The evolution of the synapsid tusk: insights from dicynodont therapsid tusk histology

Abstract

The mammalian tusk is a unique and extreme morphotype among modern vertebrate dentitions. Tusks—defined here as ever-growing incisors or canines composed of dentine—evolved independently multiple times within mammals yet have not evolved in other extant vertebrates. This suggests that there is a feature specific to mammals that facilitates the evolution of this specialized dentition. To investigate what may underpin the evolution of tusks, we histologically sampled the tusks of dicynodont therapsids: the earliest iteration of tusk evolution and the only non-mammalian synapsid clade to have acquired such a dentition. We studied the tissue composition, attachment tissues, development and replacement in 10 dicynodont taxa and show multiple developmental pathways for the adult dentitions of dicynodont tusks and tusk-like caniniforms. In a phylogenetic context, these developmental pathways reveal an evolutionary scenario for the acquisition of an ever-growing tusk—an event that occurred convergently, but only in derived members of our sample. We propose that the evolution of an ever-growing dentition, such as a tusk, is predicated on the evolution of significantly reduced tooth replacement and a permanent soft-tissue attachment. Both of these features are fixed in the dentitions of crown-group mammals, which helps to explain why tusks are restricted to this clade among extant vertebrates.

Quantitative evidence for dimorphism suggests sexual selection in the maxillary caniniform process of Placerias hesternus

“…dicynodont groups [36], those found on Placerias and other kannemeyeriiforms lack enamel and have a cone-in-cone growth arrangement [29,37], much like the tusks of living mammalian synapsids such as walruses and elephants. Placerias, like all dicynodonts [38], has a projection of rugose bone on the maxilla lateral to the tusk, known as the "caniniform process".…”

“…Additionally, we ask whether there is any quantitative indication for the presence of two morphs in three other well-represented bones (the fibula, quadrate, and premaxilla) in the fossil assemblage. Tusks in dicynodonts have been speculated to be used for feeding, digging, and display [36]. In Placerias, we surveyed the presence and morphology of tusks in maxillae, and the morphology of dissociated tusks, to test the hypothesis that the caniniform replaced the tusks for these functions, and discuss evidence that the tusk is not under strong selection or essential for individual survival.…”

A new kannemeyeriiform dicynodont (Synapsida) from a Late Triassic vertebrate assemblage in west Texas, U.S.A.

“…A slightly larger, isolated tusk, TTU-P10407, is broken on both the proximal and distal ends, but also shows no wear facets. The broken sections show the dense internal structure of the dentin and lack of enamel typical of mature dicynodont tusks (Green, 2009, 2012; Whitney et al, 2021). It is slightly flattened on the posteromedial margin, forming a slight ridge towards the posterior edge.…”