Protemnodon sp.

Kerr, Isaac A. R., Camens, Aaron B., Van Zoelen, Jacob D., Worthy, Trevor H. & Prideaux, Gavin J., 2024, Systematics and palaeobiology of kangaroos of the late Cenozoic genus Protemnodon (Marsupialia, Macropodidae), Megataxa 11 (1), pp. 1-261 : 194

Protemnodon sp. from Lachitu Cave

An upper cheek tooth specimen (PM 26591) was referred to Protemnodon sp. by Koungoulos et al. (2024), wherein it was considered to be a dp3 and to probably represent an undescribed species. The specimen comes from a Pleistocene deposit in Lachitu Cave on the far northwestern coast of Papua New Guinea, and represents both the first lowland occurrence of a Pleistocene species of Protemnodon in New Guinea and the northernmost record of any megafaunal marsupial ( Koungoulos et al. 2024). PM 26591 was considered a dp3 rather than an m1 on the basis of: its slightly narrower anterior region relative to the posterior, with a rounded, more pointed anterior cingulid; a strongly sloping and relatively straight buccal margin of the protoconid and hypoconid; a prominent premetacristid; distinctly anteriorly concave lophid crests; a lingually positioned point of greatest curvature in the protolophid and hypolophid; and a high and convex dorsal edge of the lingual portion of the anterior cingulid with a concave and low buccal portion ( Koungoulos et al. 2024, p. 4). While these differences do demonstrate that PM 26591 may represent a dp3, we believe that this is far from certain and note many features that link the tooth with the m1 and m2 of P. tumbuna .

We consider it likely, or at least possible, that the tooth is an m1 based on its morphology and proportions. The differences then cited by Koungoulos et al. (2024) to illustrate how PM 26591 differs from specimens of P. tumbuna may then be equally well or better explained by the specimen representing an m1 rather than it representing an undescribed species. PM 26591 was described as differing from the dp3 of P. tumbuna specimen PNG 82- 40-13 as follows: ‘...the protoloph[id] and hypoloph[id] are subequal in width rather than the latter being considerably wider; the lophid ridges appear somewhat higher and noticeably narrower relative to the base, and the buccal profile protoconid/hypoconid to the crown base is more strongly sloped with the dorsal-most portion presenting mildly convex whilst the ventral-most appears mildly concave (this pattern being more pronounced for the hypoconid),’ ( Koungoulos et al. 2024, p. 4).

There are certain features of PM 26591 that suggest it to be an m1. Firstly, PM 26591 is large (11.36 mm long), more similar in size to the m1 and m2 of P. tumbuna than to the dp3. If it is an m1 or m2, then the difference in size between PM 26591 and the teeth of the comparative specimens becomes negligible, or at least much less significant. The protolophid and hypolophid (or trigonid and talonid) are subequal in width, which is typical of the m1 and m2 of most macropodines, as in P. tumbuna referred specimen PNG 82-40-9 ( Fig. 82g ). In P. tumbuna the paracristid is more lingually situated relative to the protoconid in the m1 and m2 than in the dp3. The paracristid of PM 26591 is centrally rather than buccally situated, very similar to that in the m1 and m2 of PNG 82-40-9, suggesting that it is not a dp3. PM 26591 is described as differing from the dp3 of P. tumbuna in having a narrower protolophid and hypolophid relative to the lophid bases. However, as is visible in the dp3 and molars of P. tumbuna specimen PNG 82-40-9, the protolophid and hypolophid are narrower relative to the lophid bases in the m1 and more so in the m2. This may explain the perceived difference in PM 26591. The anterolingual margin is rounded and convex in the m1 and m2 of P. tumbuna , and is straighter and flatter in the dp3. The anterolingual margin of PM 26591 is slightly convex in the manner of the m1 of PNG 82-40-9, not flattened and narrowed, leading to a central and anteriorly protruding rounded point at the anterior tip of the paracristid, in the manner of the dp3.

We agree that the buccal margin of the lophids is unusually sloped, but note that the buccal margins of the lophids of the molars are slightly more sloped in the m1 and m2, as the protoconid and hypoconid are more lingually situated. So, the degree to which PM 26591 differs from the compared specimens in this regard would likely be reduced if it is indeed an m1 or m2. It is possible that this is the unusual dp3 of an undescribed species, but there is perhaps better reason to believe that it is an m1 or m2. If it is the latter, the case for it as evidence of an undescribed species of Protemnodon is considerably weakened, without repeating the comparative analysis. We consider the specimen to probably represent an m1, and allocate it to P. sp. cf. P. tumbuna .

Figures

FIGURE 82. dentary and lower dentition of P. tumbuna: (a–c) surface scan images of partial right dentary of AM F83612 in (a) occlusal, (b) buccal, and (c) lingual views; (d–f) surface scan images of partial left juvenile dentary, with i1 and dp2–m1, of AM F83614 in (d) occlusal, (e) buccal, and (f) lingual views; (g) right dp2–m2 of PNG 82-40-9 in occlusal view; and (h) right p3–m4 of AM F83612 in occlusal view.