Cladogenesis and replacement in the fossil record of Microsyopidae (?Primates) from the southern Bighorn Basin, Wyoming

The early Eocene of the southern Bighorn Basin, Wyoming, is notable for its nearly continuous record of mammalian fossils. Microsyopinae (?Primates) is one of several lineages that shows evidence of evolutionary change associated with an interval referred to as Biohorizon A.Arctodontomys wilsoni is replaced by a larger species,Arctodontomys nuptus , during the biohorizon interval in what is likely an immigration/emigration or immigration/local extinction event. The latter is then superseded byMicrosyops angustidens after the end of the Biohorizon A interval. Although this pattern has been understood for some time, denser sampling has led to the identification of a specimen intermediate in morphology betweenA. nuptus andM. angustidens , located stratigraphically as the latter is appearing. Because specimens ofA. nuptus have been recovered approximately 60 m above the appearance ofM. angustidens , it is clear thatA. nuptus did not suffer pseudoextinction. Instead, evidence suggests thatM. angustidens branched off from a population ofA. nuptus , but the latter species persisted. This represents possible evidence of cladogenesis, which has rarely been directly documented in the fossil record. The improved understanding of both evolutionary transitions with better sampling highlights the problem of interpreting gaps in the fossil record as punctuations.