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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 C 2 Extant Phylogenetic Bracket ([I C, ([I C C .-( w .rl .rl C C CI) CI) c ([I ([I C 0 CI) C m C 2 X 0 0 X a, b-4 4-r a, --3 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
WITMER-ANTORBITAL CAVITY OF ARCHOSAURS Levels of Inference A Extant Phylogenetic I Bracket 1 I P oe gl 9 &*4 g A? *A ,.ta Lnial~r I ~y IUYGI IGLI~ Bracket bracket ancestor decisive assessment soft tissue present (0) osteological correlate inferred soft tissue I Z@ or no equivocal assessment bracket ancestor Bracket w W decisive \no + a negative assessmer ancestor FIGURE 4. Levels of inference. The soft-tissue assessment at the outgroup node determines the level of inference and hence the relative amount of phylogenetic support such an inference draws. A, level I inference: as both extant members of the EPB possess the suspected soft tissue (and its causally associated osteological correlates), the assessment at the outgroup node is decisive and positive. B, level I1 inference: as only one component of the EPB has the soft-tissue attribute, the assessment is equivocal or ambiguous. C, level I11 inference: as neither component has the suspected soft tissue, the assessment is decisive and negative. Level I1 and level I11 inferences might be justifiable if a compelling morphological evidence argument can be advanced.
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