Lakotalestes luoi, 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 : 43-44

cf. Lakotalestes luoi sp. nov.

Fig. 8B .

Material.—OMNH 62854, a partial right lower molar from OMNH V1254, Fuson Member, Lakota Formation (Lower Cretaceous: upper Valanginian–lower Barremian), Meade County, South Dakota, USA.

Description.—OMNH 62854 is a moderately to well-worn right lower molar lacking the talonid, which appears to have been abraded off (L = [0.69 mm], W = 0.84 mm). The roots are broken, but their bases show that the distal root was much smaller than the mesial root and was positioned lingually, under the distolingual base of the metaconid. This is a distinguishing feature of the Dryolestidae ( Simpson 1927) . The buccal side of the crown extends farther downward than the lingual side, reflecting a lower buccal than lingual alveolar border. This was described as a dryolestid apomorphy by Prothero (1981), though a similar condition presents in the Spalacotheriidae . Like other dryolestids, the trigonid is mesiodistally compressed compared to the condition in the Paurodontidae or stem Zatheria McKenna, 1975 (angle = 31º; Fig. 8B 1 ). Each of the cusps shows apical wear, but it is clear that all were pointed and cusp-like; judged by what remains, they appear to have been gracile. Both the paracristid and protocristid are well developed and bear strap-like wear facets. Wear along the protocristid is especially strong, but because it is oblique to the occlusal plane this does not appear to have significantly affected the apex of the metaconid. The protoconid, which is also beveled on its mesial face (owing to wear on the paracristid), is most dramatically affect- ed by apical wear, being only slightly taller than paraconid and metaconid. The metaconid has a broader base than the paraconid but the cusps are equal in height. The paraconid has a slight mesial tilt, but is best described as semi-erect rather than procumbent, best seen in lingual view ( Fig. 8B 2). Remarks.—OMNH 62854 cannot be directly compared with Lakotalestes luoi , which is based on an upper molar. It appears to have come from a slightly smaller individual than that represented by the holotype of the species (tooth lengths cannot be directly compared, as the talonid of OMNH 62854 is missing), but they are similar enough in size to have belonged to the same species. Though the two specimens are from different localities positioned at different horizons (and, presumably, differing somewhat in age), comparisons below suggest that OMNH 62854 is morphologically appropriate to have belonged to Lakotalestes luoi or a similar taxon. Taken together, the restricted stratigraphic interval represented by these two localities, the very small number of mammalian specimens recovered, and the favorable morphological comparisons between the upper and lower molars lead us to retain this tentative referral as a working hypothesis, at least until the record from the Lakota Formation improves. These comparisons follow the character polarities and tree presented by Martin (1999); numbers in parentheses refer to characters in his data matrix ( Martin 1999: 83).

OMNH 62854 lacks apomorphies seen in the major clade of Dryolestidae identified by Martin (1999), that including the last common ancestor of Dryolestes and Laolestes , and all of its descendants. The paraconid is cusp-like, not shovel-like (9), as in Dryolestes and Peraspalax Owen, 1871 ; similarly, the metaconid is pointed and not mesiodistally elongate (11) as it is in Phascolestes Owen, 1871 , Guimarotodus Martin, 1999 , or Laolestes . Other apomorphies seen in one or more genera of this group are also lacking: extreme mesiodistal compression (20; Phascolestes ), metaconid inflated and paraconid flexed (12, 16; Guimarotodus ), and bifid metaconid ( Laolestes ).

Conversely, OMNH 62854 is similar to the clade ( Krebsotherium ( Crusafontia + Amblotherium )) in its pointed (not stocky) cusps (7). It lacks the flexed paraconid (16) seen in Krebsotherium (also seen in Guimarotodus , a presumed homoplasy). OMNH 62854 is similar to the clade ( Crusafontia + Amblotherium ) in having a semi-erect paraconid Thomas Martin personal communication, 2011) and slen- der cusps, differing from Crusafontia in lacking a mesial cingulid. Significantly, the paraconid and metaconid of the Lakota specimen are of equal height (19), as is the case in Amblotherium spp. The identity of OMNH 62854 will remain unknown until and unless similar lower teeth are found in association with upper molars. However, as the upper molar OMNH 62673 is most comparable to those seen in species of Amblotherium , Miccylotyrans , and Kurtodon , (which are suspected synonyms and which occupy the same position on the tree published by Martin 1999), OMNH 62854 is morphologically appropriate, at least in known respects, to belong to Lakotalestes luoi .

Figures

Fig. 8. Dryolestid mammals from the Lakota Formation (Lower Cretaceous: upper Berriasian–lower Barremian), South Dakota, USA. A. Lakotalestes luoi gen. et sp. nov., OMNH 62673 (holotype), from OMNH locality V1243; right upper molar in occlusal (A 1, stereopair), buccal (A 2), mesial (A 3), and distal (A 4) views, with interpretive illustration (A 5) of occlusal view. B. cf. Lakotalestes luoi, OMNH 62854, from OMNH locality V1254; right lower molar (trigonid only) in occlusal (B 1, stereopair) and lingual (B ) views.

Fig. 1. Map of exposures of the Lower Cretaceous Inyan Kara Group in southwestern South Dakota and northeastern Wyoming (USA), which includes the Lakota Formation (Berriasian–Barremian).Stars indicate OMNH localities which yielded specimens described in this study. Modified from Sames (2011a), as based on Waagé (1959) and Sohn (1979).