Gnaw marks on Eocene seeds: evidence for early rodent behaviour

Gnawed holes in fossil seeds of the freshwater aquatic floating plant Stratiotes, collected from the Solent Group of Late Eocene (Priabonian) age at two localities in the Hampshire Basin, UK, are recognised as the oldest direct evidence of seed predation by rodents. The type of gnawing is similar to that made by Wood Mice, Apodemus sylvaticus, today and a more reliable means of differentiating the resultant pattern from those of other gnawing types is described. By using the size relationship between modern gnaw marks and the rodent incisors that made them, isolated fossil incisors from the same strata at sites near to those where the seeds were found are matched closely to the fossil gnaw marks. By comparison with incisors in situ in jaws from the Continental European Palaeogene, and by eliminating other contenders amongst the known Solent Group rodent fauna, it is concluded that the fossil Stratiotes seeds were predated by the extinct glirids Glamys priscus and Glamys devoogdi. Different patterns of gnaw marks associated with different hole sizes are interpreted as different stages in the opening of the seeds and as demonstrating learning behaviour on the part of the predators. Similarity of fossil astragali (ankle bones) from the Solent Group, identified as Glamys devoogdi, to the recent glirid Eliomys quercinus suggests a scansorial rather than arboreal locomotor mode for the former, in keeping with a ground-level foraging strategy as indicated by the habitat of the Stratiotes plant. The direct evidence of feeding from the fossil seeds and the clear link to the specific predators are an important indication of ancient dietary adaptation independent of that deduced from tooth morphology. It supports the change in dominant frugivorous feeding type amongst European rodents from soft fruit-eating pseudoparamyine paramyids and pseudosciurids in the Middle Eocene to hard seed-eating glirids in the Late Eocene, coincident with the global climatic deterioration shown by oxygen isotopes. One new ichnogenus with one new ichnospecies is erected for one type of gnawing in fossil seeds: Glirotremmorpha entectus igen. et isp. nov., from the English Late Eocene.