Cranial Bosses of Choerosaurus dejageri (Therapsida, Therocephalia): Earliest Evidence of Cranial Display Structures in Eutheriodonts

Abstract

Choerosaurus dejageri, a non-mammalian eutheriodont therapsid from the South African late Permian (~259 Ma), has conspicuous hemispheric cranial bosses on the maxilla and the mandible. These bosses, the earliest of this nature in a eutheriodont, potentially make C. dejageri a key species for understanding the evolutionary origins of sexually selective behaviours (intraspecific competition, ritualized sexual and intimidation displays) associated with cranial outgrowths at the root of the clade that eventually led to extant mammals. Comparison with the tapinocephalid dinocephalian Moschops capensis, a therapsid in which head butting is strongly supported, shows that the delicate structure of the cranial bosses and the gracile structure of the skull of Choerosaurus would be more suitable for display and low energy combat than vigorous head butting. Thus, despite the fact that Choerosaurus is represented by only one skull (which makes it impossible to address the question of sexual dimorphism), its cranial bosses are better interpreted as structures involved in intraspecific selection, i.e. low-energy fighting or display. Display structures, such as enlarged canines and cranial bosses, are widespread among basal therapsid clades and are also present in the putative basal therapsid Tetraceratops insignis. This suggests that sexual selection may have played a more important role in the distant origin and evolution of mammals earlier than previously thought. Sexual selection may explain the subsequent independent evolution of cranial outgrowths and pachyostosis in different therapsid lineages (Biarmosuchia, Dinocephalia, Gorgonopsia and Dicynodontia).

Endocranial Casts of Pre-Mammalian Therapsids Reveal an Unexpected Neurological Diversity at the Deep Evolutionary Root of Mammals

“…In Moschops , the bones of the braincase are very thick, likely as an adaptation to headbutting ( Fig. 8 a) [Benoit et al, 2016b]. The Moschops endocast shows no direct evidence of softtissue imprints on the braincase ( Fig.…”

“…In other NMT, when present, the pineal tube is oriented close to vertical and the parietal foramen opens in the middle of the skull roof [Kemp, 1969[Kemp, , 1979[Kemp, , 2009Jerison, 1973;Hopson, 1979;Quay, 1979;Quiroga, 1980Quiroga, , 1984Kielan-Jaworowska et al, 2004;Rowe et al, 2011;Rodrigues et al, 2014;Castanhinha et al, 2013;Laass, 2015a]. The caudal orientation of the pineal tube in dinocephalians and biarmosuchians has been tentatively interpreted as an adaptation to headbutting as it places the parietal foramen away from the combative surface of the skull [Benoit et al, 2016b]. In mammals, the pineal tube and foramen are absent [Benoit et al, 2016a].…”

“…Given their evolutionary success, it is not unreasonable to hypothesise that a certain degree of behavioural flexibility existed in NMT, which would have given them the ability to adapt their survival strategy to accommodate environmental variety [Laass et al, 2010]. In addition, NMT had a variety of complex behaviours also represented in mammals, such as headbutting, gregariousness, and parental care [Brink 1955[Brink , 1965Botha-Brink and Modesto 2007;Fiorelli et al 2013;Viglietti et al, 2013;Benoit et al, 2016b;Jasinoski and Abdala, 2017].…”

“…1 ), show extensive ossification of the braincase [Boonstra, 1968;Sigogneau, 1970;Rubidge and Sidor 2001;Sidor and Smith, 2007;Ivakhnenko, 2008;Benoit et al, 2016b]. In Dinocephalia, a group of typically large-bodied therapsids with interdigitating incisors and an ornamented and pachyostotic skull, the braincase is fully ossified [Boonstra, 1968;Ivakhnenko, 2008;Benoit et al, 2016b].…”

“…resentatives of the more derived Burnetiamorpha, the skull also bears pachyostosed outgrowths and a caudally expanded ossified sphenethmoidal complex that encloses the braincase [Sigogneau, 1970;Sidor and Smith, 2007;Ivakhnenko, 2008;Benoit et al, 2016b]. The convergent evolution of the fully enclosed braincase in mammals, dinocephalians, and biarmosuchians is a unique opportunity to enhance understanding of the endocast, and by inference the brain, in early therapsids.…”