GHOST LINEAGES AND MAMMALNESS - ASSESSING THE TEMPORAL PATTERN OF CHARACTER ACQUISITION IN THE SYNAPSIDA

The origin of mammals has been characterized as a gradual process, a c laim based primarily on a well-preserved series of extinct nonmammalia n synapsids (''mammal-like reptiles'') that span some 200 million year s. In contrast to the origin of many other higher taxa, the origin of mammals from within cynodont-grade therapsids is not considered to coi ncide with a major morphological change, but rather to be simply the c ulmination of a series of more and more mammal-like transitional forms . To test these assertions, an asymmetrical cladogram extending from p rimitive ''pelycosaurs'' to morganucodontid mammaliaforms was created. Three different methodologies were then used to compare the amount of morphological change between nodes on this cladogram with the minimum missing time interval between each node, as inferred from sister taxo n-based ghost lineages. In general, a statistically significant positi ve relationship was found, indicating that greater numbers of derived features tend to be correlated with longer ghost lineages. A significa nt correlation between the number of accumulated apomorphies and branc hing events was also found. Although the rate of character change was variable, in no case was a long ghost lineage associated with few apom orphies. These correlations are consistent with the hypothesis that ra pid accumulation of derived features occurred relatively infrequently within the synapsid lineage leading toward mammals and that gradual ch aracter evolution predominated.