Meldimys musak, Rana & Kumar & Escarguel & Sahni & Rose & Smith & Singh & Singh, 2008
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publication ID |
https://doi.org/ 10.4202/app.2008.0101 |
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persistent identifier |
https://treatment.plazi.org/id/03DA87A9-FFD2-FFA8-3916-50EDB581986A |
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treatment provided by |
Felipe |
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scientific name |
Meldimys musak |
| status |
sp. nov. |
Meldimys musak sp. nov.
Figs. 2–5 View Fig View Fig View Fig View Fig ; Table 1.
Derivation of the name: From Sanskrit musak , mouse.
Type material: Holotype: GU/RSR/VAS 609, an isolated right upper second molar ( RM2 ) . Paratypes: GU/RSR/VAS 608, RdP4; GU/RSR/ VAS 606, LP4 ; GU/ RSR/VAS 449 RM1 ; GU/RSR/VAS 128 and 607 LM1; GU/RSR/VAS 450 and 454, RM2 ; GU/RSR/VAS 451–453, RM3 ; GU/RSR/VAS 455–456, LM3; GU/RSR/VAS 604, Rm1; GU/ RSR/VAS 605, Lm1; GU/RSR/VAS 603, Lm2 .
Horizon and locality: Lower Eocene (Ypresian) Cambay Shale, Vastan lignite mine (N 21 ° 25’59”, E 73 ° 06’59”), 40 km northeast of Surat, Gujarat GoogleMaps , western India.
1 mm metacone paracone anteroposterior “groove”
posteroloph hypocone protocone
Diagnosis.—Teeth similar in size and occlusal morphology to M. louisi . Differs from M. louisi in having P4 with strong metaloph; upper molars with a small median conule between protoloph and metaloph and less buccally projecting mesostyle; M1 with less prominent parastyle positioned almost in line with paracone rather than being buccally extended; M2 with less rounded occlusal outline and a protocone usually more anteriorly situated compared to the more central protocone of Meldimys louisi ; posterior width of M3 less short than in M. louisi ; M3 with sub−complete metaloph and reduced lingual sinus (between hypocone and protocone); m1–2 with antero−posteriorly reduced trigonid and weaker anterolophid and “waist” (= slight constriction between trigonid and talonid), and no paralophid (= metalophulid I) and metalophulid II; talonid cusps considerably lower than trigonid cusps.
Description.—The dP4 and M1–M3 are triple−rooted; P4 is double−rooted, its postero−labial root fused with the lingual root. The m1–2 are double−rooted. The p4 and m3 are not represented in our collection. The dP4 is triangular, transversely elongated (L/W = 0.86), quite narrow lingually and somewhat heart−shaped with a faint notch (ectoflexus) in the labial margin. Its paracone is large, slightly labially placed in comparison to the metacone and also posteriorly shifted, leaving a wide shelf anterior to the protoloph. The hypocone represented by an elongated ridge is much lower than the protocone. The protoconule is anteriorly placed rather than aligned with the paracone and protocone.
P4 is somewhat globular with a convex anterior contour and a centrally slightly concave posterior contour. The protoloph is nearly transverse, the metaloph strong and oblique and the paracone posteriorly shifted as in dP4. It differs from dP 4 in being more ovoid, and in having a thick and labially extended anteroloph, a short posteroloph restricted to the lingual half, and a more noticeable hypocone. Its lingual aspect is broad and rounded rather than narrow as in dP4.
Upper molars have occlusal surfaces somewhat concave in lateral profile, forming a shallow “groove” along their antero−posterior median line ( Figs. 3 View Fig , 4C–G View Fig , and 5A–D View Fig ). M1 is narrow lingually, its labial half has a rounded contour. The anteroloph, posteroloph, protoloph, and metaloph are distinct. The protocone is the largest cusp; the paracone is massive and tetrahedral but slightly transversely elongated, much larger than the metacone, and medially shifted relative to the labial margin of the tooth. The protocone and paracone are equally high from the crown base. The parastyle is strong and extends more labially than the paracone. The mesostyle is also quite prominent and labially projecting relative to paracone and metacone. The hypocone is moderately developed and noticeably lower than the protocone. The metaconule is better developed than the protoconule. A long sloping shelf occurs anterior to the protoloph. In unworn teeth a short crest originates from the protocone and descends into the trigon; a distinct conule is present in the middle of trigon. A weak sinus is lingually situated between protocone and hypocone.
M2 is slightly larger than M1 with a somewhat rounded contour and better developed hypocone than in other upper teeth. The paracone is labio−lingually elongated, the parastyle and metastyle distinct and the mesostyle labially protruded. The metaconule is much larger than the protoconule. A crest arises from the protocone and descends into the trigon basin extending beyond the protoconule and metaconule and directed towards the mesostyle. A small conule in the trigon basin between the protoconule and metaconule is more distinct than in M1. It is usually isolated but lies close to the buccal end of the crest that descends into the basin. The anteroloph and posteroloph are prominent. The lingual sinus between the hypocone and protocone is more pronounced than in M1.
M3 is triangular with subequal length and width. Its paracone is large and labio−lingually elongated as in the more anterior molars. The hypocone is small but distinct in most teeth and slightly labially positioned relative to the protocone ( Fig. 4B, C View Fig ). Anteroloph and posteroloph are prominent; the latter is generally smooth but somewhat uneven in GU/RSR/ VAS 452 and 455 ( Fig. 4A, C, D View Fig ). The difference in size of the metaconule and protoconule is less than in the anterior molars though the metaconule is still larger than the protoconule. The posterior part of the occlusal surface is characterized by the presence of a small depression bounded anteriorly by the metaloph and posteriorly by the posteroloph, both of which converge on the metacone on one side and the hypocone on the other. The median crest that descends into the trigon basin from the protocone and a small conule in the center of the trigon basin are generally very clearly seen in all M3s. The median crest extends almost up to the mesostyle in at least one tooth (GU/RSR/VAS 451; Fig. 4B View Fig ), in which the central conule appears to be a part of this crest. The mesostyle and parastyle are nearly as distinct as in anterior molars. The lingual sinus between the hypocone and protocone is weak.
Our collection has only three lower cheek teeth, all of which are quite worn and or partly damaged. In m1, the trigonid is antero−posteriorly reduced, the anterolophid is weak, and talonid cusps are considerably lower than trigonid cusps. They lack a paralophid (= metalophulid I), while the metalophulid II is either absent or not discernible due to strong wear.
The m1 has a distinct “waist” (constriction) between the trigonid and the wider talonid ( Fig. 5A View Fig ). The metaconid is the highest cusp, and entoconid the smallest and lowest. The protoconid is larger than the metaconid and somewhat posteriorly placed in comparison to the latter. The hypoconid is massive, and hypoconulid tiny, almost merged into the posterolophid. An ectolophid is short but well developed, particularly posteriorly; it bears a distinct mesoconid which is connected to the hypoconid but separated from the protoconid. An anterolophid is poorly developed. A faint hypolophid can be seen in one of the two m1's (GU/RSR/VAS 604; Fig. 5A View Fig ). A deep talonid notch is present between the entoconid and metaconid along the lingual margin of tooth.
The m2 differs from the m 1 in having more widely separated protoconid and metaconid, a hypoconid smaller than the protoconid, better developed ectolophid and anterolophid, a much shallower talonid notch between entoconid and metaconid and a clearer yet weak hypolophid, and in lacking a “waist” between the trigonid and talonid. It has a faint metastylid.
Comparisons.—The new Indian rodent is referred to the subfamily Ailuravinae (Family Ischyromyidae ) based on distinctive occlusal morphology of its cheek teeth—upper molars with relatively less developed hypocone (primitive character), tetrahedral paracone, strongly developed parastyle, distinct and externally extended mesostyle and a short crest ascending from the protocone into the trigon basin, and lower molars with a large mesoconid connected to the hypoconid by a strong ectolophid, and an isolated entoconid connected to a smaller hypoconulid. Most of these characters are not present in chapattimyids and yuomyids (Ctenodactyloidea), which include all middle Eocene rodents reported thus far from India and Pakistan ( Hussain et al. 1978; Kumar et al. 1997a, b). Similarly they are not seen in other contemporary ctenodactyloids such as ctenodactylids known from Central Asia (e.g., Dawson et al. 1984). Among the five known ailuravine genera ( Ailuravus , Euromys , and Meldimys from Europe and Mytonomys and Eohaplomys from North America), the Indian ailuravine from Vastan is closer to European forms than to North American forms, and particularly closer to Euromys and Meldimys than to Ailuravus . It differs from Ailuravus , the type genus of Ailuravinae , in being smaller and having cheek teeth with rounded occlusal outlines and less sharp−pointed cusps. It shows relatively less developed parastyle and hypocone on upper cheek teeth and less developed trigonid basin and hypoconulid on lower teeth. Ailuravus is characterized by upper molars with very well developed parastyle and small but fairly developed and moderately high hypocone clearly separated from the protocone by a sinus ( Escarguel 1999: pl. 1: a–c; pl. 2: a–h), and by lower molars with very well developed trigonids having distinct anterolophid and metalophulid II, a distinct hypoconulid and a lophulid between the hypoconulid and the entoconid ( Escarguel 1999: pl. 1: j–r).
The Indian ailuravine is closely comparable to Euromys and Meldimys known from the lower Eocene localities of Mutigny (dated at 51.8±0.16 Ma; Escarguel et al. 1997), Avenay (dated at 52±0.17 Ma; Escarguel et al. 1997) and Condé−en−Brie (dated at 52.1±0.37 Ma; Escarguel et al. 1997) in the Paris Basin ( France), and is most similar to the latter. Features including rounded occlusal outlines, lower crown height, a hypocone noticeably lower than the protocone, a markedly concave occlusal surface creating a “groove” along the antero−posterior median line, and protocone and tetrahedral paracone reaching nearly the same level above the crown base, are clearly indicative of its closer affinity to Meldimys than to Euromys . In contrast to Euromys , the Indian ailuravine has lower molars with a better developed trigonid and a hypoconulid more strongly connected to the hypoconid. The upper molars of the Indian ailuravine differ from those of Euromys mainly in having poorly developed hypocone and lingual sinus and in the relative size of the metaconule and protoconule; in the new taxon the metaconule is considerably larger than the protoconule whereas in Euromys the two conules appear subequal. Among the four known species of Euromys , E. cardosoi ( Estravís, 2000) (for generic assignment see Escarguel 1999) is similar in tooth size to M. musak sp. nov., but M. musak appears morphologically more similar to E. thaleri (Michaux, 1964) ( Escarguel 1999) .
Prior to this report, Meldimys was represented by a single species, M. louisi (considering that the species E. cardosoi from the earliest Eocene of Silveirinha, Portugal, cannot be referred to this genus; Escarguel 1999, contra Estravís 2000), which has cheek teeth remarkably similar to those of the new Indian species M. musak . M. musak is presently represented by dP4, P4–M3, and m1 and m2 but m1–m2 are too worn or damaged to allow detailed comparisons with those of M. louisi . The dP4 of M. musak , though also quite worn, is almost indistinguishable from that of M. louisi ( Fig. 6A View Fig ) illustrated and described by Escarguel (1999: pl. 3: d) except in having a weaker central notch in the labial outline of the occlusal surface and a somewhat more posteriorly positioned paracone. The dP 4 in M. louisi is generally heart−shaped ( Fig. 6A View Fig ; Escarguel 1999: pl. 3: d; pl. 4: f) due to its narrow lingual aspect and a prominent central notch in the broader labial aspect, but this character (heart shape) is variable—dP4 of Meldimys is not always heart–shaped ( Escarguel 1999: pl. 2: o). In fact a more or less heart−shaped dP4 is “archetypal” for the Ailuravinae and is probably plesiomorphic within rodents. This trait, amongst others (all plesiomorphic), led Hartenberger (1995) to propose inclusion of the Ailuravinae , together with Alagomys and Tribosphenomys , within the Alagomyidae . This proposition was based on a very limited cladistic analysis of selected taxa and characters and is now considered untenable ( Dawson 2003; Marivaux et al. 2004).
The only known P4 of M. musak sp. nov., is more rounded than dP4 with a width/length ratio of 1.3. Compared to this, the P4 of M. louisi is generally ovoid with rounded margins and maximum length and width along the median lines ( Fig. 6B View Fig ; Escarguel 1999: pl. 2: p; pl. 3: e), but some are transversely elongated with high (1.4) width/length ratio and others rectangular with low (1.2) width/length ratio ( Escarguel 1999: pl. 4: a, g). The anteroloph and posteroloph in P4 of the new species are marginally higher, the metaloph is stronger and the hypocone is clearly more lingually placed relative to the protocone than in M. louisi . In addition, in the single known P4 of M. musak the postero−labial root is fused with the lingual root, whereas in M. louisi the lingual root is separate and the two labial roots are fused together. However, based on a single specimen it is difficult to assess if this rather unusual configuration is really apomorphic in M. musak or just a teratological, unrepresentative state for this isolated tooth.
Among the anterior upper molars of M. musak sp. nov., M1 differs from that of M. louisi mainly in having a less prominent parastyle positioned almost in line with the paracone rather than being anterolabially extended ( Fig. 3C View Fig ). The M1 parastyle in M. louisi is so strong that the anterolabial aspect of tooth appears distinctly protruded or extended ( Fig. 6C View Fig ). M2 of M. musak differs from that of M. louisi in having a normally positioned protocone rather than a more centrally positioned one as in most specimens of M. louisi ( Michaux 1968; Escarguel 1999). Moreover, in M. louisi these teeth have a more rounded occlusal outline than in M. musak . M3 differs in having posterior width less short than anterior width ( Fig. 4 View Fig ) rather than considerably shorter posterior width in two out of three M3s of M. louisi ( Fig. 6E View Fig ; Escarguel 1999: pl. 3: c, i). The metaloph on M3 of the new species is more or less continuous and better developed than in M. louisi , in which the metaconule appears as an isolated cone. Further, in M. musak the metaloph and posteroloph converge on the metacone and hypocone to form a small enclosure ( Fig. 4B View Fig ), which is not seen in M. louisi . Most upper molars of M. musak possess a small but distinct conule in the middle of the trigon basin between the metaconule and paraconule ( Figs. 3A View Fig 1 View Fig , 4F View Fig 1 View Fig , G 4 View Fig , and 5A–D). This character is lacking in M. louisi . The m1–2 of M. musak differ from corresponding teeth of M. louisi mainly in having an antero−posteriorly reduced trigonid and in lacking a paralophid (= metalophulid I) and metalophulid II.
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