A relatively up-to-date assessment of ceboid lower-level systematics (1) divides the present-day ceboid fauna into 16 genera, 76 species, and 123 subspecies. Not only do the ceboids or Neotropical primates (i.e. New World monkeys, including marmosets and tamarins) constitute in their number of genera one of the larger assemblages of placental mammals in South America (surpassed only by rodents, bats, and carnivores) (2) but among eutherian or placental mammals, the ceboids have one of the oldest histories in South America with 26-27 million year old fossils of the primitive ceboid Branisella (3) documenting that ceboids were already established in South America during the Oligocene. Among extant eutherians, only caviomorph rodents and edentates have older documented histories in South America, the former now being represented by a 34 million year old Eocene fossil (4,5) and the latter by early Cenzoic Paleocene fossils (6). Possibly, island-hopping across the Atlantic on floating vegetation brought the founder populations of both caviomorph rodents and platyrrhine primates from Africa to South America in the late Eocene (4,5). In order to explore the long phylogenetic history of platyrrhine primates in the Neotropics, we are determining the cladistic relationships that exist among the extant ceboid genera. Such knowledge on the temporal order of ancestral branchings separating these ceboid genera from one another is needed not only to solve problems of higher-level ceboid systematics but also to be able to place the problems of lower-level ceboid systematics in their broader phylogenetic context.
Traditional classifications employed for the Neotropical ceboid primates usually divide them into two families: Cebidae and Callitrichidae (7-9). Cebidae contains all the ceboids except the small-bodied clawed marmosets and tamarins, which are placed in Callitrichidae. Callimico (Goeldi's monkey) is placed in Cebidae because, despite its clawed fingers and small body, Goeldi's monkey has other morphological features that appear more like those of larger-bodied monkeys than those of marmosets and tamarins. A modified version of this traditional classification has three families: Cebidae (with Callimico removed from it), Callimiconidae (restricted to Callimico), and Callitrichidae (10). As these classifications do not attempt to portray strictly monophyletic groups, they do not provide a basis for depicting the phylogenetic relationships which exist among the genera of Neotropical primates.
More recently, on the basis of cladistic analyses of morphological characters, Rosenberger (11), Ford (12), Rosenberger et al. (13), and Kay (14) have agreed that Callimico is closer phylogenetically to marmosets and tamarins than to any remaining ceboids. The classification proposed by Rosenberger et al. (13) for the platyrrhines, in which each taxon is presumed to be a monophyletic group, places Callimico together with the four genera of marmosets and tamarins in the subfamily Callitrichinae. Also the traditional cebid-callitrichid dichotomy is radically altered: Cebinae, consisting of Cebus (capuchin monkeys) and Saimiri (squirrel monkeys), is the sister of Callitrichinae and these two subfamilies constitute the family Cebidae. The remaining monkeys constitute the family Atelidae (13). However, the cladograms of Ford (12) and Kay (14), if translated into taxonomic classifications, would yield arrangements for ceboid families that differ from each other and from Rosenberger's.
The current views of ceboid systematics are based primarily on morphological studies (e.g. 1, 12-15). These views divide the extant ceboids into 16 genera, although some authors (11, 16) have suggested that the pygmy marmoset Cebuella could be included in the marmoset genus Callithrix. There is general agreement that the various ceboid genera form seven monophyletic groups or clades that separated from one another by early to middle Miocene times, i.e. between 20-14 million years ago (MYA) (12-15,17). Using the classification of Rosenberger et al. (13), we can designate each of the seven clades by the lowest ranking monophyletic taxon that contains all extant members of the clade. One clade, the Callitrichinae, groups together Cebuella, Callithrix, Leontopithecus (lion tamarins), Saguinus (tamarins), and Callimico. Another, the Atelinae, groups together the largest-bodied, prehensile tailed monkeys, consisting of Ateles (spider monkey), Lagothrix (woolly monkeys), Brachyteles (woolly spider monkeys), and Alouatta (howler monkeys). A third clade, the Pitheciini, groups together the specialized seed predators consisting of Pithecia (saki monkeys), Chiropotes (bearded saki monkeys) and Cacajao (uakari monkeys). The remaining four clades consist of single genera: Cebus, Saimiri, Aotus (night monkeys), and Callicebus (titi monkeys). The morphological studies agree in depicting Callimico as the sister group to all other callitrichines, in having a very close relationship between Callithrix and Cebuella within the callitrichine clade, and also between Chiropotes and Cacajao within the pitheciin clade. However, the morphological studies disagree on which two of the four ateline genera are most closely related and on the respective positions of Saguinus and Leontopithecus within the callitrichine clade.
The morphological studies also disagree on the order of cladistic relationship among the seven clades. For instance, Rosenberger et al. (13) place the pitheciins relatively near the atelines (Fig. 1A), Ford (12) places the pitheciins next to the atelines (Fig. 1B), but Kay (14) has Aotus, Saimiri, callitrichines, and atelines all closer to one another than to the pitheciins (Fig. 1C). Rosenberger et al. (13) place cebines (Cebus and Saimiri) with the callitrichines, as already noted, and place an Aotus-Callicebus clade as the sister of the pitheciin clade (Fig. 1A). Ford (12) proposes two different schemes: one has a Cebus-Saimiri clade as the sister of all other extant ceboids, and the second has only Cebus as the sister of all other extant ceboids with Saimiri as the sister of an Aotus-Callicebus clade (Fig. 1B). Both of Ford's schemes have Callitrichinae as the sister of the pitheciin-ateline clade. Kay (14) separates at the base of the tree first Callicebus, next Cebus, and then the pitheciin clade from remaining ceboids, has Saimiri as sister of the callitrichine clade, and has a trichotomous separation of Aotus, the ateline clade, and the Saimiri-callitrichine clade (Fig. 1C).
Fig. 1. Branching patterns of the seven ceboid clades as depicted by A) Rosenberger et al. (13); B) Ford (12); C) Kay (14); and D) DNA sequence evidence from two nuclear genetic loci, the epsilon-globin gene locus and the interstitial retinoid-binding protein gene (IRBP) locus.
We have confirmed with DNA sequence evidence from two nuclear genes that the 16 extant ceboid genera form the seven clades found in morphological studies (Fig. 1D). The datasets providing this DNA evidence consist of aligned 1.8-4 kb long epsilon-globin locus sequences (18-20) and aligned 1.8 kb long intron 1 sequences from the interstitial retinol-binding protein gene (IRBP) (20,21). In humans, the epsilon gene is part of the beta-globin gene cluster, located on chromosome 11 (22); IRBP is a single copy gene with four exons and three introns and is located on chromosome 10 (23). Presumably these two genes are unlinked in ceboids as in humans, but their chromosomal locations in ceboids have not yet been determined.
Fig. 2. Branching patterns of ceboid species obtained from a dataset consisting of the epsilon-globin locus. Figures at the top of the branches are bootstrap percent values of the parsimony method and figures at the bottom of the branches are parsimony strength of grouping (Bremer Decay Index) values.
Fig. 3. Branching patterns of ceboid species obtained from a dataset consisting of the interstitial retinoid-binding protein gene (IRBP) locus. Figures at the top of the branches are bootstrap percent values of the parsimony method and figures at the bottom of the branches are parsimony strength of grouping (Bremer Decay Index) values.
The cladistic results obtained with these epsilon and IRBP nucleotide sequences (Figs. 2 and 3) agree with those cladistic arrangements that the different morphological studies agree upon, except the epsilon and IRBP evidence has neither confirmed nor strongly opposed the morphological evidence that Callimico is sister to all other callitrichines. Our epsilon and IRBP results on the temporal ancestral branching order among the 16 extant ceboid genera are consistent (18,24) with paleontological evidence (12-15,17) that places the time of the radiation from which emerged the 7 ceboid stem-lineages to the 16 extant genera at about 20 to 14 MYA. As depicted in the provisional cladistic classification shown in Table 1, our epsilon and IRBP results have provided congruent parsimony evidence for grouping Aotus, Saimiri, Cebus, and the callitichines into a cebid clade, for having Callicebus as the sister of pitheciins, and having Alouatta as the sister of atelins (Ateles, Lagothrix, Brachyteles). Moreover, Lagothrix and Brachyteles form a clade within the atelin clade, and Saimiri and Cebus form a clade within the cebid clade. Each clade supported congruently by the separate epsilon and IRBP datasets was more strongly supported (as judged by bootstrap and strength of grouping values) by tandemly combined epsilon and IRBP sequences (Fig. 4).
Fig. 4. Branching patterns of ceboid species obtained from a dataset consisting of sequence from two nuclear genetic loci, the epsilon-globin gene locus and the interstitial retinoid-binding protein gene (IRBP) locus. Figures at the top of the branches are bootstrap percent values of the parsimony method and figures at the bottom of the branches are parsimony strength of grouping (Bremer Decay Index) values.
We plan to gather extensive additional DNA sequence data in order to test the congruent branching arrangements of the epsilon and IRBP maximum parsimony trees, especially those arrangements that are controversial when compared to the cladistic evidence from previous morphological studies (11-14). Thus we plan to further test the cladistic validity of proposed taxonomic groupings (20,21,24) that reflect congruent epsilon and IRBP results (but not all morphological results) such as a subfamily Cebinae (Cebus, Saimiri), a family Cebidae (Cebinae, Aotus, Callitrichinae), a subfamily Pitheciinae (Callicebus, Pitheciini), a subtribe Brachytelina (Brachyteles, Lagothrix), and a tribe Atelini (Ateles, Brachytelina). Also, to resolve the few discordancies between the epsilon and IRBP results, our most conserted efforts will focus on establishing a) the exact branching pattern of Cebinae, Aotus, and Callitrichinae within Cebidae, b) the precise positions of Callimico, Saguinus, and Leontopithecus within Callitrichinae, and c) the phylogenetic position of pitheciines with respect to atelines and cebids. To pursue these aims, it will be important to enlarge the DNA sequence data from additional ceboid species and outgroup species (from a broader range of non-ceboid primates) and from noncoding regions (introns) of additional unlinked nuclear genes.