A supernumerary distal upper molar, always expressed bilaterally, occurred in Pleistocene (one of 204) and Recent (five of 1,667) specimens of the Spermophilus richardsonii complex. Among Recent specimens with the abnormality, three of four with known provenance came from adjacent colonies with hybrids of S. richardsonii (Richardson\u27s ground squirrel) and S. elegans (Wyoming ground squirrel). Elsewhere, the trait was rare in S. richardsonii (one of 947) and absent in S. elegans (out of 668). Affected individuals exhibited normal P3-M2, but two aberrant teeth distal to M2. Aberrant teeth exhibited a mosaic of features typical of M2, M3, and atypical of any normal tooth. The trait was inferred to be hereditary
