# Ornithodesmus

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, making it one of the first dromaeosaurs known to science. 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 1.8 meters (5.9 feet) 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 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", 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. 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. 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.
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. 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. Darren Naish and colleagues in 2001 argued against a dromaeosaurid identity for Ornithodesmus, suggesting instead it was related to the ceratosaurs or coelophysids. However, Naish and Martill later changed their opinions, publishing a paper in 2007 that agreed with previous studies and classifying Ornithodesmus as a dromaeosaurid. 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.