Palaeoxonodon sp.

Sigogneau-Russell, Denise, 2003, Holotherian mammals from the Forest Marble (Middle Jurassic of England), Geodiversitas 25 (3), pp. 501-537 : 508-511

Palaeoxonodon sp.

MATERIAL EXAMINED. — BMNH J.702, J.290, J.117, right lower molars; BMNH J.802, J.53, left lower molars.

BMNH J.702 ( Fig. 10A ) is a relatively large tooth with a stocky trigonid and no cingulum beyond a slight crest running lingually between the para- and metaconid. These two characters (trigonid proportions, no lingual cingulum) characterize Palaeoxonodon ooliticus ; but the paraconid is smaller than the metaconid and the latter is hardly hollowed labially as in the second species. The talonid is elongated and its “basin” occlusally a b c d e oriented. The sharp cusp f is followed by a short antero-labial cingulum more like in P. freemani n. sp. Roots are equal.

Wear is present on the anterior face of the protoconid and extends down to the anterior sulcus; facet A (paraconid) is in a slightly different plane. The facets affecting the posterior face of the protoconid and the labial face of the metaconid are nearly aligned as is often the case in P. freemani n. sp.; the metacristid is heavily eroded. No wear is clear on the talonid either labially or occlusally. Three teeth display a complete lingual cingulum, as well as a distal metacristid. J.802 ( Fig. 10C ) is a large tooth; unfortunately the three cusps are broken but the paraconid was smaller than the metaconid. The talonid is of the Palaeoxonodon type, even having two cuspules on the metacristid and an undulating entocristid. The posterior face of the metaconid is not hollowed and is situated in the same plane as the posterior face of the protoconid. Anteriorly, cusp f is again followed by a partial labial cingulum, both characters more like P. freemani n. sp. There seems to have been an A facet. Otherwise only the paraconal sulcus shows trace of wear. Most characters point to a close affinity to P. freemani n. sp.

J.290 is smaller, and very partially preserved, but the lingual cingulum is well marked and so is the distal metacristid. J.53 (a trigonid) is even smaller (size of the Palaeoxonodon species ) and “only” the paraconid is broken. The anterior cingulum ends in a point (cusp e) but cusp f remains faint; the metaconid is slightly hollowed but there is one single posterior wear facet; anterior facet A is present. The roots are fused proximally. It is tempting to consider these specimens, especially J.802 and J.290 as variants of the Palaeoxonodon type. Finally J.117 is a relatively large posterior half of a right tooth, again with a distal metacristid and the paraconal sulcus hollowing the posterolabial face of the metaconid. The paraconid is relatively high, there is no cingulum at least posteriorly, and the talonid (quite rolled) seems to have been shorter than in Palaeoxonodon , the occlusal surface not being prolonged by a distal curvature. Roots seem to have been slightly unequal.

COMMENTS

I hesitated to create a second species of Palaeoxonodon . In Guimarotodus inflatus Martin, 1999 , the lower molar lingual cingulum is present only on the posterior teeth (but under the metaconid); it is then conceivable that this was the case in Palaeoxonodon and that the two types of teeth – without a lingual cingulum or with a partial lingual cingulum – represent the variation along the jaw. All the more so that some of the other differences cited between the two species suffer exceptions within both groups of specimens, with some uncertain teeth: on BMNH M 51823 and BMNH J.220 (? P. freemani n. sp.), the paraconid is at least equal to the metaconid; on J.569, J.618 and J.715 ( P. freemani n. sp.), the anterior lingual cingulum is very faint. Another possibility is that this cingulum would be one diagnostic criterium of the genus Palaeoxonodon versus Amphitherium , so that the teeth without cingulum (including the holotype of Palaeoxonodon ) would represent in fact Amphitherium . In the Amphitherium jaw BMNH M 36822 ( Simpson 1928: fig. 8-1) ( Fig. 10 ) however, on which teeth have no cingulum, the paraconid is equal to or smaller than the metaconid, a character observed here on teeth with a cingulum. Finally, a very tempting hypothesis is that “ Palaeoxonodon ” is but a species of Amphitherium .

The evaluation of the relations between Palaeoxonodon and Amphitherium is of importance since the former has been uniformly classified as a Zatheria (e.g., McKenna & Bell 1997), while Amphitherium has been considered to be on the dryolestoid side ( Prothero 1981; McKenna & Bell 1997; Martin 1999; Butler & Clemens 2001). However, Mills (1964), Kermack et al. (1968), Crompton & Jenkins (1968), Hopson & Crompton (1969), Crompton (1971), Freeman (1979) and Sigogneau-Russell (1999) viewed the genus closer to Peramuridae .

When Freeman (1976a) created the genus Palaeoxonodon , which he attributed to? Peramuridae , he diagnosed it on the presence of a cuspule on the metacristid and on the weakly developed antero-buccal cingulum (though both characters can also be found in Peramus ). He concluded that Palaeoxonodon is intermediate between Amphitherium (one talonid cusp and no basin) and Peramus (two to three cusps and a small basin). But in 1979, when he enriched the mammalian fauna from Kirtlington, he mentioned a partial? Amphitherium left lower molar M 36516, suborder Amphitheria (including Peramuridae , Amphitheriidae and Paurodontidae ), which he distinguished from Palaeoxonodon by “its larger size, the slightly convex lingual face of the protoconid and a prominent antero-buccal cusp”.

Size cannot be considered a generic character. The six molars of the jaw of Amphitherium BMNH M 36822 ( Fig. 11 ) have a flat lingual face of the protoconid. The prominence of the antero-buccal cusp (rather an antero-buccal cingulum in fact) as a diagnostic character of Amphitherium is weakened by the condition of m1 of the same specimen, where this cuspule is barely perceptible, while it is strong in several Palaeoxonodon specimens. Indeed, before going any further, variability along the jaw in Amphitherium , already mentioned by Crompton (1971) and Prothero (1981), should be stressed. On the jaw mentioned above (M 36822), the relative length of the talonid as well as its inclination, the distinctness of the metacristid, the relative size (equal to or smaller than the metaconid) and inclination of the paraconid, the extension of the antero-buccal cingulum (very weak cuspule f on m1), the situation of the metaconid relative to the protoconid with individualization of an antero-labial metaconid facet, as well as the proportion of the roots (unequal labially on m3) vary along the dental series.

Later, Prothero (1981) stated that Palaeoxonodon shows “further development of the talonid” relative to Amphitherium , with an “incipient hypoconulid and entoconid”. The first assumption is not confirmed by measurements (see Annexe, Table 1; Figs 4-6 ). Moreover, in Palaeoxonodon , as in Peramus , the talonid looks longer than it really is, because of the very sloping posterior face of the metaconid. The second assumption was based on fig. 6, pl. 16 of Freeman (1979); the “entoconid” is an artefact of lighting for photography, which has not been confirmed by scrutiny of the original specimen. The accessory cusps were not present either on the second tooth described by Freeman himself (1979).

Sigogneau-Russell (1999) retained the generic distinction on the basis of the smaller size of Palaeoxonodon (again not a generic character), a metaconid better detached from the protoconid, hence a more accentuated posterior hollowing of the metaconid, and the less dorsally recurved talonid. Considering the metaconid situation in the same Amphitherium jaw BMNH M 36822, it appears in fact that the first feature varies, with a better detachment of this cusp in the posterior teeth. Similarly the hollowing of the metaconid is less accentuated in Palaeoxonodon freemani n. sp. than in P. ooliticus , and this character varies along the Peramus jaw BMNH M 47339 . Finally, as already mentioned, the dorsal curvature of the talonid varies along the series in Amphitherium , as well as in the teeth attributed to Palaeoxonodon , and so does its posterior extension, though it tends to be more accentuated in Amphitherium . Butler & Clemens (2001) consider that in Palaeoxonodon as in Peramus , the molars are longer and narrower than in Amphitherium and that there is no overlap of the talonid-trigonid of adjacent teeth, the former being enclosed between “the lingual basal cusp and the mesio-buccal cingulum” (p. 15). The “lingual basal cusp” (e) is in fact absent on the holotype of Palaeoxonodon and is very rarely expressed in the hypodigm; when detectable, it represents only the anterior border of the lingual cingulum and not a real cusp as in Peramus , so that the talonid interlocks between the paraconid and f as in Amphitherium . As for the extension of this overlapping over the adjacent molar, it was probably variable along the series. It is interesting to add that, in Amphitherium and Palaeoxonodon and contrary to the condition in Peramus , the distal face of the trigonid is not wider than the mesial face. Finally, my maintaining of two distinct genera is mainly based on the ignorance of critical data, including the dental formula.

There remains the question: on which side of the cladogram are amphitheriids situated? I shall discuss the matter after study of the upper molars.

Figures

FIG. 10. — Lower molars; A, Palaeoxonodon sp., BMNH J.702, × 22; B,?Kennetheredium sp., BMNH J.515, × 20; C, Palaeoxonodon sp., BMNH J.802, × 25; a, labial view; b, lingual view; c, posterior view; d, anterior view; e, occlusal view.

FIG. 11. — SEM photographs of Amphitherium rixoni Butler & Clemens, 2001, BMNH M 36822; A, labial view of the molar series; B, lingual view of the same; C, occlusal view of the same; D, occlusal view of M1, M2, trigonid of M3. Scale bars: 1 mm.

FIG. 4. — Diagram of the width (W) versus length (L) of the lower molars of Palaeoxonodon Freeman, 1976b. Measurements units in 1/100 of mm.

FIG. 5. — Diagram of the length of the talonid (T1) versus total antero-posterior length (L) of the lower molars of Palaeoxonodon Freeman, 1976b. Measurements units in 1/100 of mm.

FIG. 6. — Diagram of the length of the talonid (T2) versus trigonid width (W) of the lower molars of Palaeoxonodon Freeman, 1976b. The use of tooth width instead of length is aimed at reducing the effect of variation in the number of molars present in the jaw (suggestion P. M. Butler). Measurements units in 1/100 of mm.