Promimomys cf. davakosi ( VAN DE WEERD, 1979 )

Promimomys cf. davakosi ( VAN DE WEERD, 1979)

Text-figs 11–13

M a t e r i a l. 25 m 1, 10 m 2, 17 m 3, 23 M1, 25 M2,

36 M3, fragments of mandibles, and palates.

D e s c r i p t i o n. Primitive brachyodont vole species with very low dentine tracts, molar teeth lacking cement, early root formation, and nearly uniform enamel thickness on leading and trailing edges of occlusal triangles. Advanced over the more underived Promimomys by expressing a Mimomys -ridge (kante) and distinct crenulations on the anteroconid of juvenile m1s. Lower molars have two roots, and the upper molars, three roots.

Lower molars. m1 ( Text-fig. 11a–c). The anteroconid shows well developed reentrant folds. The anteroconid cap bears shallow juvenile folds (crenulations) in the youngest specimens. Both Mimomys and islet reentrants and a Mimomys -ridge are well expressed, and extend down the crown nearly to the enamel-dentine juncture. LRA4 is relatively deep. Its mean elevation above a lingual enamel baseline drawn below LRA1 is 0.95 mm (n = 21). The enamel islet is closed at a very early ontogenetic stage. The islet is oval or slightly curved in molars with higher crowns and becomes more circular with wear. The islet disappears with wear at crown heights less than 0.9 mm. T1 and T2 are widely confluent at all wear stages. In medium-worn molars, tips of lingual reentrants curve anteriorly. Especially strong provergence (anterior curving) is shown by LRA2. Much weaker curving is shown by LRA3, and even less so in LRA1. Dentine tracts are low (see Tab. 1), form a sequence from highest to lowest as ASD-HSD-HSLD. ASD commonly shows curving of an incipient mimosinuid at the base of the Mimomys ridge.

m2 ( Text-fig. 11d–g). The pairs of occlusal triangles T1-T2 and T3-T4 are widely confluent at all wear stages. The posterior root of m2 has a fork-like shape and is placed upon the dorsal surface of the lower incisor (pleurorhiz condition).

m3 ( Text-fig. 11h–k). The confluence of occlusal elements is similar to m2.

Upper molars. M1 ( Text-fig. 11l–o). Occlusal dentine triangles are confluent with the strongest confluence between dentine fields of T3 and T4. Reentrant folds are wide and “V-shaped.”

M2 ( Text-fig. 11p–s). The tip of BSA2 protrudes labially only slightly further than that of BSA1. The reentrants mirror those of M1.

M3 ( Text-fig. 11t –w). An anterior enamel islet forms in the antero-labial reentrant (BRA1) at a relatively deep level, occurring above the base of BRA 1 in the range of 0.15–0.77– 1.4 mm (min-mean-max; n = 36). The height of the labial dentine tract (hyposinus) ranges from 0.02 to 0.1, with a mean of 0.05 mm (n = 28). A few specimens (n = 3) show a complete reduction of this islet formation. The posterior enamel islet closes at early wear stages by isolation of LRA3. The islet has a curved shape at earlier stages and becomes more circular in more worn molars. The islet extends nearly to the base of the crown.

Schmelzmuster. The basically undifferentiated enamel thickness shows a slightly negative differentiation, with thinning in the deep parts of reentrant folds. In molars used for microstructure studies, the differentiation quotient, BTQ, equals (m1) = 113.33; (M1) = 113.33; (M2) = 123.33. The Schmelzmuster was studied in the first lower molar ( Text-fig. 12), and the first and second ( Text-fig. 13) upper molars. There is no difference in the structure of enamel between the lower and upper molars. The type of microstructure is eopachyknem in transition to protopachyknem ( Rabeder 1981): leading edges consist of radial enamel, trailing and closing edges consist of radial and primitive tangential enamel ( Text-fig. 12c, d). In addition, primitive lamellar enamel was found on m1 and M2. The most conspicuous zone of lamellar enamel is located on the apical part of the anteroconid of m1 ( Text-fig. 12a). We propose to define apical enamel as a separate opening enamel type as opposed to the closing enamel on the posterior wall of the posterior loop. The smaller zones of lamellar enamel are also observed on the leading edge of BSA3 ( Text-fig. 12b) in m1, and in the leading edge of incipient BRA 3 in M2 ( Text-fig. 13c).

C o m p a r i s o n. Mimomys moldavicus KORMOS, 1932 . The Siberian form is notably more advanced as compared to the range of Early Pliocene forms attributed to Promimomys (= Mimomys ) moldavicus in Romania, Moldova, and southwestern Ukraine. The type material of this species shows very low indentations of the enamel-dentine boundary, poorly dissected anteroconid in m1, and poorly differentiated T3 and T4 coupled with shallow or absent LRA 3 in M1 ( Radulescu and Samson 1989, Crespo et al. 2023).

Promimomys antiquus ZAZHIGIN, 1980 . Promimomys from Novaya Livenka is notably more advanced in hypsodonty than the monotypical P. antiquus from a borehole in the Ob’ River Plateau, which shows a very low or absent anterosinuid, less developed reentrants and a more triangular anteroconid ( Zazhigin 1980: 96, fig. 19: 4).

Mimomys davakosi VAN DE WEERD, 1979 . The Siberian vole is very similar in size and tooth morphology to the form from Greece. The observable difference is in somewhat higher hypsodonty expressed in the height of the anterior dentine tract (Ea or anterosinuid), with a range of 0.55–0.7– 0.95 (min-mean-max; n = 23) compared to 0.0–0.26–0.47 (min-mean-max?; n = 21) or mean value of 0.41 (n = 14) in the type locality of Ptolemais 3 ( van de Weerd 1979, Fejfar et al. 1990). Another hypsodonty-related distinction is somewhat deeper enamel islets in m 1 in the Siberian form: islets are obliterated by wear below crown heights of 0.9– 0.8 mm. In the type sample of P. davakosi , enamel islet disappears between crown heights of 1.15 and 0.89 mm ( van de Weerd 1979). The maximum crown height in the form from Greece, ca. 2.02 mm ( van de Weerd 1979: fig. 9) or lower (Hordijk and de Brujin 2009), is slightly lower than in Novaya Livenka (Hmax = 2.15).

Mimomys vandermeuleni FEJFAR, MEIN et MOISSENET, 1990 . The Siberian form is more hypsodont than the Spanish form (anterosinuid ranging from ca. 0.5–0.6 mm). Another important distinction is deeper juvenile folds (crenulations) on the anteroconid of the vole from Almenara ( Fejfar et al. 1990). The main difference indicating separate lineages is the predominance of two-rooted M 3 in P. vandermeuleni in contrast to the three-rooted condition in P. cf. davakosi from Siberia.

C o m m e n t s. Attribution of a vole species in West Siberia to the species described from northern Greece is forced by a surprising similarity of morphology of both forms, but our identification of the Novaya Livenka species is tentative. We suspect an independent taxon in Siberia after at least one million years of protomimomyid vole evolution in Eurasia, with several regional radiation centers. At the current state of knowledge, we refrain from a description of a new species, awaiting more data and recognition of more distinctive characters, most importantly on enamel microstructure, on Ruscinian Promimomys forms from the western Palearctic. The geological age of the type locality of P. davakosi, Ptolemais 3 in Greece is precisely bracketed by radiometric data between 4.9–4.81 Ma ( Hordijk and de Bruijn 2009). This seems to be older than the estimates of the Krutaya Gorka formation by Zykin (2012), 4.5–4.3 Ma. This age difference may account for a higher hypsodontу stage in the Siberian vole.

Nevertheless, the similarity in dental evolutionary stage of the Novaya Livenka Promimomys to P. davakosi provides important biochronological information that obviously needs an accurate age calibration in future.