Fig. 12

Bony secondary palate evolution in synapsids. a Drawings showing the bottom surface of the snout in four synapsids, one lacking a bony secondary palate, the others possessing a bony secondary palate. Note that the internal nostril (the opening through which air passes as it moves from the nasal cavity to the trachea and lungs, highlighted with gray shading) is much further back on the skull in the synapsids with a secondary palate, effectively isolating the path air takes as it enters the body from the mouth. In contrast, the forward position of the internal nostrils in the synapsid lacking a secondary palate means that air must pass through the mouth on its way to the lungs. b Simplified phylogeny of synapsids showing the distribution of bony secondary palates among major groups, as well as comparative drawings of the snouts of various synapsids (internal nostrils highlighted with gray shading). Note that the distribution of secondary palates on this phylogeny implies that the character evolved three separate times in synapsid history. Also note that some synapsids with incomplete secondary palates, such as Ictidosuchoides and Procynosuchus, help to document the evolution of this character (see text for details). Groups shown in bold include living representatives. Drawings were redrawn and modified from Efremov (1940), Romer and Price (1940), Kemp (1969), Hopson and Barghusen (1986), Rybczynski (2000), Angielczyk and Kurkin (2003), Maddin et al. (2008)