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

parsimonious scenarios are outlined for each phylogenetic hypothesis. By the
twa outgroup rule (Maddison et al. 1984), the first scenafios assume that an
arctometatarsus was not present in the comrnon ancestor of melurosaurs more
denved than Ornitholestes, although the alternative view is subsequently
explored.
In one phylogeny (Figure 5.3; Holtz 2000), the simplest pattern hoids that the
arctometatarsus arase once in birds (in Mononychus), once in the common
ancestor of tyrannosaurids and bullatosaurians (ornithomirnids plus troadantids),
and once in elmisautids (Elmisaurus in Figure 5.3). Three independent gains of
the arctometatarsus, with O tosses, is the most parsimonious scenario for its
evolution that cm be derived from this hypothesis (Figure 5.3).
The second most parsimonious scenano involves 4 evolutionary steps (Figure
5.3). The structure couId have arisen oniy twice, in Mononychus, and in the
wmmon ancestor of tyrannosaurids and Emisaurus. If this is the case, at least
two fosses are implied, in therizïnosaunds and oviraptorids.
This phylogeny implies that the potential for arctometatarsus expression
evolved initially in the cornmon ancestor of birds and tyrannosaurids. If the
morphology was present in this comrnon ancestor, there are several patterns of
possible loss and emergence. In Holtz's phylogeny (Figure 5.3; Holk 2000),
identical permutations occur in the dromaeosaurid-avian clade and the
theflzinosaur-oviraptorosaur dade. The oviraptorosaur Hmisaurus and the bird
Moononychus are the sate arctometatarsalians in either dade. 1 use birds and
dromaeosaurids as an example.