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 : 41-43

Lakotalestes luoi sp. nov.

Fig. 8A .

Etymology: In honor of Zhe-Xi Luo, for his contributions to knowledge of Mesozoic mammals.

Holotype: OMNH 62673 , right upper molar from the distal part of the series, perhaps M5.

Type locality: OMNH V1243 , Fall River County, South Dakota, USA. Type horizon: Chilson Member , Lakota Formation ( Lower Cretaceous : upper Berriasian–Valanginian) .

Diagnosis.—Small dryolestid; upper molars differing from those of Dryolestes , Krebsotherium , and Laolestes in having stylocone placed mesially, adjacent to parastyle and buccal terminus of relatively straight paracrista, rather than in a more median position; differs from all Dryolestidae except Portopinheirodon (which differs in having median ridge mesially placed and closely paralleling paracrista), Kurtodon pusillus (which differs in lacking the metacone) and Amblotherium nanum (which differs in having a weaker median ridge) in lacking cusp “C”. Most similar to Miccylotyrans minimus , differing in: slightly larger size, broader ectoflexus, lacking cusp “C” and parastylar hook; and in having a stronger, continuous (from paracone to median cusp) median ridge; well-developed, tall median cusp; and broad-based central cusp placed on median ridge.

Description.—OMNH 62673 is a small (L = 0.86 mm, W = 1.40 mm) right upper molar with light wear, missing the apex and part of the lingual face of the paracone, and the stylocone-parastyle; the enamel is slightly damaged in the regions of metastyle and metacone, but the presence of these structures is clear. In occlusal profile the crown is buccolingually wide, a well-marked ectoflexus is present, and the parastylar lobe projects mesiobuccally rather than mesially, suggesting that the tooth represents a distal locus in the maxilla ( Fig. 8A 1 ). Comparison to the most morphologically similar species, Miccylotyrans minimus Simpson, 1927 , Amblotherium pusillum ( Owen, 1866) , A. gracile Marsh, 1879a , and Kurtodon pusillus Osborn, 1887 , indicates that OMNH 62673 is M5 or, less likely, M6. Like other dryolestids, three roots are present. Of these, the lingual root is notably robust, being as broad mesiodistally as the two buccal roots combined ( Fig. 8A 2). A groove is present on the buccal face of the lingual root, which bends mesially toward the tip—another indication that the tooth occupied a distal position in the molar series.

The paracone has a rounded (not angular) lingual profile; its mesial face is vertical whereas the distal face slopes slightly, so that the paracone appears to be tilted mesially. The apex of this cusp is missing, but it was clearly the tallest cusp on the tooth. The paracrista and metacrista form sharp, raised crests to enclose a trigon basin. Enamel is missing from part of the metacrista, but a distinct swelling shows that the metacone was

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buccolingually elongate and well developed. Cusp “C”, usually present on upper molars of dryolestids ( Prothero 1981), is lacking, but the metastyle is developed as a distinct cusp (well seen in buccal view; Fig. 8A 2) at the buccal margin of the stylar shelf, just mesial to the buccodistal corner of the tooth.

A well-marked median ridge extends buccally from the paracone, terminating in a distinct, mesiodistally developed median cusp that is situated at the deepest part of the ectoflexus, somewhat lingual to the buccal margin of the tooth. The most distinctive feature of the upper molar of Lakotalestes luoi gen. et sp. nov. is the presence of another cusp, herein termed the central cusp, on the median ridge about two-thirds of the distance from paracone to median cusp. The central cusp is nearly as tall as the median cusp and is much broader-based, occupying the greater part of the trigon basin.

The paracrista extends buccally from the paracone for about two-thirds of the crown width, terminating at the distolingual base of the parastylar lobe. The stylocone is not preserved, but its presence may be inferred from the upward curvature of preserved adjacent enamel and by the presence of a crest that joined the stylocone to the more distally-lying median cusp ( Fig. 8A 3). The parastyle is also missing; judged by the parastylar lobe that remains toward the base of the crown, the parastyle did not form a hook-like structure.

Remarks.— Lakotalestes luoi gen. et sp. nov. appears structurally closest to Miccylotyrans minimus (from the Kimmeridgian–Tithonian Morrison Formation, USA at Como Quarry 9; Simpson 1927), and, to a lesser extent, Kurtodon pusillus and Amblotherium spp. (known by upper dentitions from the Berriasian Purbeck Limestone Group, UK at Durlston Bay; Simpson 1928; Amblotherium is well represented by lower dentitions from the Morrison as well). Both Miccylotyrans and Kurtodon may be based on upper dentitions belonging to species of Amblotherium ( Martin 1999) and were tentatively recognized as such by Kielan-Jaworowska et al. (2004). We have retained usage of separate names in order to promote specificity in the differential diagnosis and comparisons. Lakotalestes is similar to Miccylotyrans in that the paracrista terminates at the base of the stylocone, whereas in Kurtodon and Amblotherium spp. the crest ascends the flank of the cusp, generally terminating at its apex. Like Lakotalestes , Miccylotyrans has a median cusp (though it is smaller and placed just distal to the small ectoflexus), also reportedly present, in diminutive form, in Amblotherium nanum ( Owen, 1871) and Kurtodon pusillus (see Simpson 1928). A strong, complete median ridge is present in Lakotalestes . A complete median ridge is also variably present in Miccylotyrans ; the ridge is present but incomplete buccally in Kurtodon , faint to absent (depending on tooth position) in Amblotherium nanum , and lacking in A. pusillum . Also variable in their presence among these taxa are cusps on the metacrista: Lakotalestes has a metacone but no cusp “C”; both cusps are present in Miccylotyrans and Amblotherium pusillum but lacking in A. nanum and Kurtodon . Lakotalestes is unique in the development of a broad-based central cusp ( Fig. 8A 1 , A 2). A cusp is also present in this position on the single known upper molar of the dryolestoid Donodon perscriptoris Sigogneau-Russell, 1991c , described and placed in its own monotypic family by Sigogneau-Russell (1991c). Donodon does not otherwise invite close comparison.

In his cladistic analysis of relationships among Dryolestidae (including only taxa known by both upper and lower dentitions), Martin (1999) recognizes a basal dichotomy of included genera; Amblotherium lies in a clade with the Iberian Krebsotherium Martin, 1999 and Crusafontia Henkel and Krebs, 1969 . Krebsotherium (monotypic), represented by K. lusitanicum Martin, 1999 , is known from the early Kimmeridgian of Guimarota, Portugal ( Martin 1999). The type of Crusafontia , C. cuencana Henkel and Krebs, 1969 , is known from lower Barremian horizons at Galve and Uña, Spain ( Henkel and Krebs 1969; Krebs 1993); a referred species, C. amoae Cuenca Bescós, Badiola, Canudo, Gasca, Moreno-Azanza, 2011 , was recovered from a slightly older (Hauterivian–Barremian) horizon of the El Castellar Formation at Galve ( Cuenca Bescós et al. 2011). Both the clade Krebsotherium ( Crusafontia ( Amblotherium )) and its constituents are diagnosed by apomorphies of the lower molars ( Martin 1999). As inferred for Lakotalestes , these taxa are primitive relative to other dryolestids in retaining a stylocone that is small; the stylocone is somewhat more distally placed in Krebsotherium than in Crusafontia or Lakotalestes , but variably retains connection to the paracrista (the paracrista extends to the apex of the stylocone in both species of Crusafontia whereas it terminates at the base of that cusp in Lakotalestes ). In addition to its distally placed stylocone, Krebsotherium is dissimilar to Lakotalestes , Amblotherium spp. , Miccylotyrans , Kurtodon , and Crusafontia in having a mesiodistally broader paracone profile and in lacking an ectoflexus. Cusp “C” and the metacone are variably present, at least, on molars of C. amoae ; the condition in C. cuencana is uncertain because of wear on the only known upper molar. The median cusp is absent from the known upper molar of C. cuencana and variably present in C. amoae ; a median ridge is lacking from known specimens of both species.

In their recent description of a highly specialized dryolestoid from the Late Cretaceous of South America, Rougier et al. (2011) published a phylogeny of the Dryolestoidea including a broad sample of Laurasian taxa. This study is the most recent (and most comprehensive to date) cladistic analysis of relationships among Dryolestidae , highlighting the endemism that South American forms had evolved by the end of the Cretaceous. However, the contents and topology of both the Dryolestidae and Paurodontidae Marsh, 1887 resulting from their analysis are very different from traditional hypotheses (e.g., Prothero 1981; Martin 1999; Luo et al. 2002). For example, the Paurodontidae are restricted to just three of the sampled taxa, with Henkelotherium Krebs, 1991 (previously considered a paurodontid) placed instead at the base of the Dryolestidae . The relationships of characteristic dryolestid genera within the family also differ markedly from those proposed by Martin (1999), with Laolestes Simpson, 1927 sharing a closer relationship with Amblotherium instead of with Dryolestes . It should be noted that Bremer support is, however, very weak at all nodes in this region of their cladogram ( Rougier et al. 2011: fig. S1). Given the somewhat unstable relationships within Laurasian Dryolestoidea and the proposed similarities between Lakotalestes and Amblotherium , more complete material from the earliest Cretaceous Lakota Formation could help provide additional resolution.

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

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).