Reply to comment on “Primates in the Eocene” by Gilbert and Maiolino (2015)

Abstract

My recent review titled BPrimates in the Eocene^ emphasised time as an essential variable in the study of primate evolution and primate phylogeny. The Eocene is important for primate evolution because this is the geological epoch when true primates (sometimes called euprimates) first appeared. The Eocene is interesting too because the earliest primates include two groups, Tarsioidea and Adapoidea, that are so similar early in their history that members of one group have often been mistaken for members of the other. Similarity in time, place, and morphology to the point of confusion is compelling—if not infallible—evidence of a close phylogenetic relationship. And still, in spite of this early linkage, some authors, such as Gilbert and Maiolino (2015), would classify Tarsioidea and Adapoidea in separate suborders, Haplorhini and Strepsirrhini respectively, in the order Primates. Maiolino et al. (2012) described the morphology of pedal distal phalanges in the early Eocene primate Notharctus tenebrosus and concluded that Notharctus had a grooming claw on the second toe, on pedal digit II. In their comment, Gilbert and Maiolino (2015) claim that pedal distal phalanges bearing grooming claws are readily separated from other ‘unguis’ forms on the basis of their facet-shaft angle (FSA), and volar feature length (VFL) as a proportion of total phalanx length (TPL). However, FSA and VFL/TPL are related to the positioning of distal phalanges in a grasping foot and by themselves indicate little about phalanx shape. As background it is important to note that Koenigswald, Habersetzer, and Gingerich (2012) described and analysed a broad range of primate distal phalanges, recognizing scutiform, falciform, piciform, and columnar distal phalanges on the basis of morphology, with scutiform phalanges interpreted as nail-bearing in prosimians, falciform phalanges interpreted as claw-bearing, piciform phalanges interpreted as bearing grooming claws, and columnar phalanges interpreted as nail-bearing in anthropoids. Our principal components analysis of pedal distal phalanx measurements indicated that Notharctus had grooming claws on digits II and III (von Koenigswald et al. 2012). Discriminant analysis of feet with and without grooming claws, analysed phalanx by phalanx, indicated that Notharctus lacked grooming claws. Discriminant analysis of feet with and without grooming claws, analysed foot by foot rather than phalanx by phalanx, indicated thatNotharctus falls in an ambiguous middle ground between primates with grooming claws and primates that lack them. Gilbert and Maiolino (2015) question my recent characterisation of the morphology of pedal distal phalanges in Notharctus as ‘ambiguous,’ arguing, as they did before (Maiolino et al. 2012), that morphology shows Notharctus to have had a grooming claw on pedal digit II. However, the strongest case for ‘ambiguity’ in the interpretation of Notharctus grooming claws is provided by Maiolino et al. (2012) in their figure 13 (reproduced here as Fig. 1). Notharctus pedal distal phalanges all have ‘SW-3/4/TPL’ values in the ungulae range, and most of these are beyond the tail of the grooming claw range. The one Notharctus ungule known to represent the pedal distal phalanx of digit II (arrows in Fig. 1) is far beyond the range of values observed for grooming claws. Gilbert and Maiolino (2015) can call the distal phalanx of pedal digit II in Notharctus a claw, but calling it a claw does not make it a claw—and it is not a claw unless it has the form of a claw in all three dimensions. A spoon looks like a fork in * Philip D. Gingerich gingeric@umich.edu