Dolomys occitanus ( Thaler, 1955 )

Popov, Vasil V., 2004, Pliocene small mammals (Mammalia, Lipotyphla, Chiroptera, Lagomorpha, Rodentia) from Muselievo (North Bulgaria), Geodiversitas 26 (3), pp. 403-491 : 463-470

Dolomys occitanus ( Thaler, 1955) ( Figs 33-35 )

Mimomys occitanus Thaler, 1955: 1255-1257 . — Chaline 1974: 343-353, figs 3-8, 9 5,6. — Terzea 1981: 119-124, figs 2, 3, 4A-D, 5A, B. — Chaline et al. 1981: 821-826.

Mimomys hassiacus atavus Fejfar, 1961b: 57-60 , taf. 18, figs 5, 6; abb. 5a, c, 6d.

Mimomys stehlini – Chaline & Michaux 1975: 749- 757, pl. I, text-fig. 1.

Dolomys odessanus – Topachevsky & Nesin 1989: 49, fig. 18.

Promimomys konstantinovae – Topachevsky & Nesin 1989: 82-86, fig. 31.

Dolomys occitanus – Maul 1996: 348. — Fejfar & Reppening 1998: 163, abb. 3, 5. — Dahlmann 2001: 73, abb. 20: 37-52

MATERIAL EXAMINED. — 23 m 1: morphotype “ Mimomys ”: 17 m 1 (9 fragments, 8 intact [Ms7-14]; morphotype “ Dolomys ”: 6 m 1 [Ms1-6, 42], 19 M3 [Ms23-40]) .

MEASUREMENTS (Min-M-Max, SD, N). — Lm1 = 2.67-2.91-3.20, 0.169, 8; am1 = 1.00-1.26-1.37, 0.163, 7; Wm1 = 1.07-1.23-1.65, 0.191, 8; LM3 = 1.50-1.76-2.00, 0.127, 17.

DESCRIPTION

There is no crown-cementum in the side folds of the molars. In most specimens, the enamel is not differentiated, but in some teeth it is slightly thinner on the tips of the salient angles. The crown is moderately high but the dentine tracks of the linaea sinuosa are low.

m1: the variable shape of occlusal surface can be classified into two morphotypes, called “ Dolomys ”- and “ Mimomys ”-morphotypes. The “ Dolomys ”-morphotype ( Fig. 33 A-G) is characterized by an open isle fold. The Sb3-wall is in a low position. The “ Mimomys ”-morphotype bears Mimomys -ridge, prism-fold and isle-fold or enamel islet ( Fig. 33 H-O).

M3: in unworn or slightly worn specimens, the BRA1 and LSA3 are deep, but after some wear they become reduced by insulation. The anterior islet vanishes quickly with further wear, while the posterior one is a long persistent structure, presented even in some worn teeth (Ms40). The clear relationship between the stage of wear and the number of islets indicates that all teeth belong to one species. Most teeth bear two roots. The anterior root is a compound structure, formed by the fusion of two roots. In some specimens the anterior fangs, although partially fused, are still clearly visible (Ms23, 25, 26, 35, 37-39). One tooth shows a very small third root (Ms23).

REMARKS

There is a great controversy regarding the taxonomic status of the “ Dolomys ” and “ Mimomys ” morphotypes of m1s referred to this species. Sulimski (1964) described them as two separate species – Dolomys cf. hungaricus and Mimomys cf. stehlini Kormos, 1931 . Chaline (1974) found in the large assemblage from Sète that these types are connected by intermediate forms, depending mainly on the stage of wear (see also Van de Weerd 1979: table 9), and he therefore regarded the whole material as one highly variable species, Mimomys occitanus . It is a primitive, more or less mesodont form with a relatively high share of specimens in which the isle-fold remains open even in the advanced stages of crown wear – the so called “ Dolomys ”-morphotype. According to Chaline (1974) and Chaline & Michaux (1975) two evolutionary lineages derived from this variable species: one leading through Mimomis stehlini Kormos, 1931 , M. polonicus Kowalski, 1960 , M. pliocaenicus (Forsyth Major, 1889) , and M. savini Hinton, 1910 to Arvicola mosbachensis (Schmidtgen, 1911) , the other one through Propliomys hungaricus and Pliomys episcopalis Méhely, 1914 to P. lenki (Heller, 1930) [= P. coronensis ( Méhely, 1914) ]. The former phyletic sequence is referred to the “ Mimomys ”-lineage, the later one to the “ Dolomys ”-lineage. In fact, the second lineage represents the evolution of Propliomys - Pliomys Méhely, 1914 ( Kretzoi 1955, 1962; De Bruijn & Van der Meulen 1975; Chaline 1975). This disagreement in the nomenclature within the second lineage reflects the similarity in teeth morphology of the earliest forms of these related genera ( Dolomys and Propliomys-Pliomys) ( Nehring 1898; Méhely 1914; Hinton 1926; Kormos 1934b). On the other hand the co-occurrence of the “ancestral” (“ Dolomys ”-morphotype of M. occitanus ) and the derived forms (typical Propliomys with relatively high enamel free areas on m1) in Muselievo does not confirm that they belong to the same phyletic lineage. The lineage Propliomys- Pliomys apparently constitutes an early specialized branch of the evolution of voles and cannot be regarded to derive from Mimomys occitanus as though by Chaline & Michaux (1975).

According to Van de Weerd (1979), M. occitanus is an intermediate stage of evolution from Promimomys Kretzoi, 1955 to the advanced forms of Mimomys Forsyth Major, 1902 . During this evolution, morphological changes of m1 with a variable direction have been proposed: from forms with an isolated islet ( Promimomys and Mimomys davakosi Van de Weerd, 1979 ) through an open islet (most specimens in M. occitanus ) to an isolated islet again ( M. stehlini ) ( Van de Weerd 1979). This variable trend however seems unlikely, having in mind that the direction of an evolutionary change is usually irreversible.

The analysis of the detailed morphology of the enamel islet in the anteroconid of the m1 of Dolomys , Promimomys and Mimomys presented by Maul (1996) provides a new perspective for the solution of this evolutionary and taxonomic problem. Maul (1996) found that M. occitanus differs from the earliest members of the genera Promimomys and Mimomys in having a low positioned wall of the Mimomys -isle (Sb3-wall). This feature is considered a derived one, leading to an evolutionary trend toward lowering of the wall. With the decrease in the height of the wall more Dolomys -morphotypes appear in a population which have greater functional advantage because of their longer cutting edges. In this context, it seems likely that M. occitanus is an ancestral species to forms with a lower Sb3-wall like Dolomys nehringi and does not belong to the Promimomys-Mimomys evolutionary lineage comprising voles with a higher Sb3-wall ( Maul 1996). For the time being, this hypothesis is the best solution of the problem and the attribution of the species “ occitanus ” to the genus Dolomys is followed here. According to Fejfar (2001), in contrast to the previous views ( Chaline 1974; Chaline & Michaux 1975; Van de Weerd 1979), the evolutionary chain leading to M. stehlini is as follows: M. davakosi Van de Weerd, 1979 - M. gracilis ( Kretzoi, 1959) - M. stehlini .

According to the structure of M3, the genus Dolomys is rather specific and comprises such large species as D. milleri Nehring, 1898 and D. nehringi Kretzoi, 1959 (Chaline 1975) . The occlusal pattern of m1 of these species (see for example Kretzoi 1962: abb. 4; Rabeder 1981: abb. 182) is quite different from the “ Dolomys ”- morphotypes of Dolomys occitanus . In this context the opinion of Maul (1996) that D. occitanus is an ancestor of D. nehringi seems unlikely. The occurrence of M 3 in the locality very similar to D. nehringi confirms this opinion.

The same line of reasoning can be developed against the Chaline’s hypothesis that Dolomys - morphotypes of D. occitanus represent an ancestral form for Propliomys hungaricus . The “ Dolomys ”-morphotype is quite different from the m1s of the true Propliomys hungaricus from Csarnóta-1 ( Kormos 1934b: fig. 46; Terzea 1981: fig. 5s, d) and their co-occurrence in Muselievo (see above) indicates that this genus cannot be considered as a derivate of D. occitanus . For the time being, it can be said only that the ondatrine taxa, occurring in the European Pliocene [ Dolomys occitanus ( Thaler, 1955) , Dolomys milleri ( Nehring, 1898) , Dolomys nehringi Kretzoi, 1962 , Dolomys adroveri ( Fejfar, Mein & Moissenet, 1990) , and Propliomys hungaricus (Kormos, 1934) ], represent a mosaic of species related to D. occitanus , which radiated quickly during the Ruscinian and early Villanyian (MN14-16-zones) ( Fejfar & Repenning 1998).

As a whole, the population from Muselievo corresponds to the variability of Dolomys occitanus from the type locality (Grotte 1 de Sète) ( Chaline 1974). In the scatter diagram composed for Lm1 and Em1 ( Chaline 1974: fig. 2) the specimens from Muselievo would occupy the upper half of the cluster of the population from Sète. Hence, the material under study represents a slightly more advanced form. The m1s from Muselievo resemble also D. occitanus from Ciuperceni- 2 in both size and morphology ( Terzea 1981, 1997; and pers. comm.) and differ only in having somewhat higher dentine tracks. Most probably the material from the nearby Romanian locality Dranic, determined as Mimomys moldavicus (Radulescu & Samson 1996) belongs to D. occitanus too, having in mind that M. moldavicus (Kormos, 1932) is a rather primitive form with extremely low dentine tracks, referable to Promimomys (Fejfar et al. 1990) .

In general, the material from Muselievo is very similar to m1 and M3 sample of Dolomys occitanus from Wölfersheim ( Germany) ( Fejfar & Repenning 1998; Dahlmann 2001). The difference concerns only the somewhat longer m1 and the hyposinusids slightly lower in the sample from Muselievo.

The “ Mimomys ” morphotypes of D. occitanus are superficially similar to M. davakosi but differ in having higher dentine tracks ( Van de Weerd 1979). This difference is clearly visible in relation to the material from Ptolemais 3, the type locality of M. davakosi , especially in respect to the anterosinuid. Moreover, M. davakosi , being a typical member of the Mimomys lineage, shows a higher position of the Sb3-wall (cf. Maul 1996: fig. 3).

Figures

FIG. 33. — Dolomys occitanus (Thaler, 1955), first lower molars in occlusal view; A, right, Ms42; B, left, Ms1; C, left, Ms2; D, right, Ms3; E, right fragment, Ms5; F, left fragment, Ms6; G, right fragment, Ms4; H, right fragment, Ms7; I, left, Ms8; J, right, Ms9; K, right, Ms10; L, right, Ms11; M, left, Ms12; N, right, Ms13; O, left, Ms14. Scale bar: 1 mm.

FIG. 34. — Dolomys occitanus (Thaler, 1955), first lower molars in buccal view; A, right, Ms42; B, left fragment, Ms1; C, right fragment, Ms7; D, left fragment, Ms2; E, left fragment, Ms8; F, right fragment, Ms11; G, right fragment, Ms13. Scale bar: 1 mm.

FIG. 35. — Dolomys occitanus (Thaler, 1955), third upper molars; A, left, Ms23, occlusal view; B, right, Ms24, occlusal view; C, right, Ms25, occlusal view; D, right, Ms26, occlusal view; E, left, Ms27, occlusal view; F, right, Ms28, occlusal view; G, left, Ms29, occlusal view; H, left, Ms30, occlusal view; I, right, Ms31, occlusal view; J, left, Ms32, occlusal view; K, right, Ms33, occlusal view; L, left, Ms34, occlusal view; M, right, Ms35, occlusal view; N, left, Ms36, occlusal view; O, left, Ms37, occlusal view; P, left, Ms38, occlusal view; Q, left, Ms39, occlusal view; R, right, Ms40, occlusal view; S, left, Ms23, lingual view; T, the same, buccal view; U, right, Ms24, lingual view; V, the same buccal view. Scale bar: 1 mm.