The phylogenetic position of Morotopithecus
Introduction
Hominoid fossils from two early Miocene localities (>20.6 Ma) near the Moroto volcano in the Karamoja District, Uganda have been assigned to a new genus and species of Miocene hominoid, Morotopithecus bishopi (Gebo et al., 1997). This new designation is based on a combination of previously known craniodental and vertebral specimens with new femoral and scapular material (Fig. 1a–d). The craniodental features of Morotopithecus are generally primitive whereas some of the postcrania exhibit features associated with orthogrady and suspension that resemble thecondition found in extant apes (Sanders & Bodenbender, 1994; Gebo et al., 1997; MacLatchy & Pilbeam, 1999; MacLatchy et al., 2000).
Previous phylogenetic analysis of Morotopithecus craniodental material has placed the taxon near the base of the hominoid radiation, sister to all living hominoids (Begun & Güleç, 1998). This interpretation is consistent with the dating of Morotopithecus fossil material and the divergence time of the hominoids as inferred from multiple genetic datasets (Caccone & Powell, 1989; Sibley et al., 1990; Kumar & Hedges, 1998). However, the newly described postcranial specimens have never been included in any formal phylogenetic analysis. Given the importance of postcrania to alternative hypotheses of hominoid evolution and the relative scarcity of diagnostic postcranial material in the ape fossil record, the addition of these data to a phylogenetic analysis could yield interesting insights.
For example, the phylogenetic placement of Morotopithecus based on currently available data depends heavily on whether its postcranial similarities to living hominoids are homologous or homoplastic. If the similarities are synapomorphic with living hominoids, then younger Miocene hominoids (e.g., Sivapithecus, Proconsul, Afropithecus and Kenyapithecus) that lack such features either “re-evolved” a more pronograde quadrupedalism, or are more distantly related to extant hominoids than is Morotopithecus. If the similarites are homoplastic, it would bolster claims that hominoid postcranial similarities are convergent (e.g., Larson 1998). A phylogenetic analysis including all of the Morotopithecus material may therefore illuminate not only the relationship of this taxon to other living and fossil apes but also may help to discriminate between these alternative hypotheses of ape evolution.
In addition, the analysis of Morotopithecus can serve to illustrate how character and taxa selection may affect the robusticity of results derived from these data. Debate about Miocene ape phylogenetics has produced a number of competing hypotheses concerning the specific affinities of fossil apes to living apes (Andrews & Martin, 1987; Schwartz, 1990; Andrews, 1992; Begun, 1992; Moyà-Solà & Köhler, 1995; Begun et al., 1997; Cameron, 1997; Harrison & Rook, 1997; Begun & Güleç, 1998; Andrews & Bernor, 1999). These differences are partially due to disagreements about how anatomy should be described as characters and which taxa are included in analyses.
In this paper we address the following issues:
- 1.
Using previously published character sets, what is the phylogenetic position of Morotopithecus?
- 2.
How does adding Morotopithecus to published hominoid morphological datasets affect previously inferred phylogenies?
- 3.
What effect do missing characters or different character partitions have on our interpretation of Miocene hominoid relationships?
- 4.
What effect, if any, does the analysis of Morotopithecus have on assessing the likelihood of hominoid postcranial parallelism?
Section snippets
Data
In order to sample the most variation in character description, we chose to derive our character list from three published sources (Begun et al., 1997; Moyà-Solà & Köhler, 1995; Cameron, 1997), and from our own analysis of the Morotopithecus postcranium. However, these three datasets often differ in their description of the same anatomical regions. In order to avoid any prejudgement of which character description is “better”, we created three different datasets (described in detail below) to
Analyses
Parsimony analyses of these datasets were performed in PAUP∗ (Swofford, 2002) using a heuristic search method of 1000 random replicates, and resolution of ambiguous nodes via ACCTRAN (Farris, 1970). All most-parsimonious trees were saved. If more than one most-parsimonious tree was found, a 50% majority-rule consensus tree (MRCT) was computed. Bootstrap estimates (heuristic search, 1000 random replicates) were performed for datasets 1a, 2a, 3a, and 4 (Felsenstein, 1985). Character state
Results
Analysis of dataset 1a recovered nine most-parsimonious trees (MPTs), and analyses of datasets 2a and 3a recovered one MPT each (Fig. 3b, Fig. 4b, Fig. 5b). All analyses consistently placed Morotopithecus as a sister taxon to the extant great apes, with Hylobates sister to this clade. To make Morotopithecus a sister taxon to all living hominoids requires eleven additional steps using datasets 1a and 3a, and fourteen additional steps using dataset 2a. Morotopithecus was also consistently more
Discussion
The phylogenetic positioning of Morotopithecus in all of the analyses suggests it is a primitive member of the great ape clade, and not a more primitive sister taxon of the crown hominoid clade as previously suggested (Gebo et al., 1997;MacLatchy et al., 2000). However, Morotopithecus is dated at >20.6 Ma, while inferences derived from molecularly-derived trees and calibrated using fossil dates indicate later dates for the branching of gibbons from the other hominoids: 14.3 Ma (Kumar & Hedges,
Conclusions
Phylogenetic analyses of new and previously discovered material attributed to Morotopithecus suggest that it is a primitive member of the great ape clade. This finding is consistent even with the use of alternate character descriptions, with postcranial characters only, or with characters found in Morotopithecus only. However, this finding is inconsistent with the dating of Morotopithecus at >20.6 Ma, and the molecular estimates of the divergence of hylobatids at 18 Ma or less. Thus, a
Acknowledgements
The authors would like to thank D. Pilbeam, D. Lieberman, M. Ruvolo, D. Begun, and two annonymous reviewers for providing useful comments on this manuscript. D. Begun deserves special thanks for detailed clarification of some of the characters used in our analysis. We thank the Uganda National Council for Science and Technology and the Commissioner of Antiquities, Uganda National Museum, for permission to conduct research in Uganda. We also thank the staff of the Department of Zoology, Makerere
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