Hakusanobaatar matsuoi, 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 : 381-386

Hakusanobaatar matsuoi sp. nov.

Figs. 2–7 .

Etymology: In honor of Dr. Hidekuni Matsuo, who contributed greatly to paleontological study of the “Kuwajima Kaseki−kabe” site and, as a leader of the research group, to management of the research on the fossils from the Kuwajima Formation.

Holotype: SBEI 1736 , isolated right lower incisor, left I2, left and right M1, fragmentary left upper jaw with I3, and P1 to P5, and fragment of right lower jaw with p3 and p4 (all are thought to be of the same individual); Figs. 2–4 .

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).

Referred specimens.—SBEI 581, fragmentary left lower jaw with damaged p4 ( Fig. 5A ); SBEI 582, damaged right upper premolar (probably P2; Fig 6B ); SBEI 1519,?left p3 ( Fig. 6A ); SBEI 1520, damaged left p4 ( Fig. 5C ); SBEI 1526, fragment of right lower dentary with incisor ( Fig. 5B ); and SBEI 1949, tentatively assigned poorly preserved upper premolar (two?labial cusps of probably right P5; Fig. 6C ).

Diagnosis.—Moderate−sized eobaatarid multituberculate with dental formula?3.0.5.?2/1.0.3.?2. Enamel is possibly not limited to the outer surface of the lower incisor; p3 is double−rooted and its crown is oval rather than triangular or rectangular in lateral view; p4 has ten serrations and one posterior labial cusp. Upper I2 has one main cusp and one accessory cusp; I3 is thin in lateral view and is leaf−shaped in anterior view; P1 to P3 have triangularly arranged three cusps (1:2); cusp formula of P4 is 3:5; cusp formula of P5 is 2:6:?2; M1 has postero−lingual wing and cusp formula is 3:4. Differs from other eobaatarids ( Eobaatar , Monobaatar , and Sinobaatar ) in cusp formulae of P4 and P5. Differs from? Janumys in the cusp formula of P4 and in having postero−lingual wing on M1. Description.—Parts of dentaries, incisors, p3s and p4s of lower jaws, and I2, I3, P1 to P5, and M1s of upper jaw are preserved among the specimens of Hakusanobaatar matsuoi . SBEI 1736 has the upper dentition but skull elements, including maxilla and premaxilla, are not preserved ( Fig. 2A ). The lower molars and upper M2 have yet to be discovered. Dental formula is considered to be?3.0.5.?2/1.0.3.?2 based on available materials.

Fragmentary dentaries are preserved in SBEI 581, 1526, and 1736 ( Figs. 2C , 4A , 5A, B ). There is no specimen in which the anterior and posterior parts of dentary including condyle and coronoid process are preserved. A mental foramen, at 1.1 mm posterior to incisor and 1.4 mm above ventral margin of the dentary, is situated closer to the incisor than to p 2 in SBEI 1526 ( Fig. 5B ). On SBEI 2352 (a resin cast of SBEI 581 made before the anterior part of the dentary was lost), a mental foramen is situated at 1.5 mm anterior to the alveolus of p2 and 1.5 mm above the ventral margin of the dentary, though the dentary is slightly deformed ( Fig. 7 ). This part is now missing in SBEI 581 ( Fig 5A ). The masseteric fossa extends anteriorly below the posterior root of p4 ( Figs. 5A, B , 7 ). Anterior to the p4, somewhat damaged alveoli for single−rooted p2 and double−rooted p3 are present in SBEI 2352 ( Fig. 7 ). These were mentioned by Takada et al. (2001: fig 2), although this part is also now missing in SBEI 581.

Lower incisors are preserved in SBEI 1526 and 1736 ( Figs. 4B , 5B ). The lower incisor is slender with a rounded labial surface and more flattened lingual surface, and thinner anteriorly. The ventral margin of the lingual surface is slightly swollen and bends lingually. Enamel may have been present on the inner as well as outer surface.

Lower p3s are preserved in SBEI 1519 and 1736 ( Figs. 2C , 4A , 6A ). The crown shape of p3 is oval rather than triangular or rectangular and is slightly attenuated antero−ventrally. The lower p3 is double−rooted; the anterior root is robust whereas the posterior one is thin and projects obliquely from a higher position than the anterior one. There are two small serrations on p3 ( Fig. 6A ). Each serration is accompanied by a short and indistinct ridge that extends antero−ventrally. In anterior view, there is no trace of a depression in the crown but the anterior margin is indented upward, indicating the presence of p2. The apex of p3 reaches the anterior margin of p4 ( Figs. 2C , 4A ).

Two damaged and one complete p4 are preserved in SBEI 581 and 1520, and 1736, respectively ( Figs. 2C , 4A , 5A, C ). The crown shape of p4 is parallel−sided in lateral view and is not fully rectangular, nor is it fully arcuate. Its antero−posterior length is not much greater than its height. The U−shaped anterior triangular lobe (exodaenodont lobe in many references, such as Kielan−Jaworowska et al. 1987) points ventrally and is large relative to crown size. The p4 of SBEI 1736 has ten serrations, of which at least eight of them, except for the first (most anterior) and the last (most posterior), are accompanied by ridges ( Fig. 4A ). Because of wear it is not obvious whether the last serration had originally been accompanied by a ridge that is now obliterated. The other specimens are damaged and it is impossible to count serrations and ridges. SBEI 581 has at least six ridges ( Fig. 5A ), and SBEI 1520 has at least seven ( Fig. 5C ). There is one posterior labial cusp on the distal margin of p4, positioned approximately midway between the base of the crown and the last serration ( Figs. 2C , 4A , 5A , 7 ). Dorsal to this cusp, a wear facet, which reaches the last serration in height and extends to anterior end of the cusp, is observed on SBEI 1736 ( Figs. 2C , 4A ). The posterior root of the p4 is long antero−posteriorly relative to the crown length, and is more than twice as long as the anterior one ( Figs. 5A , 7 ).

An isolated left I2 is preserved in SBEI 1736 and its base is preserved in the matrix that contains other upper teeth ( Figs. 2A , 3C ). I2 is a single−rooted, small and conical tooth with one main cusp and one tiny cusp projecting distally from about midway along the main cusp.

The left I3 is preserved in SBEI 1736 ( Figs. 2A , 3A ). I3 is probably situated at the lateral margin of the premaxilla, not medially. I3 is thin in lateral view, tapering toward the tip, and is leaf−shaped in anterior view. It is single−rooted and bears weak ridges on its crown.

Three anterior upper premolars, identified as P1–P3, are preserved in SBEI 582 and 1736 ( Figs. 2A , 3A , 6B ). The three teeth have similar shapes, with three cusps arranged triangularly: one on the labial side and two on the lingual. On each tooth the three cusps are subequal in size. P2 differs in having a tiny cusp anterior to the labial cusp. All cusps are ornamented with radiating (in occlusal view) ridges. The sizes of P1 and P2 are similar, and P3 is smaller than the other two. P3 has a distinct cingulum that extends posteriorly. On the premolar (probably right P2) of SBEI 582, there is an incipient antero−lingual cingulum ( Fig. 6B ). The anterior part of P2 overlaps the posterior part of P1, and the posterior part of P2 slightly overlaps P 3 in SBEI 1736 ( Figs. 2A , 3A ). The posterior cingulum of P3 is overlapped by the anterior part of P4.

A left P4 is preserved in SBEI 1736 ( Figs. 2A , 3A ). There are two cusp rows on P4; cusp formula is 3:5 (labial:lingual). The tooth is morphologically similar to P4 of Eobaatar , though the cusp formula is different. The height of cusps of the labial row does not vary greatly, though the second cusp is larger than the other two. The third labial cusp is clearly separated from the second, whereas the first and second cusps are close to each other. The cusps of the lingual row increase in height posteriorly, with the fourth cusp being the highest; the fifth cusp is small. There is a tiny cuspule situated between the cusp rows at the anterior margin of the tooth. The three posterior cusps of the lingual row are higher than those of the labial row. All cusps are ornamented with fine ridges. The lingual wall of the tooth forms a shearing surface.

The left P5 is preserved in SBEI 1736 ( Figs. 2A , 3A ). The crown is almost rectangular in occlusal view. The cusp formula is 2:6:?2 (labial:medial:lingual). The labial two cusps are situated lateral to the notch between the first and second cusps of the medial cusp row, and to the third cusp, respectively. A cuspule is present posterior to the second labial cusp. The medial cusp row is diagonally oriented postero−labially from the antero−lingual corner of the crown. The third medial cusp is the highest in the row, with the cusps decreasing in height both anteriorly and posteriorly. The cusps of the medial main cusp row are higher than the labial cusps. All cusps are ornamented with fine ridges. On the postero−lingual corner of the tooth, there is a terrace−like flattened region with a transverse groove. At least two cusps of the lingual cusp row were probably present in this region but have been lost by wear or by postmortem erosion.

Left and right M1s are preserved in SBEI 1736 ( Figs. 2B , 3B , 4C ). The cusp formula is 3:4. All cusps have approximately the same height, but the fourth lingual cusp is slightly larger than the others. There is a cuspule anterior and slightly medial to the first labial cusp. The cuspule is somewhat ridge−like and not fully separated from the first cusp. A crescentic wing without any cusp is present at the postero−lingual corner of the tooth. The anterior margin is slightly oblique to the longitudinal axis of the tooth. The labial cusps are positioned about opposite the embrasures between the cusps of the lingual row. Posterior to the third labial cusp there is a small flattened surface. The posterior ends of the cusp rows are connected by ridges.

Measurements.—See Tables 1, 2.

Remarks.— Hakusanobaatar matsuoi differs from cimolodontans in having five upper premolars (see Kielan−Jaworowska et al. 2004), and should be placed in the “Plagiaulacida”. It is clearly distinguishable from “plagiaulacidans”, except for eobaatarids and Arginbaatar Trofimov, 1980 , in having a much reduced p3 ( Kielan−Jaworowska et al. 2004). The lower p4 of Hakusanobaatar matsuoi is not fully arcuate in lateral view, which distinguishes H. matsuoi from cimolodontans and Arginbaatar (see Trofimov 1980; Kielan−Jaworowska et al. 1987; Kielan−Jaworowska et al. 2004). Hakusanobaatar is distinguished from albionbaatarids by P1 to P3 with only three cusps and by the morphology of P5 ( Kielan−Jaworowska and Ensom 1994; Kielan−Jaworowska et al. 2004).

Compared with eobaatarids, H. matsuoi is almost the same size as Eobaatar magnus Kielan−Jaworowska, Dashzeveg, and Trofimov, 1987 , and is slightly smaller than Sinobaatar lingyuanensis Hu and Wang, 2002 ( Tables 1 and 2). Hakusanobaatar matsuoi shares a similar morphology of p4 with Eobaatar and Sinobaatar , being slightly more arcuate than those of plagiaulacids and other primitive “plagiaulacidans” in lateral view (see Kielan−Jaworowska et al. 1987; Hu and Wang 2002a, b), and the much reduced p3 is similar to those of Sinobaatar and, possibly, Eobaatar (see Kielan−Jaworowska et al. 1987; Hu and Wang 2002a, b). It also shares similar P1 to P3 morphology with Eobaatar and Monobaatar in having three main cusps (see Kielan−Jaworowska et al. 1987), but this feature is present in “plagiaulacidans” of other families such as the Arginbaataridae (e.g., Trofimov 1980; Kielan−Jaworowska et al. 1987; Kielan−Jaworowska et al. 2004). These dental similarities suggest that H. matsuoi is phylogenetically related to the Eobaataridae .

p3 p4

L H L H Hakusanobaatar matsuoi SBEI 581 32 SBEI 1519 0.9 1.1 SBEI 1736 1.0 1.4 3.5 2.1 Tedoribaatar reini SBEI 1570 3.7 2.4 Sinobaatar lingyuanensis IVPP V 12517 1.2 1.9 4.1 2.5 Eobaatar magnus PIN 3101−57 3.5 2.0 PIN 3101−60 3.0 2.1

Hakusanobaatar matsuoi is distinguished from Eobaatar by the following characters (see Kielan−Jaworowska et al. 1987): P4 with cusp formula 3:5 (those of Eobaatar have only four lingual cusps); P5 has three cusp rows (only two are present in Eobaatar ; tooth designation of P5 of Eobaatar in Kielan−Jaworowska et al. 1987 is, however, somewhat questionable). The cusp formulae of P4 and P5 and morphology of P5 also distinguish H. matsuoi from Sinobaatar (see Hu and Wang 2002a, b). Hakusanobaatar matsuoi is also distinguished from Monobaatar by the cusp formula of P4 (see Kielan−Jaworowska et al. 1987). Hakusanobaatar matsuoi differs from? Janumys in the cusp formula of P4 and in having a postero−lingual wing on M1 ( Eaton and Cifelli 2001). Hakusanobaatar matsuoi can not be sufficiently compared with the other two poorly known eobaatarid and?eobaatarid genera, Loxaulax and Parendotherium ; it is, however, reasonable to recognize H. matsuoi as a new genus and species of the Eobaataridae .

The holotype of H. matsuoi gen. et sp. nov. ( SBEI 1736 ) has the best preserved upper dentition among known eobaatarids and provides a complete premolar series ( Figs. 2A , 3A ). It shows the precise dental characters of eobaatarid upper cheek teeth, especially those of the premolars, and provides the key to resolving homology of “plagiaulacidan” and cimolodontan premolars that has yet to be sufficiently understood .

Figures

Fig. 2. Eobatarid multituberculate mammal Hakusanobaatar matsuoi gen. et sp. nov., SBEI 1736, holotype; Lower Cretaceous Kuwajima Formation, Shiramine, Japan. SEM photograph of resin casts. A. Left upper dentition; A1, labial view (I3, P1–P5 and the base of I2); A2, occlusal view (only cheek teeth), left to anterior. B. Isolated right M1, postero−labial view. C. Right lower jaw fragment with p3 and p4, labial view.

Fig. 3. Eobatarid multituberculate mammal Hakusanobaatar matsuoi gen. et sp. nov., SBEI 1736holotype; Lower Cretaceous Kuwajima Formation, Shiramine, Japan. A. Left upper dentition; A1, I3 and P1 to P5 in labial view; A2, I3 in anterior view; A3, P5 in lingual view, right to anterior; A4, P1 to P4 in occlusal view, left to anterior; A5, P5 in occlusal view, left to anterior.B. Isolated left M1; B1, in labial view; B2, in occlusal view, left to anterior. C. Isolated I2 in lateral view.

Fig. 4. Eobatarid multituberculate mammal Hakusanobaatar matsuoi gen. et sp. nov., SBEI 1736, holotype; Lower Cretaceous Kuwajima Formation, Shiramine, Japan. A. Right lower jaw fragment with p3 and p4, labial view. B. Isolated right lower incisor; B1, labial view; B2, somewhat occlusal view. C. Isolated right M1; C1, occlusal view, right to anterior; C2, labial view.

Fig. 5. Eobatarid multituberculate mammal Hakusanobaatar matsuoi gen. et sp. nov.; Lower Cretaceous Kuwajima Formation, Shiramine, Japan. A. SBEI 581, fragment of left lower jaw with damaged p4; A1, labial view; A2, lingual view. B. SBEI 1526, fragment of right lower dentary with incisor; B1, labial view; B2, lingual view. C. SBEI 1520, damaged left p4; C1, labial view; C2, lingual view.

Fig. 6. Eobatarid multituberculate mammal Hakusanobaatar matsuoi gen. et sp. nov.; Lower Cretaceous Kuwajima Formation, Shiramine, Japan. A. SBEI 1519,?left p3; A1,?lingual view; A2,?labial view; A3, anterior view. B. SBEI 582, damaged right upper premolar (probably P2); B1, labial view; B2, occlusal view, left to anterior; B3, lingual view. C. SBEI 1949, poorly preserved upper premolar fragment (two?labial cusps of probably right P5); C1,?labial view; C2, occlusal view,?right to anterior.

Fig. 7. Eobatarid multituberculate mammal Hakusanobaatar matsuoi gen. et sp. nov.; Lower Cretaceous Kuwajima Formation, Shiramine, Japan; SEM photograph of SBEI 2352 (a resin cast of SBEI 581 before the anterior part of the dentary was lost), left lower jaw fragment with damaged p4. A. Lingual view. B. Labial view; arrows indicate alveoli of a single−rooted p2 and a double−rooted p3. C. Occlusal view; arrows indicate the alveolus of a p2 and the anterior alveolus of a p3.