Expansion, speciation and a change of trophic niche: a case study of the Early Pleistocene ground squirrels Spermophilus polonicus and S. praecox

ABSTRACT

During the Early Pleistocene, two ground squirrel species, Spermophilus praecox (northern Black Sea area) and S. polonicus (Central Europe) coexisted in geographically well-separated ranges. Studied samples from Tiligul (Ukraine) and Kamyk (Poland) have approximately the same biostratigraphical age: the early Odessa faunal complex (Tiligul) and the Villanyian-Biharian transition (Kamyk). A significant difference in trophic niches and a high similarity of phylogenetically significant tooth characters have been established for S. praecox and S. polonicus. Seemingly archaic features of S. polonicus are identified as adaptations to a predominance of non-abrasive parts of plants in the diet, while phylogenetically significant characters are either close to those of S. praecox (e.g. patterns of bunodonty) or more advanced (e.g. the relationship between the height of cingula and main lophs). The diet of S. praecox included a higher amount of the abrasive food and underground parts of plants. The effects of this abrasive diet are seen in the early flattening of occlusal surfaces. We suggest that S. polonicus branched off from early S. praecox between 2.5 and 2.0 Ma; the speciation was triggered by an expansion to the northwest, probably, along the Dniester valley.

KEYWORDS:

• Spermophilus praecox
• S. polonicus
• Early Pleistocene
• reconstruction of past ecological niches
• occlusal surface

Acknowledgments

The authors thank to Simon A. Parfitt for language corrections and to anonymous reviewers for their work on improving the manuscript.

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes

1. The suggestion is based on the correspondence between the presence/absence of discrete characters and tooth shape parameters believed by Gromov et al. (1965) to enable the shift from crushing/grinding teeth in archaic ground squirrels to grinding/shearing teeth in more advanced forms. For example, the reduction of mesostiles, metastylids and ectostylids obviously corresponds to progressive shortening of ground squirrel molars and especially premolars.

2. This locality is listed in Sinitsa et al. (2019) as Morskoy, although authors who described the locality and its fauna (Topachevsky and Skorik 1977; Tesakov 2004; Krokhmal’ and Rekovets 2010; Popova and Nadachowski 2018), mentioned it as Tiligul and only Rekovets (1994) as Tiligul (Morskoy).

3. a seemingly opposite situation of the m3 (Figure 10) is a result of the difference in sample size, as it was mentioned above.

Additional information

Funding

This work was supported by International Union for Quaternary research, Commission “Humans and Biosphere”, under Grant 1606.