Cheek tooth morphology and ancient mitochondrial DNA of late Pleistocene horses from the western interior of North America: Implications for the taxonomy of North American Late Pleistocene Equus.

The research article discusses a study that analyzed horse tooth fossils and mitochondrial DNA from the late Pleistocene era in North America in an attempt to better understand the various horse species that lived in this region before the major extinction at the end of that era.

Objective of the Study

• The objective of the study was to clarify the horse species (from the genus Equus) that inhabited the Western Interior region of North America prior to the end-Pleistocene extinction.
• This was achieved by investigating both the cheek tooth morphology (structure and form) and ancient mitochondrial DNA of late Pleistocene horse specimens.

Methods and Findings

• The researchers identified a caballine species (Equus ferus) and a non-caballine species (E. conversidens) from various localities across the Western Interior based on the data analyzed.
• Another non-caballine species (E. cedralensis) was identified from southern localities based solely on cheek tooth morphology.
• The division into caballine and non-caballine species was seen in the Bayesian phylogenetic analysis of ancient mitochondrial DNA and in the geometric morphometric analyses of the upper and lower premolars.

Significance of E. conversidens

• Teeth identified morphologically as E. conversidens that yielded ancient DNA fall under the New World stilt-legged clade (a group of organisms believed to have evolved from a common ancestor).
• This follows a similar distinction recognized in previous studies and is therefore how the group is labeled.

Geographic Variation in Morphology

• Geographical variation in morphology in the caballine species is suggested by statistically different occlusal enamel patterns in the Yukon Territory specimens compared to specimens from other regions.
• The study proposes that further research is needed to determine if this represents ecomorphological variation (changes in morphology due to environmental factors) or some level of geographic and genetic isolation of these Arctic populations.