Sahnitherium rangapurensis, Rana & Wilson, 2003

Rana, Rajendra S. & Wilson, Gregory P., 2003, New Late Cretaceous mammals from the Intertrappean beds of Rangapur, India and paleobiogeographic framework, Acta Palaeontologica Polonica 48 (3), pp. 331-348 : 339-340

Sahnitherium rangapurensis sp. nov.

Table 1, Fig. 6 .

Derivation of name: rangapurensis from the village of Rangapur near the type locality in the state of Andhra Pradesh, India .

Holotype: ITV/ R /Mm−1, isolated right upper molar, M1 or M2.

Type locality: Terminal Cretaceous (Maastrichtian) Intertrappean horizon, Rangapur, Andhra Pradesh, India.

Distribution.—Late Cretaceous (Maastrichtian) Intertrappean beds of Rangapur, Andhra Pradesh, India.

Differential diagnosis.—Similar in size and in some morphological aspects to Deccanolestes hislopi but differs in that it is not as transversely wide; the postmetacrista is stronger, longer, and carries the so−called cusp “C”; a stylocone is not present; the metastylar area is more posterolabially expanded and larger than the parastylar region; the ectoflexus is shallower; the labial cusps are labiolingually compressed, convex in labial view, and with steeper lingual slopes; paraconule larger and more anterolingually placed; and the protocone height is greater and its labial aspect meets the crown at a steeper angle. The taxon is also diagnosed by a paracone slightly higher and more lingually expanded than the metacone; bases of the labial cusps slightly appressed (moderate zalambdodonty); conular region not anteroposteriorly constricted; conules wing−like with weak internal crests; protocone well−developed and somewhat anteroposteriorly expanded; the apex of the protocone anterolabially recumbent; and no cingula present.

Description.—ITV/R/Mm−1 is identified as a right upper molar, either M1 or M2 ( Fig. 6 ). The posterolabial and lingual roots are preserved. The enamel surface of the crown is pitted in some regions, and a small break can be seen in labial view at the anterolabial end of the crown ( Fig. 6C ). The break removed an anterolabial root and perhaps a parastylar hook and parastyle, if they existed. Otherwise, the labial margin of the crown, including the region where a stylocone would be preserved, is unaffected.

Although the break slightly altered the buccal length and perhaps the anterior width, the crown was clearly not transversely wide ( Table 1). The main cusps of the crown are moderately tall and conical. The paracone is the tallest cusp, but it is only slightly taller than the metacone and protocone. The apices of the labial cusps are somewhat labiolingually compressed. The bases of these cusps are fused just above the level of the stylar shelf (moderate zalambdodonty). Centrocristae are weak. The base of the paracone is somewhat lingually expanded. The labial aspect of the paracone is flat to convex and gently slopes toward the labial margin of the tooth encroaching on the parastylar region. The labial aspect of the metacone is less convex and slopes more steeply toward the labial margin of the crown. The metastylar region slopes and then flattens but is not gutter−like. Also, this region is slightly posterolabially expanded. The postmetacrista is a distinct ridge that runs along a shallow gradient posterolabially toward the metastylar region. A metacrista cusp (cusp “C”) is present. The preparacrista is a less distinct ridge that runs along a steep gradient anterolabially toward the parastylar region. A faint trace of a raised ectocingulum is preserved on the labial margin of the stylar shelf, but a stylocone or accessory cusps are not present. The parastylar region (except a parastylar hook if one existed) is significantly smaller than the metastylar region. The ectoflexus forms a shallow curve.

The paraconule is larger than the metaconule. It is shifted anteriorly and positioned midway between the paracone and protocone. The metaconule is positioned closer to the metacone than to the protocone. The postmetaconule wing terminates at or before the lingual base of the metacone. The preparaconule wing extends labially to the parastylar region. The preparaconule crista runs labially and dorsally along the anterior margin of the crown and becomes a faint trace near the parastylar region. Faint traces of internal wings are present. The trigon basin is deep, especially so toward the labial cusps. The step down to the trigon basin from the metaconule is small and from the paraconule it is negligible. The base of the protocone is somewhat lingually expanded and anteroposteriorly widened. The apex of the protocone is tall and anterolabially deflected. The labial face of the protocone is nearly perpendicular to the rest of the crown, as are the lingual faces of the paracone and metacone ( Fig. 6D ). In occlusal outline, there is little to no anteroposterior waisting of the crown in the conular region ( Fig. 6A ). No cingula are present.

Discussion.—The specimen (ITV/R/Mm−1) is similar in size and in some morphological aspects to the M1 of Deccanolestes hislopi (VPL/JU/NKIM/10), but it differs in a number of important ways outlined in the diagnosis. Based on evaluation of upper molars from other Cretaceous eutherians, we judge these differences to be greater than preservational, interspecific, or even intrageneric variation. Thus, we erect a new genus and species for the single isolated upper molar because we are unaware of any other Cretaceous or Paleocene boreosphenidan or australosphenidan with this morphology.

Figures

Fig. 6. Sahnitherium rangapurensis. Right M1 or M2 (ITV/R/Mm−1), holotype in occlusal (A, B), labial (C), anterior (D), and posterior (E) views. Scale bar 0.5 mm.