Micromammal and macromammal stable isotopes from a MIS 6 fossil hyena den (Pinnacle Point site 30, south coast, South Africa) reveal differences in relative contribution of C4 grasses to local and regional palaeovegetation on the Palaeo-Agulhas Plain
Introduction
Quaternary paleoenvironments in Africa are often contextualized within a framework of glacial/interglacial variation, where aridity during glacial periods significantly impacts the biogeography and diversity of both flora and fauna (e.g (Schreiner et al., 2013). However, local proxy records and vegetation modeling both suggest highly variable regional paleoenvironmental responses to changes in major abiotic factors across the continent (Blome et al., 2012; Cowling et al., 2008; Scholz et al., 2007). This is especially true in many parts of southern Africa where distinctive local climatic and environmental features likely had unique responses to changing global factors, because these features arise independently of the primary drivers of climate/environmental change in the rest of Africa (Barrable et al., 2002; Chase and Meadows, 2007; Muller and Tyson, 1988; Reason and Rouault, 2005; Stuut et al., 2004). Marine isotope stage 6 (MIS 6) is a long glacial phase, but to date there are few proxy records from this time in the Cape Floristic Region (Marean et al., 2014).
In order to address the gap in information on MIS 6 conditions for the southern Cape of South Africa, we present here tandem micromammal and macromammal stable carbon and oxygen isotope data for one of only two fossil-bearing localities on the south coast of South Africa that is well-dated to the Middle Pleistocene (Jacobs, 2010; Marean et al., 2014; Rector and Reed, 2010), the MIS 6-age fossil hyena den Pinnacle Point 30 (PP30). Tandem micromammal and large mammal isotope data complement each other for a number of behavioural and ecological reasons, and have the potential to provide a more complete picture of local and regional vegetation and moisture than either data set alone. Isotopic data from micromammals and large mammals represent the prey of very different accumulators - in the case of PP30, brown hyena (Parahyena brunnea) and owls (likely the spotted eagle owl, Bubo africanus) (Matthews et al., 2020 [this issue]). Owls and hyenas sample distinct faunal communities (large versus small mammals), have very different ranges, and create accumulations that differ significantly in taxonomic representation. The predation ranges of raptors such as owls are comparably small (r = ∼3 km, or ∼28.27 km2) when compared to the ranges of large carnivores such as hyenas (Andrews, 1990; Matthews, 2004 ; Mills, 1990).
Furthermore, these herbivorous mammalian taxa have distinctive life histories and foraging ranges. Small fauna tend to have restricted birthing seasons and lifespans, and similarly restricted home ranges. Large herbivores, even those not part of migration ecosystems, tend to have significantly larger ranges and to grow and develop more slowly. Thus, micromammals and large mammals sample different components of both site-local and regional environments, and time intervals. In ecotones or other localities where habitat heterogeneity is likely to have been present, direct comparison of the micromammal and large mammal stable isotope data may provide a more comprehensive view of paleovegetation that reflects taxon-specific behavior, herbivore foraging ranges and predator ranges.
The isotopic character of an animal’s preferred food is reflected in that animal’s isotopic profile, and thus the habitat from which the predator selects prey items is reflected in the isotopic composition of the fossil assemblages. This is the foundation of studies that utilize faunal isotopes to infer paleoenvironment (Ecker et al., 2018; Lee-Thorp, 2002; Lee-Thorp and Talma, 2000). We argue that these differing prey communities of owls and hyenas also likely sample different parts of the ancient ecosystem, with differing stable isotopic characteristics, and thus by considering accumulations of both we significantly enrich our understanding of past environments during the poorly sampled MIS 6.
Stable carbon and oxygen isotope data obtained from fossil tooth enamel have long been applied as proxies for paleoenvironmental conditions. Most of these data are from large mammalian fauna. Sampling of very small mammalian fauna to produce stable isotope data sets that reflect environmental conditions over a narrower geographic range is a relatively recent phenomenon (Gehler et al., 2012; Hopley et al., 2006; Hynek et al., 2012; Jeffrey et al., 2015; Thackeray et al., 2008; Yeakel et al., 2007a). This research has been driven in part by the realization that micromammal isotope data can provide statistically robust datasets and that small body size is less of a constraint for δ18O than previously believed (Luz and Kolodny, 1985). Furthermore, methodological advances in mass spectrometry, such as laser ablation, now permit sampling of very small specimens (Cerling and Sharp, 1996; Lindars et al., 2001; Passey and Cerling, 2006; Richards et al., 2008; Sharp and Cerling, 1996).
Section snippets
The site
PP30 (Fig. 1) is a paleontological locality that occurs within a cave in a calcrete layer stratified above the Table Mountain Sandstone cliffs at Pinnacle Point, South Africa (Rector and Reed, 2010). It was excavated as a salvage operation when it was discovered by trenching to lay pipes for a development. Excavations followed most of the procedures typical for excavations at Pinnacle Point (Bernatchez and Marean, 2011; Marean et al., 2004; Oestmo and Marean, 2014) though some strict
Materials: micromammals
A large sample of both cranial and post-cranial micromammal remains were recovered from PP30, both in situ and thus plotted by total station (electronic survey instrument), and in the screened sediments. Identification of murid taxa was made using the M1 and M1 teeth (in situ and isolated teeth) with the exception of the Otomyinae where the more diagnostic M3 was used, rather than the M1. Soricids were quantified by mandibles and maxillae rather than single teeth as loose teeth were generally
Micromammals
Twenty-seven isolated and in situ micromammal teeth were sampled using LA-GC-IRMS. Data from five specimens were excluded due to char on the edges of the ablation pits and two others on the basis of large blank fractions, which potentially compromised the accuracy of isotope measurements. The remaining dataset is comprised of 28 isotope measurements made on 21 individual specimens.
Replicate measures were made on all 5 incisors sampled (2 sets of incisor replicate data excluded for char), as
Discussion
Because the fossil assemblages from PP30 represent a fairly constrained period of deposition, the micromammal and large faunal material can be treated as contemporaneous and the isotope data as proxies of paleoenvironmental conditions near to the site at ∼151 ka, a phase of MIS6 indicated by deep sea core and ice core records to be a strong phase of glacial cooling. However, taken separately, analyses of the PP30 fossil micromammal and the PP30 fossil large mammal carbon and oxygen isotope
Conclusions
The stable isotopes, δ13C and δ18O, of micromammals and macromammals from an MIS 6 paleontological site at Pinnacle Point (PP30) were compared. The vast majority of micrommamals were accumulated by owls, while the macromammals were accumulated by brown hyenas. The results are consistent with sampling of the palaeovegetation at different geographic scales. Taking into account the differences occasioned by the analytical methods – LA-GC-IRMS and H3PO4-IRMS – micromammal stable isotope data act as
Acknowledgments
The authors would like to thank the MAPCRM crew for their invaluable assistance; B. Genari for laboratory and catalogue assistance at the Diaz Museum; A. Miller, A. Sommerville, and A. Michaud for advice and assistance with the sample preparation and data collection of the Wilderness modern micromammal data; K. Knudsen for laboratory space in the Archaeological Chemistry Laboratory (ASU). We also thank B. Passey for access to and training on the LA-GC-IRMS at Johns Hopkins University, and H.L.
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