Arvicola jacobaeus, Cuenca-Bescós & Agustí & Lira & Rubio & Rofes, 2010

Cuenca-Bescós, Gloria, Agustí, Jordi, Lira, Jaime, Rubio, Maria Melero & Rofes, Juan, 2010, A new species of water vole from the Early Pleistocene of Southern Europe, Acta Palaeontologica Polonica 55 (4), pp. 565-580 : 570-572

Arvicola jacobaeus sp. nov.

Figs. 4–7.

Etymology: From St. James or Santi Jacobi, the pilgrims’ road to Santiago de Compostela that traverses the Sierra de Atapuerca and the symbol of the pilgrims (the scallop Pectem jacobaeus ).

Holotype: Right mandible, with the complete horizontal ramus, the incisor and the three molars (m1, m2, m3). Catalogue number: ATA04 TE 13 H31 z1150−1160, MPZ 2008/380.

Type locality: Sima del Elefante site, Atapuerca, Burgos, Spain.

Type horizon: Lower Red Unit (TE−LRU), levels TE8–TE13, of Sima del Elefante site, pre−Jaramillo faunas, Early Pleistocene.

Material.—121 dental elements MPZ 2008/342–2008/462 (Appendix 1).

Diagnosis.—Medium−sized hypselodont Arvicola species with abundant cementum in the re−entrant folds, and thick enamel band with differentiation of the Mimomys − type. The enamel−free areas are present in both the salient angles of the posterior lobes of the lower molars (m1, m2, m3), as well as in both sides of the anterior lobe and in the T1 of the M1 and in the lingual side of the anterior lobes of M2 and M3. The occlusal surface of m1 is characterised by the absence of the enamel islet and Mimomys −kante. The distal triangles are closed to slightly confluent. The occlusal surfaces of m2 and m3 are of the Mimomys − type, with triangles that are slightly (in m2) to broadly (in m3) convergent; M1 has the anterior lobe and T1 slightly confluent, while T2 may or may not be connected with T1; M2 and M3 are of the Mimomys − type. The occlusal surface of M3 shows two to three fields, the anterior lobe 1(AL1), the T2 and the rest of the molar, where budding T3 and T4 are variable in size. The M3, in basal view, has a thick enamel wall.

Differential diagnosis.— Arvicola jacobaeus sp. nov. is somewhat smaller (length and width) than the extant species of Arvicola ; the occlusal surface of the m1 is derived, similar to that in Arvicola sapidus , though the M3 is primitive, similar to that in Mimomys savini ; the enamel thickness is of the Mimomys − type, though fairly thick all around the enamel wall; the roots have disappear though in M3 the base of the crown is slightly close, and the enamel thicker in basal view.

doi:10.4202/app.2009.0027

Arvicola mosbachensis , described by Maul et al. (2000), presents morphological resemblances in the occlusal morphology of the molars with those of A. jacobaeus . The differences between A. jacobaeus and A. mosbachensis are: the smaller size of the m1 and the M3 of A. mosbachensis ; the lack of Mimomys —fold in m1, the lack of T5, the high confluence between T3, T4 and PC 1 in the M3, and the thicker, though highly variable enamel–thickness wall of A. jacobaeus . As in A. jacobaeus , some specimens of A. mosbachensis display incipient root development and thicker enamel wall in basal view, though A. jacobaeus presents this plesiomorphy only in the M3. On the other hand, by comparison with Arvicola terrestris , A. sapidus , and Microtus , the M3 of A. jacobaeus has an enamel wall markedly thick in the basal part, a stage prior to the initiation of the root formation ( Fig. 7K, L ). On the contrary, in A. mosbachensis , all the molars show the thick enamel wall in basal view ( Maul et al. 2000).

Description.—The mandible has the typical arvicoline shape. The mandibular corpus is stout and deep, with a short diastema. A strong symphyseal, or digastric, eminence is its most remarkable feature. In medial view, a large mental spine or genial tubercle for the insertion of the genioglossus muscle is situated just above the posterior part of the symphyseal process, and thus above the digastric eminence and below m1 ( Figs. 5 , 6 ). The symphyseal area is caudally extended, indicating a strong fibrous joint ( Fig. 6 ). In lateral aspect, the large mental foramen opens below the mesial border of m1. The lateral surface has an enlarged masseteric fossa of which the rostral end is situated below T4 of m1; its dorsal end is limited by the coronoid crest. The posterior part of the mandible is not preserved.

The molars of A. jacobaeus have abundant cementum in the re−entrant angles, and lack roots, folds and enamel islets. Moreover, the enamel differentiation is of the Mimomys − type and in general fairly thick throughout the perimeter of the tooth in its occlusal surface.

The lower first molar (m1): this has a posterior lobe; three alternating, closed to slightly open triangles, T1–T3; and an anteroconid complex with two open triangles, T4 and T5, broadly confluent with the anterior lobe ( Fig. 7A–C ).

The second and third molars (m2, m3): have similar occlusal surfaces to those of other species of Mimomys and Arvicola ( Fig. 7D–F ). In m3 the dentine fields vary from being broadly connected to slightly closed ( Fig. 7F ).

The upper first molar (M1): has an anterior lobe and four triangles, T1–T4, which are slightly interconnected. T1 has an enamel−free area in the lingual salient angle ( Fig. 7G–I ).

The second upper molar (M2): is of the general type in arvicolines.

The upper third molar (M3): this molar has three dentine fields: the anterior lobe, the T2, and a third one consisting of the rest of the molar, the T3 and T4 that comprise a single field together with the posterior lobe. There are two lingual and two buccal re−entrant angles ( Fig. 7J–M ) Two M3 present a thickening of the wall on their basal surface ( Fig. 7K, L ).

Figures

Fig. 7. Occlusal views (except some root or basal views) of isolated teeth of the cricetid rodent Arvicola jacobaeus sp. nov. from Sima del Elefante level TE9. A. Right m1 (TE9 1 1997 MPZ 2008/343). B. Left m1 (TE9 1 1998 MPZ 2008/344). C. Left m1 (TE9 s/n 1998 MPZ 2008/372). D. Right lower m2 (tooth in mandible with the m1 of A) E. Left lower m2 (tooth in mandible with the m1 of B). F. Right lower m3 (TE9 1 1998 MPZ 2008/345). G. Left M1 (TE9 s/n 1998 MPZ 2008/374) H. Left M1 (TE9 s/n 1998 MPZ 2008/376). I. Root view of left M1 (TE9 s/n 1998 MPZ 2008/375). J. Left M2 (TE9 s/n 1998 MPZ 2008/377). K. Root view of right M3 (TE9 1 1998 MPZ 2008/346). L. Detail of the incipient root development in the M3 in K. M. Left M3 (TE9 1 1997 MPZ 2008/342). Abbreviations: L, total length; W, total width; P, posterior length; 2,3, triangles 2,3. The arrow in L shows the thicker enamel and the closening of the crown in basal view.

Fig. 5. Occlusal view of the right mandible with m1, m2, m3 of the cricetid rodent Arvicola jacobaeus sp. nov. from Sima del Elefante level TE 13. Holotype (ATA04 TE13 H31 1150−1160, MPZ 2008/380).

Fig. 6. Medial view of the right mandible with i1, m1, m2, m3 of the cricetid rodent Arvicola jacobaeus sp. nov. from Sima del Elefante level TE 13. Holotype (ATA04 TE13 H31 1150−1160, MPZ 2008/380). The photograph was made during the field campaign of 2005 by Javier Trueba.