Symmetrodonta, Simpson, 1925

Sigogneau-Russell, Denise, 2003, Holotherian mammals from the Forest Marble (Middle Jurassic of England), Geodiversitas 25 (3), pp. 501-537 : 524

? Symmetrodonta indet.

MATERIAL EXAMINED. — BMNH J.438, left? lower molariform; BMNH J.253,?right tooth.

BMNH J.438 ( Fig. 18 C-E), a left? lower molariform, most probably represents a milk tooth of a symmetrodont. It is complete but the enamel is poorly preserved and there are no remains of roots. The crown is long and low; the lingual cingulum is slightly convex dorsally but not rising in the middle. The?paraconid is lower and more labial than the?metaconid. If the tooth is properly oriented (the symmetry of the protoconid cristae makes this orientation uncertain), there is a particularly high e cusp; but is it in fact a d cusp? ( NB: the holotype tooth of Tinodon micron Ensom & Sigogneau-Russell, 2000 , presented as a left tooth, is in fact probably a right). Labially the crown is broken too high to allow a statement about a cingulum. Wear surfaces are uncertain. This tooth evokes the anterior molars or Tinodon Marsh, 1879 but differs by a lower crown and a cingulum not being angled in the middle. This morphology also recalls some of the deciduous symmetrodont teeth published by Cifelli (1999: fig. 6A, B, E).

The?posterior half of a?right tooth J.253, though of a similar type to J.438, does not have the appearance of a milk tooth; the preserved root is long and vertical. The metaconid is small as is the talonid cusp. The lingual cingulum is straight and there is no labial cingulum. Wear shows on the whole posterior face, creating even a sulcus between proto- and metaconid.

These two badly preserved specimens, whether deciduous molars or not, are the only suggestion of the presence of symmetrodont-like forms in the fauna.

Figures

FIG. 18. — SEM photographs of lower molars; A, B, Cladotheria indet., BMNH J.490, lingual and posterior views; C-E,?Symmetrodonta indet., BMNH J.438, oclusal, labial and lingual views. Scale bars: 1 mm.