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

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.

The origin and evolution of Cynodontia (Synapsida, Therapsida): Reassessment of the phylogeny and systematics of the earliest members of this clade using 3D‐imaging technologies

“…This stereotyped pattern can be confirmed for Lycosuchus (Olroyd & Sidor, 2022), Alopecognathus , Olivierosuchus , and Theriognathus (based on AMNH FARB 8226). In early cynodonts, the topography of the angular is simplified compared with most other non‐mammalian therapsids, but at least two fossae are present facing posteriorly to posteroventrally and antero‐ to posteroventrally (Olroyd & Sidor, 2022). This simplified morphology can be confirmed for Abdalodon muchingaensis (Huttenlocker & Sidor, 2020), Charassognathus , Procynosuchus , Galesaurus , and Thrinaxodon (Olroyd & Sidor, 2022).…”

“…In early (non‐eucynodont) cynodonts the remnant of the angular keel is a small finger of bone curving along the anteroventral edge of the medial face of the free‐hanging reflected lamina (Olroyd & Sidor, 2022). This morphology can be confirmed for Abdalodon muchingaensis (Huttenlocker & Sidor, 2020), Cynosaurus (based on BP/1/4469), Vetusodon (based on SAM‐PK‐K10702), Progalesaurus , Nanictosaurus (based on both RC 47 and TM 279), Platycraniellus (based on TM 25), and Charassognathus , Procynosuchus , Galesaurus , and Thrinaxodon (Olroyd & Sidor, 2022). In contrast to the reduced morphology of the angular keel in non‐eucynodont cynodonts, an angular keel is completely absent in eucynodonts (Olroyd & Sidor, 2022).…”

“…In non‐cynodont therapsids such as therocephalians, an angular keel is present and extends ventral to the margin of the lower jaw (Olroyd & Sidor, 2022). This morphology can be confirmed for Lycosuchus (Olroyd & Sidor, 2022), Alopecognathus , Olivierosuchus , and Theriognathus (based on AMNH FARB 8226).…”

Gorgonops and Endothiodon (Synapsida: Therapsida) from the Madumabisa Mudstone Formation: evidence of a previously unreported tetrapod biozone in the Mid-Zambezi Basin of southern Zambia

“…It bears the characteristic cruciate ornament of gorgonopsians and forms most of the angular in lateral view. As in other gorgonopsians, a lateral angular foramen is present at the dorsal end of the lamina's vertical ridge and the free posterior margin of the lamina lies ventral to its dorsal notch, leaving a long angular gap posteriorly (Olroyd & Sidor, 2022; Sigogneau-Russell, 1989). A damaged articular, including the hook-like retroarticular process, is present on the posteriormost end of the lower jaw, although this area is weathered laterally.…”

New fossils, new ideas: selected contributions of vertebrate paleontology to scientific knowledge

“…Homologous with the ectotympanic of modern mammals, the function of the reflected lamina in non-mammalian synapsids has remained mysterious, likely due to the lack of a specific, consistent terminology for describing it and its surrounding spaces as a plethora of new early synapsids have been discovered. In “Nomenclature, comparative anatomy, and evolution of the reflected lamina of the angular in non-mammalian synapsids,” Olroyd and Sidor (2022), remedy this problem by revising the terminology surrounding these important evolutionary features. Aided by tomographic data to investigate the exceedingly fragile reflected laminae of several non-mammalian synapsids, they establish the ancestral morphology of the lamina and trace its modifications through evolution, moving closer to ruling out certain functional hypotheses, at last approaching the question of what role(s) the lamina and associated spaces played in the evolution of chewing and/or hearing in mammals.…”