The intraspecific variation documented here for Sigmodon curtisi has important implications for the taxonomic identification of fossil cotton rats. Multiple characters previously considered diagnostic are here shown to vary during the age of the individual. None of the observations presented here call into question the validity of any species of Sigmodon, but do suggest a couple categories of individuals that may be difficult to identify taxonomically to species based on published descriptions:
• mature individuals of S. curtisi and small individuals of S. leucotis,
• young individuals of
• S. curtisi and young individuals of S. hudspethensis, and
• mature individuals of
• S. curtisi and mature individuals of S. lindsayi.
Hopefully this study soon will be supplemented by similar work documenting the ontogenetic variation in other fossil cotton rats. Such studies could provide details to reject my concerns, or could help establish limits to our ability to identify species of various dental wear stages.