Alterodon, MAJOR

MacPhee, R. D. E. & Flemming, Clare, 2003, A Possible Heptaxodontine and Other Caviidan Rodents from the Quaternary of Jamaica, American Museum Novitates 3422, pp. 1-43 : 8-11

STATUS OF ALTERODON MAJOR

The dubious notion that Jamaica supported an endemic octodontid during the late Quaternary may be traced to Anthony’s (1920) exceedingly brief description of the holotype of Alterodon major , a species based on a single, apparently hypsodont cheektooth (AMNHP 17638) from Wallingford Roadside Cave (Manchester Parish) in central Jamaica (figs. 3, 4; see MacPhee [1984] for description and dating of faunule). In light of the new fossils described elsewhere in this paper, it is appropriate to briefly reconsider the status of Alterodon , with special reference to Spencer’s (1987) arguments in favor of an octodontid affiliation for this genus.

Anthony (1920) was impressed by the fact that the holotype of Alterodon bore, in his view, a certain resemblance to the cheekteeth of Octodontidae , which are characteristically bilobate or dumbbell­shaped in cross­section. However, Anthony’s remarks amount to no more than the mere recognition of vague similarity: in his only other published remarks on the matter, Anthony (1926: 206) simply noted that Alterodon may have unspecified ‘‘affinities to Octodontidae ?’’.

Reference to Octodontidae was enough to get Alterodon into the biogeographical literature as an insular member of that family (e.g., Darlington, 1957), but not enough to get anyone interested in re­examining the holotype in print until MacPhee et al. (1983) did so. These authors concluded that Anthony’s interpretation was fundamentally flawed, because the holotype tooth of Alterodon is incomplete: AMNHP 17638 as preserved (fig. 3) has a fracture surface on the cementum lining its apparent distal aspect, which in our view is secure evidence that the tooth originally possessed at least one more lamella. This would make it unlike any accepted octodontid but suggestively like Clidomys , abundant remains of which occur in Wallingford Roadside Cave ( MacPhee, 1984). However, as the partial tooth pattern of the Alterodon holotype did not precisely match that of any teeth assigned to Clidomys (cf. fig. 4), MacPhee et al. (1983) concluded that Alterodon might represent a different but closely related species of clidomyine. In a subsequent revision, MacPhee (1984) reconsidered this point and placed Alterodon major in synonymy with C. osborni , arguing that despite its shape the former was just another dental variant of the latter.

Spencer (1987) disagreed with each of these actions, arguing that MacPhee et al. (1983) misinterpreted the amount of damage to the Alterodon holotype. Far from being extensively damaged, he argued, the tooth has lost nothing but the greater part of a thick outer covering of cementum. That is, it is nearly complete as preserved, and therefore cannot have been multilamellar. Spencer (1987: 108–109) went on to note that thick cementum sheathing is characteristic of the cheekteeth of the capromyid Isolobodon and ‘‘many octodontids’’, and that therefore ‘‘ Alterodon major could as easily be a capromyid like Isolobodon ’’ or ‘‘a remnant octodontid as maintained by Anthony.’’

Although there have been several new Clidomys discoveries in the past few years (e.g., McFarlane et al., 1998; Morgan and Wilkins, 2003), investigators have not turned up any more teeth with an Alterodon ­like morphology. Spencer’s (1987) argument must therefore be analyzed on the basis of the existing fossil evidence. While it is true that cementum sheaths covering the cheekteeth of extant octodontids and their unquestioned relatives vary in thickness, none displays a substantial local thickening of the sort seen in the Alterodon holotype —situated exactly where the interlamellar plaque of cementum ought to be if the tooth belonged to a clidomyine, as MacPhee et al. (1983) originally contended. There is an additional aspect to this. On re­examining the few intact isolated cheekteeth of Clidomys in the AMNHP collection, we noticed that, disregarding the interlamellar plaques, the cementum covering is actually very thin or absent on all surfaces. Indeed, on most teeth in the collection cementum is not present at all on free surfaces, apparently because of overly vigorous preparation on Anthony’s part. Nevertheless, it is evident that the cementum covering on the Alterodon holotype could never have been of substantial, even thickness. On this analysis the type of Alterodon actually looks very much like other teeth in the Clidomys hypodigm, and we see no reason to infer that it once had a much thicker circumferential coating of cementum.

The other feature that Spencer (1987) em­ phasized is tooth pattern. Numerous examples of ‘‘unusual’’ occlusal­surface patterns in isolated cheekteeth from the Clidomys type site show that clidomyines were dentally variable (fig. 4A–J; MacPhee, 1984), a point which Morgan and Wilkins (2003) have additionally documented. To this we add the further observation that in Clidomys cheekteeth the enamel sheath of the mesialmost or distalmost lamella is occasionally interrupted or much thinner on one side than the other. In the holotype of Alterodon there are two interruptions that define a tiny splint of enamel (fig. 3B). One’s initial impression is that the interruptions are simply artifacts, but they lack fracture surfaces and can be traced continuously to the root end, where it is clear that they are developmental in origin. Such a minor, variable feature does not show beyond doubt that Alterodon is a clidomyine, but it is one more way in which its holotype fits within accepted variation in that group. How this variability ought to be reflected at lower taxonomic levels is possibly still an issue in need of resolution (see table 1), but at present there is no warrant to infer the presence of a family otherwise unrepresented in the Jamaican fossil record.