Patterns of Dental Evolution in Early Eocene Anaptomorphine Primates (Omomyidae) from the Bighorn Basin, Wyoming

Abstract

The subfamily Anaptomorphinae contains the oldest and most generalized members of the tarsier-like primates and is the basal group of the extinct family Omomyidae. The best and most continuous record of anaptomorphine history is from rocks of early Eocene (Wasatchian) age in the Bighorn Basin of northwest Wyoming where eight genera and 14 species are recognized. Three of these species are new (Teilhardina crassidens, Tetonius matthewi, Absarokius metoecus), and four other new species are described from elsewhere (Tetonius mckennai, Absarokius gazini, A. australis, Strigorhysis huerfanensis). Teilhardina tenuiculaandAbsarokius noceraiare new combined forms.Absarokius noctivagusis considered to be a synonym ofA. abbotti, andMckennamorphusis a synonym ofPseudotetonius.The evolution of dental characters in three principal morphologic clades of anaptomorphines from the Bighorn Basin is documented with the aid of numerous new specimens (75% of the sample is new) and with precise stratigraphic data. These major clades areTeilhardina–Anemorhysis, Tetonius–Pseudotetonius, andAbsarokius. In each of these clades, evolution appears to have occurred gradually. In the first two clades it was mainly anagenetic, although each one included a minor branching event. InAbsarokius, evolution was instead characterized by cladogenesis followed by continued (and continual) anagenetic change in each of the new lines. Anagenetic gradual evolution produced the new genusPseudotetonius(fromTetonius) and possiblyAnemorhysis(fromTeilhardina). Similarly, theAbsarokius metoecuslineage probably gave rise to late Wasatchian–early BridgerianStrigorhysis. Evolution fromTetoniustoPseudotetoniushas been clarified by establishment of five arbitrary stages of evolution (Tetonius–Pseudotetoniusintermediates). Estimates of relative proportions of time represented by paleosols in different parts of the Willwood section suggest that cladogenetic speciation inAbsarokiuswas almost certainly more rapid than anagenesis inTetonius–Pseudotetonius.Anagenetic character evolution and speciation in the anaptomorphine primates was typified first by increase, then decrease in variability, which resulted in measurable apomorphic morphologic change. Cumulation of changes of this sort created more extensive differences of importance at the species and genus levels. Introduction of changing character states and their tempo was staggered temporally, and new characters (and new taxa) are least separable from their antecedent states when they first appear. This evidence is in sharp contrast to predictions of the punctuated equilibria model of evolution. Because the emergence of diagnostic characters occurs gradually (in evolutionary terms) and not all at once (in temporal terms), and because diagnostic characters are the essence of the diagnosis (and thereby taxonomy), the implications of gradual evolution for both systematic paleontology and biostratigraphy are profound. Stasis exists in the evolution of individual characters over certain periods, but this study offers no evidence supporting either organismic stasis or even stasis in the dental evolution of the Anaptomorphinae over a period of about 4.8 million years.