Neohelos stirtoni Murray, Megirian, Rich, Plane, and Vickers−Rich, 2000a

Black, Karen H., Archer, Michael, Hand, Suzanne J. & Godthelp, Henk, 2013, Revision in the diprotodontid marsupial genus Neohelos: Systematics and biostratigraphy, Acta Palaeontologica Polonica 58 (4), pp. 679-706 : 691-693

Neohelos stirtoni Murray, Megirian, Rich, Plane, and Vickers−Rich, 2000a

Fig. 6 , Table 1.

Neohelos sp. B ; Murray et al. 2000b: 38–72, figs. 29–51.

Holotype: CPC 22200, cranium with left and right P3, M1–4; missing part of right squamosal, parietal, zygomatic arch and left and right I2–3.

Type locality: Small Hills Locality, 26 km east southeast of Camfield Station Homestead, Bullock Creek , Northern Territory (approx. Latitude 17 ° 07’ S, Longitude 131 ° 31’ E) .

Type horizon: On the basis of stage−of−evolution biocorrelation, the limestones of the Camfield Beds are believed to be Moddle Miocene in age ( Archer et al. 1997; Murray et al. 2000a).

Additional material.—To the referred material listed in Murray et al. (2000a), we add the following Riversleigh specimens. From Gag Site: QM F40165, right maxilla with P3, M1–3; QM F40168, LP3; QM F40166, M1. From HH Site: QM F40167, LP3; QM F40170, Rm3; QM F40169, Rp3. From GS Site: QM F40117, left dentary fragment with m3 and protolophid of m4.

Revised diagnosis.— Neohelos stirtoni differs from Ne. tirarensis and Ne. solus in the following combination of features: larger; higher crowned dentition; bladed parametacone on P3; distinct, posteriorly increasing molar gradient; canine absent. Neohelos stirtoni differs from Ne. solus in having broader, squarer molars with less arcuate protolophs and less convex paracone and metacone buccal margins; in lacking the posterolingual crest that ascends the metaloph on M1–2; in having a continuous, arcuate lingual cingulum on M1 and in having a lower paralophid and broader protolophid on m1. Neohelos stirtoni differs from Ne. davidridei in: having a stronger anterior protoconid crest and associated cuspule on p3; having a proportionately less elongate P3; and in lacking an incipiently divided parametacone on P3.

Description

Upper dentition.—Description of the upper dentition is primarily based on QM F40165 ( Fig. 6 ), a right partial maxilla with P3, M1–3. The dentition is generally unworn, except for slight wear on the parametacone and protocone of P3, the parastyle of M1 and the anterior faces of the lophs on M1–2.

P3: P3 with four cusps: a large anterior parastyle; a tall, central parametacone; a moderate lingual protocone; and a small posterolingual hypocone. The P3 is relatively small (17.1 mm in length), but falls within the size range displayed by the Bullock Creek Ne. stirtoni material (15.1–20.4 mm, mean 18.2 mm) found by Murray et al. (2000a). The parastyle is erect and widely separated from the base of the parametacone. A swelling exists at the buccal base of the crown opposite the parametacone, but a distinct mesostyle is absent, as is a posterobuccal cingulum. There is a strong lingual emargination between the bases of the parastyle and protocone. In many respects, the P3 of QM F40165 shows a similar development of features to the premolar figured by Murray et al. (2000a: fig. 28C). QM F40168, a partially encrypted LP3 also from Gag Site, is slightly longer (17.5 mm) and much broader (16.1 mm) than QM F40165, and differs in the following features: the anterior parastylar border is gently rounded (rather than pointed as in QM F40165), as is the protocone base; the hypocone is a small swelling of the

http://dx.doi.org/10.4202/app.2012.0001

posterolingual cingulum, rather than a distinct cusp; and the mesostyle and posterobuccal cingulum are better developed and the mesostyle is distinctly cuspate.

M1–3: These molars are similar morphologically to specimens of Ne. stirtoni from Bullock Creek and, although small (e.g., M1 length 17.8 mm), fall within the size range for that sample (17.2–21.7 mm). There is a distinct, posteriorly increasing molar gradient compared to Ne. tirarensis .

Lower dentition.— p3: QM F40169 ( Fig. 6 ), a right p3, is a sub−ovate, anteriorly tapering tooth. There is slight wear on the protoconid apex, which is situated just posterior to the midline of the crown. A weak crest extends anteroventrally to approximately three quarters of the distance to the base of the crown, where it becomes weakly cuspate. It then curves lingually, forming a shallow anterior fossa between it and the base of the protoconid. A posterior protocristid bifurcates at the posterior tooth border into a well−developed posterolingual cingulum and a weaker posterobuccal cingulum. The lingual fossa is well defined by the postprotocristid buccally, the posterolingual cingulum, and a vertical buttress of the protoconid anteriorly.

m2: QM F40170, a right m2 from the HH LF is a large, sub−rectangular molar with a linear protolophid and slightly curved and obliquely offset hypolophid. The protolophid is slightly narrower and taller than the hypolophid. Both the anterior and posterior cingula are well developed but positioned low on the crown, rising towards the midline. The transverse valley is open buccally and lingually, and is U−shaped in lateral view.

m3–4: Description is based on QM F40117 ( Fig. 6 ), a left dentary fragment with m3 and the protolophid of m4, from the GS LF. The m3 is similar to QM F40170 but larger, and the hypolophid is more obliquely offset with respect to the protolophid. The tip of the protolophid slightly overhangs its base posteriorly. The m4 is similar to m3, except that the protolophid is wider and taller buccally.

Remarks.— Murray et al. (2000b) referred specimens AR 13791 ( QM F40172), a partial right dentary with m2–4 and AR13969 ( QM F40173), a partial right dentary with p3–m1, both from SB Site, to Neohelos sp. B (= Neohelos stirtoni ). A reappraisal of this material suggests QM F40172 is more appropriately assigned to Ngapakaldia bonythoni (Black 2010) and QM F40173 to Ne. tirarensis . Murray et al. (2000b: 65) note that the molar dimensions of AR13791 and QM F40173 are similar to those of Bematherium (synonymized with Ng. bonythoni ; see Black 2010), but differ in having a well−developed paralophid crest on m1. This is true of QM F40173, which is unquestionably Neohelos . However, QM F40172 does not preserve an m1, but does retain the alveoli of the anterior and posterior root of p3, which suggests that the p3 was approximately 9.8 mm in length—dimensions that fall within the range of Ng. bonythoni (9.0– 10.8 mm), but not Ne. stirtoni (11.5–17.9 mm). In terms of both the morphology of the dentary and dentition and molar dimensions, QM F40172 is most similar to specimens of Ng. bonythoni , which are common in Riversleigh’s FZ A deposits, including SB, WH, Jeanette’s Amphitheatre and Hiatus sites (Black 2010).

In regard to QM F40173, the lower premolar lacks the small anterior cuspid, the sharp anterior blade of the protoconid and the anterolingual fossa, which are characteristic of Ne. stirtoni lower premolars. In terms of both size and morphology, it is most similar to Ne. tirarensis material from Riversleigh’s CS Site.

Geographic and stratigraphic range.— Middle Miocene; Bullock Creek LF, Camfield Beds, Northern Territory , and several FZ C deposits of the Riversleigh World Heritage Area , northwestern Queensland, Australia .

Figures

Fig. 6. Diprotodontid marsupial Neohelos stirtoni Murray, Megirian, Rich, Plane, and Vickers−Rich, 2000a, material from Middle Miocene deposits of the Riversleigh World Heritage Area, Queensland, Australia. A. QM F40165, partial right maxilla with P3–M3 from the Dwornamor LF (Gag Site); occlusal stereopair (A1), buccal (A2), and lingual (A3) views. B. QM F40169, right p3 from the Henk’s Hollow LF; occlusal stereopair (B1), lingual (B2), and buccal (B3) views. C. QMF40117, left dentary fragment with m3 and protolophid of m4, from the Golden Steph LF; occlusal stereopair (C1), lingual (C2), and buccal (C3) views. Scale bars 20 mm.