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 The latter is then superseded by
Microsyops 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 between
A. nuptus
and
M. angustidens
, located stratigraphically as the latter is appearing. Because specimens of
A. nuptus
have been recovered approximately 60 m above the appearance of
M. angustidens
, it is clear that
A. nuptus
did not suffer pseudoextinction. Instead, evidence suggests that
M. angustidens
branched off from a population of
A. 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.