Masillamys, TOBIEN, 1954
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publication ID |
https://doi.org/ 10.2478/if-2019-0028 |
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persistent identifier |
https://treatment.plazi.org/id/0394CF6E-FFEE-C223-FC4A-FB524D0AB344 |
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treatment provided by |
Diego |
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scientific name |
Masillamys |
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Masillamys beegeri TOBIEN, 1954
Text-fig. 5 View Text-fig , Pls 1, 2
H o l o t y p e. A poorly preserved skeleton with the extracted left upper and lower toothrows, housed at the Hessisches Landesmuseum Darmstadt (HLMD) under reference number HLMD-Me 1.
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. Strong and high main cusps on the lower and upper molars. Entoconid especially well developed, so that the lingual opening of the central depression is narrow and V-shaped. Lower molars clearly longer than wide; lingual cusp opposite to buccal. ( Tobien 1954: 18–19, translation adapted from German).
E m e n d e d d i a g n o s i s. Masillamys species larger than M. krugi with lower p4 shorter than m1 and ectocingulid absent; P4 strongly shorter than M1.
D i m e n s i o n s o f t h e h o l o t y p e. Lower teeth (length × width in mm) – p4: 2.23 × 2; m1: 2.59 × 2.41; m2: 2.72 × 2.45; m3: 2.87 × 2.13; upper teeth (length × width in mm) – P3: 0.87 × 0.75; P4: 1.84 × 2.41; M1: 2.33 × 2.62; M2: 2.33 × 2.52; M3: 2.26 × 2.13.
D i f f e r e n t i a l d i a g n o s i s. Masillamys beegeri differs from M. krugi in its more bulged and higher main cusps on upper and lower molars, and with the sinus better marked; lophs and lophids thicker, the bottom of flexids less flat; in the absence of the mesostylid; in the p4 smaller than m1; on p4, the entolophid turns backwards to join the hypoconulid area, whereas it joins the distal ectolophid in M. krugi and M. mattaueri. Differs from M. mattaueri in the hypocone nearly as large as the protocone on M1 and the higher endoloph; the complete thick entolophid on molars.
R e f e r r e d m a t e r i a l. Senckenberg Research Institute and Natural History Museum Frankfurt (SMF). Masillamys beegeri adult: SMF-ME 11295: consisting in Plate A of a complete skeleton, left side; Plate B, skull, right side; and C, teeth extracted from B (m1, m2 and m3). SMF- ME 3407: complete skeletton but skull damaged; upper teeth absent; three lower teeth badly exposed, buccally (dp4– m2). SMF-ME 11115: skeleton with skull and toothrows compressed and distorted.
Masillamys beegeri juvenile: SMF-ME 1287: complete skeleton, right side (Plate A), with DP3, DP4, M1 and M2 badly exposed; dp4, m1 and m 2 in buccal views; M3 and m3 can be seen on X-ray pictures ( Koenigswald et al. 1992: 222, fig. 327). SMF-ME 3567: crushed skull, right side, without upper teeth, only a part of M3; dp4, m1 and m2 exposed (buccal view).
Hessisches Landesmuseum Darmstadt (HLMD). Masillamys beegeri holotype: HLMD-Me 1: a poorly preserved skeleton of an adult individual with left upper (P3 ( DP3 ?) –P4, M1–M2–M3) and lower (p4–m1–m2–m3) toothrows extracted .
M e a s u r e m e n t s. The adult skull (SMF-ME 11295) length (from nasal to occipital) is about 5.5 cm, whereas it is about 3.6 cm for the juvenile (SMF-ME 1287). The length of the adult dentary (incisor excluded) is about 2.5 cm and 1.8 cm for the juvenile. The height of the dentary (from top of the coronoid apophysis to the lowest angle) is about 1.95 cm for the adult and 1.25 cm for the juvenile. The length of the diastema is 0.45 cm in the adult SMF-ME 11295. The radius of curvature of the upper incisor in the adult SMF-ME 11295 is 0.8 cm and of the lower one is 1.6 cm.
The adult specimens SMF-ME 3407 and SMF-ME 11295 have the same humerus (maximum) length of 2.8 cm and femur (maximum) length of 4 cm. In addition, the radius of SMF-ME 11295 is 2.2 cm and the tibia is 3.8 cm long. Their trunk length (measured along the cervical-thoracic-lumbar spine) is about 12.5 cm for both specimens. In comparison, the juvenile specimen SMF-ME 1287 has a trunk length of about 7.3 cm, a humerus of 2.1 cm, a radius of 1.5 cm, a femur of 2.8 cm, and a tibia of 2.3 cm in length. The description of postcranial specimens of the Messel rodents is beyond the scope of the present paper and will be exposed in a separate work.
For teeth measurements see Tab. 1 and Text-fig. 4. View Text-fig
D e s c r i p t i o n. Skull. Among the studied specimens, two are well preserved enough to provide accurate character description: one adult ( SMF-ME 11295 ) (Pl. 1) and one juvenile ( SMF-ME 1287 ) (Pl. 2). A few additional details were observed on the crushed juvenile skull SMF-ME 3567 . The individual SMF-ME 11295 is preserved on two plates in the collection: the isolated right side of the skull (11295 B) and the left side of the skull as well as the complete skeleton (11295 A). Also, on the crushed specimen SMF-ME 11115 , it is possible to distinguish the presence of a wide i.o.f .
Escarguel (1999) reported the debate around specimen SMF-ME 1287, found in 1982, after Tobien (1954) wrote his paper on the Messel rodents. That specimen was successively considered as a representative of the genus Masillamys , then of the species parvus, either included in the genus Microparamys ( Hartenberger 1968) or Microparamys (Sparnacomys) ( Hartenberger 1971) , and finally as Masillamys beegeri ( Koenigswald et al. 1992). Direct examination of the specimen confirmed the later identification as M. beegeri, and the presence of DP3–DP4, as well as unerupted M3/3 suggest further, that the individual was a juvenile.
The snout of SMF-ME 1287 bears an i.o.f. (preserved only on the right side) that is clearly larger than in the ‘protrogomorphous’ condition, and lacks the plateau on the ramus ascendens of the zygoma, which is typical for ‘sciuromorphous’ rodents. Like in the adult specimen SMF- ME 11295, the muscular scars for the medial masseter muscle of SMF-ME 1287 are limited to the ventral and lateral aspects of the zygoma. Therefore, like R. Lavocat, G. Storch and C. Seiffert (op. cit.), we interpret this condition as ‘hystricomorphous’.
Even if the bones are more or less fractured, and some have moved and shifted from their original position due to compression and crushing, it is possible to describe accurately the anterior part of the skull, and notably the snout and the infraorbital area of SMF-ME 11295 (Pl. 1, Figs 1, 2). The well-preserved nasal extends a little posterior to the i.o.f. level. The snout is relatively short (length of the premaxillary above the incisor = 1.53 cm; length of the premaxillary at the base of the i.o.f. = 0.83 cm; maximum height about 1 cm). The i.o.f. is widely open. Its opening reaches the level of the upper surface of the incisor in the adult. In the juvenile, it seems slightly smaller and its opening reaches the base of the incisor alveolus; but the maxillary is broken at its upper part above the i.o.f., and its fragments are overlapping. Moreover, the maxillary base of the border of the i.o.f. is tilted medially, as a result of compressions, which reduces the apparent size of the opening. Despite this, the i.o.f. is large and hystricomorphous in adults as well as in juveniles. The lacrimal is positioned at the mesiodorsal corner of the i.o.f. The suture between the maxillary and the jugal in the arch bordering the i.o.f. laterally is partly exposed on the left side SMF-ME 11295, but it is hardly distinguishable at the base of the zygomatic arch. The jugal occupies the main lateral side of the arch, and the squamosal reaches the middle of its dorsal side. Even fractured, the masseteric plate of the maxillary is distinct mesioventrally.
The lateral profile of the glenoid fossa appears at the distal end of the squamosal, above the mandibular condyle on the right side of SMF-ME 11295. On the left side of SMF-ME 11295 and on the juvenile SMF-ME 1287, the tympanic ring lies behind and against the dentary; it is strong on the latter and as high as the dentary height, whereas it is less high relative to the ramus ascendens on the adult. Therefore, during ontogeny, the bullae does not grow substantially, contra the other parts of the skull and mandible. It is not possible to distinguish the petrosal, this a being badly crushed or broken on the right side of SMF-ME 11295. On the juvenile SMF-ME 3567, the right petrosal has been turned inside out and its cerebellar face is exposed, with a wide fossa subarcuata, and a hollow corresponding to the foramina acusticum, but it is still not possible to give more details without a better preparation of the specimen, or a scan of it.
The dorsal row of bones, behind the nasal, shows a flat frontal (as seen on the juvenile SMF-ME 3567 ); at the middle of the orbit fossa (orbit length about 1.43 cm), this frontal is enlarged in a short scalloped postorbital process (seen on all specimens). The temporal fossa is longer than the orbit, but the mosaic of bones involved in this area are mostly obscured, so that the foramina cannot be observed along the lateral walls of the skull. The frontal-parietal suture is visible on the two juvenile specimens. However, the shape of the parietals and their relationships with the squamosal and occipital bones are not clear. On the right side of SMF-ME 11295 , a bone seems to be a laterally displaced interparietal or occipital. It is limited distally by a crest, connected to the probable lambdoid crest on each side of which a tear-shaped hollow is marked. It is limited mesially by an incomplete suture .
Dentary. The incisor is long, ending far behind m3. Even faintly damaged, the diastema appears short (about 0.45 cm whereas the total length of the jaw, from the mesial margin of the incisor alveolus to the distal end of the angular process, is 2.5 cm). On the right side of SMF-ME 11295, the anterior part of the body is shorter than the ramus ascendens (length from the mesial end of the diastema to the mesial corner of the masseteric tubercle [LmD] / LmD + L ramus ascendens = 0.40). The masseteric tubercle reaches mesially the contact between m1 and m2. The main foramen mentale is located at mid diastema, well before the p4 mesial root; on the juvenile, it appears closer to the mesial root of dp4. On the adult, one (or two?) additional foramina, smaller and more distally positioned are present.
Teeth. The teeth of the type (HLMD-Me 1) are used here as a reference, and they are then compared to the teeth of other specimens referred to M. beegeri, from the SMF collections .
Upper teeth. See Text-fig. 5b, c View Text-fig .
DP3. It is tiny and has one buccal cusp as seen on the juvenile specimen SMF-ME 1287. On the latter, its apex is strongly worn.
DP4. Present on SMF-ME 1287, its occlusal features are partly hidden. From the lateral view of the skull, it can be observed the two divergent buccal roots (characters of DP 4 in most of rodents), which are no more covered by bone. Details on the occlusal face are provided by a scan of the specimen. The anteroloph is long, slightly curved, and ending buccally at the base of the paracone. The anterosyncline is well developed (long and wide). The paracone bears a long sloping postparacrista, ending with a flattened area in contact with the mesostyle base. This is outlined mesiobuccally by a short ectocingulum, and distobuccally by a low ectomesostyle.A short and low mesoloph is present. The protoloph is thick and joins the preprotocrista. The paraconule, smaller than the metaconule, is protruding in the anteroflexus. Pre- and post-protocristae make a wide-open ‘V’ with the protocone apex. The endoloph joins the small hypocone, which connects the long posteroloph, ending before the buccodistal corner of the metacone. The buccal profile of the metacone is stronger than the paracone. There is a premetacrista, shorter than the postparacrista. The thick metaloph lowers before reaching the strong metaconule; the latter is separated from the lingual cusps and lophs. The presence of a dental bud remnant of a probable P4 is detected on the X-ray picture, between the roots of DP4.
P3. Present on the type specimen, in front of P4, its unique cusps is pointed; the short lingual cingulum is thin and situated at one third of the height of the crown. It is interpreted as a P3, because it is unworn, and followed by a P4, contra the worn DP3 on the specimen SMF-ME 1287 , which is followed by a DP4. However, as the retention of DP3 is frequent on rodents, we cannot ascertain this .
P4. On the type specimen, this tooth is more trapezoidal than triangular. The anteroloph is short, limited to the middle of the mesial border of the tooth, and the anterosyncline is reduced. The paracone is quite stronger than the metacone (seen also on both sides of SMF-ME 11295), enlarged by its thick postparacrista and thick buccal protolophule. Four low extra-ridges descend from it to the mesosyncline. The protruding paraconule, small and rounded, is adjoined to the middle of the anteroloph, and separated from the buccal metalophule by a shallow notch. It is better connected to the thick preprotocrista. A well-defined extra-ridge descends from the paraconule to the centre of the mesosyncline. The protocone is swollen, and its thick anterior and posterior arms form a wide ‘V’. A low and short endoloph links the posterior arm to the hypocone. The hypocone is distinct but weakly bulged and fused with the posteroloph, which ends at the base of the metacone. The sinus is narrow and shallow.
The mesiodistally stretched mesostyle is slighly displaced buccally, closer to the end of the postparacrista than to the weak premetacrista. A low ridge is developed at the place of the buccal mesoloph. The bulged metacone is prolonged in a thick buccal metalophule II, which is separated from the strong metaconule by a narrow and relatively deep notch. This metaconule connects to the postprotocrista by a slender ridge. The metaconule bears a mesial outgrowth towards the centre of the mesosyncline. It is of same height as that of the well-developed metalophule I, which runs parallel but lower and narrower than the metalophule II. There are two extraridges on the buccal slope of the protocone, and one thinner on the lingual metalophule, all running towards the basin of the mesosyncline.
M1–M2. These teeth are worn horizontally on the lingual half of the crown, and more vertically on the buccal half (the buccal surfaces of the paracone and metacone), the wear facets being oriented towards the basin.
On the type, M1 differs from M 2 in:
– a smaller and closer together protocone and hypocone;
– a less angulated junction preprotocrista-anteroloph;
– a wider buccal opening of the mesosyncline;
– a more mesiodistally stretched mesostyle (or twinned as seen on the buccal profile of the M1 of SMF-ME
11295 B, with two mesolophs (only one on M2));
– a stronger metaconule;
– a more lingual metacone with a shorter posteroloph.
In both cases, there is a relatively deep (about 2/5 of crown height) pinched and narrow sinus.
The parastyle is swollen on both teeth; the low anteroloph joins a bulging (the anterostyle) at the extremity of the preprotocrista. The paracone and metacone have a same size. The buccal protoloph is weakly connected to the paraconule. The latter is bulged and protruding in a ridge weakly separated from the anteroloph. It joins the centre of the protocone. There are two or three low (worn) extra-ridges from the buccal protoloph and the paraconule to the centre of the basin. Like on P4, the postparacrista is thick and strong, whereas the premetacrista is weaker, both being separated from the mesostyle by narrow and shallow notches. The buccal metalophule II is as high and strong as the buccal protoloph, reaching the level of the metaconule to which it is weakly fused. The metaconule is conspicious on M1, weakly divided into two elements. It is less clear on M2, where it appears weaker, but it is much worn. There are several lingual connections of the metaconule: with the postparacrista, with the endoloph or with the hypocone (M1). Low extra-ridges, worn and hardly distinct, are present along the mesial slope of the metaloph and could be the equivalent of the ‘metalophule I’ of P4. There is a thickening at the end of the posterior arm of the hypocone, better seen on M2 (posterostyle?). The posteroloph is low, ending at the base of the metacone. There are a few extraridges in the anterosyncline and the enamel surface of the crown appears rough, despite some wear.
M3. The protocone and its arms are longer than on M1–2; the parastyle, with a mesiodistal wrinkle, and the anterostyle are present. The protruding paraconule is thin and reduced. The protoloph is connected both to the anterior arm (preprotocrista) and to the apex (protocrista) of the protocone. The mesostyle is followed distally by additional mesostyle and mesoloph. The metacone is low and reduced, displaced distally, making an arcuate cingulum with the shortened posteroloph and posterior arm of the hypocone. The buccal metalophule is low and directed distomesially into the basin. The mesolophs, the metalophule and a ridge from the hypocone-posteroloph area, converge, without connecting, into the mesosyncline to a rounded metaconule. The latter is linked to the postparacrista by a thin ridge (lingual metalophule?). The hypocone is very reduced, but distinct, and a sinus is well present, filled lingually by a small conule (derived from the short endoloph situated above?).
Lower teeth. See Text-fig. 5a. View Text-fig
p 4 View Text-fig . This tooth is markedly shorter than the m1. The metaconid, mesiomedian, is the highest and strongest cuspid. Two mesiodistal ridges descend from the apex of the metaconid to the centre of the talonid basin, the more buccal being the strongest. A long and high postmetacristid descends to the narrow mesoflexid opening, which separates the postmetacristid extremity from the entoconid. Buccally, there is a cristid symmetrical to the lingual postmetacristid, but lower, and which ends as a low ectocingulid; it is anteriorly thick (instead of the linguobuccal premetacristid?), and becomes thinner at the base of the hypoconid. Parallel to this cristid, but higher and more lingual, a thick cristid occupies the place of the protoconid (protocristid) at the anterior part of the ectolophid. The buccomesial connections of the protocristid and of the ectocingulid to the metaconid are somewhat worn: it is difficult to decide if a buccal and a lingual metalophulid I are present. The thick protocristid is followed by a premesoconid thickening; then the mesoconid bears lingually a short and angled mesolophid. As the tooth is weakly worn, one can see that the mesoconid is well defined from the thinner and lower ectolophid, the mesial and distal parts of the ectolophid being underlined by shallow and short notches. Two buccal ridges descend one from the distal end of the protocristid, the other from the premesoconid area.
The ectolophid connects the anterior arm of the hypoconid, together with a buccal part of the entolophid, bearing a postmesoconid swelling. The protoconid is slighly higher than the entoconid; it is prolonged in a thick posterior arm separated by a narrow and very shallow notch from the strong hypoconulid, higher than the following short posterolophid. A shallow notch separates this posterolophid from a thick and short posterior arm of the entoconid. The lingual entolophid turns posteriorly to join the hypoconulid. A few weak extra-ridges are seen on the mesial flank of the entoconid and on the distal slope of the metaconid, to the talonid basin.
The enamel of the lingual and buccal flanks of the crown is rough.
m1–m2. The teeth are worn horizontally on the buccal half of the crown, and more vertically on the lingual half (the buccal surfaces of the metaconid and entoconid), the wear facets being oriented towards the basin. The m1 anterior half is as wide as the posterior half of the p4. The ‘trigonid’ of m1 is much higher than the basin and posteroflexid, whereas it is only slighly higher on m2, and of same height on m3.
The metaconid is the highest cusp; its posterior arm is sharply interrupted at its distal end; on m2 only, it ends into a short mesolophulid-like linguo-buccal ridge. The postmetacristid is long and high, ending at the lingual opening of the ‘mesoflexid’, much narrower at its base. The mesostylid is absent.The anteroconid is indistinct from the anterolophid, ending at mid width; it is higher than the bottom of the talonid basin, and higher than the entolophid. It is connected to the anterior arm of the protoconid. The anterosynclinid is narrow and shallow. A linguobuccal premetacristid is present, linked to and aligned with the anterolophid. The buccal metalophulid, developed from the apex of the protoconid, is transverse. It is connected to the lingual metalophulid on m1 and m2, making a complete metalophulid I.
The protoconid is slighly distal to the metaconid. The extremity of the long, thick and oblique postprotocristid is swollen and forms a premesoconid spur, prolonged in a long extra-ridge towards the centre of the talonid basin on m2. The mesial part of the ectolophid and mesoconid are mesiodistally oriented, and the distal part runs more obliquely. The mesoconid is moderately bulged; its buccal flanc bears two (m1) or one (m2) thin and plunging ectomesolophids. One low mesolophid converges towards the centre of the talonid basin associated with extra-ridges. Another extra-ridge develops from a postmesoconid spur. On the buccal flank, the sinusid reaches the mid-height of the crown. When the enamel is not too heavily worn (as is the case for m1), the sinusid is bounded buccally by one distal ectostylid (m2). The distal ectolophid is short (longer on m2 than on m1). The prehypocristid is short and thick. The entoconid is the smallest cuspid. The entolophid is low at its buccal half. It is complete, and connects directly to the posthypocristid on m1, and to the postmesoconid swelling on m2. The hypoconulid is bulged at the extremity of the posterior arm of the hypoconid. The posterolophid is short, with a short and lower part closing the posterosynclinid. This lophid is related to the posterior arm of the entoconid.
From the lingual metalophulid area to the centre of the basin, there are two short mesiodistal ridges, while another, longer, run from the buccal metalophulid area. These extraridges converge towards the centre of the talonid basin, where they meet another one on m2, buccolingual, running from the postmesoconid area. There are also short wrinkles from the entolophid and hypoconulid in the posterosynclinid.
m3. On the single m3, which is less worn than the m1–2, the extra-ridges are better exposed in the metalophid and talonid basin area, as well as in the antero- and posterosynclinids. This m3 differs from the m 2 in its reduced posterolophid and more lingual hypoconulid. A short buccal entolophid is distinct from the connection between the entoconid and the premesoconid ridge.
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Forschungsinstitut und Natur-Museum Senckenberg |
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