Chapter 2 Fossil History , Zoogeography and Taxonomy of the Pitheciids

Introduction An important shift in thinking has become cause for renewed scrutiny concerning the course of platyrrhine evolution and the shape of New World monkey (NWM) classification. For the first time in nearly 200 years, Aotus is being moved across a major taxonomic divide. It is being considered as a genus aligned with marmosets and tamarins rather than titi monkeys and saki–uacaris and, more generally, atelines. The emergence of this debate reflects the impact of molecular cladistics since the early 1990s. The conversation has turned from a prolonged controversy (e.g. Rosenberger 1981, 2002) over marmosets and tamarins, a dispute that was fundamental to modernizing our views of NWM evolution and appears to be resolved for the moment. Now, beginning with the successful molecular cladistic analysis of Schneider et al. (1993), the deliberation is over Aotus: is it a pitheciine, an atelid, a stem platyrrhine or a cebid? The prevailing opinions that warrant close examination are the last and the first – Aotus is either a cebid or a pitheciine. However, the crux of the matter is that the hypothesis of Aotus as a “cebid” is based almost entirely on genes; the idea that it is a pitheciine is based entirely on morphology. While some morphologists align Aotus more closely with the molecular trees (e.g. Kay 1990; Horovitz 1999; see also Meldrum & Kay 1997; Kay et al. 1998a), we believe this assessment does not adequately account for anatomical evidence bearing on Aotus, Callicebus, Pithecia, Chiropotes and Cacajao, no less their fossil relatives (Rosenberger 2002). How is the Aotus matter different? Aotus has rested comfortably near Callicebus in morpho-space ever since higher level classifications of the platyrrhines were developed in the early 1800s (Rosenberger 1981). There was not a hint that Aotus could be related to anything but a pitheciine or ateline until the 1990s (see Tejedor 2001). As a consequence, morphologists challenged by the molecular evidence regarding Aotus have no fallback position from which our information can be reinterpreted. The molecules conflict with the morphology rather directly. The discord goes beyond that: the molecules clash with ecology and behavior. Aotus and Callicebus are bound together by a unique combination of attributes: social monogamy, biparental care with extensive input by males, no sibling care, long call advertising, territoriality, locomotion and feeding (e.g. Robinson et al. 1987; Wright 1996; FernandezDuque 2007; Norconk 2011). One is hard-pressed to find any two genera of modern NWMmore alike than Aotus and Callicebus, except for the obvious dyads that split cladistically relatively recently, and only arguably into distinct genera – Callithrix and Cebuella, and Cacajao and Chiropotes. Given the narrow scope of this chapter, a complete analysis of the problem is impossible. For one, it would require a full explication of the fossil record pertaining to Aotus, Callicebus and the saki–uacaris. Instead, to introduce these taxa in condensed form, and to clarify our use of taxonomic terms, we present a classification of pitheciines (Table 2.1), extending the scheme of Rosenberger et al. (1990). In an effort to summarize our assessment, we also advocate a stance rather than illuminate the conjectures and refutations: Aotus is a pitheciine, not a cebid. Overall, our aim is to present a synopsis of three aspects of the problem that must be accounted for in order to unravel the Aotus puzzle: (1) the morphological evidence linking Aotus, Callicebus and saki–uacaris cladistically; (2) a critical assessment of the molecular evidence; and (3) a synthesis of the evolution of pitheciine feeding adaptations which we further promote as heuristic evidence that Aotus is, in fact, pitheciine – phylogeny and adaptation are two sides of the same evolutionary coin, to paraphrase Fred Szalay.