Craniodental anatomy in Permian–Jurassic Cynodontia and Mammaliaformes (Synapsida, Therapsida) as a gateway to defining mammalian soft tissue and behavioural traits

Abstract

Mammals are diagnosed by more than 30 osteological characters (e.g. squamosal-dentary jaw joint, three inner ear ossicles, etc.) that are readily preserved in the fossil record. However, it is the suite of physiological, soft tissue and behavioural characters (e.g. endothermy, hair, lactation, isocortex and parental care), the evolutionary origins of which have eluded scholars for decades, that most prominently distinguishes living mammals from other amniotes. Here, we review recent works that illustrate how evolutionary changes concentrated in the cranial and dental morphology of mammalian ancestors, the Permian–Jurassic Cynodontia and Mammaliaformes, can potentially be used to document the origin of some of the most crucial defining features of mammals. We discuss how these soft tissue and behavioural traits are highly integrated, and how their evolution is intermingled with that of craniodental traits, thus enabling the tracing of their previously out-of-reach phylogenetic history. Most of these osteological and dental proxies, such as the maxillary canal, bony labyrinth and dental replacement only recently became more easily accessible—thanks, in large part, to the widespread use of X-ray microtomography scanning in palaeontology—because they are linked to internal cranial characters. This article is part of the theme issue ‘The mammalian skull: development, structure and function’.

Modularity, homology, heterochrony: Gavin de Beer's legacy to the mammalian skull

“…de Beer would have regarded the reptilian origin of mammals as a singular evolutionary event. He never could have anticipated that selection for a more active terrestrial lifestyle would be evidenced in multiple origins of ‘mammalness’ (see Julian Benoit [ 74 ] in this issue). Indeed, the middle ear itself evolved independently at least four times in early terrestrial tetrapods [ 75 , 76 ].…”

The mammalian skull: development, structure and function

“…Norton et al . [ 12 ] provide the broadest evolutionary landscape; they explore evolutionary accretion of mammalian characters through the analysis of the cranial structure in cynodonts and mammaliaforms. That reminds us that mammals are synapsids and their skull structure is deeply rooted in the Paleozoic, about 320 Ma [ 13 ].…”

Weighing in on miniaturization: New body mass estimates for Triassic eucynodonts and analyses of body size evolution during the cynodont‐mammal transition

“…The diversity of Late Triassic probainognathians from Brazil and Argentina currently counts more than 20 species (Martinelli & Soares, 2016; Martinelli & Soares, 2016, Martinelli et al, 2017, b; Abdala et al, 2020; Schultz et al, 2020), including the prozostrodontian Brasilodon quadrangularis , which represents the sister taxon of Mammaliaformes (e.g., Bonaparte et al, 2003, 2005; Luo, 2007; Martinelli et al, 2017, b; Norton et al, 2023; Soares et al, 2014; Stefanello et al, 2023). Previous studies on these cynodonts have provided great insight into the cynodont‐mammal transition through different approaches (Bonaparte et al, 2003, 2005, 2012; Botha‐Brink et al, 2018; Guignard et al, 2018; Guignard et al, 2019a, 2019b; Martinelli & Bonaparte, 2011; Martinelli & Rougier, 2007; Rodrigues et al, 2013a, 2013b; Ruf et al, 2014).…”