Infernolestes rougieri, Cifelli & Davis & Sames, 2014

Cifelli, Richard L., Davis, Brian M. & Sames, Benjamin, 2014, Earliest Cretaceous mammals from the western United States, Acta Palaeontologica Polonica 59 (1), pp. 31-52 : 39-41

Infernolestes rougieri sp. nov.

Fig. 7 .

Etymology: In honor of Guillermo Rougier, for his contributions to knowledge of Mesozoic mammals.

Holotype: OMNH 62671 , right lower molar, identified as m1.

Type locality: OMNH V1243, Fall River County, South Dakota, USA.

Type horizon: Chilson Member, Lakota Formation (Lower Cretaceous: upper Berriasian–Valanginian).

Diagnosis.—Moderate-sized spalacotheriid differing from other spalacotheriids except Heishanolestes in having an m1 with an obtuse trigonid angle, in having the paraconid/ metaconid closely appressed to the protoconid (crown transversely narrow), and in possessing a wide, flat lingual cingulid; differs from other spalacotheriids except Yaverlestes in possessing a long mesial shelf; differs from other spalacotheriids except Spalacotherium evansae and Yaverlestes in bearing an incomplete buccal cingulid; differs from other spalacotheriids except Spalacotheridium , Symmetrodontoides , and Heishanolestes in possessing a flat (not dorsally flexed) lingual cingulid; differs from other spalacotheriids except Spalacotheridium and Spalacolestes in having a relatively shorter metaconid; differs from Heishanolestes in having a less obtuse trigonid angle and less robust trigonid cusps especially the paraconid).

Description.—OMNH 62671 is a small molar (L = 1.25 mm, W = 0.71 mm) from the mesial end of the tooth row, probably representing the m1. It is preserved in a small fragment of dentary, but no traces of alveoli remain mesial or distal to this tooth. The three trigonid cusps form an obtuse angle (111º), with the protoconid by far the most robust and tallest cusp. The crown is virtually unworn, with the exception of slight apical wear on the metaconid and some breakage of the tip of the paraconid. The protoconid has an almost flat lingual face, and the cusp reclines distally so that its apex is closer to the metaconid than to the paraconid. The paraconid and metaconid originate very close to the base of the protoconid, with the paraconid roughly half the height of the metaconid. The paraconid is conical, and its apex is connected to the apex of the protoconid by a weak paracristid. The metaconid is also conical, and is slightly more than half the height of the protoconid. The metaconid is closely appressed to the protoconid, such that the protocristid is substantially higher than the paracristid; the postvallid shearing surface is also better developed than the prevallid surface. The lingual cingulid is very broad and bears two tiny cuspules mesial to the midpoint. This cingulid is also approximately horizontal in lingual view and roughly straight in occlusal view (i.e., not concave; Fig. 7A ). There is a wide mesial shelf bearing a small but abraded cusp e, situated in a mesiodistal line with the paraconid and metaconid. Distally, cusp d is larger than cusp e but is still a small cusp. It is connected to the base of the metaconid by a weak crest, but this crest does not climb the distal face of the metaconid. The buccal cingulid is strong distally, weaker mesially, and incomplete at the midpoint of the crown ( Fig. 7C ), though it is possible the gap in the cingulid is a result of preservation. The cingulid flexes ventrally at the midpoint, the apex of this arch corresponding to the hiatus in the cingulid. The buccal margin of the crown is set substantially lower (ventral) relative to the lingual side. The obtuse angle formed by the trigonid cusps and the great height differential between the paraconid and metaconid lead us to suggest that OMNH 62671 represents an m1.

Remarks.— Infernolestes gen. nov. is placed within the Spalacotheriidae based on the presence of derived characters and the absence of features typical of the Tinodontidae Marsh, 1887 and Zhangheotheriidae Hu, Wang, Luo, and Li, 1997 (see Rougier et al. 2003; Tsubamato et al. 2004; Li and Luo 2006). Infernolestes differs from tinodontids and zhangheotheriids (and resembles spalacotheriids) in the marked difference in height at the alveolar margin of the crown, with the buccal side extending farther ventrally. There is also substantial height difference between the paraconid and metaconid and the accompanying placement of the paracristid and protocristid, with the metaconid and protocristid much higher. The primary shearing crest on the crown, formed by the protocristid, is developed flush with the distal face of the trigonid; this crest functions as the leading edge of postvallid shear, which is present and developed at eruption in Spalacotheriidae (apomorphy). The paracristid is, in contrast, much more poorly developed. There appears to be a slight displacement of the mesial face of the trigonid, with a groove present between the paraconid and protoconid extending to the base of the cusps ( Fig. 7A ). Unfortunately, the tooth is unworn (other than some apical wear) and evidence of occlusal contact at these surfaces is absent. It is not entirely clear if the morphology seen in Infernolestes represents an intermediate condition between the development of shearing surfaces in plesiomorphic groups, which require remodeling through wear (see Crompton and Jenkins 1968; Cifelli et al. 1998; Luo and Ji 2005), and the derived spalacotheriids, which have a functional surface present at eruption (see Cifelli and Madsen 1999). However, the balance of character evidence supports placement of this taxon within the Spalacotheriidae .

Infernolestes does differ from nearly all spalacotheriids (and instead resembles tinodontids and zhangheotheriids) in the obtuse angle of the trigonid and the relatively poor transverse development of the trigonid (especially the short length of the pre- and postvallid shearing surfaces; Fig. 7A ). Howev- er, a close point of comparison exists with the morphology of the m 1 in Heishanolestes Hu, Fox, Wang, and Li, 2005 ( Hu et al. 2005: fig. 4B), which is clearly a spalacotheriid based on the remainder of the molar series. These two taxa also share a wide lingual cingulid (to the exclusion of all other described spalacotheriids), but Infernolestes lacks the specialized, robust cusps of the m 1 in Heishanolestes and, in general proportions, more closely resembles the m1 of other North American spalacotheriids (such as Spalacolestes Cifelli and Madsen, 1999 and Spalacotheridium Cifelli, 1990 ; see Cifelli and Madsen 1999). Currently, the oldest known spalacolestine spalacotheriid is Yaverlestes ( Sweetman 2008) , from the Barremian of Britain. However, mesial lower molars (the only material available for Infernolestes ) are generally not informative as to subfamilial relationships, and we therefore conservatively refer the Lakota taxon to the Spalacotheriidae only.

Two additional specimens referred to spalacotheriid taxa bear mention in this context. Fox (1976) described Symmetrodontoides canadensis Fox, 1976 on the basis of a dentary fragment preserving three molars, which he identified as the m3–5. He referred a second dentary fragment bearing a single, low-crowned molariform tooth to the same species, identifying this tooth as the m1 ( Fox 1976: fig. 4). The cusps form a severely obtuse angle (much more so than in Infernolestes ), while the distal molars are more typical of a spalacotheriid. However, this specimen more likely represents a deciduous premolar (see discussion in Cifelli 1999b), and the holotype of Symmetrodontoides canadensis can be reinterpreted with confidence to contain the m1–3. Gill (2004: fig. 4A, B) referred an isolated molariform from Cliff End (Valanginian of Britain) to Spalacotherium cf. tricuspidens ; it is unclear if this specimen actually represents the m1 of a species of Spalacotherium Owen, 1854 , but it does bear close resemblance to the m1 of Heishanolestes ( Hu et al. 2005: fig. 4B). The specimen is lower-crowned and has a more obtuse trigonid angle than Infernolestes , with bulbous cusps and a paraconid set more mesially (as in Heishanolestes). The mesial shelf is very short and cusp d is apparently lacking; this, coupled with very weak cingulids may suggest that this low-crowned tooth represents a different iteration of a spalacotheriid deciduous premolar.

Stratigraphic and geographic range.— Lower Cretaceous (upper Berriasian–Valanginian), South Dakota, USA. Currently known only from the type locality and horizon .

Figures

Fig. 7. The spalacotheriid “symmetrodont” mammal Infernolestes rougieri gen. et sp. nov. from OMNH locality V1243, Lakota Formation (Lower Cretaceous: upper Berriasian–Valanginian), South Dakota, USA. OMNH 62671 (holotype), right lower molar in occlusal (A, stereopair), lingual (B), and buccal (C) views.