Both stem-diapsids and primitive synapsids are represented in the Moscovian (families
Protorothyrididae and Ophiacodontidae, respectively) and in the Kasimovian (families
Petrolacosauridae and Edaphosauridae, respectively). The divergence between mammals
and sauropsids, placed at about 310 Ma in the Pennsylvanian, has been widely used to
calibrate molecular clocks (Kumar and Hedges 1998; Hedges 2001; Van Tuinen et al.
2001). The timing of this event is not affected by the controversial locations of turtles and
Figure 11.10 Stratigraphical plot of strict consensus of 100440 most parsimonious trees at 945 steps
(CI=0.2447; RI=0.6835; RC=0.17) derived from analysis of cranial characters and resolved down
to species level (Ruta et al. 2003).
MARCELLO RUTA AND MICHAEL I.COATES 251