Stelakoala, Black, 2016

Black, Karen H., 2016, Middle Miocene origins for tough-browse dietary specialisations in the koala (Marsupialia, Phascolarctidae) evolutionary tree: description of a new genus and species from the Riversleigh World Heritage Area, Memoirs of Museum Victoria 74, pp. 255-262 : 256-257

Phascolarctidae Owen, 1839

Stelakoala gen. nov.

Type and only species. Stelakoala riversleighensis gen. et sp. nov.

Generic diagnosis. High crowned M 1 with: large protostylid; strong pre- and postprotostylid cristids; large entostylid ridge with well-developed buccal spur; twinned lingual ribs on the entoconid and a single rib on the metaconid; a metastylid fold wherein the postmetastylid cristid is continuous with the preentocristid; a metaconid positioned close to the lingual tooth margin. Stelakoala is unique among phascolarctids in having the following features: a greater separation between the metaconid and protoconid apices with respect to trigonid width; possession of a strong premetacristid; and an incipiently cuspate, short, crescentic preprotostylid cristid that is not continuous with an anterior cingulum. Among known koalas it is most similar to species of Phascolarctos and Litokoala , particularly with respect to the development of the protostylid and entostylid and in its apomorphic possession of a metastylid fold.

Remarks. The Jim’s Jaw Site M 1 was previously regarded to be a new species of Litokoala in an unpublished thesis by Black (1992) (see also Litokoala new sp. 1 of Archer et al., 2006). This assignment was based on overall similarities in molar morphology to that of Litokoala kutjamarpensis (sensu Louys et al., 2007; previously L. kanunkaensis Springer, 1987 ), although Black (1992) did also note similarities to Phascolarctos spp. in the shared presence of twinned lingual ribs on the entoconid. The M 1 is recognized here as a new genus and species of koala because it possesses a combination of derived features found in Litokoala and/or Phascolarctos , as well as autapomorphic features that preclude its assignment to either genus.

Etymology. Stela is the latin word for ‘a pillar or support bearing markings’ in reference to the buttressing of the lingual faces of the entoconid and metaconid by the development of lingual ribs.

Stelakoala riversleighensis sp. nov.

( Figures 2–3 ; Table 1)

Holotype. QM F57737 , isolated right M 1.

Type locality. Jim’s Jaw Site, northern section of the Gag Plateau ( Fig. 1 ), Riversleigh World Heritage Area , northwestern Queensland .

Age. Faunal Zone C deposit; middle Miocene ( Archer et al., 1989, 1991; Arena, 2005; Travouillon et al., 2006; Woodhead et al., 2016).

Specific diagnosis. The species diagnosis is that for the genus until other species are known.

Specific etymology. In reference to the Riversleigh World Heritage Area where this taxon has been found.

Description. Robust M 1 (length 6.0 mm; anterior width 3.6 mm; posterior width 4.2 mm) with five major cuspids (metaconid, protoconid, and protostylid on the trigonid; and entoconid and hypoconid on the talonid) and four smaller cuspids (anterior paraconid, lingual metastylid, posterolingual entostylid, and cuspate entostylid ridge). The protoconid, protostylid and hypoconid are the most heavily worn of the cuspids, with the entoconid and metaconid enamel only slightly breached at their apices. This pattern of greater wear on the buccal conids relative to the lingual conids is also found in P. cinereus .

The cristid obliqua, the anterobuccal base of the entoconid, the posterobuccal base of the metaconid and the base of the posterior arm of the protostylid are strongly crenulated with large ridge-like crenulations ( Fig. 2 ). A well-developed paraconid occupies the anterolingual corner of the tooth, and is connected to the protoconid by the preprotocristid. There is no connection between the apices of the protoconid and

1 metaconid, yet weak ridges (ribs) on their lingual and buccal faces, respectively, meet in the deep longitudinal valley separating these cuspids. A short, arcuate valley is developed between the posterior base of the paraconid, the preprotocristid and the anterobuccal base of the metaconid. The apex of the protoconid is positioned slightly buccal of the tooth midline ( Fig. 2B , 3 ). The postprotocristid runs posteriorly and slightly lingually into the central basin where it joins the cristid obliqua along the transverse axis. The metastylid and entostylid are moderately developed. The premetacristid is a short linear crest which fades into the anterior base of the metaconid. The postmetacristid extends posterolingually from the metaconid apex to the lingual margin, swelling at this point to form the metastylid. If not for the tooth being broken at this point, the short buccal postmetastylid cristid would meet the preentocristid slightly buccal to the top of the transverse valley forming a ‘metastylid fold’ ( Fig. 2B ).

The protostylid is moderately developed and its base occupies one third the width of the trigonid. The valley separating the protoconid and protostylid is very narrow, unlike that seen in Litokoala kutjamarpensis and Phascolarctos species. The posterior ridge of the protostylid terminates in the transverse valley between the protostylid and hypoconid becoming slightly crenulous at its tip. A relatively deep pocket is formed between the posterior protostylid ridge, the cristid obliqua, the postprotocristid and the bases of the protoconid and protostylid. The short anterior ridge of the protostylid extends anteriorly, then curves anterolingually, terminating at the anterobuccal base of the protoconid. It does not extend from the protostylid apex but from the anterior base of the protostylid and when viewed buccally, appears as a small cuspule at the anterior base of the protostylid ( Fig. 2D ). An anterior cingulum is not developed. Short buccal ribs extend from the apex of the metaconid, protoconid and entoconid. Lingual ribs are present on all major cusps. The lingual rib of the entoconid bifurcates, one arm continuing lingually, the other anterolingually fading down the lingual tooth margin ( Fig. 2B–C ), giving the appearance of a ‘twinned’ entoconid similar to that found in Phascolarctos . Also as in Phascolarctos spp. , moderate depressions occupy the lingual face of the entoconid between the lingual ribs and the preentocristid (anterolingually) and postentocristid (posterolingually) (see entoconid lingual shelf of Black et al. 2014a; fig. 2).The preentocristid is a relatively short, crescentic crest extending anterobuccally then curving anterolingually where it bifurcates, a short ridge continuing anterolingually to meet the postmetacristid at the lingual margin. A second ridge extends anterobuccally and becomes part of the crenulation pattern, terminating in the transverse valley. The postentocristid, a more linear crest, runs posterolingually to the apex of the entostylid at the posterolingual corner of the tooth. A short lingual ridge fades down the lingual margin from the entostylid apex. A short posterobuccal ridge extends from the entostylid apex and is continuous with the posterior cingulum (which continues buccally to meet the posthypocristid). From the junction of the posterobuccal ridge of the entostylid and the posterior cingulum the well-developed, slightly crescentic entostylid ridge extends anterobuccally along the longitudinal valley between the entoconid and hypoconid and is cuspate at the posterobuccal base of the entoconid. A short posterobuccally directed ridge from the apex of the entostylid ridge spreads into three buccally directed spurs which terminate at the posterolingual base of the hypoconid ( Fig. 2B ). The cristid obliqua extends anterolingually from the hypoconid apex to meet the postprotocristid in the central basin slightly lingual to the longitudinal axis in the transverse valley.

Figures

Figure 1. Map and regional schematic of the Riversleigh World Heritage Area (After Arena, 2005, and Megirian, 1992). The Type locality of Stelakoala riversleighensis gen. et sp. nov., Jim’s Jaw Site, is located on the northern Gag Plateau (highlighted red).

Figure 2. Stelakoala riversleighensis gen. et sp. nov. holotype (QM F57737) from Jim’s Jaw Site, Riversleigh World Heritage Area, Qld. A-A’, Occlusal stereopair; B, labelled occlusal view; C, lingual view; D, buccal view. Abbreviations: co, cristid obliqua; end, entoconid; esd, entostylid; er, entostylid ridge; hyd, hypoconid; lr, lingual ribs; med, metaconid; msd, metastylid; pad, paraconid; ppsd, preprotostylid cristid; prd, protoconid; psd, protostylid.

Figure 3. The evolution of primary M1 trigonid cuspids (metaconid, protoconid and protostylid) in phascolarctids. A, Schematic diagram of a phascolarctid right M 1 illustrating how dimensions and percentages represented in Table 1 were obtained (adapted from Black et al., 2014a); B, Graphical representation of the relative positions and distance between the primary M 1 trigonid cuspids as a percentage of trigonid width; C, phylogenetic relationships of phascolarctids (from Black et al., 2012a). Abbreviations: end, entoconid; hyd, hypoconid; med, metaconid, prd, protoconid; psd, protostylid. Data for Madakoala, Perikoala, Litokoala, Nimiokoala and Phascolarctos, is based on M. devisi, Pe. robustus, L. kutjamarpensis, N. greystanesi and P. cinereus, respectively, with mean values used for the latter two species (Table 1).