The functional significance of morphological changes in the dentitions of early mammals

Abstract

The Mesozoic marked a time of experimentation in the tooth morphology of early mammals. One particular experiment involved the movement of three points, or cusps, on the surface of a molar tooth from a line into a triangle. This transition is exemplified by two extinct insectivorous mammals, (cusps in a line) and (cusps in a triangle). Here we test whether this difference in cusp arrangement, alongside cusp heights and angles between cusps, is associated with differences in the ability of the teeth to fracture proxy-insect prey. We gathered measurements from molar teeth of both species and used them to create physical models. We then measured the force, time and energy at fracture and peak force, and the amount of damage inflicted by the models on hard and soft gels encased in a tough film that mimicked the material properties of insects. The model required less force and energy to fracture hard gels and reach peak force compared with required a similar time, force and energy to fracture soft gels but reduced the time, force and energy to reach peak force. More importantly, also inflicted more damage to both the hard and the soft gels. These results suggest that changes in dental morphology in some early mammals was driven primarily by selection for maximizing damage, and secondarily for maximizing biomechanical efficiency for a given food material property.

Molar biomechanical function in South African homininsAustralopithecus africanusandParanthropus robustus

“…Dental replicas were placed in the grips of a motorized FLS II portable mechanical tester (Lucas Scientific) and lowered into the gelatine blocks until fracture at a constant displacement of 5.7 mm min −1 (figure 2). As in previous experiments, masticatory kinematics was simplified and modelled as a simple translation of the tooth in the occlusal direction [9,50,53,75,76]. Force and displacement were recorded at a data acquisition rate of 50 samples/second.…”

Revisiting life history and morphological proxies for early mammaliaform metabolic rates

“…Newham et al 3 had 69 data points for Q i , 11 were collected de novo, and 58 were collected by Seymour et al 2012 7 (who measured the foramen area directly or from a digital photo of the surface of the bone). They found that Morganucodon had a lower Q i than expected for a mammal (or for ectothermic varanid reptiles) of a similar size 1 , and considerably closer to small non-varanid reptiles. Seymour et al 7 used mammals from ten orders, including bats, in three mammalian infra-classes.…”

“…Morganucodon and Kuehneotherium are two Late Triassic-Early Jurassic stem mammals that are often portrayed as possessing key mammalian characteristics such as multi-cusped molars 1 , respiratory turbinates, and Harderian glands (associated with grooming and maintaining insulatory pelage 2 ). Newham et al 3 recently used synchrotron X-ray tomographic imaging of incremental tooth cementum to reconstruct the longevity of large series of Morganucodon and Kuehneotherium .…”

The anteriorization of tooth position underlies the atavism of tooth morphology: Insights into the morphogenesis of mammalian molars

“…Slightly complex teeth with secondary cusps anterior and posterior to the first cusp appeared in nonmammalian cynodonts, which are considered ancestral to mammals, and molar teeth with three main cusps arranged in the form of a triangle and additional cusps were acquired in early mammals (Feldhamer et al 2003;Kemp 2005;Tims 1903;Ungar 2010;Weller 1968). A variety of molar forms in mammals were derived from the tribosphenic molar (Conith et al 2016;Davis 2011;Hofer 1960;Thenius & Weller 1968). The tribosphenic molar is characterized by the protocone cusp on the upper molar and the talonid basin on the lower molar, providing shearing and crushing functions (Conith et al 2016;Davis 2011;Feldhamer et al 2003;Simpson 1936;Ungar 2010).…”