Therizinosauria

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Therizinosauria Russell, 1997 (=Segnosauria Barsbold & Maryanska, 1990)

Quite simply, therizinosaurs are the strangest dinosaurs ever discovered, if not the most peculiar tetrapods as a whole. They still leave dinosaur paleontologists baffled and at a loss to explain their phylogeny. The initial material was mistaken for that of a chelonian (Maleev 1954, Sukhanov 1964), due to the extreme elongation of the manal digits and their associated claws. Displaying a mix of characters from across Dinosauria, therizinosaur skeletons look as if they were assembled by an inebriated designer who got quite confused as to what belonged where. As a result, a plethora of varying phylogenies have been proposed. Perle (1979), Barsbold & Perle (1980) and Barsbold (1983) argued that therizinosaurs were in fact aberrant theropods. Perle (1981) later reversed himself and aruged instead that therizinosaurs were best considered Theropoda incertae sedis. Paul (1984) suggested the therizinosaurs were a relictual clade representing the prosauropod/ornithischian transition. Gauthier (1986) argued that therizinosaurs constituted a distinct saurischian lineage more closely related to Sauropoda than Theropoda. Barsbold & Maryanska (in Dodson et al 1990) concurred with this assessment. Most recently, Russell (1997) has presented compelling data to support the theropod affinities of therizinosaurs, and the osteology of the basal most forms yet uncovered, Beipiaosaurus inexpectus (Xu, Tang, & Wang 1999) underwrite the relationship of therizinosaurs to Maniraptora (Paul 2002). Further studies (e.g. Elzanowski 1999) concur with this classification. Sereno (1997, 1998, 1999) considers Therizinosauria to be related to the ornithomimosaurs, although this conclusion is not generally accepted.

While holophyly of Therizinosauria is nearly axiomatic, the characters underwriting this conclusion include:

a)Skull and mandible shallow, long, edentulous rostrally

b)External naris very elongate

c)Palate highly vaulted, displaying elongated and caudally displaced vomers

d)Pterygoids reduced rostrally

e)Premaxillary/maxillary secondary palate well developed

f)Basicranium and otic region extremely pneumatized and enlarged

g)Dentary curves downward rostrally

h)Sacral count, six, all firmly ossified

i)Distal and proximal ends of the humeri greatly expanded

j)Pelvic girdle opisthopubic

k)Ilia are broadly separated

l)Preacetabular process of ilium deep, displaying a cranioventral process which flares at right angles to the iliac blade

m)Postacetabular process reduced, with knoblike caudal protuberance

n)Astragalar condyles reduced

Adding to the mix, there is now conclusive evidence that at least some therizinosaurs were feathered (Xu, Tang, & Wang 1999, Paul 2002). Such integument is preserved along the antebrachial margins of the forelimb in Beipiaosaurus.

JGK