Nomenclature, comparative anatomy, and evolution of the reflected lamina of the angular in non-mammalian synapsids

Abstract

ABSTRACT The reflected lamina of the angular has long been portrayed as a key component in understanding the evolution of the mammalian mandible and middle ear, yet the function of this structure in non-mammalian therapsids remains uncertain. We undertake a broad scale survey of two aspects of the reflected lamina that have been poorly characterized: the ridges and fossae on its lateral surface and the extent of the underlying angular cleft. These two features were visualized in a wide range of therapsids using CT data from the literature and blue light surface scanning. Most species exhibit a clade-specific pattern of surface topography, although dicynodonts are highly variable. A striking similarity in the general number and orientation of fossae and ridges is seen between the otherwise disparate therapsid clades, and we propose homologies in these surface features across Therapsida. These features may serve as useful phylogenetic characters in analyses of the relationships among the major therapsid clades. The angular cleft does not underlie all of what is often referred to as the ‘reflected lamina’ in the literature, and an updated definition of this term is provided. Several therapsid groups independently expanded the angular cleft to further separate the reflected lamina from the angular body. We discuss some functional consequences of our findings and note inconsistency between proposed clade-specific muscle attachment sites and the presence of homologous fossae and ridges across much of Therapsida.