Masillamys, TOBIEN, 1954

Vianey-Liaud, Monique, Marivaux, Laurent & Lehmann, Thomas, 2019, A Reevaluation Of The Taxonomic Status Of The Rodent Masillamys Tobien, 1954 From Messel (Germany, Late Early To Early Middle Eocene, 48 - 47 M. Y.), Fossil Imprint 75 (3 - 4), pp. 454-483 : 465-469

Masillamys krugi TOBIEN, 1954

Text-fig. 6 , Pl. 3

H o l o t y p e. HLMD-Me 910, poorly preserved fragmentary skeleton with complete left upper jaw and lower toothrow, and right P3–P4.

Ty p e l o c a l i t y a n d d i s t r i b u t i o n. Messel Fossil Pit ( Germany). Transition late Ypresian-early Lutetian, MP 11 , 47– 48 m. y.

O r i g i n a l d i a g n o s i s. Similar to M. beegeri, but relief of the tooth crowns flat and low. Lower molars wider and shorter than in M. begeeri, with main cuspids lower and not protuberant. Entoconid weak. Lingual opening of the central valley of the lower molars wide and open. Enamel wrinkling finer and denser than in M. beegeri. Hypolophid and ectolophid of the lower molars low, but distinct. ( Tobien 1954, translation adapted from German).

E m e n d e d d i a g n o s i s. Teeth close in size to M. beegeri, but with different ratios between loci: lower molars wider and shorter than in M. begeeri, lower p4 (2.36 × 2.1) slighly longer than m1 (2.3 × 2.36), and m2 (2.38 × 2.39) shorter than m3 (2.69 × 2.26); main tubercles not strongly protruding; entoconid weak; lingual opening of the central valley at the lower molars wide and open, with a flattened mesostylid, at the distal extremity of the postmetacristid; long oblique postprotocristid nearly aligned with the mesial ectolophid and slender mesoconid.

Enamel wrinkling along the loph(id)s thinner and higher; antero- and posteroloph(id)s, and ectolophid relatively higher; on P4, nearly complete pericingulum: short anteroloph, protocone, pre- and post-protocristae + endoloph and posteroloph, paracone + postparacrista + mesostyle elements + premetacrista + metacone; relief of the tooth crowns flat, and thinner ridges than in M. beegeri. Tibia longer than the femur (i.e. crural index> 1).

D i f f e r e n t i a l d i a g n o s i s.Thespecies Masillamys krugi differs:

– from M. beegeri in overall smaller size and tibia longer than femur; less bulged paracone and metacone, and thinner and higher lophs on upper teeth; on lower molars, presence of mesostylid and mesial ectolophid oblique in the continuity of the postprotocristid, and weaker entoconid; p4 longer than m1; on p4, the entolophid joins the distal ectolophid;

– from M. mattaueri in higher tooth crown; tubercles less bulged; p4 longer relative to m1; and on p4, the entolophid complete and higher, joining the distal ectolophid, whereas it is interrupted, lower and joins the prehypocristid or the premesoconid ridge on M. mattaueri.

R e f e r r e d m a t e r i a l. Hessisches Landesmuseum Darmstadt (HLMD). HLMD-Me 910, holotype: poorly preserved fragmentary skeleton with complete left upper P3 to M3 and lower p4 to m3, and right P3–P4.

HLMD-Me 11015: Better preserved, although distorted, except for the lower jaws: complete skeleton with an incomplete skull; on the left side, two lower jaws are available, one offering the buccal aspect of p4 and m1; on the right side, teeth are not visible.

HLMD-Me 7441: Complete and crushed skeleton with teeth seen from their buccal side (left dp4 (p4 erupting below), m1 to m3; upper toothrow complete). The attribution to M. krugi is not certain: the erupting p4 seems as long as m1, but given that the tooth is still in its crypt, it has not been possible to carry out precise measurement, and to analyse all features.

M e a s u r e m e n t s. HLMD-Me 11015 (adult): length of the dentary from the angle to the border of the incisor alveolus = 1.78 cm; height of the dentary = 1.52 cm; length of the diastema = 0.33 cm.

HLMD-Me 7441 (juvenile with p4 erupting): length of dentary = 2.30 cm; height of dentary = 1.72 cm; length of the diastema = 0.44 cm.

The best preserved skeleton (adult HLMD-Me 11015) shows a humerus of 2.1 cm, a radius of 1.6 cm, a femur of 2.6 cm, and a tibia of 3.0 cm (maximum) length. According to Tobien (1954: 23), the trunk of the holotype (along the cervical-thoracic-lumbar spine) measures 16 cm long, which would be longer than in M. beegeri. We re-measured the trunk of the holotype and found a length of about 10 cm, whereas the trunk in HLMD-Me 11015 is about 8.5 cm long.

For teeth measurements see Tab. 1 and Text-fig. 6.

D e s c r i p t i o n. Skull and dentary. The skulls are particularly damaged but the dentaries are well exposed nonetheless (Pl. 3). If the juvenile HLMD-Me 7441 really belongs to M. krugi, this individual has a larger dentary than the adult HLMD-Me 11015, both being smaller than the adult of M. beegeri, but not much for the juvenile. It is not possible to describe their cranial characters in detail, except that the i.o.f. appears large. The components of the horizontal ramus of the zygomatic arch are seen on the right side of HLMD-Me 11015 (Plate B), the squamosal ending at mid-length, as in M. beegeri; the anterior part of the jugal turning along the vertical part of the arch, and the maxillary is well developed at its ventral part.

Teeth. The description is mainly based on the teeth of the type specimen (HLMD-Me 910), as the teeth of the other specimens referred to this species (HLMD-Me 11015 and HLMD-Me 7441) were not extracted and are only partially visible. The comparison of the length of the premolar with that of the first molar allowed us to identify HLMD-Me 11015 (Pl. 3) as M. krugi; as its p4 is not yet erupted, hindering precise measurement, this attribution is less certain for HLMD-Me 7441.

Upper teeth. See Text-fig. 6a.

P 3 . Only the left P3 is preserved ( Text-fig. 6a 1 , a 4 ). It is tiny, more reduced compared to P4 than in M. beegeri. The buccal cusp is only slightly higher than the lingual cingulum, which is longer relative to the cusp than in M. beegeri, where it is weaker and shorter.

P4. The left P4 is slightly more damaged than the right one, mainly at the level of the anteroloph. This tooth is trapezoidal, with rounded angles. The anteroloph is low, thin and short, but longer than in M. beegeri, and does not reach the mesiobuccal border of the tooth. The anterosyncline is reduced as the anteroloph is stuck against the mesial flank. The paracone is faintly stronger than the metacone. The postparacrista is longer but slender than in M. beegeri. Long extra-ridges descend from the paracone, the postparacrista and the buccal part of the protoloph, all towards the mesosyncline. The protoloph is straight and thin in its lingual part, before joining the extremity of the preprotocrista. There is no trace of a paraconule. The protocone is swollen, its thick anterior and posterior arms being nearly aligned mesiodistally. It is difficult to distinguish an endoloph at the distal extremity of the postprotocrista: there is here a swelling corresponding to the hypocone and that continues without interruption into a long posteroloph. There is no development of sinus.

The mesostyle is preceded and followed by equally developed additional mesostyles. All are aligned and slightly displaced buccally, and connected to the postparacrista and to the strong premetacrista, respectively. A buccal mesoloph is well developed from the median mesostyle, converging to the extra-ridges in the mesosyncline. The strong bulged metaconule is connected to the thick metalophule I. An additional ridge lines mesially this metalophule I, from the middle of the metacone. The metaconule joins the postprotocrista through a slender ridge. The metaconule bears a long distal outgrowth towards the posteroloph, and its mesial base displays two thick granules. The metalophule II has two parts: the mesialmost to the metalophule I is short and ends free, whereas the distalmost is directed distally and joins the posteroloph. There are two wrinkles on the buccal slope of the protocone, converging towards the other extra-ridges in the mesosyncline. Two granules make a link between the hypocone and the distal flange of the metaconule.

M1–M2. As the M1 is partly damaged, it is difficult to evaluate the shape of the protocone. M2 differs from M1 mainly in its more lingual metacone, which is weaker than the paracone, and its shorter posteroloph. In both cases, the sinus is shallow, widely open and not pinched. Their parastylar area is not swollen; the anteroloph is thick without anterostyle at the junction with the preprotocrista on M2 (damaged area on M1). The hooklike postparacrista is strong, and makes an arch with the paracone. It is separated from the mesostyle by a narrow notch. The protoloph is continuous from the paracone to the centre of the protocone. From its buccal part, three distomesial extra-ridges runs into the anterosyncline, and also mesiodistal ridges are running to the basin of the mesosyncline. Then the paraconule extends forward, to the anteroloph; two other extra-ridges occur from the paraconule to the centre of the basin. The lingual part of the protoloph bears also extra-ridges directed towards the basin. The mesostyle is slightly stretched mesiodistally and bears two mesolophs; it is followed distally by a short ectocingulum, related to the thin premetacrista, which descends from the metacone. The buccal metaloph is as high and strong as the buccal protoloph on M1, and slightly lower on M2. From it, three (M1) or two (M2) short and thick ridges descend in the mesosyncline, and two (M1) or one (M2) in the posterosyncline. The third mesial extraridge connects to the base of the metaconule (equivalent of metalophule I?). The metaconule is slightly stronger than the paraconule, projecting thick mesial and distal extensions. The lingual part of the metaloph is weak and very low, ending against the postparacrista. Distal to this junction, there is a spur from this arm and another from the hypocone, both ending free. Despite some wear, the enamel surface of the crown is rough, and a network of thin accessory enamel wrinkles is visible.

M3. Even damaged, the size of the protocone and that of its arms are like on M2; the anterostyle is present. The protruding paraconule is thin and reduced; it shows the same extra-ridges seen on the M1–2, but shorter. The tooth is fractured along the protoloph. Nonetheless, it appears that the protoloph is continuous and connected to the protocone (even if it is not sure that it is the apex or the anterior arm, due to wear). The mesostyle and mesoloph are followed distally by at least two additional mesostyles and mesolophs. The metacone is not higher than the other elements of the bucco-disto-lingual cingulum. The buccal metalophule (I or II?) is low and directed distomesially into the basin. The hypocone is much reduced, but given that its contact with the protocone is broken, no observation of the endoloph and sinus can be made.

Lower teeth. See Text-fig. 6b and Pl. 3.

p4. This tooth is nearly as long as m1. The metaconid, mesiomedian, is still the highest and strongest cuspid, but it is less prominent than in M. beegeri. Like in M. beegeri, two mesiodistal ridges descend from the apex of the metaconid to the centre of the talonid basin; the lingual-most ridge is the strongest. The postmetacristid is less high and shorter, than in M. beegeri, ending with a low and short mesolophulid at the level of a wide lingual opening of the mesosynclinid.

The protoconid is less reduced at the anterior part of the ectolophid than on M. beegeri, and is connected to the apex of the metaconid by a ‘lingual metalophulid’ I. A short linguobuccal premetacristid runs mesially, parallel to this metalophulid. There is no continuous ectocingulid. The thick oblique postprotocristid is followed by a premesoconid thickening; then the mesoconid bears lingually a very short mesolophid. The very short mesial and distal parts of the ectolophid are oblique and aligned obliquely with the weak mesoconid. One buccal ridge descends from the mesial end of the mesoconid (ectomesolophid) towards a buccal spur developed along the buccomesial flank of the hypoconid. The ectolophid connects the prehypocristid, together with the entolophid, which bears a weak postmesoconid swelling. Two short extra-ridges (the longest mesial, the shortest distal) run from the entolophid. Protoconid and entoconid have the same development. The posthypocristid is linked to a regular posterolophid, on which no hypoconulid can be distinguished. Numerous extra-ridges are seen in the mesosynclinid, notably along the main mesiodistal ridge. The buccal slope of the ectolophid area is wrinkled.

m1–m2. There is less difference in height and strength between the lingual and buccal halves of the wear surface than for M. beegeri, suggesting that horizontal wear is predominant. There is few difference between the height of the ‘trigonid’ of m1, which is almost at the level of the talonid basin and the posterosynclinid. The enamel outline of the crown is rough and irregularly wrinkled.

Although the metaconid is the highest and strongest cuspid of lower molars, it is lower and blunter, and its posterior arm lower and shorter than in M. beegeri. This creates a wide mesostylar opening for the mesosynclinid before the entoconid. The mesostylid area is worn; if unworn, it would be occupied by a wide mesostylid or a mesostylar flange.

The anteroconid is indistinct on the anterolophid, which ends at mid mesial width. Buccally, it joins the mesial flank of the protoconid, closing the anterosynclinid there; this anterosynclinid is more expanded mesiodistally than in M. begeeri. The linguobuccal premetacristid, fused with the mesial flank of the metaconid, is also aligned with the anterolophid. It bears a small anterolophulid, parallel to three (m2) to four (m1) other ridges, the third being the ‘lingual metalophulid’ I, which joins the extremity of the buccal metalophulid, making an angle on m1, less pronounced on m2. This buccal metalophulid, developed from the apex of the protoconid, is postwardly directed on m1, but less on m2. Two main extra-ridges run from the metalophid mesiobuccal to distolingual in the talonid basin, where they meet several small extra-ridges from the mesostylid, the ectolophid (mesolophid) or the entolophid areas.

The buccal and lingual main cuspids are more facing each other than on M. beegeri. The extremity of the long thick oblique postprotocristid is swollen in a premesoconid bearing only a short lingual spur on m1. The mesial part of the ectolophid and the mesoconid are obliquely oriented, in the prolongation of the postprotocristid. The distal part of the ectolophid, following the postmesoconid swelling, turns more mesiodistally. The mesoconid is moderately bulged; its buccal flank bears one strong plunging ectomesolophid, which joins a mesiobuccal spur of the hypoconid. The wide sinusid is deep (to mid-height of the crown); its mesial slope bears two wrinkles. The entoconid is the smallest cuspid of lower molars, smaller than the hypoconid. The entolophid joins the distal ectolophid, but is briefly interrupted by a narrow notch separating a short buccal part and a longer lingual one on m1; the buccal part includes the postmesoconid swelling. Two mesial short extra-ridges are seen along the entolophid on m2, as well as very short ones into the posterosynclinid, while they are less distinct on the more worn m1. The prehypocristid is weaker than on M. beegeri. The hypoconulid goes from slightly swollen to indistinct. It prolonges in the relatively short posterolophid, related to the posterior arm of the entoconid, thereby closing the posterosynclinid.

m3. On the weakly worn m3, the numerous extra-ridges are well exposed along the metalophid and in the talonid basin area, as well as in the antero- and posterosynclinids. As for M. beegeri, the m3 differs from the m 2 in its reduced posterolophid and in its more lingually positioned hypoconulid. Here, the entolophid is uninterrupted and bristling with its long mesial and distal extra-ridges. Two or three wrinkles descend distally on the posterior slope of the posthypocristid.

D i s c u s s i o n. When Tobien (1954) first described Masillamys beegeri and M. krugi, he gave a detailed description of both species (then represented by only one specimen each), and provided arguments in favor of their distinction. Later, Hartenberger (1968, 1993: 166) considered M. krugi as a junior synonym of M. beegeri, based on the ‘morphological variation observed in the specimens of Vielase [originally identified as Masillamys cf. beegeri by Legendre et al. (1992); but see below]’. Concerning the specimens from Vielase, Hartenberger noted later that they were smaller and somewhat less advanced over the specimens of M. beegeri from Messel. In particular, ‘the lower p4 is more reduced, and the crenulations in the molars are less developed’ ( Hartenberger 1993: 166).

Subsequently, Escarguel (1999) assigned the specimens from Vielase to M. mattaueri (see below), and suggested that this species (early Eocene, MP 10/11 according to Escarguel 1999) was in the evolutionary lineage leading to M. beegeri from Messel (early-middle Eocene boundary, MP 11). Moreover, Escarguel (1999) gave detailed descriptions of the tooth morphology for the different populations he referred to M. mattaueri from France. Like Hartenberger (1968, 1993), he suggested that the differences between M. beegeri and M. krugi can be found in the different populations of M. mattaueri, and thus that they represent one species only. It is not the case in the type population of M. mattaueri, in which we do not found the thin and high lophs and extra-ridges observed in M. krugi (see description below). Nonetheless, after a careful review of the diagnosis given by Tobien (1954) and the direct comparison of the original and new dental material from Messel, it appears that several discrete and biometrical characters convincingly support the species distinction between M. beegeri and M. krugi (see ‘Differential diagnosis’ above). In particular, we observed that the character ‘p4 longer than m1’ has been noticed in none of the M. mattaueri populations, making it an autapomorphy of M. krugi. Likewise, the large mesostylid seen on the lower molars of M. krugi, but absent in M. beegeri, is only present (and if so, quite small) on a few lower m3s of the M. mattaueri population from Mas de Gimel and Naples, and one lower dp4 from Grauves (= Cuis) according to Escarguel (1999).

Finally, considering also its tibia distinctively longer than its femur, we recommend keeping M. krugi as a separate species, as originally described by Tobien (1954).

Figures

Text-fig. 6. Teeth of Masillamys krugi TOBIEN, from Messel (Hesse, Germany, MP 11). a) HLMD-Me 910, holotype, upper teeth; a1 – left upper tooth row, P3–P4–M1–3, occlusal view; a2 – id. lingual aspect, a3 – buccal aspect of P4, a4 – right upper P4–P3, lingual aspect. b) HLMD-Me 910, holotype, right lower tooth row, p4–m1–3; b1 – occlusal view, b2 – buccal aspect, b3 – lingual aspect. Scale bar 1 mm.

Text-fig. 3. Front views of hemi-maxillaries from Prémontré (Paris Basin, MP 10), showing the large infraorbitary foramen ofa) SLP29PR-1312,storedintheMontpellierUniversitycollections (ISE-M), Hartenbergeromys hautefeuillei; b) SLP29PR-960, Pantrogna marandati. Scale bar 1 mm.

Text-fig. 1. Dental terminology (modified, after Vianey-Liaud and Marivaux 2019). a) upper molar; a1 – names of flexi on upper molar; a2 – names of cusps and ridges on upper molar. Anlph = anteroloph, Anst = anterostyle, Ectcing = ectocingulum, Endlph = endoloph, Hy = hypocone, Hyplph = hypolophule, Me = metacone, Mecl = metaconule, Melph I = lingual metalophule I, Melph II = lingual metalophule II, Mest = mesostyle, Meslph = mesoloph, Metst = metastyle, Pa = paracone, Pacl = paraconule, Parst = parastyle, Postmecr = postmetacrista, Postpacst = postparacrista, Postst = posterostyle, Postlph = posteroloph, Posthycr = posthypocrista, Prc = protocone, Prcr = protocrista, Prehycr = prehypocrista, Premecr = premetacrista, Prepacl = preparaconule, Preprcr = preprotocrista, Prlph = protoloph. b) lower molar; b1 – names of flexi and some ridges and conules on lower molar; b2 – cusps and ridges on lower molar. Ancd = anteroconid, Anlphd = anterolophid, Anlphld = anterolophulid, Antsnd = antesinusid, Ectmeslphd = ectomesolophid, Ectlphd = ectolophid, Ectsd = ectostylid, Entcd = entoconid, Encld = entoconulid, Enlphd = entolophid (lingual entolophulid + buccal entolophulid), Hyd = hypoconid, Hycld = hypoconulid, Mbcing = mesiobuccal cingulid, Med = metaconid, Mescd = mesoconid, Meslphd = mesolophid (lingual mesolophulid + buccal mesolophulid), Mestd = mesostylid, Prcd = protoconid, Prcdsp = protoconid spur, Prehycrd = prehypocristid, Premecrd = premetacristid, Posthycrd = posthypocristid, Postlphd = posterolophid, Postmcrd = postmetacristid, Postprcd = postprotocristid, Sind = sinusid.

Text-fig. 4. Bivariate graph (length/width) of the teeth of the three species of Masillamys from Messel.