Skull Characters and Conclusion

The study of paromomyid skulls underscores the difficulty of interpreting partial and distorted specimens. Studying a crushed skull of Phenacolemur jepseni showing parts of the middle ear, Szalay (1972) inferred the presence of a petrosal bulla, in agreement with his hypothesis of a petrosal bulla in Plesiadapis (Szalay, 1969). He also tentatively suggested that the base of a ridge on the promontorium continuous with a longitudinal septum could house a bony canal for a promontory artery (Szalay, 1972). The basicranial morphology of Phenacolemur was subsequently reconstructed with a promontory canal (Szalay, 1975; Szalay and Delson, 1979). A petrosal bulla and an osseous promontory canal would have been primate-like characters. However, on another fragmentary skull of the closely related Ignacius, it was shown that the "canal" for the promontory artery was in fact imperforate, and it was suggested that Ignacius had an ascending pharyngeal artery entering the brain cavity through a middle lacerate foramen (MacPhee et al., 1983).

However, much better preserved specimens later recovered from calcareous nodules and extracted by acid-attack allowed progressively better interpretations (Bloch and Silcox, 2001; Kay et al., 1990, 1992). Study of these new beautiful fossils revealed that the bulla of Ignacius is not petrosal, but made by an independently derived entotympanic bone (Kay et al., 1992). This contrasts with dermopterans, which have an ectotympanic bulla (Hunt and Korth, 1980; Wible and Martin, 1993). On their specimen, Kay et al. (1990, 1992) could see only a crest below the ectotympanic, which they interpreted as the crista tympanica. They concluded that Ignacius had a tympanic ring fused with the bulla, a morphology that might have been a shared derived similarity between Ignacius and dermopterans (absence of annular bridge, Beard and MacPhee, 1994). However, one of the partial skulls described by Bloch and Silcox (2001) clearly shows part of an ectotympanic ring: the crista seen on the other specimen had to be the remnant of an annular bridge (Bloch and Silcox, 2001). With a ringlike ectotympanic suspended by an annular bridge, isolating an epitympanic recess, Ignacius appeared similar to Plesiadapis, possibly primate-like, and very distinct from dermopterans.

The internal carotid artery (ICA) provided provisional support for a paro-momyid-dermopteran link. Szalay (1972) had guessed that a large part of the blood supply to the brain would be carried through the vertebral arteries in Phenacolemur and Plesiadapis as in other primitive mammals. However, subsequent analyses of arterial circulation in mammals have shown that large promontory and stapedial arteries were probably primitive in eutherian mammals (Wible, 1987). When a carotid foramen and canal were found in Ignacius, they were so small that Kay et al. (1992) interpreted them as carrying only nerves, as in lorisids. The complete loss of the ICA could then be a derived similarity shared with the similar loss in dermopterans. This gave some support for a plesiadapiform-dermopteran link (non-microsyopoid plesiadapiforms, Kay et al., 1992). However, study of better-preserved specimens led Bloch and Silcox (2001) to conclude that Ignacius still preserved a small promontory artery. More importantly, the lateral course of the internal carotid artery and nerves in Ignacius was different from the medial course of these nerves in dermopterans, strongly suggesting that the reduction of the ICA in the two groups was convergent (Bloch and Silcox, 2001). Wible and Martin (1993) had pointed out that a partial involution of the internal carotid system is not unusual in eutherians. Bloch and Silcox conclude the most complete study of paromomyid skulls done until now with: "there remain no unequivocal cranial synapomor-phies linking paromomyids and dermopterans to the exclusion of other archon-tans" (Bloch and Silcox, 2001). Their arguments appear quite convincing.

The absence of finger gliding in paromomyids destroys the most compelling evidence put forward by Beard (1990, 1993a,b) in favor of paromomyids being dermopterans. In view of the quasi-impossible dental morphological transformation implied by this hypothesis, and the absence of any significant cranial character supporting it (Bloch and Silcox, 2001), the primatomorph hypothesis should be abandoned. The primatomorph hypothesis is not definitively refuted, and the problem of dermopteran origins is not solved. An origin within the radiation of archontan claw climbers remains likely. However, the paro-momyid connection is eliminated, and we do not know either the living sister group of Dermoptera, nor the fossil sister group of Galeopithecidae, which could be Plagiomenidae (which have a very peculiar basicranium; MacPhee et al., 1989), Mixodectidae, or yet another unknown group. Under these conditions, the concept of Primatomorpha should be abandoned until a better connection suggests dermopterans to be the living sister taxon of primates.

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