Memoir cover 0.tif - Ohio University College of Osteopathic Medicine

SOCIETY OF VERTEBRATE PALEONTOLOGY, MEMOIR 3
rotation around
outgroup node
Extant Phylogenetic Bracket
FIGURE 2. Inverted Pyramid of Inference. Inferences about the softtissue
relations of bony structures are the foundation and justification
for many paleobiological inferences. The pyramid is inverted to highlight
the point that mistakes in soft-tissue inference cascade up the hierarchy,
magnifying the error. (Modified from Witmer, 1992a, 1995a.)
spurious hypotheses about the structure and evolution of whole
communities. As we will see, complicated paleoecological and
evolutionary scenarios have been tied explicitly to particular
notions about the soft-tissue relations of the antorbital cavity of
archosaurs. Thus, a seemingly arcane issue can have important-and
unexpected-ramifications (see Witmer, 1992a,
1995a for other examples).
The Extant Phylogenetic Bracket Approach
The next question is, how do we obtain this requisite softtissue
information? A method is outlined here and is presented
in detail elsewhere (Witmer, 1992a, 1995a; see also Bryant and
Russell, 1992, for an independently-devised but generally similar
approach). The approach is firmly grounded in basic phylogenetic
principles, somewhat resembling two-pass systems of
a posteriori character optimization (Wiley et al., 1991; Witmer,
1995a). The only source of direct information about soft tissues
and their relationships to the skeleton are extant taxa, and thus
they must be components of the analysis. The most relevant
extant taxa are the two most-proximal living outgroups of the
fossil taxon of interest (Fig. 3A). Figure 3B presents a topology
in which there has been a rotation about the outgroup node such
that the extant taxa now flank the fossil taxon. This rotation is
simply a heuristic device that highlights a central objective of
a
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Extant Phylogenetic Bracket
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FIGURE 3. The Extant Phylogenetic Bracket Approach. A, phylogenetic
relationships of a fossil taxon and its first two extant sister groups.
B, rotation about the outgroup node in A brings the extant taxa to the
periphery, forming the Extant Phylogenetic Bracket (EPB). C, cladogram
showing the inference of soft-tissue attributes in fossil taxa using
the EPB approach. Similarities in soft tissues and osteological correlates
between the extant taxa are hypothesized as having been present in their
common ancestor, the bracket ancestor (hatched arrows). If the fossil
taxa possess the osteological (skeletal) correlates, then the hypothesis
of homology survives the congruence test. If the soft tissues and osteological
correlates indeed are causally associated, then the soft tissue
can be inferred in the fossil taxa with confidence. (Modified from Witmer,
1992a. 1995a.)
bracket
ancestor