This character is of developmental interest, especially given the ontologi-cal significance, relative to the adult condition. The example provided by Raff (1996) will suffice to establish to some degree the likely significance of these two deciduous dental characters. He reminds us that early in the 19th century, ascidians (marine invertebrates that as adults are saclike filter feeders) were classified as molluscs, but they were later shown to have larvae unlike any mollusc. Their larvae would resemble tiny tadpoles, complete with dorsal chord and notochord. These larvae are built along the same overall body plan as vertebrates and as such were reclassified. While perhaps an extreme example relative to the deciduous dentition, it does, however, highlight the need to examine processes that can be partly identified by examining stages of ontogeny.
The character state definitions and taxa allocation used here have been taken directly from Strait et al. (1997) and Strait and Grine (2001). Three states are recognized. The primitive condition is defined by the absence of a mesial marginal ridge (MMR), the protoconid is situated well mesial, and the anterior fovea has a wide lingual opening (= 0), as observed in Ardipithecus, Gorilla, and Pan. The next condition is defined by the primitive condition, but with the MMR being slightly developed (= 1), as seen in the "anamensis group," Praeanthropus, Australopithecus, H. ergaster, and H. sapiens. Finally, the MMR is strongly developed, the protoconid is aligned with the metaconid, and the fovea is closed (= 2), as observed in P. robustus and P. boisei.
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