Extended embryo retention, caecilian oviparity and amniote origins

A recent discussion of the evolution of the amniotic egg (Laurin and Reisz, 1997) was criticized by Wilkinson and Nussbaum (1998), and these criticisms provoked a rebuttal (Laurin et al., 2000). Here we show that the objections raised by Laurin et al. (2000) do not substantiate the conclusions of Laurin and Reisz (1997). We also discuss additional evidence on the ancestral ontogeny of caecilians from the literature. This evidence is inconsistent with the view that extended embryo retention is the ancestral condition for caecilians and that it is a parsimonious interpretation of the condition of the ancestral amniote as argued by Laurin and Reisz (1997) and by Laurin et al. (2000). The available data are more consistent with the traditional hypothesis that the amniotic egg originated as an adaptation of eggs to the terrestrial environment. We also discuss problems in the definition of ontogenetic characters reflecting variation with respect to extended embryo retention, and we present new observations on the early development of the caecilian Gegeneophis ramaswamii Taylor.