Abstract
The origin of the amniotic egg was a major event in vertebrate evolution and is thought to have contributed to the spectacularevolutionary radiation of amniotes. We test one of the most popular scenarios proposed by Carroll in 1970 to explain the originof the amniotic egg using a novel method based on an asymmetric version of linear parsimony (aka Wagner parsimony) foridentifying the most parsimonious split of a tree into two parts between which the evolution of the character is allowed to differ.The new method evaluates the cost of splitting a phylogenetic tree at a given node as the integral, over all pairs of asymmetryparameters, of the most parsimonious costs that can be achieved by using the first parameter on the subtree pending from thisnode and the second parameter elsewhere. By testing all the nodes, we then obtain the most parsimonious split of a tree withregard to the character values at its tips. Among the nine trees and two characters tested, our method yields a total of 517 parsi-monious trend changes in Permo-Carboniferous stegocephalians, a single one of which occurs in a part of the tree (among stem-amniotes) where Carroll’s scenario predicts that there should have been distinct changes in body size evolutionary trends. Thisrefutes the scenario because the amniote stem does not appear to have elevated rates of evolutionary trend shifts. Our nodalbody size estimates offer less discriminating power, but they likewise fail to find strong support for Carroll’s scenario.