INTRODUCTION

Comparative development can be used to understand the derivation of form and also to understand homology versus the convergent evolution of structures. Indeed, Shubin (1994) states: ‘Ontogenetic data can greatly aid in the formation of hypotheses of homology … comparative studies of ontogenetic sequences, and the analysis of interspecific patterns of variability can all provide information about potential homologies.’ Phylogenetic homology (Roth 1984, 1994) refers to those structures derived from the same feature in a common ancestor and shared by two or more taxa (synapomorphies). Biological homology, however, refers to characters that share a set of specific developmental constraints (Wagner 1989). Understanding both phylogenetic and biological homologies is necessary to fully discern the evolutionary origins of characters. In this chapter, we examine the developmental patterns of the hindlimb and calcar and their implications for the phylogenetic relationships among bats.

THE HINDLIMB: A DERIVED STRUCTURAL COMPOSITE

The skeletal system of bats is unique in form and function, predominately due to suites of characters derived for flight. For example, the hindlimb of bats is unlike that of most other mammals and its fundamental morphology is a compromise between adequate structural support and reduced weight for flight (Howell & Pylka 1977). The bat hindlimb has evolved specializations involving bone reductions and joint rotations (MacAlister 1872; Vaughan 1959), as well as highly derived characteristics such as the passive digital lock (Schaffer 1905; Bennet 1993; Quinn & Baumel 1993; Schutt 1993, 1998) and the calcar (Vaughan 1959, 1970a, b; Koopman 1988; Schutt & Altenbach 1997; Schutt & Simmons 1998).