Sunday, January 8, 2012

Late Triassic Dinosaurs of the British Isles



Fig 1.- Thecodontosaurus antiquus.

The British Late Triassic land vertebrate fauna is essentially known from disarticulated bones found in various Mesozoic fissure fills of underlying Lower Carboniferous limestone in England and Wales. The fauna include the very first British dinosaurs of Rhaetian age (~ 200 MYA), living alongside a variety of reptiles including sphenodontians, running crocs of the sphenosuchian group (such as Terrestrisuchus gracilis), trilophosaurs (such as Variodens inopinatus), and gliding lizards such as Kuhneosaurus. Dinosaurs were not the dominant group that will rule the land during the Jurassic and Cretaceous periods, constituting only about 10% of the vertebrate fauna found in those fissure fills. One archosaurian form, named Agnostiphys cromhallensis from the Cromhall Quarry in Avon, has clearly some dinosaurian characteristics. It was described in 2002 by N.C. Fraser and co-workers based on a left ilium, left maxilla, a right humerus, a pair of astragalus and an isolated tooth. The validity of the taxon is disputed as the different parts may come from different animals (thus making the original description a chimera) and the remains are too scant to decide if it was an archosaur closely related to dinosaurs or a true primitive dinosaur, possibly a herrerasaur.


Sauropodomorphs from Rhaetian fissure fills: Thecodontosaurus, Asylosaurus, Pantydraco

The other dinosaurian remains of the Rhaetian fissure fillings all belong to basal sauropodomorphs, the group of saurischian dinosaurs which will eventually lead to the Jurassic and Cretaceous long-necked herbivorous giants called Sauropods to which the North American Brachiosaurus, Diplodocus and Apatosaurus are the most popular members. From a locality known as Durdham Down, Clifton, near Bristol in England, hundreds of bones have been excavated in the 1830s and originally described by Riley and Stutchbury as three separate taxa: Thecodontosaurus antiquus based on a right dentary with 21 teeth, Paleosaurus cylindricon (Now Paleosauriscus cylindicon) and P. platyodon (now Rileyasuchus platyodon), the latter two, each based on a single tooth. The other bones were subsequently distributed by Owen (1842), Huxley (1870) and Marsh (1892) among the three taxa. Huxley (1870) was the first to recognize the dinosaurian nature of the remains. The different disarticulated bits had subsequently quite diverging and complicated systematic histories, some being referred to a phytosaur (i.e. Rileya), others to theropods and ornithosuchians. Sadly, a portion of the materials, including the holotype dentary of Thecodontosaurus, was destroyed during the WWII bombing of Bristol. Recent efforts try to put order in the systematic mess accumulated over more than a century, but opinions went from attributing all the Durdham Down dinosaurian bones to the single species of basal sauropodomorph, Thecodontosaurus antiquus (Benton et al., 2000) with different morphotypes representing sexual dimorphism, to reserving the generic name to the original jaw and its neotype only (Galton, 2005). In his last magisterial review, Pete Galton (2007) examined all the surviving materials of Durdham Down (including the mislabeled bones which were for a while thought to come from Australia, leading Seeley to described them in 1861 as a new species, Agrosaurus macgillivray, which would have been the earliest known dino from down under) and illustrations made of materials lost during the war. He concluded that the Durdham Down collection is a mixture originating from a handful of species: Thecodontosaurus antiquus, to which the jaw and gracile bones were attributed, a new erected taxon, Asylosaurus yalensis for the only significant articulated skeletal part (an almost complete forearm and associated dorsal vertebrae and ribs), and a few number of unnamed basal sauropodomorphs, including a possible anchisaur. New materials of Thecodontosaurus have been discovered in fissure fill at Tytherington Quarry, Avon, in 1975 are are being prepared and awaiting formal description. Compared to the later huge sauropods, Thecodontosaurus was a rather small dinosaur measuring no longer than 2.5 meters in length that was probably bipedal. His leaf shaped teeth indicated that it was a herbivore.

Fig 2.- Pantydraco caducus.

From the more or less contemporary fissures fill of Panty-ffynnon Quarry in South-Western Wales, came several partial skeletons including an almost complete skull and associated partial skeleton of the unfortunately named Pantydraco caducus. The remains are from a juvenile basal sauropodomorph. Like Thecodontosaurus, Pantydraco was a bipedal herbivore, measuring probably no more than 2 meters in length.

Fig 3.- Camelotia borealis.

The large Late Triassic sauropodomorph, Camelotia

All the sauropodomorphs from the rhaetian fissure fills were rather small in size and gracile in appearance, but a larger animal announcing the heavy sauropods from the Jurassic, existed at that time, as proven by a partial disarticulated skeleton found in sedimentary beds at the base of the Westbury formation at Wedmore Hill, Somerset. The animal was named Camelotia borealis, after the legendary castle of King Arthur.  Remains of Camelotia are so fragmentary that it is hard to derive its affinities but it might have been related to the quadrupedal Melanosaurus, unless it was a very primitive sauropod. Camelotia is estimated to have measured about 10 meters in length.


Original artworks on Paleoexhibit are copyrighted to Nobu Tamura. Do not use without permission (Email: nobu dot tamura at yahoo dot com)

References:

Benton M.J. , Juul, L., Storrs, G.W. & Galton, P. M. 2000. Anatomy and systematics of the prosauropod dinosaur Thecodontosaurus antiquus from the Upper Triassic of Southwest England. Journal of Vertebrate Paleontology 20: 77–108.

Fraser, N. C., K. Padian, G. M. Walkden, & A.L.M. Davis , 2002. Basal dinosauriform remains from Britain and the diagnosis of the Dinosauria. Palaeontology , 45 (1): 79-95.

Galton. P. M. 2005. Basal sauropodomorph dinosaur taxa Thecodontosaurus Riley & Stutchbury, 1836, T. antiquus Morris, 1843 and T. caducus Yates, 2003: their status re: humeral morphs from the 1834 fissure fill (Upper Triassic) in Clifton, Bristol, UK. Journal of Vertebrate Paleontology 25(3, suppl.):61A

Galton, P. M. 2007. Notes on the remains of archosaurian reptiles, mostly basal sauropodomorph dinosaurs, from the 1834 fissure fill (Rhaetian, Upper Triassic) at Clifton in Bristol, southwest England. Revue de Paléobiologie 26(2):505-591.

Galton, P. M., Yates, A. M.  and Kermack, D. M. 2007. Pantydraco n. gen. for Thecodontosaurus caducus Yates, 2003, a basal sauropodomorph dinosaur from the Upper Triassic or Lower Jurassic of South Wales, UK. Neues Jahrbuch für Geologie und Paläontologie Abhandlungen 243(1):119-125.

Naish, D. and Martill., D. M. 2007. Dinosaurs of Great Britain and the role of the Geological Society of London in their discovery: basal Dinosauria and Saurischia. Journal of the Geological Society, London 164:493-510

Yates. A.M. 2003. A new species of the primitive dinosaur Thecodontosaurus (Saurischia: Sauropodomorpha) and its implications for the systematics of early dinosaurs. Journal of Systematic Palaeontology 1(1):1-42.

2 comments:

  1. Lovely summary - the graphics are fantastic too! Will you be writing similar articles for theropods and other ornithischian groups too?

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  2. @ protohedgehog: Thank you! Articles are already done and posted on my blog for the ornithischians (with the exception of Echinodon). Eusauropods and Theropods are in production. :)

    ReplyDelete