Interpterigoid vacuities in the primary palate of cynodonts (Therapsida): Heterochrony and evolutionary significance

AGUSTIN MARTINELLI

Congreso; E-ICES 4, International Center for Earth Sciences; 2008

In this contribution, I explore the evolutionary meaning of the interpterigoid vacuities, located in the primary palate of the skull, in the lineage of probainognathian cynodonts basal to mammals. The extensive fossil record of this lineage and the presence of ontogenetic sequence in some fossil taxa permitted the analysis for detecting heterochronic mechanisms in non-mammaliaform cynodonts and their bearing for the establishment of derived cranial features of basal mammaliaforms. The re-acquisition of interpterygoid vacuities in derived non-mammaliaform cynodonts, which have high resemblance with the pattern present in basal-most forms and immature individuals of more derived forms, could be the result of heterochronic processes. The fact that the descendant adult morphology resembles that of the juvenile ancestor suggests that the re-appearance of vacuities in these derived cynodonts could be explained by paedomorphosis. This feature possibly has been achieved in advanced cynodonts either by a neotenic slowing down of growth rate or by a progenetic earlier onset of maturity, truncating the growth period. The presence of interpterigoid vacuities in basal-most and derived cynodonts results in transversely wide pterygoids, and consequently a broad floor of the mesocranium similar to that of basal mammaliaforms (e.g., Morganucodon), although mammaliaforms lack interpterygoid vacuities. Therefore, the presence of interpterygoid vacuities represents an ancestral condition that facilitated the development of the mammalian basis cranii constituted by the reduced pterygoids, the basisphenoid, and posteriorly the development of the presphenoid by the ventral extension of the braincase. Possibly, the main factors in the remodeling of this area are the forward and ventral expansion of the cranial cavity and the subsequent development of the sphenoid sinus. This is a major transformation of the skull architecture of the mesocranial region in basal mammaliaforms.