Neohelos solus, Black & Archer & Hand & Godthelp, 2013

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 : 681-684

Neohelos solus sp. nov.

Figs. 1 , 2 , 9B , Table 1.

2000 Neohelos sp. A ; Murray et al. 2000a: 31–37, figs. 24–27.

Etymology: From Latin solus , alone, the only, which alludes to the fact that this species does not form part of the chronological morphocline, to which all other Neohelos species belong.

Holotype: QM F30878, a left partial maxilla with P3, M1–3.

Type locality: Cleft of Ages Site, Riversleigh World Heritage Area fossil deposit; Queensland, Australia.

Type horizon: COA Site is a fissure fill deposit located on the southern section of the Gag Plateau ( Creaser 1997). On the basis of vertebrate stage−of−evolution biocorrelation it is tentatively regarded as Middle Miocene (FZ C) in age.

Referred specimens.—From COA Site: QM F40164, Lm1; QM F40158, RM3; QM F40159, Rm1; QM F40160, RM1; QM F40161, RM2; QM F40162, LM1; QM F40163, LM4; QM F56232, RP3; QM F56233, RP3; QM F56234, LP3; QM F56136; Lp3; QM F12432, Lm3; QM F12433, LM4; QM F12434, LM1; QM F20486, Lm1; QM F20488, LM2; QM F20489, Lm3; QM F20490, Rm3; QM F20491, Rm1; QM F20584, RP3; QM F20585, LM2; QM F20709, Rm3; QM F20830, RM2; QM F20831, LM3; QM F20832, Lm3; QM F20838, Lm1; QM F20852, RM1; QM F22765, left partial maxilla with M2–3; QM F22766, Rm2; QM F22767, RM3; QM F22771, RM4; QM F22773, RP3; QM F22774, Rp3; QM F23195, RM1; QM F23197, RM1; QM F23198, Rp3; QM F23199, Rp3; QM F23274, LP3; QM F23407, LM1; QM F23408, RM3; QM F23472, RM1; QM F24270, LM1–3; QM F24298, RP3; QM F24299, RM3; QM F24300, Rp3; QM F24432, Rm2; QM F24433, Rm3; QM F24435, Lm1; QM F24440, Lm4; QM F24667, Rp3; QM F24731, RM1; QM F24741, LM3; QM F29738, Lp3; QM F29739, RM1; QM F29740, RM1; QM F30231, left dentary fragment with m1–2 and partial m3; QM F30305, left maxilla with M1–3; QM F30306, Rm2; QM F30554, Rm1; QM F30556, LM1; QM F30558, RM1; QM F30560, Rm1; QM F30734, RP3; QM F30819, right dentary fragment with m3–4; QM F31356, M1; QM F31357, LM1; QM F31359, Rp3; QM F31364, Rp3; QM F31366, Rm1; QM F36232, Rp3; QM F50481, P3; QM F50487, Rm1; QM F50488, P3; QM F50490, LM2; QM F50492, LM2; QM F50493, LM3. From KCB Site: QM F56138, LP3–M1.

Diagnosis.— Neohelos solus differs from other species of Neohelos in the following combination of features: small size (except some Ne. tirarensis ); weak transverse parametacone crest on P3 that does not meet a corresponding crest from the protocone; a tendency to have a more sharply delineated anterobuccal crest on P3; weaker posterobuccal cingulum on P3 that generally lacks a cuspate mesostyle; P3 with a more steeply sloping buccal parametacone surface; proportionately narrower upper molars; shorter, more arcuate protoloph with a deep cleft on its posterior surface on M1–2; a posterolingual metaconule crest that is continuous with the posterior cingulum on M1–2; a discontinuous lingual cingulum on M1; more distinct postparacrista and premetacrista that meet in the interloph valley; more trapezoidal M 1–2 in occlusal outline and more convex buccal margins of the paracone and metacone; weaker stylar cusps that are positioned lower on the molar crown; and a higher paralophid and shorter protolophid on m1. Neohelos solus differs from Neohelos davidridei in having: an undivided parametacone; a P3 with a shorter parastyle that is less separated from the parametacone base; and a p3 with greater emargination between the anterior and posterior tooth moieties.

Description

Holotype.—QM F30878, partial left maxilla with P3, M1–3 ( Fig. 1 ). The dentition is relatively well preserved, except for the absence of enamel on the posterolingual corner of P3 and

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some slight fracturing of enamel on the posterobuccal and anterobuccal margins of M2 and M3, respectively. The cheek tooth row is relatively straight along its lingual margin, but slightly convex along its buccal margin. A slight, posteriorly increasing molar gradient is evident. A large, ovate (9.5 × 4.5 mm) infraorbital foramen is positioned 14.4 mm above the anterior root of P3. A round (4.9 mm diameter) infraorbital canal opens 34.5 mm posterior to the infraorbital foramen on a rounded sub−orbital shelf that is scarred by nutrient foramina.

P3 ( Fig. 1 ): The premolar is a small, sub−ovate tooth with four main cusps: a tall, central parametacone; moderate lingual protocone; small, erect, anterior parastyle; and a weak posterolingual hypocone. The apices of all these cusps show moderate wear. A distinct anterobuccal crest from the parametacone apex terminates in the transverse valley separating the bases of the parastyle and parametacone. A stronger posterobuccal parametacone crest extends to the posterior tooth border where it meets the parastyle of M1, and is continuous with a weak posterolingual cingulum. A buccal cingulum is absent, however, a swelling at the base of the crown, opposite the parametacone apex, may represent a weak mesostyle. The enamel in this area is strongly ridged. The lingual base of the protocone is bulbous. The buccal base of the protocone is separated from the parametacone by a moderately deep cleft. There is no evidence of a transverse crest linking the apices of the protocone and parametacone, but there may have been a weak anterolingual parametacone crest. The anterolingual cingulum is thick but low, dominated by a series of vertical ridges in the enamel, and continuous with a vertical crest which ascends the lingual face of the parastyle.

M1 ( Fig. 1 ): M1 is elongate and trapezoidal in occlusal outline, with a narrow anterior protoloph and a wider posterior metaloph. Both lophs are moderately worn, particularly on their anterior faces. The protoloph is crescentic (compared with the relatively linear metaloph), creating a deep cleft on its posterior face at the midline of the tooth. The transverse median valley is moderately deep, convoluted, and open buccally. A weak lingual cingulum is formed by the junction of a posterolingual protocone crest and anterolingual metaconule crest. The enamel on the lingual faces of the protocone and metaconule is heavily ridged. A well−developed posterior crest descends the lingual face of the metaconule and becomes continuous with the posterior cingulum. A weaker anterior cingulum is also present. The parastyle and metastyle are distinctly cuspate, but situated low on the crown. The metastyle is connected to the metaloph by a weak, elongate postmetacrista.

M2 ( Fig. 1 ): M2 is similar to M1, except that it is larger overall and proportionately wider, with a wider protoloph and metaloph. The parastyle and metastyle are reduced, as are the preparacrista and postmetacrista. The buccal tooth margin is more bulging and convex and the transverse median valley is more deeply convoluted. The posterior face of the metaloph is steeper and narrower.

M3 ( Fig. 1 ): Similar to M2, except that the metaloph is much narrower than the protoloph and more obliquely orientated, resulting in a more trapezoidal occlusal outline. The transverse median valley is wider and open lingually and the parastyle and metastyle are further reduced.

Referred material.—Dentary: Description based on QM F30231 ( Fig. 2 ) and QM F30819. The partial left dentary QM F30231 preserves the area of the horizontal ramus below m1 to the posterior border of m3, but is missing the inferior bor− der and much of the surface bone on its lingual face. The dentary is moderately deep and the lateral surface of the horizontal ramus is broadly rounded below m3. The medial surface of the horizontal ramus is relatively flat. QM F30819, a right partial dentary, preserves the area posterior to m3 and anterior to the (secondary) masseteric foramen. The dentary is moderately deep (42.3 mm taken between m3 roots) and broadly rounded (23.6 mm) with a broad lateral shelf beside the molar row. Medially, the dentary drops away steeply below m3–4. The ascending ramus originates 15 mm lateral to the interloph valley of m4 and rises at an angle of 70 ° relative to the occlusal molar plane. The post−alveolar shelf is 14.5 mm long, yet the post−alveolar process is weak. The pterygoid fossa extends anteriorly below the level of the post−alveolar process. In cross section, the internal mandibular canal is large and ovate (9.1 mm high × 5.6 mm wide). A small masseteric foramen (1.8 mm diameter) is situated 17.0 mm posterior to the anterior border of the masseteric fossa. A smaller (1.5 mm diameter) secondary masseteric foramen lies 4.6 mm posterior to the first. Both foramina are confluent with the mandibular canal internally.

p3: Description based on QM F31359, QM F36232 ( Fig. 2 ) and QM F31364 (right p3s) and QM F56136 (left p3). The p3 is a two−rooted, sub−ovate tooth, dominated by a central protoconid which is connected to a shorter posterior cuspid by a concave crest. In all specimens, the anterior border of the protoconid descends steeply to the base of the crown. In QM

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F31359 and QM F31364, it is convex in lateral profile, whereas in QM F36232 and QM F56136 it is straighter and terminates in a slight swelling at the base of the crown. A well−developed lingual fossa is present in all, and is defined anteriorly by a lingual cristid from the protoconid apex, posteriorly and lingually by a well−developed cingulum, and buccally by the posterior protoconid crest. In QM F31364, the lingual fossa is deeper and better delineated owing to a steeper posterior protoconid crest and lingual protoconid cristid.

m1: QM F31357 (left m1, Fig. 2 ), QM F31366, and QM F50487 (right m1s). The m1 is a two−rooted, sub−rectangular tooth with a narrow, elongate trigonid (consisting of a transverse protolophid and an anteriorly directed paralophid) and a broader talonid (consisting of a transverse hypolophid). QM F31366 and QM F31357 are unworn specimens and both possess high paralophid crests. In QM F31366 the paralophid is continuous with the anterolingual cingulum, however in QM F31357, an anterolingual cingulum is absent. In all specimens, the preentocristid is well−developed and terminates in the interloph valley, and a slightly cuspate buccal cingulum is present, albeit to varying degrees. QM F50487 is a slightly broader tooth overall.

m2–3: Description based on QM F30231 ( Fig. 2 ), a left dentary fragment with m1–2 and partial m3, and QM F30819, a right dentary fragment with m3–4. The m2 and m3 are two−rooted, sub−rectangular teeth with broad anterior protolophids and narrower posterior hypolophids. Low, broad anterior and posterior cingula are present. The interloph valley is broadly V−shaped (in lateral view) and open, owing to the absence of buccal and lingual cingula. QM F50408, a left m3, is similar to QM F30819, but larger overall.

m4: Based on QM F30819. Similar to m3, yet slightly narrower with a more reduced hypolophid.

Remarks.—QM F30878 ( Fig. 1 ), here designated as the holotype of Neohelos solus , was nominated by Murray et al. (2000b) as a reference specimen for Neohelos sp. A . Extended descriptions of the following referred material (representing i1, p3, m1–4, P3, M1–4), can be found in Murray et al. (2000b): QM F20488; QM F20584; QM F20585; QM F20828; QM F20852; QM F23197; QM F23274; QM F24230; QM F30305; QM F30557; QM F30734; QM F69257. Additional referred material recovered since submission of MAGNT Report 6 is described in Appendix 3 in so far as it differs from the holotype, and includes QM F56232−4, QM F31356, QM F50481, QM F50486, QM F50488, QM F50490 and QM F50492−3, all from the COA LF, and QM F56138, from the KCB LF.

Geographic and stratigraphic range.—Middle Miocene; COA and KCB sites (FZ C), southern section of the Gag Plateau, Riversleigh World Heritage Area, northwestern Queensland, Australia.

Figures

Fig. 1. Diprotodontid marsupial Neohelos solus sp. nov. holotype, QMF 30878, partial left maxilla with P3–M3, from the Middle Miocene Cleft Of Ages LF, Riversleigh World Heritage Area, Queensland, Australia. Occlusal stereopair (A), lingual (B), and buccal (C) views. Scale bar 20 mm.

Fig. 2. Diprotodontid marsupial Neohelos solus sp. nov. from the Middle Miocene Cleft Of Ages LF, Riversleigh World Heritage Area, Queensland, Australia. A. QMF30231, partial left dentary with m1–2, partial m3; occlusal stereopair (A1), buccal (A2), and lingual (A3) views. B. QMF31357, Lm1; occlusal stereopair. C. QMF36232, Rp3; occlusal stereopair (C1), lingual (C2), and buccal (C3) views. Scale bars 10 mm.

Fig. 9. Comparison of diprotodontid marsupials Neohelos tirarensis Stirton, 1967 (QMF 30438) (A) and Neohelos solus sp. nov. (QMF29739) (B) first upper molars (M1) in occlusal (A1, B1) and lingual (A2, B2) views. In addition to differences in size and relative width the following distinctions are evident: 1, elongate protoloph compared with; 2, short, arcuate protoloph; 3, shallow posterior face of protoloph compared with; 4, deep cleft on posterior face of protoloph; 5, broad, high stylar cusp E compared with; 6, weak, low stylar cusp E; 7, lingual cingulum low compared with; 8, lingual cingulum ascends lingual surface of metaconule; 9, weak postparacrista and weak/absent premetacrista compared with; 10, distinct postparacrista and premetacrista that meet in the interloph valley. Scale bar 10 mm.