THE UNIVERSITY OF CALGARY Eric Snively A ... - Ohio University

did not quantify the degree of proximal constridion of MT III that diierentiates an
arctometatarsus from other morphologies. A theropod MT Ill with any degree of
proximal constriction, such as that displayed by Allosaurus (Figure 2.1 : e), wuld
qualify as arctometatarsalian if the definition is interpreted with some latitude (for
example, Hutchinson and Padian 1997). This unquantified morphological
definition has become confiated with the hypothesis of a polyphyletic origin of the
structure, given the controversial constituency of Holtz's Arctometatarsalia
(Tyrannosauridae + Bullatosaura; Holîz 1994b, Hutchinson and Padian 1997,
Sereno 1 999). Sereno challenged the monophyly of the Arctometatarsalia (Hoitz
1994b) at the 1994 Society of Vertebrate Paleontology meeting, in part because
Deinonychus and Allosaurus appear to display some proximal narrowing of MT
111.
Most parties to this controversy initially overlooked Holtz's (1 994a) character
of plantar constriction of MT III, which separates the arctometatarsus from other
theropod morphologies. Thus, Holtz's (1 994a,b) characterization of MT III
morphology that typified the Arctometatarsalia was a three dimensional
assessment, and accounted for more than a simple narrowing of the proximal
end of this elernent. Sereno (1999) redressed this imbalance by incorparating
plantar constriction and further qualitative character states into a phylogenetic
analysis. This systematic reassessment of the Theropoda (Sereno 1999)
dismpted the monophyly of HolKs (1994b, 1996) Arctometatarsalia.
Regardless of the ultimate phytogenetic distribution of the arctometatarsus,
the ambiguity surrounding its morphology invites further elucidation. The