Peramuridae, Kretzoi, 1946

Sigogneau-Russell, Denise, 2003, Holotherian mammals from the Forest Marble (Middle Jurassic of England), Geodiversitas 25 (3), pp. 501-537 : 528

? Peramuridae

MATERIAL EXAMINED. — BMNH J.752, left upper molariform.

One last interesting tooth is the left upper molariform BMNH J.752 ( Fig. 20 ): very small, it has only two roots, the posterior one showing only a sulcus along its length and being dominant lingually. The tooth is very narrow so that there is no trigon basin. The paracone is high, closely followed postero-lingually by a small metacone; there is only a faint indication of a low “c” cusp but the distal labial lobe is not quite complete. A shelf-like labial cingulum leads to a very extensive parastyle. The paracrista crest leading to the tiny stylocone is vertical; from the stylocone a short horizontal crest connects to the parastyle.

The first interpretation is that this tooth would be a milk molar. However, in dryolestids ( Martin 1999) DP4 and even more DP3 are longer than a molar; if J.752 is identified as a milk molar of Palaeoxonodon , it is too small to be either tooth. Apparently DP2 would be shorter, but none is known in dryolestids: only a fragment is preserved in GUI MAM 34/76; from what is left it can be assumed that the tooth measured slightly more than a half molar; J.752 would then fit in Paleoxonodon only as a DP2. The large parastyle extension also fits the deciduous interpretation (though the morphology seems overly complex for a DP2).

But other features do not fit: lingual position of the metacone (no different from that of molars in DP3 and DP4, Martin 1999: fig. 39A), possible presence of a facet 4, reduction of the stylocone (not occuring on DP3 and DP4, Martin 2002: fig. 39A, E-H), vertical preparacrista.

These features are strongly reminiscent of M1 of Peramus (in fact J.752 has the morphology and proportions intermediate between the P5 and M1 of this genus). Could it be that the structure of the peramurid upper molars is plesiomorphic (the situation of the metacone and the lingual cingulid have already been suspected of such, Fox [1975], Sigogneau-Russell [1999] but contra Butler & Clemens [2001]), and that the amphitheriid structure is derived from that scheme, which would still show in its DPs? That would also explain the configuration of the talonid similar in both lines.

However, J.752 is smaller than M1 of Peramus , has a larger parastyle, and lacks the lingual cuspules. Also the metacone is lower situated relative to the paracone. The Kirtlington tooth appears even closer to SA 37, from the Early Cretaceous of Morocco ( Peramus sp. , Sigogneau- Russell 1999), being only a bit smaller; the metacone is not quite so lingual, and the “c”- metastylar area possibly less developed; unfortunately the parastylar area is not fully preserved on SA 37, which may also represent a milk molar.

In conclusion, if amphitheriids were dominant in this Bathonian fauna, there are some indications that Dryolestida were already individualized, and scant suggestion that “Peramura” had detached from the “stem-lineage of Zatheria”. At the same time there also were evolutionary tentatives among holotherians (J.490; Kennetheredium n. gen.) which are not so far known in later faunas. Bushiness is a common step in the early evolutionary history of clades, after which natural sorting permits the flourishing of only a limited number of branches. Finally the quasi-absence of symmetrodonts suggests that these forms were direct ecological competitors of amphitheriids, or even of dryolestoids.

Figures

FIG. 20. —?Peramurid upper molar, BMNH J.752, × 34; A, lingual view; B, labial view; C, posterior view; D, anterior view; E, occlusal view.