Collimys caucasicus TESAKOV, 2017
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publication ID |
https://doi.org/ 10.2478/if-2017-0021 |
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persistent identifier |
https://treatment.plazi.org/id/03AF8797-FFD9-FFA1-FF65-FA81B899FC4E |
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treatment provided by |
Diego |
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scientific name |
Collimys caucasicus TESAKOV |
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Genus Collimys DAXNER- HÖCK, 1972
Collimys caucasicus TESAKOV , sp. nov. Pl. 10, Text-fig. 13f, g View Text-fig
H o l o t y p e. Right m1, GIN-1143-001, 1.85 × 1.17 mm (Pl. 10, Fig. 1), housed in the collection of Geological Institute of the Russian Academy of Sciences.
T y p e l o c a l i t y. Volchaya Balka, bed 7, Late
Miocene, North Caucasus, Russia.
E t y m o l o g y. After the geographic provenance of the new taxon.
D i a g n o s i s. Hypsodont Collimys preserving short mesolophs/mesolophids.
D i f f e r e n t i a l d i a g n o s i s. C. caucasicus sp. nov. differs from most congeneric species in higher hypsodonty ( Text-fig. 13 View Text-fig ) and strong reduction of mesoloph/mesolophids. From Pseudocollimys steiningeri DAXNER- HÖCK, 2004 it differs in larger size ( Tab. 7), lower hypsodonty, and less reduced mesolophs/mesolophids.
A d d i t i o n a l m a t e r i a l. 1 m 1, 1 M3: Volchaya
Balka; 1 m 1, 1 m2, 1 M1, 1 M2, 2 M3: Gaverdovsky .
D e s c r i p t i o n. The hypsodont cricetine with alternating dental prisms, deep reentrant synclines, flat occlusal surface after a slight wear, and thick enamel wall. Molars with two roots in the lower jaw and three roots in the upper dentition.
m1. The molar is composed of ta posterior loop integrating a hypoconid and broad posterolophid, alternating entoconid, protoconid, and metaconid, and anteroconid. Tips of lingual anticlines are squared in contrast with the more rounded labial ones. The mesolophid is reduced in size (Pl. 10, Figs 1–2) and becomes an indistinct sloping spine reaching the side of the molar. An ectostylid is present in two of the three specimens, in one case it is doubled (Pl. 10, Fig. 3). A rounded anteroconid is broadly fused with the metaconid. In the less worn specimen it is slightly bifid anteriorly (Pl. 10, Fig. 1). The rounded lingual side contrasts with the labial which is expanded by a well-developed anterolophid. The anteroconid of the type specimen bears two strongly obliterated enamel islets indicating its initial complex structure of three lobes subsequently fused by wear.
m2. The only available molar is heavily worn (Pl. 10, Fig. 4).
M1. The molar is composed of alternating loops of anterocone, protocone, paracone, hypocone, and metacone. The mesoloph is well developed but does not reach the labial side of the molar; at the current stage of wear (Pl. 10, Fig. 5) it is fused with the metacone thus closing a fossette. The anteromesoloph is reduced in length and fused with the anterocone.
M2. The molar is composed of four main cusps, and a moderately developed anterocone on a long labial anteroloph (Pl. 10, Fig. 6). The mesoloph is short, with wear its tip fuses with the anterior face of the metacone enclosing a fossette. The posterior metalophule is prominent and delimits a shallow posterosinus.
M3. The anterior part of the molar is composed of a large protocone and labial anteroloph. The large paracone connects to the protocone after considerable wear (Pl. 10, Fig. 7). The anterosinus tends to be closed forming a fossette by the anterior lophule (protolophule I) (Pl. 10, Figs 8–9). The mesoloph is long and reaches the labial margin of the crown. The paracone spur may reach the mesoloph and close a fossette in the anterior part of the mesosinus. With only slight wear the mesoloph, hypocone, posteroloph and a small, cusplet-like metacone delimit a fossette in the posterior part of the mesosinus. When in an unworn condition, the posterosinus contains a minute basin (Pl. 10, Fig. 7) which becomes a very small fossette when worn (Pl. 10, Fig. 9).
C o m p a r i s o n. In comparison with late Middle Miocene (“MN 7”) forms from south Germany, C. transversus HEISSIG, 1995 from Steinheim and C. gudrunae RUMMEL et PRIETO, 2009b from Petersbuch 31, the Caucasian Collimys is significantly more hypsodont, and shows a large reduction of mesolophs(ids) and ectomesolohids in m1, and a nearly complete reduction of anteromesoloph in M1, and lacks a lingual anteroloph in M2. It is also larger than C. transversus .
In comparison with the group of forms from faunas of the Astaracian-Vallesian transition in south Germany, Switzerland, and Hungary (“MN 8”/MN 9), C. hiri PRIETO et RUMMEL, 2009 from Hammerschmiede, C. longidens KÄLIN et ENGESSER, 2001 from Nebelbergweg, and C. dobosi HIR, 2005 from Felsőtárkány 3/2, C. caucasicus sp. nov. differs in higher hypsodonty, a single anterolophulid in m1, stronger reduction of mesolophs(ids) and other elements of dental complexity.
From C. primus DAXNER- HÖCK, 1972 from Kohfidisch and Eichkogel (MN 11) and Richardhof-Wald (MN 10) it differs in its slightly higher hypsodonty ( Text-fig. 13 View Text-fig ), a flatter occlusal surface, and strong reduction of mesolophs(ids) and anteromesolophs in M1.
From C. cf. primus from Dorn-Dürkheim ( Franzen and Storch 1975) it differs in having a less reduced mesolophid in m1, and mesoloph in M2. Otherwise, this form is the best morphological match for the described species.
From Colloides xiaomingi from Vallesian – early Turolian correlated faunas of northern China ( Qiu and Li 2016) it differs in smaller and less divided anteroconids in m1, and less reduced mesolophs (-lophids).
C o m m e n t s. After the first description of the peculiar hypsodont hamsters of the genus Collimys in Late Miocene deposits of the Vienna basin ( Daxner-Höck 1972), the group was recognised in multiple late Middle Miocene ( Heissig 1995, Kälin and Engesser 2001, Hír 2005, Prieto and Rummel 2009a, b) and Late Miocene ( Franzen and Storch 1975) localities in central Europe. Probably descending from Democricetodon ( Heissig 1995) , the group splits into separate phyletic lineages all demonstrating trends towards higher hypsodonty and larger dimensions, and different degrees of reduction in elements of molar complexity. The largest so far known sample of Collimys ( C. dobosi ) described by Hír (2005) from latest Middle Miocene of Hungary showed a considerable amount of morphological variation in dental characters thus emphasizing the risk of coining taxa based on minor differences in small samples. The description of Pseudocollimys from the late Vallesian locality of Schernham ( Daxner-Höck 2004a) showed that some of these lineages could develop a notable hypsodonty and almost completely reduce the mesolophs (-ids) and other smaller elements. The rank of genus for Pseudocollimys is questioned here. Two other records referable to Pseudocollimys originated from the MN 10 correlated locality of Suchomasty in the Czech Republic (labeled as “ Cricetidae aff. Pseudomeriones gen. et sp. indet.” in Fejfar 1990: fig. 14: 1–3) and the MN 11 locality of Dorn-Dürkheim in Germany ( Franzen and Storch 1975). One specimen of an incomplete M1 from the latter site ( Franzen and Storch 1975: taf. 5, fig. 41) more closely corresponds with the morphology of Collimys primus . This broadly synchronous form, the type species Collimys primus from late Vallesian to early Turolian (MN 10 – MN 11) of Austria, shows a much less derived morphology (Daxner- Höck 1972, Daxner-Höck and Höck 2015) and apparently represents a Late Miocene survivor of a distinct and more conservative lineage/species group, most likely that of C. hiri-C. dobosi – (MN 9) of Prieto and Rummel (2009b). In addition to the original type series of Collimys primus from Eichkolgel, Daxner-Höck and Höck (2015) illustrated supplementary material on C. primus (M1, M2) from Kohfidisch (MN 11), and m2 from Richardhof-Wald (“MN 10a”). The latter locality is believed to be older than the latest Vallesian fauna of Schernham (“MN 10b”; Daxner- Höck et al. 2016) that yielded Pseudocollimys . These data indicate a co-existence of two collimyine lineages in the late Vallesian of Austria. This co-occurrence could possibly have continued into the early Turolian as likely indicated by the material from Dorn-Dürkheim.
The Caucasian form shows a notable similarity to Pseudocollimys steiningeri , but although being slightly larger, it demonstrates less advanced states of most dental characters. The striking difference is in the well developed mesolophs/mesolophids resembling a condition of less hypsodont Collimys species. Thus being dentally intermediate between the two Late Miocene collimyines from Central Europe, the Caucasian form is most likely the youngest of them. This mosaic of different stages of dental complexity and hypsodonty not forming a chronological sequence probably indicates a separate position of C. caucasicus sp. nov., as an advanced stage of a distinct lineage of collimyines in the Caucasus, or a direct successor of C. primus . It can be hypothesized that Pseudocollimys steiningeri with a reduced depth of anterolingual synclines in M1 may be derived from brachyodont Collimys showing this character, such as C. transversus from Petersburg 26 ( Prieto and Rummel 2009b: fig. 2d). The limited material hampers evaluation of this character, moreover C. transversus from the type locaity of Steinheim does not show any reduction in the depth of this syncline ( Heissig 1995: taf. I, figs 5, 7, Kälin and Engesser 2001: fig. 36d).
The recent find of collimyine hamsters in Late Miocene faunas of northern China ( Qiu and Li 2016) reveals a previously unrecognised vast distribution area of this group in Eurasia. The Caucasian form shows a similarity to Colloides xiaomingi from the Vallesian-early Turolian correlated faunas of northern China ( Qiu and Li 2016) in size and hypsodonty level and overall dental structure of a medium-hypsodont collimyine. It is, however, markedly different in the less reduced mesolophids (-lophs) and smaller anteroconid in m1. The teeth described as M 3 in Chinese form may actually represent M2. The genus rank status of the Chinese form is due to a discussion. A series of forms with a gradually increasing hypsodonty may suggest the attribution of Colloides xiaomingi to the genus Collimys .
The hypsodonty and alternating dental elements with transverse enamel ridges indicate propalinal mastication and probably mark an early stage of obligate herbivory in at least advanced forms of Collimys . The similar biomechanical constraints on dentition with propalinal movements in muroid rodents form vole-like prismatic molars in many independent lineages in the Middle – Late Miocene ( Fejfar et al. 2011).
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