TWO NEW SPECIES OF SHREWS (MAMMALIA: EULIPOTYPHLA: SORICIDAE) FROM THE LATE OLIGOCENE CABBAGE PATCH BEDS OF WESTERN MONTANA: EXCEPTIONAL PRESERVATION REVEALS NICHE PARTITIONING

Shrews (Soricidae) have been present in North America since the Middle Eocene but remained rare until the early Miocene when they underwent a taxonomic radiation. In fact, only three genera of soricids have been described from the late Oligocene of North America, mostly based on teeth and partial jaws. The late Oligocene (~28 -23 Ma) Cabbage Patch (CP) beds of western Montana preserve exceptional fossils of two of these taxa, the two longest-lived genera of North American soricids. These fossils include a mostly complete skull of Domnina and a partial skull of Pseudotrimylus as well as lower jaws and associated dentitions.

The Domnina from the CP beds likely represents a new species. Six upper antemolars, five lower antemolars, and a distinct postentoconid valley on the m1 differentiate this species from D. hutchisoni, D. gradata and Domnina dakotensis. Though it most closely resembles D. greeni, the Domnina from the CP beds is larger. It is also larger than Domnina thompsoni. The Pseudotrimylus material from the CP beds represents a new species, one the two largest known for this genus. It is similar in size to P. mawbyi from which it differs greatly in the morphology of the molars. The m1 of the new taxon is 20% larger than the largest m1 of P. roperi available for comparison. All other species of Pseudotrimylus are even smaller. The mental foramen is located below m1 rather than m2 in this new species of Pseudotrimylus that bears five upper antemolars.

The exceptional preservation of the specimens from the CP beds offers an opportunity to investigate the little known paleoecology of fossil soricids. The two taxa provide evidence for some ecological diversity within a family of mostly insectivorous ground-dwelling animals. The Pseudotrimylus specimen is larger and more robust than the Domina specimen. This robustness is particularly apparent in the dentary and the rostrum. Additionally, the teeth of Pseudotrimylus are more bulbous than those of Domnina. These morphological characteristics concentrated in the jaws and teeth of the animal suggest different feeding strategies and a partitioning of the resources amongst these two taxa found within the same horizons of the CP beds. These ecological inferences will be further tested using morphometric comparisons between extant and modern shrews.