Research Article: The ecomorphology of southern African rodent incisors: Potential applications to the hominin fossil record

Date Published: February 20, 2019

Publisher: Public Library of Science

Author(s): Oliver C. C. Paine, Jennifer N. Leichliter, Nico Avenant, Daryl Codron, Austin Lawrence, Matt Sponheimer, Cyril Charles.

http://doi.org/10.1371/journal.pone.0205476

Abstract

The taxonomic identification of mammalian fauna within fossil assemblages is a well-established component of paleoenvironmental reconstructions. However, many fragmentary specimens recovered from fossil sites are often disregarded as they can be difficult to identify with the precision required for taxonomic methods. For this reason, the large numbers of isolated rodent incisors that are often recovered from hominin fossil bearing sites are generally regarded as offering little interpretive value. Ecomorphological analysis, often referred to as a “taxon-free” method, can potentially circumvent this problem by focusing on the adaptive, rather than the taxonomic significance of rodent incisor morphology. Here, we determine if the morphology of the upper incisors of modern southern African rodents reflects dietary behavior using discriminant function analysis. Our model suggests that a strong ecomorphological signal exists in our modern sample and we apply these results to two samples of isolated incisors from the hominin fossil bearing sites, Sterkfontein and Swartkrans.

Partial Text

Fossil assemblages often contain elements that are of little or no interpretive value for traditional paleoecological investigations. In particular, methods relying on precise taxonomic identification of fossil specimens can rarely use fragmentary, or otherwise adiagnostic material. Such is the case with most rodent-based reconstructions of hominin paleohabitats, which rely on the preservation of the molar tooth rows for species, genus, and/or subfamily-level identification (e.g., [1–6]). While such material exists, in many fossil localities there is often an abundance of isolated incisors that heretofore have been of little interpretive value due to their poor taxonomic control. Because rodent incisors are ever-growing, they are not rooted in the alveoli in the same manner as other teeth. As such, rodent incisors often freely slide in and out of the maxillary and premaxillary alveoli when soft tissue is lost after death—a major reason why so many are found in isolation in fossil deposits. In the course of our (OP and JL) work with fossil rodent collections from South African cave deposits we have come across an abundance of isolated incisors. The main objective of this study is to determine if these abundant fossil elements can yield an ecomorphological signal to augment previous interpretations of hominin paleohabitats.

We sampled 41 modern South African rodent species curated in the collections of the Ditsong Museum of Natural History, Pretoria, South Africa (six Families and 31 Genera, N = 163; S1 Table). For each species, we measured two adult males and two adult females, with the exception of the Cape mole-rat, Georychus capensis, for which reliable measurements could only be attained for three individuals. Specimens were photographed and measured at the museum using a Keyence VHX-2000E digital microscope with a VH-Z00R zoom lens (0x to 50x) and internal measurement software, and with Mitituyo Absolute AOS Digimatic calipers (for measuring mesiodistal diameter) (Fig 1).

Fourteen of the 36 measures tested for a phylogenetic signal produced K values < 1 and all 14 were ratio measurements (S2 Table). These ratio measurements were used in our stepwise QDA and a model was constructed (using OA/MD, OD/DE/ BD/MD, and RC/MD) that misclassified 37 of the 163 training specimens (22.7%; Fig 2 and Table 1). Canonical 1 explains 90% of the variance and is driven by BD/MD and OA/MD; Canonical 2 explains 10% of the variance and is driven by BD/MD and OD/DE (Table 2). Isolated rodent incisor morphology is a potential source of paleoenvironmental information. It may offer many of the same benefits of taxonomically-based small mammal studies, such as relatively high spatial resolution, while greatly increasing the available sample and limiting assumptions about the behavior of fossil taxa. Our model was able to successfully place modern specimens into broad dietary categories, and further refinement of our understanding of modern African rodent ecology should sharpen this model’s interpretive power.   Source: http://doi.org/10.1371/journal.pone.0205476

 

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