Enamel-reduction and orthodentine in Dicynodontia (Therapsida) and Xenarthra (Mammalia): an evaluation of the potential ecological signal revealed by dental microwear
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2010-08-07
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Abstract
Mastication produces dental scar patterns (microwear) linked to diet in extant mammals, providing a baseline for reconstructing paleodiet in extinct taxa. Although microwear on enamel is a well-accepted proxy for dietary inference, microwear on orthodentine has received less attention. Because some synapsids lack enamel on their teeth in maturity (e.g., xenarthran teeth, proboscidean tusks), our understanding of paleoecology from microwear in these animals is limited. This dissertation addresses the central question: are orthodentine microwear patterns reliable indicators of ecology in synapsids? Chapter 1 evaluates the distribution of enamel-reduction across Synapsida to select appropriate taxa to target for this research. Enamel-reduction exhibits homoplasy and has independent origins in 13 synapsid taxa. Members of Dicynodontia, Proboscidea, and Xenarthra were chosen for orthodentine microwear analysis because these groups are species-rich with total enamel loss on teeth in maturity.
Chapter 2 establishes the correlation between orthodentine microwear and diet in extant xenarthrans using 255 tooth specimens from 21 species. Tooth sampling loci were standardized across taxa. Mean scratch number was significantly higher for extant xenarthran folivores and frugivore-folivores than carnivore-omnivores or insectivores. This supports utility of microwear in distinguishing herbivorous from non-herbivorous taxa. Orthodentine microwear was examined in the Pleistocene ground sloth Nothrotheriops shastensis (4 teeth) to extend this correlation to extinct taxa. Statistically, microwear in N. shastensis was most similar to extant folivores, corroborating independent evidence for herbivory in this taxon.
Chapter 3 examines the necessity of standardized tooth sampling (used in chapter 2) by quantifying intertooth variation of microwear in each of the four extant xenarthran dietary groups. Distribution of microwear features between tooth loci is more conserved in folivores than other groups. Significant variation in the latter may stem from uneven distribution of bite-force in long-faced animals (two-toed sloths, armadillos) relative to short-faced folivores (three-toed sloths). These results support standardized tooth comparison in future analyses of paleodiet in extinct xenarthrans.
Chapter 4 tests hypotheses of possible tusk function (sexual display, feeding) of extinct Permo-Triassic dicynodonts through phylogeny and orthodentine microwear analysis. Optimization of tusk function in living mammals supports evolution of tusks through sexual selection, with an associated feeding function arising second in some taxa. Tusk orthodentine microwear was compared between extant analogues (Loxodonta, Odobenus) and extinct dicynodonts to assess use of tusks for feeding in dicynodonts. Microwear on tusks of extant analogues was significantly different between taxa that use tusks for feeding (Loxodonta) versus those that do not (Odobenus), which supports a link between feeding behavior and tusk microwear. However, only three of 24 sampled dicynodont tusks had original, unaltered microwear. This low yield precluded statistical comparison with extant taxa, so the utility of orthodentine microwear in reconstructing tusk function in extinct taxa remains uncertain.
This dissertation supports the utility of orthodentine microwear as a proxy for ecology in extant mammals. Analysis of fossil material supports this method as a proxy for paleodiet in Xenarthra, although more research is needed to test whether microwear can reconstruct tusk feeding function in extinct taxa.
Life history of dicynodonts includes elucidating not only feeding behavior but also growth, among other traits. Chapter 5 uses bone histology to reconstruct growth patterns in a large late Triassic dicynodont, Placerias hesternus. Cortical microstructure of 21 dissociated long bones supports rapid growth in early ontogeny, with slowed osteogenesis in late ontogeny. Although microstructure of primary bone is similar between P. hesternus and smaller, geologically older dicynodonts, extent of secondary remodeling in long bones is significantly higher in the former, supporting differential growth between taxa. These data need to be analyzed using a more complete ontogenetic series to understand more about the evolution of growth strategy in Dicynodontia.
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Xenarthra, Dicynodontia, Mammalia, Synapsida, orthodentine, dental microwear, Enamel-Reduction, Paleoecology
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Degree
PhD
Discipline
Marine, Earth and Atmospheric Sciences
