Oklatheridium sp.
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publication ID |
https://doi.org/ 10.4202/app.2011.0037 |
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publication LSID |
lsid:zoobank.org:pub:5990F90D-CD8B-4BB2-96FB-466D8794E29D |
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persistent identifier |
https://treatment.plazi.org/id/FB113777-7B42-0767-FF73-FA2629716398 |
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treatment provided by |
Felipe |
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scientific name |
Oklatheridium sp. |
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Fig. 4, Table 4.
Referred material.—PM 1287, LM4; OMNH 61642, Rm1 (trigonid only); OMNH 33940, Rmx (trigonid only); OMNH 33945, Lmx (trigonid only); OMNH 61643, Rmx (trigonid only); OMNH 63728, Rmx (trigonid only); PM 660, Rmx (trigonid only); PM 965, Rmx.
Description
Upper molar: PM 1287 was originally illustrated in Patterson (1956: fig. 4B; Fig. 4A). This specimen was referred to Pappotherium sp. by Turnbull (1971), though Butler (1978) did not believe it could belong to either that genus or Holoclemensia , and must represent the ultimate molar of something different. In their interpretation of the molar series of the Tomato Hill deltatheroidans, Davis et al. (2008: fig. 1.8) speculated that the M4 of Oklatheridium would be heavily reduced, much like the condition in the larger but morphologically similar Deltatheridium . PM 1287 retains a large paracone, stylocone, and preparacrista (characters of the mesial molars of Oklatheridium ), while the metacone and metastylar lobe are strongly reduced (a trend beginning with the M3; Fig. 3E, H View Fig ). Such strong reductions in ultimate molar proportions are not expected in Early Cretaceous lineages other than deltatheroidans. The ultimate molar of the other deltatheroidan in the sample, Atokatheridium (hypothesized doi:10.4202/app.2011.0037
by Davis et al. 2008: fig. 1.8), probably would bear a proportionately larger protocone. This is the basis for referral of this specimen to Oklatheridium ; however, the absence of a more completely preserved molar series precludes specific assignment of this M4 within the genus.
Lower molars: Davis et al. (2008) referred a number of lower molars (all represented by trigonids) to Oklatheridium szalayi . However, the recognition of a second species of Oklatheridium necessitates reconsideration of all lower molar material. The fragmentary nature of these specimens (with the exception of PM 965, described below) makes it difficult to differentiate between size and morphological differences that are attributable to positional variation, and differences which relate to interspecific variation. A conservative approach is warranted, and all lower molars are here referred to Oklatheridium sp.
One specimen is complete (PM 965; Fig. 4C) and allows the talonid morphology of this genus to be described. This molar bears a very large and prominent paraconid, with a tall, deeply notched paracristid. The metaconid is much lower than the paraconid. In occlusal view, the trigonid cusps form a roughly equilateral triangle. Cusp f is prominent but not shelflike, and is positioned below the paracristid notch. A very small cusp e is located slightly higher but at the mesiolingual margin of the paraconid. A distinct distal metacristid is present, meeting the cristid obliqua at its mesial terminus below the protocristid notch. The highly developed prevallid shear evidenced by this specimen clearly points to deltatheroidan affinities, and it shares much of its trigonid morphology with the other specimens referred to Oklatheridium . However, the most striking feature of this molar is the size of the talonid. While smaller in area than the trigonid, it is still substantially larger, in a relative sense, than in all other known deltatheroidans. The hypoconid is the largest cusp, and is conical except for a flat internal face. The hypoconulid is as tall as the hypoconid, but is elongate and somewhat ridge−like. The hypocristid between these cusps is long enough to have accommodated a relatively large metacone, which also supports referral of this specimen to Oklatheridium . The entoconid is lower than the other cusps but is still very prominent and broad, and the entocristid is notched. The talonid basin is well enclosed and deep, and the three cusps are evenly spaced. In all, the talonid is not as predicted by Davis et al. (2008) for Oklatheridium , especially in that it is much larger than the preserved talonid in Atokatheridium ( Fig. 2E). The pattern of breakage on other trigonids referred to Oklatheridium does not preclude this sort of talonid morphology, and PM 965 is on the large end of the size range for the sample, which suggests it may represent O. minax . This species has a much larger protoconal region than other deltatheroidans, which should reflect an accompanying large talonid.
Of the remainder of the sample, only the trigonid of an m1 is readily identifiable. OMNH 61642 ( Fig. 4B) is markedly different from OMNH 33945, incorrectly identified as an m1 by Davis et al. (2008), in being lower crowned, much narrower transversely, and in having widely spaced paraconid and metaconid. It is proportionally very similar to the m1 of Atokatheridium , differing only in larger size and a somewhat proportionally larger paraconid.
Stratigraphic and geographic range.— Greenwood Canyon (Triconodont Gully), upper Antlers Formation (Aptian– Albian), north−central Texas, USA ; Tomato Hill ( OMNH V706 ), middle Antlers Formation (Aptian–Albian), southeastern Oklahoma, USA .
| OMNH |
Osaka Museum of Natural History |
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