Plesiosorex fejfari, Oshima & Tomida & Orihara, 2017

Oshima, Mitsuharu, Tomida, Yukimitsu & Orihara, Takamichi, 2017, A New Species Of Plesiosorex (Mammalia, Eulipotyphla) From The Early Miocene Of Japan: First Record Of The Genus From East Asia, Fossil Imprint 73 (3 - 4), pp. 292-299 : 293-297

Plesiosorex fejfari sp. nov.

Text-fig. 2

H o l o t y p e. Nearly complete left mandible with complete i2 and p3, fragment of p4 and incomplete m1 (NMNS PV-20155).

T y p e l o c a l i t y. Left bank of the Kiso river in Dota village, Kani City, Gifu Prefecture, central Japan ( Text-fig. 1 ).

H o r i z o n a n d g e o l o g i c a g e. Uppermost part of the Nakamura Formation of Mizunami Group; Early Miocene, correlated with European land mammal zone MN 3.

E t y m o l o g y. Dedicated to Professor Oldřich Fejfar in recognition of his outstanding work on Neogene mammals.

M e a s u r e m e n t s. See Text-fig. 3 , Table 1.

D i a g n o s i s. Large-sized species of the genus with elongated mandibular body; ventral margin of the dentary nearly horizontal; both condyloid and angular processes posteriorly elongated; p3 double rooted; neither hypoconulid nor any cingulid present on m1.

D e s c r i p t i o n. Left mandible. It is nearly complete, lacking the coronoid process. Impression of the coronoid process had been preserved on the host rock, but it was lost by the preparation of the specimen ( Text-fig. 2c ).

The mandibular body in lateral view is relatively horizontal, and it is deepest at the area anterior to p4. The mandibular ramus inclines anteriorly, and the tip of the coronoid process is rounded ( Text-fig. 2c ), as stated from the impression on the host rock. This lost ascending ramus may have been offset laterally judging from the remaining portion of the ramus. The dentary bends laterally next to m3, with a shelf set between m3 and the ascending ramus. A ridge is present between m3 and the mandibular foramen on the dorso-lingual edge of the dentary, and it becomes sharper posteriorly. The mandibular foramen opens posteriorly but slightly dorsally above that ridge. The upper pterygoid fossa is triangular in outline and shallow. Since the angular process extends postero-lingually, the lower pterygoid fossa is C-shaped in posterior view ( Text-fig. 2d ), and the bottom of the lower pterygoid fossa is relatively wide and shallow. The condyloid process is extended posteriorly and slightly dorsally, but does not reach posteriorly as far as the angular process, and is transversely almost horizontal and cylindrical in shape. The angular process is significantly elongated posteriorly. The lower sigmoid notch is relatively deep and C-shaped in lateral view. The mandibular symphysis terminates below the posterior margin of p4. The anterior and posterior mental foramina are below the anterior root of p3 and the posterior root of p4, respectively.

Dentition. Dental formula of the lower jaw is 3·0·4·3, lacking c ( Gunnel et al. 2008). The i2 and p3 are well preserved, while p4 and m1 are fragmentary and incompletely preserved, respectively. The i1, i3, p1‒2, and m2‒3 are lost, while their alveoli are preserved. A small cylindrical hole near and ventral to the i2 is considered to be the alveolus of the i1. The i2 is a large piercing, rooted tooth; it gently curves upward, and the tip is sharp. In cross section the i2 crown is D-shaped as the lingual side is worn. The i3, p1 and p2 are single rooted. The alveolus of the i3 is smaller than that of p1, wider than long, and it opens antero-dorsally. The alveolus of the p1 is smaller than that of p2, equal width and length, and it opens antero-dorsally, but slightly more dorsally than in i3. Alveolus of p2 is wider than long and opens antero-dorsally.

The p3 is composed of a large high principal cusp and a distal small cusp, and the occlusal outline of the crown is somewhat rhombus in shape. A sharp edge is developed on the proximal margin, whereas the distal ridge is weak and worn, and a small cusp is weakly developed at the posterior margin (arrow of Text-fig. 2e ). The postero-buccal part of the crown widens and slopes gently from the small cusp at the distal end of the crown. The ventral margin of enamel on the buccal surface descends distinctly posteriorly, while that of the lingual surface does not extend downwards as much. The p3 has two roots, partially visible, the anterior one directed almost vertically, while the posterior one directs postero- -ventrally. For the p4, only the lingual half of the posterior root and a small postero-lingual portion of the crown are preserved. The latter preserves a small wear surface with dentine exposed, indicating the presence of an entoconid- -like structure. This fact, in addition to the p3 having some structure on the posterior part of the crown, suggests the possibility that p4 had been a submolariform tooth, like that of P. evolutus ( Ziegler 2006) , and not a premolariform, as in P. styriacus ( Thenius 1949).

The occlusal outline of m1 is trapezoidal. The m1 is extremely large, the largest tooth of the dentition. The trigonid is composed of the protoconid, paraconid and metaconid, while the talonid is composed of the hypoconid and entoconid. Since the buccal side of m1 is damaged, protoconid and hypoconid are somewhat deformed distally. Because of this deformation, m1 length ( Tab. 1) is not accurate and is somewhat overestimated. The paraconid, metaconid, and entoconid maintain their original form and positions. The trigonid of m1 is moderately long, with an estimated trigonid/talonid length ratio of ca. 1.6, although it is inaccurate because of the damage mentioned above (Textfig. 2f). The postero-linguo-dorsal view of m1 ( Text-fig. 2g ) does not show any distortion due to the viewing angle. The trigonid is higher than the talonid, and the highest cusp of the crown is the protoconid.

Because of the damage to the protoconid, a carnassial- -like notch is not observable, but the curvature of the enamel surface in the postero-lingual direction at the postero-buccal end of the paralophid suggests the presence of the carnassial- -like notch at the middle of the paralophid. The metaconid is an isolated conical cusp, and the metalophid is not present. The hypoconid is positioned clearly distally to the entoconid, and it projects postero-buccally. The hypoconulid and all cingulid structures are absent in the m1.

The alveolus of the m2 comprises two holes: the proximal one is 0.96 mm long, 1.64 mm wide, while the distal one is 1.19 mm long, 1.61 mm wide, with a total length of 2.42 mm. Estimating from the size of the alveolus, the m2 is much smaller than m1, about one half the length (based on an estimation of m2 length ca. 2.5 mm considering the gap between alveoli of m2 and m3). There are also two holes conforming with the m3 alveolus: the proximal one is nearly ellipsoidal in outline and 0.68 mm long, 1.23 mm wide, whereas the distal one is almost circular and is 0.90 mm long, 1.06 mm wide, and the total length of the two is 1.90 mm.

C o m p a r i s o n. The family Plesiosoricidae is partially characterized by the lower jaw and teeth (Gunnell et al. 2008): i2 hypertrophied into procumbent, piercing teeth; p4 with low, short talonid; dentary with sharply upright or slightly anteriorly inclined coronoid process and transversely cylindrical condyle; symphysis procumbent and extending to anterior root of p4.

The genus Plesiosorex is partially characterized by the lower jaw and teeth as well (Gunnell et al. 2008): dental formula complete or reduced by one tooth; molars distinctly graded in size from large m1 to small m3; c1 reduced or absent; i2 enlarged, piercing teeth; p4 semi-molariform; m1 with carnassial-like shear; lower molar hypoconids reduced and centrally located on hypolophids; incisors to p2 and usually p3 single-rooted and unicuspid.

These characters are consistent with those of the specimen described above, and we conclude that it belongs to the genus Plesiosorex . Among the family, Plesiosorex aydarlensis KORDIKOVA, 2000 (E. – M. Miocene), Pseudoneurogymnurus shevyrevae GUREEV, 1979 (E. Oligocene), Pakilestes lathrius RUSSEL et GINGERICH, 1981 (M. Eocene), and Ordolestes ordinatus LOPATIN, 2006 (E. Eocene) are known in Asia ( Ziegler 2009), but Plesiosorex is the only genus known from the Early Miocene in Asia, and it is morphologically distinguishable from the Paleogene genera. Twelve species have been identified in the genus Plesiosorex globally ( Ziegler 2009).

Plesiosorex fejfari differs from all known species with known p3 or p3 alveolus ( P. soricinoides (DE BLAINVILLE, 1838) , P. germanicus ( SEEMANN, 1938) , P. coloradensis WILSON, 1960 , P. donroosai GREEN, 1977 , P. latidens ( HALL, 1929) , and P. greeni MARTIN et LIM, 2004 ) in that it has a double rooted p3. Comparisons based on other characters and with the species without p3 follow. Among European species, P. roosi FRANZEN, FEJFAR et STORCH, 2003 differs from P. fejfari in having nearly upright or only slightly anteriorly inclined coronoid process, posterior mental foramen beneath m1, and high trigonid/talonid length ratio in m1 ( Franzen et al. 2003). Plesiosorex evolutus ZIEGLER, 2006 is known only from isolated teeth and is somewhat similar to P. fejfari in size and trigonid/talonid length ratio in m1, but it differs from P. fejfari in having a m1 with cingulids, a deep notch on the entocristid, and small m1/m2 length ratio ( Ziegler 2006). Plesiosorex schaffneri ENGESSER, 1972 differs from P. fejfari in being somewhat smaller in size and in having a lower trigonid/talonid length ratio in m1 and posterior mental foramen beneath m1 ( Engesser 1972, 1979, Franzen et al. 2003: 100, Ziegler 2006: 112). Plesiosorex germanicus and P. styriacus (HOFMANN, 1893) are quite similar to each other ( Franzen et al. 2003), but both differ from P. fejfari in their smaller size and in having a relatively small trigonid/talonid length ratio in m1 and posterior mental foramen beneath m1 ( Franzen et al. 2003). Plesiosorex soricinoides (including P. cf. soricinoides from Chaveroche, France) differs from P. fejfari in having precingulid and ectocingulid on m1, wide occlusal outline and antero-posteriorly shorter trigonid on m1, posterior mental foramen beneath the anterior root of p4, and smaller size ( Engesser 1979, Franzen et al. 2003). Because P. cf. soricinoides seems to have complete dentition ( Viret 1940, Engesser 1979) based on the number of alveoli, we code it as complete lower dentition, although Ziegler (2009) considered it incomplete.

Among North American species, P. coroladensis differs from P. fejfari in having complete lower dentition, posterior mental foramen beneath the anterior root of m1, anteroposteriorly shorter trigonid in m1, and smaller m1/m2 length ratio (ca. 1.4) ( Wilson 1960, Engesser 1979). Although Wilson (1960: 21) indicated “p3 with two roots”, if the lower dentition is complete, then the number of alveoli is insufficient, so we follow Ziegler (2009: 370), in which he noted “p3 singlerooted”. Plesiosorex donroosai differs from P. fejfari in having an angular tip on the coronoid process, curved ventral margin of the mandibular body, posterior mental foramen beneath the anterior root of m1, and antero-posteriorly shorter trigonid in m1 ( Green 1977, Martin 2012). Plesiosorex greeni differs from P. fejfari in having a posterior mental foramen beneath the anterior root of m1, m1 with strong external but reduced labial cingulid, antero-posteriorly shorter trigonid in m1, and smaller m1/m2 length ratio (ca. 1.43) ( Martin and Lim 2004). Plesiosorex latidens (including Meterix sp. of Engesser 1979) differs from P. fejfari in its angular tip of the coronoid process, curved ventral margin of the mandibular body, posterior mental foramen beneath the anterior root of/ or middle of m1, and antero-posteriorly shorter trigonid in m1 ( Hall 1929, Engesser 1979).

Figures

Text-fig. 2. Plesiosorex fejfari (holotype, left mandible, NMNS PV-20155). a. buccal view; b. lingual view; c. buccal view of the impression of the posterior part of the mandible on the host rock; d. posterior view; e. p3 in postero-bucco-dorsal view; f. m1 in occlusal view; g. m1 in postero-linguo-dorsal view; d and f images are depth synthesized by digital microscope (KEYENCE VHX-900).

Text-fig. 1. Map showing the locality of Plesiosorex fejfari sp. nov. and chronological relationship of the Nakamura Formation (from Mörs et al. 2016). a. general location of Kani City; b. geographical map showing the type locality (solid star) near Dota village in Kani City; c. chronological relationships of the Miocene strata in the Kani Basin. For further information, see Mörs et al. (2016).

Text-fig. 3. Measurements of the holotype.