Neocometes aff. similis Fahlbusch, 1966
Fig. 2
View Fig
.
Material.—KIGAM VP 200315, a complete left first lower molar from the Jiga−dong site of the Bukpyeong Formation (Miocene) in Donghae City, South Korea.
Description.—The specimen is a well−preserved, two−rooted tooth although the mesiolabial part of the anterolophid is slightly damaged. The enamel is dark brown and black in color. The maximum length and width of the crown is 2.0 mm and 1.25 mm, respectively. Two relatively long roots of equal length are nearly perpendicular to the crown. The boundary between the crown and the root both on the labial and lingual sides is not straight but wavy. The total height of the tooth is 2.3 mm ( Fig. 2
View Fig
), including the roots, although the tooth pattern is lophodont and relatively low−crowned. In occlusal view, the tooth shows a pear−like outline as the mesial portion is a little narrower and more round than the distal portion. Six transverse ridges are separated by five synclines (Ia, I, II, III, IV). These ridges show a medium stage of wear, but the labial side is higher crowned than the lingual side. Although the synclines are curved mesially in the labial region in occlusal view, their lingual angles to the longitudinal axis vary at each syncline (30 ° in Ia, 75 ° in I, 85 ° in II, 90 ° in III and IV). The inclination of the ridges (difference from the perpendicular) also gradually decreases from mesial to distal. While synclines II and III are widely and deeply open both lingually and labially, synclines I and IV are slightly open only lingually and syncline Ia is completely closed by the anterolophid and the anterior transverse ridge. The anterior basin is divided by the anterior transversal extra ridge (= “Vorderer Quersporn”), but the metalophid is connected with the junction of the anterolophid and the anterior transversal extra ridge labially so that syncline I is open only lingually. Syncline Ia is asymmetrically divided into two small basins by the connection of the anterolophid with an anterior transversal extra ridge. The lingual basin is slightly larger than the labial one. The enamel ridge of the smaller basin in syncline Ia is connected with the mesial enamel ridge of the anterolophid. Syncline II is open labially and lingually between the mesolophid and the metalophid. The mesolophid is curved labially parallel to the curvature of the metalophid so that the protoconid is located more mesially than the mesoconid. The mesolophid is not connected with any ridges and overlaps the positions of the protoconid and mesoconid. Syncline III between the mesolophid and the posterior transversal extra ridge is the deepest and longest among all synclines. The entoconid–hypoconid is connected with the posterolophid labially. The latter is nearly parallel to the posterior transversal extra ridge, but relatively short. The hypoconid is more distinct than the entoconid. Syncline IV is slightly open lingually.
Comparisons.—The tribe
Platacanthomyini
(subfamily
Platacanthomyinae
) includes the living genera
Platacanthomys
and
Typhlomys
, and the fossil genus
Neocometes
. Living
Platacanthomys
and
Typhlomys
are monospecific (e.g.,
Platacanthomys lasiurus Blyth, 1859
, and
Typhlomys cinereus
Milne− Edwards, 1877), but
Platacanthomys
has one fossil species from the late Miocene (about 9 Mya, Ni and Qiu 2002) of China (
P. dianensis, Qiu 1989
) and four extinct species of
Typhlomys
were described from late Miocene and Quaternary sediments of China (
Typhlomys primitivus
,
Typhlomys
hippa−
doi:10.4202/app.2010.0013
rionum,
Typhlomys intermedius
, and
Typhlomys macrourus
; Qiu 1989; Zheng 1993). The dental morphology of these two genera is easily distinguished from that of
Neocometes
. The latter has more open synclines and the inclination of ridges and synclines to the longitudinal axis of the crown is steeper than in
Platacanthomys
and
Typhlomys ( Fejfar and Kalthoff 1999)
. KIGAM VP 200315 fits well with the dental characters of
Neocometes
, such as lingually and labially opened synclines II and III, and synclines III and IV oriented nearly 90 °.
Neocometes
is known from three species:
Neocometes orientalis
,
Neocometes similis
, and
Neocometes brunonis
.
Neocometes orientalis
from Thailand is quite different from Korean
Neocometes
because its lingual synclines tend to close ( Mein et al. 1990), thus exhibiting similarity to extant
Typhlomys ( Chaimanee et al. 2007)
View in CoL
.
Korean
Neocometes
is most similar morphologically to
N. similis
from Dolnice 2, Czech Republic ( Fejfar 1974) and Erkertshofen 2, Germany (type locality of Fahlbusch 1966). The dental structure of KIGAM VP 200315 is strikingly similar to
N. similis
(specimen number 73355) from Dolnice 2, Czech Republic ( Fejfar 1974: fig. 31−1), the only difference being that the former (length 2.0 mm) is a little larger than the latter (length 1.83 mm). Intermediate forms between
N. similis
and
N. brunonis
were called
Neocometes cf. similis
by Fejfar (1999) and Fejfar and Kalthoff (1999). They also interpreted the European Miocene record to demonstrate evolution from
N. similis
to
N. cf. similis
to
N. brunonis
. Morphological changes are modest with only increasing size of the low−crowned molars recognized. The size of KIGAM VP 200315 falls into the range of
N. brunonis
rather than
N. similis
( Fejfar and Kalthoff 1999: fig. 2). However, Daxner−Höck (1998) demonstrated that the first lower molar of
N. similis
(length 2.0 mm, width 1.3 mm) from Oberdorf 4 (MN 4) overlaps with
N. brunonis
in size. The large Oberdorf specimen is essentially the same size as Korean
Neocometes
(length 2.0 mm, width 1.25 mm). Large
Neocometes
teeth were also reported by Aguilar et al. (1997) from Ste Catherine 2 and 9 in France. They indicate that size is not an absolute criterion to identify
Neocometes species.
In addition, Fejfar (1974) divided
Neocometes
teeth into morphotype A and morphotype B by the labial connection of the metaconid–paraconid ridge with the anterolophid. Morphotype A makes up 100% of the sample of first lower molars of
N. similis
from Erkertshofen (type locality), but it is still present in 40% of
N. brunonis
from Neudorf (type locality; Fejfar 1974; Schötz 1981). However, the typical characteristic of
N. brunonis
is the separation of the anterolophid into stylids (= morphotype B, Fejfar 1999; Fejfar and Kalthoff 1999). KIGAM VP 200315 belongs to morphotype A (= archaic stage, sensu Fejfar 1974).
Applying Fejfar’s (1999) proposed chronospecies−model for the evolution of
Neocometes
in the European Miocene to Korean
Neocometes
, in which
N. similis
(MN 4) gives rise to
N. cf. similis
(MN 5), which gives rise to
N. brunonis
(MN 6 and MN 7/8), it is reasonable that KIGAM VP 200315 should be assigned to
N. aff. similis
, at least until more fossils are available.
