Thylophorops lorenzinii, Goin, Francisco J., Zimicz, Natalia, Reyes, Martín De Los & Soibelzon, Leopoldo, 2009

Goin, Francisco J., Zimicz, Natalia, Reyes, Martín De Los & Soibelzon, Leopoldo, 2009, A new large didelphid of the genus Thylophorops (Mammalia: Didelphimorphia: Didelphidae), from the late Tertiary of the Pampean Region (Argentina), Zootaxa 2005, pp. 35-46 : 38

Thylophorops lorenzinii , new species

Figs. 1–4

Holotype. MLP 08-III-10-1, a fragment of skull preserving the left maxilla, and a left dentary preserving the second premolar (p2), the deciduous premolar (dp3) and an erupting third molar (m3), all belonging to the same, juvenile specimen. Collected by Leopoldo Soibelzon in January, 2008.

Etymology. Honouring Mr. Silvio Lorenzini, outstanding amateur collector of fossil specimens in Southeastern Buenos Aires Province.

Measurements. Extra-alveolar length of upper canine (C): 10.15; CL: 4.33; CW: 3.41; height of dentary at m3: 11.87; width of dentary at same point: 8.08; length of the p2-m3 series: 40.02 mm; p2L: 7.64; p2W: 3.22; dp3L: 5.76; dp3W: 2.83; m3L (total): 9.69; trigonid m3L: 4.22; talonid m3L: 5.47; talonid m3W: 4.31; trigonid m3W: 4.38 (see also Fig. 3 ).

Diagnosis. Largest known didelphid; differs from the other species of Thylophorops in having its lower molars with proportionally large hypoconulids which are not antero-posteriorly compressed; labially salient hypoconids; p2 is proportionally larger, higher, and posteriorly wider than in T. chapalmalensis and T. perplanus .

Description of the holotype. Specimen MLP 08-III-10-1 belongs to a juvenile individual, as evidenced by the persistence of a dp3 and an erupting m3 which is placed immediately anterior to the masseteric crest (i.e. m4 is not observable). Only the anterior part of the snout is preserved ( Fig. 1 A), which is broken. The dentary ( Fig. 1 B–C) is partially broken and lacks the incisor region and the angular process; it is quite robust, especially under m3. The posterior end of the symphysis reaches the distal margin of p2. The mental foramen is in line with the mesial root of m1. The distal root of p1, and complete p2, dp3 and m3 are preserved.

The p2 is large and high. The crown is relatively narrow over the mesial root but is much wider and more robust over the distal root. The posterior cingulum ends near the labial side of the talonid.

The dp3 is the smallest tooth of this specimen; the trigonid and talonid are similar in length, but the talonid is wider. The metaconid is slightly smaller than the protoconid and is located on the posterolabial margin of the trigonid. The small paraconid is positioned anteromedially. The entoconid is the highest cusp of the talonid; the hypoconid is labially salient.

The erupting m3 ( Fig. 2 ) is very large, with its trigonid slightly longer than the talonid. All cusps are well preserved. The paraconid is triangular in occlusal view; its postero-lingual side is flat. In contrast, in T. chapalmalensis ( Fig. 4 , left), this side is rounded and the cusp is conical in occlusal view. The protoconid is the highest and largest cusp of the trigonid; it lies on the lingual side and occupies almost the entire trigonid surface. The metaconid is proportionally larger than that of T. chapalmalensis . The talonid is well developed in all dimensions. The entoconid is tall and prominent. The hypoconulid is large and well developed anteroposteriorly. The entoconid and hypoconulid are similar in size, in sharp contrast with T. chapalmalensis , in which the entoconid is much larger. The hypoconid is located on the labial side of the talonid and is noticeably salient.

Figures

FIGURE 1: Thylophorops lorenzinii sp. nov. MLP 08 - III- 10 - 1 (Holotype); (A), labial view of the anterior part of the skull and dentary; (B) occlusal view of the dentary; (C) lingual view of the dentary. Scale: 10 mm.

FIGURE 2: Thylophorops lorenzinii sp. nov. MLP 08 - III- 10 - 1 (Holotype); detail of the erupting m 3 in lingual (A), and occlusal-lingual views (B). Scale: 2 mm.

FIGURE 3: Thylophorops lorenzinii sp. nov. MLP 08 - III- 10 - 1 (Holotype); schematic drawing of the m 3 showing the cusp homologies and measurements. Abbreviations: End., entoconid; Hyd., hypoconid; Hyld., hypoconulid; Med., metaconid; Pad., paraconid; Prod., protoconid. Scale: 2 mm.

FIGURE 4: Lower left molars of Thylophorops chapalmalensis (left, detail of specimen MPH 064), T. lorenzinii sp. nov. (centre, detail of the type specimen), and T. perplanus (right, detail of specimen MLP 97 - XI- 15 - 1). Because of the erupting stage of the m 3 of T. lorenzinii in the type specimen, the anterobasal cingulum is not visible. Scale: 2 mm.