Ornithodesmus Sacrum

''Ornithodesmus'' (meaning "bird link") is a genus of small, dromaeosaurid dinosaur from the Isle of Wight in England, dating to about 125 million years ago. The name was originally assigned to a bird-like sacrum (a series of vertebrae fused to the hip bones), initially believed to come from a bird and subsequently identified as a pterosaur. More complete pterosaur remains were later assigned to ''Ornithodesmus'', until recently a detailed analysis determined that the original specimen in fact came from a small theropod, specifically a dromaeosaur. All pterosaurian material previously assigned to this genus has been renamed ''Istiodactylus''.

Description

As it is only known from isolated vertebrae, little is known about the appearance of ''Ornithodesmus''. The neural spines of the vertebrae are fused and form a blade over the 9.6 centimetres long sacrum, which is slightly arched. The bases of the neural spines form a lateral platform, and the first two vertebrae of the sequence have deep hollow cavities, which formed space for air sacs.

Based on its apparent identity as a dromaeosaur, it was probably carnivorous, and likely measured about long in life. Dromaeosaur teeth probably belonging to a velociraptorine are known from the same formation, but are too large to have belonged to ''Ornithodesmus''; rather, these must have come from a theropod closer in size to the giant '' Utahraptor''.

History and classification

''Ornithodesmus cluniculus'' was first described by Harry Govier Seeley in 1887, based on a set of six fused vertebrae from the hip (''sacrum''), specimen number BMNH R187, found by William D. Fox in the Wessex Formation of Brook Bay. Seeley thought the bones came from a primitive bird, and gave it a name meaning "bird link",Seeley, H. (1887). "On a sacrum, apparently indicating a new type of Bird, ''Ornithodesmus cluniculus'', Seeley, from the Wealden of Brook." ''Quarterly Journal of the Geological Society of London'', 42: 206-211. from Greek ὄρνις (''ornis''), "bird", en δεσμός (''desmos''), "link". The specific name ''cluniculus'' means "little buttock" in Latin, a reference to the small thighs indicated by the size of the specimen.

Later that year, John Hulke (in an anonymous paper) suggested the remains actually belonged to a pterosaur.Anonymous (1887). "Discussion (on ''Ornithodesmus'' and ''Patricosaurus'')." ''Quarterly Journal of the Geological Society of London'', 43: 219-220. Seeley himself later changed his opinion when he described the complete skeleton (specimen number BMNH R176) of a new pterosaur species he believed was closely related to ''O. cluniculus''. He named this new species ''Ornithodesmus latidens'' in 1901. Although he now considered it a pterosaur, Seeley at the time still considered ''Ornithodesmus'' close to the origin of birds, and suggested the (now defunct) theory that birds and pterosaurs shared a close common ancestry.Seeley, H. (1901). ''Dragons of the Air''. London: Methuen & Co. 239 pp. For over a century following this, the pterosaur ''O. latidens'' was used as the standard example of ''Ornithodesmus'', and the fragmentary type specimen was largely ignored. In 1913, Reginald Walter Hooley named a new family to distinguish ''Ornithodesmus'' from other large pterosaurs known at the time, Ornithodesmidae.Hooley, R.W. (1913). "The skeleton of ''Ornithodesmus latidens''; an Ornithosaur from the Wealden Shales of Atherfield (Isle of Wight)." ''Quarterly Journal of the Geological Society'', 69(1-4): 372-422.

In 1993, Stafford C. Howse and Andrew Milner re-examined the type specimen of ''O. cluniculus'' and determined that Seeley had incorrectly referred the pterosaur species to this genus. They identified ''O. cluniculus'' as a theropod dinosaur. Specifically, they suggested it was a troodontid, based on its similarity to the supposed troodontid specimen BMNH R4463.Howse, S.C.B. and Milner, A.R. (1993). "''Ornithodesmus''—a maniraptoran theropod dinosaur from the Lower Cretaceous of the Isle of Wight, England." ''Palaeontology'', 36: 425–437. However, later study by Peter Makovicky and Mark Norell showed this specimen to be a dromaeosaurid; because of this mis-identification, they suggested ''Ornithodesmus'' was likely a dromaeosaurid as well.Norell, M.A. and Makovicky, P. (1997). "Important features of the dromaeosaur skeleton: Information from a new specimen." ''American Museum Novitates'', 3215: 1-28. Darren Naish and colleagues in 2001 argued against a dromaeosaurid identity for ''Ornithodesmus'', suggesting instead it was related to the ceratosaurs or coelophysids.Naish, D. Hutt, and Martill, D.M. (2001). "Saurischian dinosaurs: theropods." in Martill, D.M. and Naish, D. (eds). ''Dinosaurs of the Isle of Wight''. The Palaeontological Association, Field Guides to Fossils. 10, 242-309. However, those scientists later changed their opinions, publishing a paper in 2007 that agreed with previous studies and classifying ''Ornithodesmus'' as a dromaeosaurid.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(3): 493-510 A 2019 analysis placed ''Ornithodesmus'' in family Unenlagiidae, otherwise considered a subgroup of Dromaeosauridae.

The more complete pterosaur specimens that had long been associated with the name ''Ornithodesmus'' were given a new name in 2001, ''Istiodactylus''.Howse, Milner and Martill (2001). "Pterosaurs." in Martill, D.M. and Naish, D. (eds.). ''Dinosaurs of the Isle of Wight''. The Palaeontological Association, London. pp. 324-335.

See also

* Timeline of dromaeosaurid research

References

External links

*

{{Taxonbar, from=Q1578419

Dromaeosaurs
Barremian life
Early Cretaceous dinosaurs of Europe
Cretaceous England
Fossils of England
Fossil taxa described in 1887
Taxa named by Harry Seeley