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HYPSILOPHODONTID CHARACTERS



Tom wrote...

> Take care, and can we move onto a new topic for the day? Like (and
> maybe I should rephrase this question this time): What anatomical
> evidence supports a monophyletic Hypsilophodontia, versus the > evidence suggesting that traditional hypsilophodonts are a > paraphyletic series relative to iguanodonts? Inquiring minds want to > know!!

Two characters support monophyly of Hypsilophodontidae:-

1) Small size
2) Obligate bipedality

Just kidding. Pasted in below find the section on hypsilophodontid monophyly from the Isle of Wight dinosaurs book Dave Martill and I are finishing. The references are not included I'm afraid. The section below is followed by a substantial body of text which argues that Hypsilophodontidae is a grade - the evidence for this is now pretty good (like, some 'hypsilophodontids' have hypsilophodontid characters when juvenile but then become iguanodontians as they mature).

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What, if anything, is a hypsilophodontid?

To Sternberg and the researchers that followed him, hypsilophodontids were small, bipedal cursorial ornithischians that lacked the specialisations of iguanodontians (such as hoofed fingers, toothless premaxillae, robust hindlimb bones and large body size) (Galton 1972, Sternberg 1940, Thulborn 1971, 1977). Relying on the absence, rather than presence, of derived characters, this loose definition meant that Hypsilophodontidae soon became a dumping group for all small ornithopods or ornithopod-like ornithischians. As a consequence, a number of primitive Asian forms, such as _Agilisaurus_ and _Yandusaurus_ from the Lower Jurassic of China (He 1979, He and Cai 1983), as well as the dryosaurids of the Upper Jurassic and Cretaceous (see pp. xx-xx) came to be regarded as hypsilophodontids.

Later, the cladistic studies of Milner and Norman (1984), Norman (1984, 1990) and Sereno (1986) showed that dryosaurids were members of the Iguanodontia. These studies and others (Norman 1998, Sereno 1997, 1999, Sues and Norman 1990, Weishampel and Heinrich 1992), however, still supported the view that Hypsilophodontidae could remain as a monophyletic assemblage containing _Hypsilophodon_, _Parksosaurus_ from Upper Cretaceous Canada and _Zephyrosaurus_ from Lower Cretaceous Montana as well as the othnielians of Upper Jurassic western USA, the Upper Cretaceous North American thescelosaurids and the primitive Asian yandusaurs and agilisaurs. Hypsilophodontidae is this sense was supposedly united by the presence of a proportionally elongate scapula, a rod-shaped prepubic process of the pubis and ossified tendons in the distal tail (Sereno 1986, 1997, 1999, Sues and Norman 1990, Weishampel and Heinrich 1992).

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"Well, this isn't 'Mission: Difficult' Mr. Hunt, it's 'Mission: Impossible'"

DARREN NAISH 
PALAEOBIOLOGY RESEARCH GROUP
School of Earth, Environmental & Physical Sciences
UNIVERSITY OF PORTSMOUTH
Burnaby Building
Burnaby Road                           email: darren.naish@port.ac.uk
Portsmouth UK                          tel: 01703 446718
P01 3QL