted the existence of a robust morph in the population, seen by him as a possible second species or, more likely, an instance of sexual dimorphism. Benton also indicated some unique derived traits, or autapomorphies for the species: a long basipterygoid process on the braincase; a dentary that is short in relation tot the total length of the lower jaw; an ilium that has a back end that is subquadrate instead of rounded.

The small size has been explained as an instance of insular dwarfism.

## Phylogeny

Riley and Stutchbury originally saw Thecondontosaurus as a member of the Squamata, the group containing lizards and snakes. This did not change when Richard Owen in 1842 coined the Dinosauria, because Owen did not recognise Thecodontosaurus as a dinosaur; in 1865 he assigned it to the Thecodontia. Only in 1870 was Thomas Huxley the first to understand it was a dinosaur, though referring it incorrectly to the Scelidosauridae. Later it would normally be placed in either the Anchisauridae or its own Thecodontosauridae. Modern exact cladistic analyses have not been conclusive. Although not actually the earliest member of the group, Thecodontosaurus is sometimes placed in a very basal position among the sauropodomorph dinosaurs. It was earlier included under the Prosauropoda but more recently it has been suggested that Thecodontosaurus and its relatives were prior to the prosauropod-sauropod split. New reconstructions show that its neck is proportionally shorter than in more advanced early sauropodomorphs.

## Paleobiology

Biomechanics suggests that it was a fast bipedal runner that relied on its weaker front limbs to grasp vegetation before processing it in its mouth. It was likely well-adapted for advanced running and high-speed running to escape predators.

## The Bristol Dinosaur Project

In 2000, the Bristol Dinosaur Project has formed the Thecodontosaurus basis for massive public engagement exercise.

## JPInstitute.com Description

Thecodontosaurus is the earliest known member of the dinosaur family called the prosauropods that many scientists believe became the huge long-necked herbivorous (plant-eating) dinosaurs called sauropods. It was very small compared to the later giant plant-eaters. Not a lot is known about this small dinosaur as very little of it was found. Thecodontosaurus, being such an early dinosaur, did not look very different from early meat-eaters - in fact, they may have all been related! It wasn't until later in the Triassic period that plant-eaters got bigger and began to walk on four legs instead of two.

This small dinosaur is another victim of WWII. The type specimen and other material attributed to this dinosaur were destroyed by Nazi bombs in 1940. Fossils found in South Africa, North America and Australia are believed to potentially be the same genus. Again, as this was such an early dinosaur, it provides evidence of early dinosaur migration.

## Links

https://web.archive.org/web/20030813040317/http://www.jpinstitute.com/dinopedia/dinocards/dc\_theco.html
https://web.archive.org/web/20080706164227/http://kids.yahoo.com/dinosaurs/47--Thecodontosaurus

## References

1. ↑ https://www.newscientist.com/article/2305123-dinosaur-ancestor-of-long-necked-diplodocus-ran-swiftly-on-two-legs/
2. ↑ https://phys.org/news/2022-01-fossils-sauropodomorph-ancestor-upright-quick.html