Bohra wilkinsonorum Dawson, 2004a

Prideaux, Gavin J. & Warburton, Natalie M., 2023, A review of the late Cenozoic genus Bohra (Diprotodontia: Macropodidae) and the evolution of tree-kangaroos, Zootaxa 5299 (1), pp. 1-95 : 33-38

publication ID	https://doi.org/ 10.11646/zootaxa.5299.1.1
publication LSID	lsid:zoobank.org:pub:9CA85AEC-7128-4118-A50D-FCD16502F5E0
DOI	https://doi.org/10.5281/zenodo.8017936
persistent identifier	https://treatment.plazi.org/id/03C24E22-F639-562D-FF01-C594C7A1F32E
treatment provided by	Plazi
scientific name	Bohra wilkinsonorum Dawson, 2004a
status

Treatment

Holotype. QM F43277 , right juvenile maxilla fragment (preserving dP2–3, M1–2; P3 removed from crypt).

Type locality. Chinchilla Sand, Chinchilla Rifle Range (site not specified), north bank of the Condamine River , Chinchilla , western Darling Downs , Queensland. The holotype was collected by Doris and Cecil Wilkinson.

Referred specimens. Probably Chinchilla. QM F675, right adult calcaneus. This specimen retains no associated locality or collection information, although it was likely collected in the late 19 th century. Chinchilla Sand is suggested by preservation. See earlier comments on specimens referred to B. sp. cf. B. bandharr putatively from Chinchilla.

Childers Cove , Victoria. NMV P221950 About NMV , left P3. Collected by David Pickering and others on 24 November 2011 . NMV P222378 About NMV , right M1. Collected by David Pickering and Wayne Gerdtz on 3 or 4 August 2004 .

Etymology. Named in honour of Doris and Cecil Wilkinson for their fossil collecting efforts at the Chinchilla Rifle Range locality.

Revised diagnosis. The largest-known species of Bohra based on molar and calcaneal dimensions. The P3 is similar in size and morphology to that of B. bandharr , but is less markedly narrowed anteriorly. The postpara- and premeta-cristae on M2 are strongly developed and incurved, more so than in any other species of Bohra . The slight convexity of the buccal side and distinct interloph concavity of the lingual side of the M1–2 crown resembles that of the much smaller B. nullarbora .

The species is also distinguished by the following combination of calcaneal features: massive, dorsoventrally compressed tuber calcanei; plantar aspect very broad and strongly flared posteriorly, particularly on the lateral aspect of the epiphysis; medial and lateral talar facets of the calcaneus discontinuous; pronounced and strongly demarked crest from the caudomesial border of the medial talar facet for the posterior talocalcaneal ligament; calcaneocuboid facets transversely broad, with oblique, moderately deep step; ventromedial facet separated from dorsomedial facet by deep fossa.

Description and comparisons. Maxilla. Essentially all that is preserved of the holotype maxilla is that portion bearing the erupted dP2–3 and M1–2, the unerupted P3, and the masseteric process ( Figure 19A–C View FIGURE 19 ). With its extremity positioned adjacent to the M2 protoloph in this juvenile specimen, the masseteric process is posteroventrally orientated and bears a low, slightly rugose crest along its anterolateral edge ( Figure 19C View FIGURE 19 ).

The juvenile maxilla is known for no other species of Bohra , and so no direct morphological comparisons are possible due to the changes that can occur from the ontogenetic stage where M2 has just come into occlusion versus a full adult where M4 is in occlusion. Nevertheless, it is worth noting that in D. bennettianus , the molars progress anteriorly relative to the position of the masseteric process between these two ontogenetic stages. When M2 first comes into occlusion, its protoloph is positioned adjacent to the masseteric process, but in adults the abutment of M2–3 is adjacent to the masseteric process. The same may well have been the case for B. wilkinsonorum . In shape, the masseteric process is more rounded than in B. illuminata and B. bila .

Upper dentition. Previously described in detail ( Dawson 2004a).

The dP2–3 remain unknown for other species of Bohra , but comparisons with the much smaller D. bennettianus and D. inustus reveal very similar general crown outlines, although those of D. bennettianus are the closest match. Their dP2 crowns are relatively flat-sided oval shapes, and they share a low weak buccal cingulum. However, B. wilkinsonorum differs by lacking the large anterior cusp of the main crest bordered posteriorly by a deep cleft ( Figure 19A–C View FIGURE 19 ), which is characteristic of both the dP2 and P3 of species of Dendrolagus . These species are also very similar in dP3 morphology, sharing: a strongly developed postparacrista, which abuts the end of the dP2 main crest; well-developed postpara- and premeta-cristae, which are anteroposteriorly orientated and confluent, rather than incurved as on M1–2; and a distinct preprotocrista that marks the lingual end of the transversely narrow precingulum. No preprotocrista is evident on the holotype M1, but there is a slight preprotocrista present on the unworn (unerupted) referred specimen, NMV P222378 ( Figure 19L View FIGURE 19 ). There is a stylar eminence in the region of cusp C on M1, but not M2 ( Figure 19C View FIGURE 19 ). Whether a urocrista (of which there is a slight manifestation in D. bennettianus and a very distinct expression in D. inustus ) was also present in B. wilkinsonorum cannot be ascertained due to extensive wear of the metaloph.

Bohra wilkinsonorum is most similar in P3 size and morphology to B. bandharr ( Tables 1 View TABLE 1 , 3 View TABLE 3 ). It differs by narrowing less markedly anteriorly, possessing three buccal cingular cuspules and bearing a slightly less pronounced posterobuccal eminence. Although the holotype P3 bears an anterior crest on the posterobuccal eminence ( Figure 19D–E View FIGURE 19 ), this is lacking from NMV P221950 About NMV ( Figure 19G–K View FIGURE 19 ) .

In M1–2 occlusal outline ( Figure 19C, F, L View FIGURE 19 ), B. wilkinsonorum resembles the much smaller B. nullarbora . They share a slightly convex, but essentially quite flat, buccal side of the crown, a trait that also characterises B. bandharr . However, the lingual sides of M1–2 of B. bandharr are also flat lingually, rather than convex adjacent to the interloph valley. The strongly developed and incurved form of the postpara- and premeta-cristae on M2 distinguishes B. wilkinsonorum from all other species of Bohra . Absence from M2 of a stylar crest in the region of cusp C is a condition shared with B. bandharr , B. illuminata and B. nullarbora , but not B. bila and B. planei , in which this crest is distinct.

Calcaneus. The referred calcaneus, QM F675, is well preserved and complete except for the caudolateral extremity of the tuber calcanei ( Figure 20B, D View FIGURE 20 ). The calcaneus is large, stout, broad, stout and dorsoventrally flattened, with clearly defined articular surfaces and rugged ligamentous insertions. The tuber calcanei is strongly flared caudally ( Figure 20B View FIGURE 20 ). From the plantar view, the medial margin diverges from the long axis more continuously along its length, in comparison to the lateral margin, which widens distinctly at the epiphysis. The rugose plantar surface is broad cranially, with a large anterolateral tubercle ( Figure 20A View FIGURE 20 ). The transverse sulcus that separates the cranial margin of the rugose surface from the cuboid articulation is slightly oblique from medial to lateral sides. The sustentaculum tali is deep (from medial view), medially expanded and very slightly convex ( Figure 20C View FIGURE 20 ).

The medial and lateral talar facets are separated by a narrow but distinct, non-articular area ( Figure 20B, E View FIGURE 20 ). The lateral facet is barrel-shaped ( Figure 20B View FIGURE 20 ). The fibular facet is broad and rounded, and projects over the deep sulci for the posterior calcaneofibular ligament ( Figure 20B, D View FIGURE 20 ). The tubercle and sulcus for the anterior calcaneofibular ligament are large, laterally expanded and rugose. The posterior margin of the oval medial talar facet projects dorsally and expands onto the dorsal surface toward the base of the calcaneal tubercle, with a marked projection for posterior talocalcaneal ligament ( Figure 20B View FIGURE 20 ). Cranial to the talar facets, the mid-dorsal surface is has a deep scar for the ligamentum cervicis tali. The calcaneocuboid articulation is transversely very broad (laterally extending beyond the margin of the fibular facet; Figure 20B View FIGURE 20 ); and dorsoventrally compressed in cranial view ( Figure 20E View FIGURE 20 ). The step between the rectangular dorsomedial and rounded dorsolateral facets is distinct, obliquely aligned and moderately smoothed ( Figure 20A View FIGURE 20 ). The ventromedial facet is transversely broad, but dorsoventrally relatively shallow ( Figure 20E View FIGURE 20 ). It is smoothly continuous with the dorsolateral facet but separated by a deep transverse groove from the dorsomedial facet.

The lateral flaring of the tuber calcanei epiphysis is more pronounced than in all other species of Bohra , especially the similarly large B. paulae ( Figure 12B View FIGURE 12 ). The transverse plantar sulcus is relatively deep and long in comparison to all other species of Bohra , in particular B. paulae and QM F51762 ( Bohra sp. indet. 1), and is much deeper than in the species of Dendrolagus ( Figure 9B View FIGURE 9 ). The expanded medial plantar margin of the sustentaculum tali accentuates the moderately deep flexor sulcus of the sustentaculum tali to a greater extent than in B. paulae , B. sp. indet. 1 and the species of Dendrolagus , though it is not as deeply flared as in B. planei . The gap between the medial and lateral talar facets resembles that observed in B. nullarbora , but differs from the condition seen in other species of Bohra and species Dendrolagus ( Figure 9A View FIGURE 9 ), in which the facets are confluent. The calcaneocuboid articulation is transversely much broader than in any other species ( Figure 20B, E View FIGURE 20 cf. Figure 9B, D View FIGURE 9 ). The profile of the calcaneocuboid step and complete separation of the ventromedial and dorsomedial facets is most like B. planei , while the rounded and laterally expanded of the dorsolateral facet is more like other species of Bohra , in contrast to B. planei ( Figure 33C View FIGURE 33 ).

Remarks. The recognition of B. wilkinsonorum as a second member of the genus Bohra on the basis of one juvenile maxilla fragment ( Dawson 2004a) was a highly perceptive deduction given that the type species was described and is still known only from hindlimb elements from a locality 665 km to the south ( Flannery & Szalay 1982). Based on the dimensions of these specimens ( Table 8 View TABLE 8 ), B. wilkinsonorum and B. paulae are the two largest species of Bohra , and one might be able to hypothesise on those grounds that the fossils belong to the same species. However, the recognition from the Darling Downs region (indeed, probably from the type locality) of a very large calcaneus clearly referable to a species of Bohra that is morphologically distinct from that of B. paulae supports its taxonomic separation. This calcaneus is much larger than, and differs in morphology from, the other two calcanei from the Chinchilla LF, which we refer to B. sp. indet. 1. Referral to B. wilkinsonorum of a loose upper premolar and molar from the putatively late Pliocene or early Pleistocene Childers Cove LF of southern Victoria ( Rich et al. 2006) represents a major range extension of 1,500 km to the south ( Figure 1 View FIGURE 1 ).