Phascolarctidae, Owen, 1839

Price, Gilbert J., Zhao, Jian-xin, Feng, Yue-xing & Hocknull, Scott A., 2009, New Records of Plio-Pleistocene Koalas from Australia: Palaeoecological and Taxonomic Implications, Records of the Australian Museum 61 (1), pp. 39-48 : 41

Phascolarctidae gen. et sp. indet.

Figs 2 , 3A

Referred material. QMF52287, left dentary fragment, QML7, Chinchilla , southeast Queensland, Australia (Pliocene) .

Description. Dentary fragment with horizontal ramus broken anterior to P 3 alveolus and posterior to M 3 alveolus; surface bone slightly root-etched; cheek teeth crowns missing, roots present, alveolar border broken buccally; ventral border slightly in-turned lingually; mental foramen anteroventral to P 3 anterior alveolus; posterior mental foramen well developed, ventral to position between posterior root of M 1 and anterior root of M 2; symphysis strongly fused, very deep, extended posteriorly to below posterior root of M 1, kinked such that posteroventral border extends below ventral border of horizontal ramus, inclined posteriorly at posteroventral margin, inclined anteriorly at a low angle (35°); genial pit, c. 3 mm largest diameter, present at posterior ventral portion of symphysis, similar symmetrical but broken pit present on opposing dentary.

Remarks. The dentary fragment is considered to be adult based on the presence of well-developed P 3 and M 3 alveoli, and a strongly fused symphysis. The dentary resembles phascolarctids in general morphology. However, QMF52287 does not appear to be referable to any phascolarctid where the dentary is known. It differs from Nimiokoala , Perikoala , Madakoala , and Phascolarctos cinereus by: (a) being significantly more robust in terms of depth and width ( Fig. 3A ); (b) possessing a more anteriorly positioned posterior mental foramen; (c) having a straighter ventral border; and (d) by possessing a relatively deeper symphysis that has a lower anterior ascending angle. QMF52287 is more gracile in comparison to Cundokoala yorkensis (a genus that is questionably distinct from Phascolarctos ; see Black, 1999) ( Fig. 3A ), but like Ph. cinereus , also differs from the former taxon in having a more anteriorly positioned posterior mental foramen, straighter ventral border, and deeper symphysis. The dentary of Litokoala is unknown, but on the basis of alveoli dimensions, is probably significantly smaller than QMF52287. QMF52287 differs from Perikoala and Madakoala in that the symphysis is more strongly fused, and the diastema is relatively longer. Well-developed symmetrical genial pits are present at the posteroventral base of the symphysis of QMF52287, but are not expressed as strongly in other koala genera. Well-developed genial pits occur in wombats such as Vombatus , but such pits are relatively larger, positioned relatively higher from the ventral border, and the pits commonly converge on each other on the left and right side of the symphyseal fusion forming a single large pit. Comparison of QMF52287 to other largesized phascolarctids such as Ph. maris , or unusual koala-like marsupials such as Koobor , is not possible because dentaries of those taxa are unknown.

Figures

Fig. 2. Phascolarctid gen et sp. indet. dentary (QMF52287) from site QML7, Chinchilla, eastern Australia. (A) External view, (B) Internal view, and (C) Occlusal view.

Fig. 3. Morphometrics of fossil koala specimens. (A) Depth versus width of dentaries of modern Phascolarctos cinereus from eastern Queensland (Appendix), fossil Cundokoala (?Ph.) yorkensis (SAMP24904) from Corra Lynn Cave (South Australia), and phascolarctid gen et. sp. indet (QMF52287) from Chinchilla, eastern Australia. Note that the depth of the Chinchilla koala dentary is a minimum measurement because the specimen is broken along the alveolar border (Fig. 2). (B) Anterior versus posterior width of Phascolarctos spp. M2 (See Appendix for list of modern Ph. cinereus specimens examined).