Tedoribaatar reini, Kusuhashi, 2008

Kusuhashi, Nao, 2008, Early Cretaceous multituberculate mammals from the Kuwajima Formation (Tetori Group), central Japan, Acta Palaeontologica Polonica 53 (3), pp. 379-390 : 386-387

Tedoribaatar reini sp. nov.

Figs. 8 , 9 .

Etymology: In honor of Dr. Johannes Justus Rein, a German geographer who first collected fossil plants from the Kuwajima Formation (reported by Geyler 1877).

Holotype: SBEI 1570 , fragment of right lower jaw with p4 ( Figs. 8 , 9 ).

Type locality: “Kuwajima Kaseki−kabe” site, Shiramine district, Hakusan City, Ishikawa Prefecture, central Japan.

Type horizon: Upper part of the Kuwajima Formation (Tetori Group), Barremian to early Aptian (Early Cretaceous).

Diagnosis.—Lower dental formula 1.0.?2.?2; lower p3 single−rooted; p4 having ten serrations and one posterior labial cusp. Differs from other “plagiaulacidans,” including eobaatarids, in having a small number of lower permanent premolars and a single−rooted p3.

Description.—SBEI 1570, fragmentary right dentary preserves p 4 in the holotype ( Figs. 8 , 9 ). The other teeth are not known. The dental formula of lower dentition is 1.0.?2.?2.

SBEI 1570 ( Figs. 8 , 9 ) does not preserve a definitive mental foramen. At 1.5 mm anterior to p4 and 1.2 mm above ventral margin of the dentary, there is a relatively large hole that might be in the position of the mental foramen. The masseteric fossa extends anteriorly below the posterior root of p4, and becomes indistinct below the anterior root of p4. Broken alveoli for a p3 and a double−rooted m1 are preserved anterior and posterior to p4, respectively ( Figs. 8 , 9 ). There is no trace of two roots in the broken alveolus of a p3, though the possibility that the p3 was double−rooted cannot be definitely ruled out. Anterior to the alveolus of p3, there is a tiny pit that is possibly an alveolus for a shed dp2. There is no trace of permanent p2.

The p4 is not fully parallel−sided and is neither fully arcuate nor rectangular in lateral view. The U−shaped anterior triangular lobe is large relative to crown size; it extends postero−ventrally. The anterior part of p4 probably slightly overhung p3. The fourth lower premolar has ten serrations, eight of which (except for the terminal ones) are accompanied by ridges. Only one posterior labial cusp is present. It is located high on the crown, somewhat above half the height of the distal margin of p4 ( Figs. 8 , 9 ). The position of this cusp is higher than that in Hakusanobaatar matsuoi . Dorsally, a wear facet extends from a position above the last serration to the anterior end of this cusp. The length of the posterior root of the p4 is modest, and is less than twice as long as the anterior one ( Figs. 8 , 9 ).

Measurements.—See Table 1.

Remarks.—The tiny pit positioned anterior to the alveolus of p 3 in SBEI 1570 is interpreted as an alveolus for a shed dp2. The mental foramen is usually larger than this pit, and is situated in lower position on the labial side of the dentary, possibly at the position of the large hole in SBEI 1570. There is a possibility that the tiny pit is a foramen for a blood vessel; however, this is unlikely because such a foramen does not normally open to the occlusal surface of the dentary. The lower cheek teeth of multituberculates are obliquely arranged to the dentary in occlusal view. Taking this into account, the position of the pit is thought to be just anterior to the p 3 in tooth row, and it is the position of a p2, if present. Therefore, this tiny pit is more likely to be an alveolus for a dp2 or p2 than a blood vessel foramen. This alveolus is very tiny and it is hard to imagine that it contained a tooth. The alveolus is, thus, thought to be for a shed dp2. Lacking the eruption of a permanent p2, the alveolus is interpreted to have become reduced its size. Tedoribaatar reini is, therefore, thought to have had only two lower permanent premolars.

Tedoribaatar reini is thought to have had only two lower permanent premolars. The pit situated anterior to the alveolus for p3 on the holotype (SBEI 1570) is interpreted as the alveolus for a shed dp2, as mentioned above, and no trace of a permanent p2 is present. Although there is a possibility that this pit is a blood vessel foramen, it still is the case that T. reini does not have p2. Cimolodontans have at most only two lower premolars ( Kielan−Jaworowska et al. 2004), but the morphology of the p4 seen in T. reini is intermediate between the typical “plagiaulacidan” and cimolodontan conditions. In lateral view p4 of T. reini is neither fully arcuate nor extended forward to overhang the crown of p3 as seen in cimolodontans. From the size of the alveolus, p3 of Tedoribaatar reini is estimated to have been larger than the peg−like p3 of cimolodontans. Tedoribaatar reini is, therefore, assigned to “Plagiaulacida”. Tedoribaatar reini has a single−rooted p3, which indicates that the p3 crown was reduced. Tedoribaatar reini differs from “plagiaulacidans” except for eobaatarids and Arginbaatar in this feature (see Trofimov 1980; Kielan−Jaworowska et al. 1987; Kielan−Jaworowska et al. 2004). The morphology of p4 of T. reini is clearly different from that of Arginbaatar , which has a highly arcuate, specialized p4, and is rather similar to those of eobaatarids (see Trofimov 1980; Kielan−Jaworowska et al. 1987). The number of serrations of p4 (ten) is in the range of Eobaataridae . Tedoribaatar reini is tentatively considered as a member of the Eobaataridae and the most derived “plagiaulacidan” multituberculate yet discovered.

Compared with eobaatarids, T. reini is almost the same size as Eobaatar magnus and Hakusanobaatar matsuoi , and slightly smaller than Sinobaatar lingyuanensis ( Table 1). Tedoribaatar reini shares a reduced p3 with Eobaatar , Sinobaatar (see Kielan−Jaworowska et al. 1987; Hu and Wang 2002a, b) and Hakusanobaatar . Tedoribaatar reini is, however, distinguished from Hakusanobaatar matsuoi , discovered from the same locality, by the higher position of the posterior labial cusp of the p4 and the antero−posteriorly shorter posterior root of the p4. A single−rooted p3 is present only in T. reini among “plagiaulacidans”, and clearly distinguishes T. reini from Eobaatar , Sinobaatar (see Kielan−Jaworowska et al. 1987; Hu and Wang 2002a, b) and Hakusanobaatar . Tedoribaatar reini also differs from Eobaatar , Sinobaatar , and Hakusanobaatar in having a lower number of lower premolars. The lack of p2 and a single−rooted p3 are clearly apomorphic characters among “plagiaulacidans”. Tedoribaatar reini is, therefore, recognized as a new genus and species of Eobaataridae , and as a species that is most closely related to cimolodontans among “plagiaulacidans”, although it can not be compared with the other three eobaatarid and?eobaatarid genera ( Monobaatar , Loxaulax and Parendotherium ) whose p4s have not been discovered.

Figures

Fig. 8. Eobatarid multituberculate mammal Tedoribaatar reini gen. et sp. nov., SBEI 1570, holotype, right lower jaw fragment with p4; Lower Cretaceous Kuwajima Formation, Shiramine, Japan. SEM photograph of the resin cast. A. Lingual view. B. Labial view. C. Occlusal view, left to anterior. D. Occlusal view of the alveoli of p2 (arrow) and p3, left to anterior.

Fig. 9. Eobatarid multituberculate mammal Tedoribaatar reini gen. et sp. nov., SBEI 1570, holotype; Lower Cretaceous Kuwajima Formation, Shiramine, Japan. Fragment of right lower jaw with p4. A. Labial view. B. Lingual view. C. Occlusal view, left to anterior.