Gomphos progressus, Li & Wang & Fostowicz-Frelik, 2016
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publication ID |
https://doi.org/ 10.4202/app.00292.2016 |
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persistent identifier |
https://treatment.plazi.org/id/03E687D4-FFF5-FFCD-FF1C-7AAEFC1B27C4 |
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treatment provided by |
Felipe |
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scientific name |
Gomphos progressus |
| status |
sp. nov. |
Gomphos progressus sp. nov.
Figs. 3 View Fig , 4 View Fig .
Etymology: From Latin progressus , progressive; in relation to the derived dental morphology seen in this species.
Type material: Holotype: left m2 ( IVPP V20259 ), paratypes: right M3 ( IVPP V20824 ); right m3 ( IVPP V20259.1 ).
Type locality: Layer 4 (lower to middle part), Wulanhuxiu , Ulan Shireh Formation, Erlian Basin, Nei Mongol, China .
Type horizon: Irdinmanhan, middle Eocene.
Other material.—DI2 ( IVPP V20259.2 ), di2 ( IVPP V20259.3 ) from the type locality and horizon .
Diagnosis.—Derived Gomphos characterized by fusion of lower molar cusps to form continuous crests. Differs from all other Gomphos species in lacking mesoconid, cristid obliqua and mesostylid. Entoconid and hypoconulid form a ridge instead of separate cusps. Further differs from G. elkema in having reduced hypoconal shelf on M3 and lacking mesostyle. Further differs from G. shevyrevae in being less hypsodont and in having buccally shifted entoconid on m3.
Description.—The M3 of Gomphos has not previously been described in detail. The tooth is strongly asymmetrical with a buccolingually wide anterior part and a posterior part which is roughly 40% narrower. Both individual cusps and the crown as a whole seem more inflated than in G. elkema (the M3 is unknown in G. ellae and G. shevyrevae ). The tooth resembles the M3 of G. elkema in lacking strong unilateral hypsodonty. The roots have been broken away. The protocone is large and prominent, being larger overall than the paracone. As in other Gomphos and Mimolagus aurorae , the protocone produces two arms (lophs), the preprotocrista and postprotocrista, which diverge anteroposteriorly, and extend buccally to delineate the trigon. The anterior and posterior lophs meet at an acute angle, that is slightly wider than in G. elkema . The preprotocrista surrounds the paracone anteriorly and continues to the anterobuccal corner of the tooth ( Fig. 3 View Fig ), while the postprotocrista joins with the metacone. The postprotocrista is shorter than in G. elkema , and deviates more posteriorly from the paracone. The metacone therefore is located more posterolingually than in G. elkema . The metaconule is small and marked only by a slight broadening of the postprotocrista, whereas in G. elkema the metaconule is larger than the metacone. The specimen from Wulanhuxiu lacks a mesostyle, although this feature is quite prominent in G. elkema . The trigon basin is wider than in G. elkema and is open buccally, forming a relatively flat, triangular valley. The small, oval hypoconal shelf, and buccolingually shorter than in G. elkema , and more similar to the equivalent structure in Mimolagus aurorae . The hypoconal shelf is positioned, slightly askew, at the posterolingual side of the tooth. The hypocone is aligned with the protocone, and the posterobuccal end of the hypoconal shelf is aligned with the metacone ( Fig. 3 View Fig ). Between the metacone and hypoconal shelf is a deep narrow groove.
The lower molars ( Table 2) are slightly larger than those of Gomphos shevyrevae (see Meng et al. 2009), and the crowns are even more swollen than in that species. The unilateral hypsodonty of the crown is very poorly expressed and is slightly weaker than in G. shevyrevae . The tooth crowns also differ slightly in occlusal outline from those of G. shevyrevae , being less perfectly square; instead, the trigonid is slightly wider than the talonid and this difference seems to persist as the crown swells towards the base. In both specimens from Wulanhuxiu the hypoconulid lies at the distal edge of the talonid. In the specimen IVPP V20259.1 ( Fig. 3C View Fig ) the hypoconulid is slightly larger, and more central, than in the holotype. For this reason, we are inclined to regard V20259.1 as m3. The holotype specimen, in which the hypoconulid is shifted more lingually, can be identified as m2. The distal margin of the talonid between the hypoconulid and entoconid is straight. This feature is more clearly visible in m3, which is the more worn of the two lower molars, although wear in both cases is light. In both lower molars, the trigonid has two large cusps, the protoconid and the metaconid. The metaconid is visibly higher, although both cusps have similar base radius. There is no trace of a paraconid or paracristid, as is typical of Gomphos ( Zhegallo and Shevyreva 1976; Dashzeveg and Russell 1988; Meng et al. 2004, 2009). Between the cusps of the trigonid a deep and wide V-shaped trigonid basin is formed. In both molars the protoconid and metaconid are aligned.
In the less heavily worn left m2, the talonid forms a large basin surrounded by a perimeter of almost totally merged cusps, namely the entoconid, hypoconulid, and hypoconid. The hypoconid is particularly prominent on the m2, although it is hard to determine the exact boundary of each talonid cusp. In m3, which is slightly more worn, the hypoconid is more isolated and forms its own wear facet ( Figs. 3 View Fig , 4 View Fig ). The posterior rim connecting the hypoconulid with entoconid is slightly indented where it intersects with the hypoconid. The hypoconulid is similar to that of Gomphos shevyrevae but smaller than in G. elkema . The hypoconulid forms most distally pronounced portion, which is gently rounded, of the distal rim of the talonid ( Fig. 3B View Fig 1, C 1 View Fig ). The edge between the entoconid and hypoconulid tends to be flat ( Fig. 3B View Fig 1, C 1 View Fig ). During the early stages of wear, the hypoconid is aligned with the protoconid, although as wear progresses this alignment becomes distorted because the protoconid “shifts” more buccally as the whole trigonid is more extended buccally, rather unlike in G. shevyrevae and other Gomphos species (see Meng et al. 2004, 2009). The hypoconid and entoconid are aligned in both molars. However, the metaconid and entoconid are aligned in m2 ( Fig. 3B View Fig 1 View Fig ) but not in m3, in which the entoconid is shifted buccally ( Fig. 3C View Fig 1 View Fig ).
The lower molars of Gomphos from Wulanhuxiu show no trace of the mesostylid, which is well developed in G. elkema and G. ellae (Meng et al. 2004; Kraatz et al. 2009). The Wulanhuxiu specimens also lack a mesoconid, a feature present in all other Gomphos species including G. shevyrevae ( Meng et al. 2009) .
Stratigraphic and geographic range.—Ulan Shireh Formation, Irdinmanhan, middle Eocene, Erlian Basin, Nei Mongol, China.
| IVPP |
Institute of Vertebrate Paleontology and Paleoanthropology |
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