Atokatheridium boreni Kielan−Jaworowska and Cifelli, 2001

Davis, Brian M. & Cifelli, Richard L., 2011, Reappraisal of the tribosphenidan mammals from the Trinity Group (Aptian-Albian) of Texas and Oklahoma, Acta Palaeontologica Polonica 56 (3), pp. 441-462 : 443-444

Atokatheridium boreni Kielan−Jaworowska and Cifelli, 2001

Fig. 2, Table 1.

Holotype: OMNH 61623 , RM2 .

Type locality: Tomato Hill (OMNH V706), southeastern Oklahoma, USA.

Type horizon: Middle Antlers Formation (Aptian–Albian).

Referred material.—OMNH 61151, LM1; 63725, LM3; 63724, Lm1; 63889, Rm1; 63890, Rm1; 34905, Rmx; 61181, Lmx; 61624, Lmx; 63891, Rmx; 63892, Lmx (all from the type locality).

Emended diagnosis.—Small deltatheroidan differing from all other deltatheroidans in smaller size, weaker stylocone, shallower ectoflexus on mesial molars, slightly narrower parastylar lobe, trend of increasing width of metastylar lobe distally through molar series (excluding the unknown but hypothesized M4), greater height differential between the paracone and metacone, transversely wider protoconal region, and a taller protocone. Differs from Oklatheridium and Sulestes in weaker conules. Differs from Oklatheridium in slightly narrower metastylar lobe on M2, and in less reduction of the metastylar lobe on M3.

Description

Detailed descriptions of the molars of Atokatheridium boreni can be found in Davis et al. (2008), and will not be repeated here (with the exception of lower molars which can be re−

doi:10.4202/app.2011.0037

ferred with confidence to the first molar locus, described below). However, the morphology is summarized to facilitate comparisons between the various tribosphenidan taxa described in this paper.

Upper molars: The three referred upper molars of Atokatheridium boreni ( Fig. 2A–C) are all to a greater or lesser extent abraded, so some morphological details may be lost or understated. The molars are transversely wide, with a wide stylar shelf (especially the metastylar lobe of the M3). The paracone is larger and taller than the metacone, and the postmetacrista is strong and deeply−notched at the base of the metacone as in other deltatheroidans. The stylocone is well developed but not as large as in Oklatheridium or Pappotherium . The preparacrista is weak, but this could be a factor of preservation. The protoconal region of the crown is mesiodistally compressed and wide. The conules are distinct but weak, and there is faint evidence of an internal crista on the paraconule. The preprotocrista extends to the parastyle, but the postprotocrista ends at the base of the metacone.

Lower molars: A. boreni is known by a single complete but abraded isolated lower molar and several fragmentary molars preserving only the trigonid ( Fig. 2E). Given the poor preservation of the sample, it is not possible to confidently identify most specimens to locus. OMNH 61624 has a very tall trigonid relative to the small talonid, and a very prominent, projecting paraconid that is much taller than the metaconid. These features suggest that this specimen might represent the m4, but there are some substantial differences between it and preserved m4s of other deltatheroidans. In Deltatheridium and Sulestes , the trigonid is extremely obtuse−angled, and in the former the metaconid is completely absent ( Rougier et al. 1998; Averianov et al. 2010). While the low, small talonid in A. boreni is open lingually and lacks an entoconid (comparing in some ways favorably with the basal tribosphenidan Kielantherium ; Dashzeveg and Kielan−Jaworowska 1984), the trigonid has a high, wall−like paracristid, reinforcing deltatheroidan affinities for this taxon. A strong distal metacristid is present. The remainder of the referred lower molars (with the exception of those identified as m1s, see below) generally agree with OMNH 61624 in morphology, though they are all either too heavily worn or poorly preserved to add anything new.

OMNH 63724 ( Fig. 2D) is identified as an m1 on the basis of a somewhat more open trigonid and on relative cusp heights. Only the trigonid is preserved, and it is transversely compressed and lingually open relative to the other referred lower molars. The paraconid is slightly taller than but not as robust as the metaconid; the paraconid is not inclined, as would be expected if the specimen were a deciduous premolar. A distal metacristid is present. The paracristid is strong and notched, and this specimen can be excluded from the other deltatheroidan in the fauna, Oklatheridium , based on size. The relative size and proportions of this specimen in reference to the rest of the referred sample agree well with the distal trend in morphology of the molar series seen in Deltatheridium ( Rougier et al. 1998) , and it is on this basis that it is excluded from the other tribosphenidans of the Trinity Group.

Comments.— Atokatheridium was referred with confidence to the Deltatheroida by Davis et al. (2008) based on the combination of a strong upper molar postmetacrista on all upper molar loci and a tall lower molar paraconid (well−developed postvallum/prevallid shear, which is typical of all deltatheroidans). Averianov et al. (2010: 318) disputed the deltatheroidan nature of this taxon, claiming that the referral “...was heavily influenced by interpretation of a relatively large incomplete [M3]...” and concluding that Atokatheridium was a stem tribosphenidan. Justification for the referral of OMNH 63725 to Atokatheridium (not Oklatheridium as suggested by Averianov et al. 2010) and of the taxon to the Deltatheroida was given in detail by Davis et al. (2008: 13: fig. 1.8), and draws from both upper and lower molar morphology at all known loci. Other recent work on the group (e.g., Rougier et al. 2004) has supported this position, and we feel no further validation is necessary.

Stratigraphic and geographic range.—Middle Antlers Formation (Aptian–Albian), southeastern Oklahoma, USA.