Pliocene Pleistocene Recent ========================================================================== +--F-----------------------------------F (_ferox_) | | +---------Mm (_muticus muticus_) | +--M------------------+ | | +---------Mc (_muticus calvatus_) | | FMSA-+ | +--Ss (_spinifer spinifer_) | | +------+ | | | +--Sh (_spinifer hartwegi_) | | +--Ssha-+ +--MSA-+ | | | | +---------Sa (_spinifer asper_) | | | +--S---+ +--Sp (_spinifer pallidus_) | | | +--Spg-+ | | | | +--Sg (_spinifer guadalupensis_) +--SA--+ +--Sepg-+ | | | +--Se-----Se (_spinifer emoryi_) | +--A---------------------A (_ater_) -------------------------------------------------------------------------- An arid environment in the central and southern United States and northern Mexico may have increased in area especially southward from Miocene times into the Pliocene (Dorf, 1959:189, 191). The combination of physiographic changes and aridity, which modified the mesic, essentially continuous, aquatic habitats, may have isolated and aided in the differentiation of the _ferox_, _muticus_ and _spinifer_ stocks. Encroachment of the Eocene seas, the maximal extent of which corresponded to the Gulf Coastal Plain and included a northerly extension as far as Cairo in southern Illinois (Mississippi embayment), possibly was an initial barrier isolating the _ferox_ stock of the east. In the late Miocene or early Pliocene, the MSA (_muticus-spinifer-ater_) stock presumably occupied a large region of the central United States, which extended southward into northern México and along the Gulf Coast at least as far as Alabama. Farther eastward, the _ferox_ stock was isolated in more mesic, probably swampy, marshy habitats.