Presenter : Savannah Olroyd
Mentor : Sam Hopkins
Major : Biology
Poster 30
Diagnostic features that show variation can be problematic when their variation interferes with clear distinction between species. Fossil identification can be further confounded when highly variable supposedly diagnostic dental characters change with tooth wear. The fossil beavers Dipoides stirtoni and Dipoides smithi are distinguished from one another by the presence or absence of striations on the fourth premolars. D. stirtoni has a parastria running down the cheek side of the upper P4 and a parastriid on the tongue side of the lower p4 in all wear stages. D. smithi lacks the parastria and only occasionally has a parastriid in later wear stages.
We have reviewed this diagnosis by examining over 200 cheek teeth of both species. No well-worn upper P4s of D. smithi have a para- stria, but the parastria is present in the earlier wear stages of all upper cheek teeth. Approximately 25% of the lower p4s of D. smithi studied have a parastriid in later wear stages, and one D. stirtoni lower p4 had no parastriid. We investigated other dental characters to see if other features were diagnostic to these species. The two are indistinguishable in tooth size, molar shape, and wear stages. The cur- rent diagnosis makes confident identification of new specimens difficult unless the specimen includes a well-worn upper P4. Complex series of tooth wear can produce a large amount of apparent dental variation in mammals with high-crowned teeth. Caution should be taken when using dental characters to assign diagnoses to such taxa.