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 : 469-471

Masillamys parvus ( TOBIEN, 1954)

Text-fig. 7

H o l o t y p e. HLMD-Me 625, poorly preserved skeleton with extracted teeth: left upper M1–M3 and lower p4–m3, right m3;

Ty p e l o c a l i t y. 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. Definitely smaller than M. beegeri and M. krugi. On lower molars, the mesial cingulum is separated from the protoconid by a deep incision. The only remain of the metalophid is a posterior arm of the protoconid, whereas a lingual metalophid is missing. The hypolophid [= entolophid] is more sharply defined than in the other two species, proceeding from the entoconid, and joining the ectolophid between the mesoconid and the hypoconid. Entoconid opposite to the hypoconid on the p4, whereas it is distinctly more mesial on the lower molars. ( Tobien 1954: 23–24, translation adapted from German).

E m e n d e d d i a g n o s i s. Relatively small sized rodent (holotype: p4 = 1.75 × 1.42; m2 = 1.93 × 1.84; m3 = 2.02 × 1.75), slighly larger than Hartenbergeromys hautefeuillei. On lower molars, lingual main cuspids more mesial than buccal ones; p4 with protruding metaconid, mediomesial, and no protoconid; posterior lobe of m3 not strongly reduced. On molars, lingual metalophulid I incomplete, buccal metalophulid I ending at midwidth; entolophid complete, attached to the distal ectolophid/prehypocristid junction; ectomesolophid present and hypoconulid reduced. On upper M2 and M3, mesostyle present and positioned only slightly more buccally than the paracone and metacone; hypocone well-developed, with strong pre- and post-hypocristae; shallow sinus present on both teeth.

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

– M. beegeri and M. krugi in its smaller size; the absence of mesiobuccal cingulid and the reduction of the protocristid on p4; the absence of lingual metalophulid on lower molars.

– H. hautefeuillei in the absence of mesiobuccal cingulid, the reduction of the protocristid and the metaconid lower relatively to the entoconid on p4; the entolophid more regularly continuous on molars, the less reduced hypocone relative to the protocone on upper molars.

R e f e r r e d m a t e r i a l. Hessisches Landesmuseum Darmstadt (HLMD). Masillamys parvus holotype: HLMD- Me 625: a poorly preserved skeleton with teeth extracted: left upper M1–M3 and lower p4–m3, and right m3.

Senckenberg Research Institute and Natural History Museum Frankfurt (SMF). Masillamys ? parvus juvenile: SMF-ME 2099: a skull badly crushed; a left row of lower teeth exposed in nearly occlusal view (dp4, m1, m2 and m3).

M e a s u r e m e n t s. The only known skeleton is that of the holotype (HLMD-Me 625), which is poorly preserved. Almost all its articulations are crushed, distorted or unrecognizable. Nonetheless, its estimated body weight of 55–60 g ( Escarguel 1999), and estimated trunk length (measured along the cervical-thoracic-lumbar spine) of less than 80 mm makes H. parvus a smaller animal than M. krugi and M. beegeri.

For teeth measurements see Tab. 1 and Text-fig. 4.

D e s c r i p t i o n. Upper teeth. See Text-fig. 7a, b.

M 2–M 3 . Two fragmentary left teeth have been described and illustrated by Tobien (1954). The left M2 lacks the protocone and its anterior arm plus a part of the anteroloph, and the posterobuccal corner of the crown. The left M3 is less incomplete, lacking only the posterolingual edge of the tooth.

On M2, the buccal part of the anteroloph lacks a parastyle. The anteroloph is regular, thin and low, leaving a well-developed – but narrow – anterosyncline. The paracone and metacone have equal size. The postparacrista and premetacrista are equally developed, both attached to the mesostyle. The latter is low and weakly stretched mesiodistally; it is connected to the mesoloph, which occupies alone the bottom of the small mesosyncline. The buccal part of the protoloph is straight and, after the protruding paraconule, it joins the protocone. From the buccal part of the protoloph, there is one extra-ridge running mesially into the anterosyncline, and three very low extra-ridges descending to the centre of the basin. One linguobuccal extra-ridge starts from the lingual protoloph to the basin. The buccal metalophule II is weak and thin, turning mesially to join the metaconule, which is low and stronger than the paraconule. It is also linked to the posteroloph. Two small low extra-ridges are present along the mesial base of the metaconule. The hypocone is small and the endoloph short; below, the sinus is not well defined. A posterostyle marks the posthypocrista/ posteroloph junction. The posteroloph is higher than the anteroloph, ending at the base of the metacone.

On the weakly worn M3, the parastyle appears slightly swollen at the lingual end of the anteroloph, which is more worn. The mesial end of the preprotocrista is swollen at the place of the anterostyle. The two arms of the protocone make a wide open angle. The buccal half of the protoloph is high and plunging to the level of the low extra-ridges. It bears a stretched and protruding paraconule, and then continues to a swelling before being weakly attached to the thick protocrista. Three extra-ridges are present on the distal slope of the buccal protoloph, parallel to the postparacrista. Another ridge runs from the level of the postparacrista to the basin. The mesostyle is low and stretched mesiodistally; it is prolonged in one short, low and thick mesoloph. The metaconule is present as a low bulged ridge facing the endoloph. The hypocone is reduced; distally, the area between the posthypocrista + the posteroloph + the metacone form a cingulum elevated above the basin. Its posterior edge being broken, precise relationships between these structures cannot be described.

Lower teeth. See Text-fig. 7c–g . The crowns are relatively low and basined, with moderately prominent main cuspids, showing acute lingual ridges.

dp4. SMF-ME 2099: As it is heavily worn, extraridges are not visible. The metaconid is the unique mesial cuspid, from which a faint mesiodistal ridge descends. The protoconid ridge is low and worn. At the opposite, the lingual postmetacristid is long and high. The entoconid is isolated from the posterolophid, and the entolophid seems absent.

p4. HLMD-Me 625: This tooth is quite smaller than m1. The metaconid, mesiomedian, is the highest and strongest cuspid. Its posterior arm descends gently towards the mesosynclinid lingual opening. The tooth is slightly damaged, but it is possible to see one main low mesiodistal ridge descending from the apex of the metaconid to the centre of the talonid basin. The postmetacristid is gently sploping. The protoconid is absent and the protocristid ridge hardly visible. A swelling indicates the presence of a mesoconid. There is a break between a spur at the junction between the ectolophid – anterior arm of the hypoconid and the thin and continuous entolophid. The posthypocristid makes an angle with the long posterolophid, which joins the distal slope of the entoconid. The hypoconulid is not distinct.

m2–m3. HLMD-Me 625: Only one fragmentary m1, one slightly damaged m2, and the two m3s are known. On the well-preserved surfaces of teeth, the enamel of the crown outline is rough. Like in M. krugi, the lingual cuspids are less high than the buccal ones. But there is neither an isolated mesostylid nor a wide lingual opening for the mesosynclinid in front of the entoconid. On m2, the postmetacristid descends first gently, then more abruptly at the end, from where starts the relatively long mesolophulid ( Text-fig. 7d ). On m3, the end of the postmetacristid makes a kind of flat, from which the mesolophulid starts ( Text-fig. 7e, f ).

The anterolophid is well-preserved only on the m3: it is slender, without an anteroconid and it ends lingually at a short mesial and linguobuccal premetacristid. It does not fuse with the protoconid, leaving a narrow buccal opening of the anteroflexid. The anterolophid is higher than the other ridges, which run towards the anteroflexid and mesoflexid. From the metaconid and anterolophid, short and low extraridges descend on the centre of the tooth. Starting from the protoconid, the short buccal metalophulid is the only component of the metalophid being well developed. The thick and oblique postprotocristid bears a premesoconid spur, before the shrinkage of the mesial ectolophid. The mesoconid is bulged (horizontally worn like the protocone and hypocone) and buccally bears the ectomesolophid, as well as lingually a long mesolophid, nearly merged with the mesolophulid. The angled entolophid connects to the distal ectolophid, where it thickens (postmesoconid swelling?). There are two extra-ridges in the anterosynclinid and a few along the lingual slope of the metaconid to the basin and some granules in the posterosynclinid.

m1–m2–m3. SMF-ME 2099 ( Text-fig. 7g ): Like on the holotype, the lingual main cusps are more mesially positioned than the buccal ones, but only slighly more than in M. beegeri. The complete anterolophid is transverse, aligned with the plunging mesial premetacristid, without an individualized anteroconid, as on the type specimen; as the teeth are nearly unworn, the buccal end of the anterosynclinid remains open. This feature was argued to differentiate M. parvus from M. beegeri and M. krugi but this opening is not deep. It is found also on the m1 of M. krugi, less worn than the m2, and on the m3 of M. beegeri, less worn than m2 and m1. This feature cannot be used in a differential diagnosis. The lingual metalophulid is stronger than on the type, and weakly joins the buccal metalophulid I on m1 and m2. The postmetacristid has the same organization as on the type with a narrow lingual opening of the mesosynclinid, but it ends into a shorter low lingual mesolophulid. Like on the type, the entolophid is complete, attached to the distal ectolophid junction with the prehypocristid. The ectomesolophid is present and the hypoconulid is reduced. The extra-ridges are better marked (but the teeth are less worn). Like for the type, the width of the posterior lobe of the m3 is only slightly reduced.

Figures

Text-fig. 7. Teeth of Masillamys parvus TOBIEN, from Messel (Hesse, Germany, MP 11), HLMD-Me 625, holotype. a) left upper M2, protocone damaged, occlusal view. b) left upper M3, occlusal view. c) left lower p4; c1 – occlusal view, c2 – buccal aspect, c3 – lingual aspect. d) left lower m2, metaconid damaged; d1 – occlusal view, d2 – buccal aspect, d3 – lingual aspect. e) right m3, occlusal view. f) left m3; f1 – occlusal view, f2 – buccal aspect, base of crown lacking, f3 – lingual aspect; g) SMF-ME 2099A: right lower tooth row with dp4 to m3, g1 – occlusal view, g2 – lingual aspect. Scale bar 1 mm.

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

Text-fig. 2. Species of Masillamys considered on the phylogenetic tree of theridomorphs (Vianey-Liaud and Marivaux 2017: fig. 7), within the basal Theridomorpha, before the polyphyletic genus Protadelomys. Position inferred from their dental features (see text).

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.