Abstract
It is contentious whether size variation among mammalian teeth is heterogeneous or homogeneous, whether the coefficient of variation is reliable, and whether the standard deviation of log-transformed data and the residual of standard deviation on mean variable size are useful replacements for the coefficient of variation. Most studies of tooth size variation have been on mammals with complex-crowned teeth, with relatively little attention paid to taxa with simple-crowned teeth, such as Pinnipedia. To fill this gap in knowledge and to resolve the existing controversies, we explored the variation of linear size variables (length and width) for all teeth from complete permanent dentitions of four pinniped species, two phocids (Histriophoca fasciata, Phoca largha) and two otariids (Callorhinus ursinus, Eumetopias jubatus). Size variation among these teeth was mostly heterogeneous both along the toothrow and among species. The incisors, canines, and mesial and distal postcanines were often relatively highly variable. The levels of overall dental size variation ranged from relatively low as in land carnivorans (Phoca largha and both otariids) to high (Histriophoca fasciata). Sexual size dimorphism varied among teeth and among species, with teeth being, on average, larger in males than in females. This dimorphism was more pronounced, and the canines were larger and more dimorphic relative to other teeth in the otariids than in the phocids. The coefficient of variation quantified variation reliably in most cases. The standard deviation of log-transformed data was redundant with the coefficient of variation. The residual of standard deviation on mean variable size was inaccurate when size variation was considerably heterogeneous among the compared variables, and was incomparable between species and between sexes. The existing hypotheses invoking developmental fields, occlusal complexity, and the relative timing of tooth formation and sexually dimorphic hormonal activity do not adequately explain the differential size variation along the pinniped toothrow.
Highlights
Variation is a prerequisite for evolution by natural selection
The purpose of this study was fourfold: (1) to evaluate the reliability and usefulness of CV, SDL, and RSD; (2) to describe and compare the variation of tooth size within and among species of Pinnipedia representing its two major extant clades, Otariidae and Phocidae; (3) to compare the variation of tooth size between pinnipeds and land carnivorans; and (4) to test hypotheses related to tooth size variation
CV and SDL showed similar levels of correlation with mean variable size, varied almost identically along the toothrow, and were very strongly correlated with each other (r = 0.99–1.00). This agrees with previous observations from land carnivorans [13,14,15] and suggests the redundancy of SDL with CV
Summary
Variation is a prerequisite for evolution by natural selection. Dentition is of fundamental importance for the study of mammalian evolution. This is because teeth are highly informative of a mammal’s taxonomic identity, ecological adaptation, and phylogenetic relationships, and because they are inorganic, which makes them durable and relatively abundant in the fossil record [4, 5]. The size of a tooth crown is fixed by the cessation of enamel apposition before tooth eruption, offering a correlate to body size [6,7,8,9], which in turn correlates with many aspects of an animal’s life history and ecology [10, 11]
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