Meeting Abstract
Tetrapods primarily present a pentadactyl plan. However, limb/digit reductions can correlate with trunk elongated or miniaturized body plans. The typical pattern of digit appearance in amniotes and anurans is posterior to anterior (5 to 1), with digit loss occurring in a reverse anterior to posterior direction (1 to 5). Only in salamanders does the pattern of digit appearance differ, following an anterior to posterior (1 to 5) direction. Within extant salamanders, the Family Amphiumidae presents a unique system to study the evolution and development of limbs due to their very elongated body as well as having reduced limb length and digit number in a graded fashion within the genus: Amphiuma tridactylum (Three-toed), A. means (Two-toed), and A. pholeter (“One-toed”). Ancestrally, salamanders have lost digit 5 in the forelimb, yet retain the pentadactyl pattern in the hind limb. Ancestrally in the genus Amphiuma, the forelimb has lost digit 4 while the hindlimb has lost digits 5 and 4 with subsequent reduction within the genus toward a total of 3, 2 and “1” digits per autopodium. Our preliminary data show variation in digit number within each species, and has led us to examine mesopodial skeletal variation to provide insight on which digits were lost. Ultimately, the mesopodial skeleton should provide support for the homology of the retained digits. Our results for A. tridactylum show an increase in carpal/tarsal skeletal variation and loss of distal carpal and tarsal 3, which also lacks digit 3 and is thus convergent with A. means. In addition, A. pholeter was found to consistently present two digits bound in syndactyly rather than the previously assumed one digit.
