Systematic Palaeontology

Marsupialia (Illiger, 1811) Cuvier, 1817

Australidelphia Szalay, 1982

Peramelemorphia (Kirsch, 1968) Aplin & Archer, 1987

Lemdubuoryctes aruensis gen. et sp. nov.

Diagnosis

Lemdubuoryctes is distinguished from all currently extant bandicoot genera (plus Chaeropus), the early Pliocene cf. Peroryctes tedfordi33, early–middle Miocene Kutjamarcoot14, Madju13, Liyamayi12, Crash12, Galadi amplus18, and late Oligocene Bulungu campbelli16 by its retention of a ‘complete’ centrocrista with continuous postparacristae-premetacristae on all upper molars. The centrocrista is incomplete on M3 of the early Miocene Galadi speciosus17, and is formed by residual buccal crests on M3 of the early Miocene Bulungu palara15. Galadi grandis18 also from the early Miocene, and Bulungu muirheadae16 the oldest known late Oligocene bandicoot, possess ‘complete’ centrocristae along their upper molar rows but differ from Lemdubuoryctes in the presence of two mental foramina on the dentary, and the absence of anterior cingulae on M2–4 respectively. The lack of an elevated talonid separates Lemdubuoryctes from the latest Miocene–early Pliocene Ischnodon. Finally, oblique orientation of the posthypocristid relative to the lower molar row contrasts with late Oligocene–early Miocene Yarala19,21.

Etymology

‘Lemdubu’ from the type locality, and ‘oryctes’ (ορυκτης, masculine) for ‘digger’; species name refers to its endemic occurrence on the Aru Islands.

Holotype

WAM 14.9.6, left maxilla containing P3 and M1–2.

Additional material

WAM 14.9.1–WAM 14.9.5 and WAM 14.9.7–WAM 14.9.20 (dentigerous elements, petrosals and calcanea).

Type locality and horizon

Liang Lemdubu cave (‘Layer 4’, Spit 24) on Pulau Kobroor, Aru Islands group, Eastern Indonesia29. Additional material was recovered from Spit 6, 14 and 18–25 (‘Layers 3–5’) at Liang Lemdubu, and Spits 33–40 (‘Layer 4’) in a second cave on Pulau Kobroor – Liang Nabulei Lisa29. A combination of radiocarbon and U-series age determinations place these deposits within the latest Pleistocene to Holocene, between ca 28,000–9,000 BP for Liang Lemdubu, and ca 16,200–12,000 BP for Liang Nabulei Lisa30,31.

Description of the new fossil taxon

Both the holotype (Western Australian Museum [WAM] 14.9.6) and referred (WAM 14.9.9) maxillae of Lemdubuoryctes (Fig. 2) display tooth eruption and molar wear indicative of adult animals (this is most extreme in WAM 14.9.9). The remnant palatal shelf on WAM 14.9.9 preserves a vacuity in the molar region. The base of the zygomatic arch is level with the alveolar margin. The antorbital fossa extends posteriorly from above the M3 to behind the M4; this differs from many extant peramelemorphians, as well as Bulungu palara15 and Galadi speciosus17, but can be intraspecifically variable13. Posterior expansion of the antorbital fossa otherwise occurs only in living Peroryctes34 (Supplementary Fig. S1), some Echymipera rufescens (e.g. Australian Museum [AM] S1866)35, Yarala burchfieldi20, and osteologically mature Madju13 specimens. The antorbital fossa of Macrotis is uniquely elevated above the tooth row11. The anterior opening of the infraorbital canal in WAM 14.9.6 extends to the P3 alveolus (or the posterior margin of the M1 in WAM 14.9.9). This is similar to most bandicoots (including B. palara)15, in which the infraorbital canal usually has an expansive exit over the M1–P3. The infraorbital canal opens immediately above the P3 in Y. burchfieldi20 and species of Galadi17,18.

Figure 2 Lemdubuoryctes aruensis referred (WAM 14.9.9) and holotype (WAM 14.9.6) maxillae. (a) SEM image of P3–M4 from WAM 14.9.9 shown in oblique occlusal view. (b) SEM image of the hypertrophied P3 and M1–M2 from WAM 14.9.6 in oblique occlusal view. Scale bars: (a) 3 mm, (b) 4 mm. Anatomical abbreviations: acd, anterior cingulid; ecf, ectoflexus; mcl, metaconule; mtc, metacone; pac, paracone; paf, area of the palatal fenestra; pmtc, premetacrista; ppac, postparacrista; pprc, postprotocrista; prc, protocone; rcc, residual centrocrista; stB, stylar cusp B; stylar cusp D. Imaging by K.P.A. Full size image

The upper premolars of Lemdubuoryctes are double-rooted with diastemata between P1 and P2 (WAM 14.9.11), suggesting an elongate maxillary rostrum. The P3 exhibits marked size dimorphism consistent with sexual variation observed in some extant peramelids, most notably species of Peroryctes and Echymipera34 (Supplementary Fig. S1). We therefore interpret WAM 14.9.6 as a probable male because the P3 exceeds the M1 in occlusal area (Fig. 2B). The P3 is smaller than the M1 in WAM 14.9.9 and thus represents a potential female (Supplementary Table S1). Both P3 morphotypes are otherwise identical in their triangular basal outline with conical central cusp, weak posterolingual cingulum and incipient anterobasal cuspule.

At up to 14.89 mm in length (Supplementary Table S1), the complete M1–M4 row of Lemdubuoryctes (WAM 14.9.9) is equal to the largest living bandicoot Peroryctes broadbenti34. The M1 (Fig. 2; Supplementary Fig. S2) is triangular in occlusal outline unlike the more quadrangular molars of peramelines, Chaeropus and Macrotis; the latter further distinguished by extreme lingual displacement of the metacone27. In Lemdubuoryctes, the metacone is positioned at the posterolabial margin of the trigon basin, which is bounded buccally by the paracone. A prominent protocone is situated lingually. There is no protoconule. The metaconule (metaconular hypocone) forms only a weak spur that connects the postprotocrista to the base of the metacone. This is compatible with extreme metaconular reduction seen in the stem peramelemorphians Yarala19,21, Bulungu15,16, and Galadi17,18, together with the early Pliocene cf. Peroryctes tedfordi33. Alternatively, living bandicoots27 as well as species of Crash12, Madju13 and Kutjamarcoot14 elaborate the metaconule into an enamel flange that is demarcated from the protocone via a vertical trough (Supplementary Fig. S1). Dasyuromorphian marsupial carnivores have a more prominent cusp-like metaconule36. Amongst bandicoots only Macrotis completely lacks a metaconular structure, but a small metaconule is present in the putative thylacomyid Liyamayi12.

The anterior cingulum on the M1 of Lemdubuoryctes is formed by the preprotocrista, which connects to the parastylar base. There is no posterior cingulum. The paracone lies directly behind the parastyle and the preparacrista runs posterobuccally towards the parastylar tip (a common trait amongst Peramelemorphia: see Supplementary Data, character 14). The postparacrista merges with the premetacrista to create a ‘complete’ centrocrista. The opposing buccal ectoflexus is shallowly incised between the remnants of stylar cusps B and D (there is no discernible stylar cusp E). Remarkably, there are broad ectolophs evident on the M2 and M3 that closely resemble those of the most ancient peramelemorphians Yarala kida21 and Bulungu muirheadae16. In other fossils, the postparacrista and premetacrista gradually retract resulting in a ridge-like centrocrista on the M3 of B. palara15, and total division of the ectoloph in Bulungu campbelli16, Galadi17,18, Madju13 and Kutjamarcoot14. Remnants of the centrocrista also occur on the M1–3 of extant P. broadbenti (Supplementary Fig. S1), less prominently on the M1–2 of Peroryctes raffrayana, and occasionally in E. rufescens34. Crests appear elsewhere on the M1–2 of Crash12 and cf. P. tedfordi33, which also has a small ‘stylar cusp C’, perhaps constituting another residual component.

The M2 and M3 of Lemdubuoryctes (Fig. 2; Supplementary Fig. S2) differ from the M1 in their lingually positioned paracone, less distinct ectoflexus, and transversely oriented preparacrista that trends towards stylar cusp B, but retains contact with the parastylar tip via a subsidiary crest. This forms a truncated anterior cingulum comparable to that on the M2–3 of P. raffrayana34.

The M4 of Lemdubuoryctes (Fig. 2; Supplementary Fig. S2) is reduced relative to the anterior molars and bears both a paracone and diminutive protocone. An anterobuccal cingulum is not visible but could be covered by matrix in WAM 14.9.9. The postprotocrista forms the posterior margin of the tooth and meets the postparacrista at stylar cups B.

Mandibular elements were referred to Lemdubuoryctes based on obvious morphological distinction from the sympatric bandicoots29 Isoodon macrourus, E. rufescens and E. kalubu. Relative hypertrophy of the p3 differentiates presumed male (WAM 14.9.1) and female (WAM 14.9.3) specimens (Fig. 3; Supplementary Fig. S3). The mandibular rami of Lemdubuoryctes (Supplementary Fig. S4) are ventrally convex and up to 9.7 mm deep below the m3 (WAM 14.9.3). The single mental foramen is level with the midline of p1, and the mandibular symphysis extends to the middle of p2. The ascending ramus in WAM 14.9.1 is angled at ~45°; the mandibular foramen opens low on the medial surface and the masseteric fossa is well defined. The i3 root on WAM 14.9.3 is separated from the canine alveolus (3.9/1.8 mm in maximum length/width) by a 2 mm diastema. Another diastema (3.5 mm) intersperses between c1 and p1 with a narrower gap between p1 and p2. The length and height of p1–3 decrease anteriorly (Supplementary Table S2) and are coupled with progressive migration of the blade-like central cusp forward over the anterior root. There are no accessory cuspids.

Figure 3 Lemdubuoryctes aruensis referred dentaries. (a) SEM image of p1–p3, and m2–m4 from WAM 14.9.1 shown in oblique occlusal view. (b) p3 and m1–m4 from WAM 14.9.3 in oblique occlusal view. Scale bars: 4 mm. Anatomical abbreviations: cob, cristid obliqua; dia, diastema; ecd, entoconid; hcd, hypoconid; hcld, hypoconulid; mcd, metaconid; pcd, paraconid; phcd, posthypocristid; prcd, protoconid, tal, talonid; tgd, trigonid. Imaging by K.P.A. Full size image

The complete m1–4 row of Lemdubuoryctes (Fig. 3; Supplementary Fig. S3) was up to 17.43 mm long (WAM 14.9.3: Supplementary Table S2), with marked constriction evident at the enamel crown–root interface (also visible on p1–3); this is typical of peramelemorphians except for Isoodon and Macrotis27. The m1 is laterally compressed with a bulbous trigonid incorporating a prominent paraconid, which is absent in Ischnodon37, living peramelines and Echymipera27. There are no median buccal cuspules between the trigonids and talonids as reported in Y. burchfieldi19. The anterior cingulid is reduced on m1–4 and the labial cingulids are weakly developed, similar to Peroryctes34 (Supplementary Fig. S1). The cristid obliqua terminates buccally against the posterior wall of the protoconid on m1–2 (rather than the metaconid as in many peramelines27,38) but is more lingually positioned on the m3, and immediately adjacent to the metacristid notch on m4. The hypoconulid is situated directly posterior to the entoconid, and sunken well below the talonid basin on all lower molars (synapomorphies for Peramelemorphia27). The posthypocristid is oblique to the molar row like that of P. raffrayana34 (Supplementary Fig. S1). The conical entoconids on m2–3 differ from the blade-like structures in P. raffrayana34 (Supplementary Fig. S1), some species of Microperoryctes (Supplementary Data, character 25), cf. P. tedfordi33 and Y. kida21. A discrete entoconid and hypoconid on the reduced m4 talonid are additional differences relative to Y. burchfieldi19 and B. campbelli16.

The petrosals of Lemdubuoryctes (WAM 14.9.13–WAM 14.9.15) were identified by their weakly inflated periotic hypotympanic sinuses (thus excluding I. macrourus27), and large size relative to those of I. macrourus, E. rufescens and E. kalubu. They also bear small rostral tympanic and caudal tympanic processes, as well as shallow mastoid sinuses and epitympanic recesses (Supplementary Fig. S5). The prominent ventral flange on the pars cochlearis is compatible with those of Y. burchfieldi21, B. palara15 and P. raffrayana34.

Calcanea were assigned to Lemdubuoryctes on the basis of their substantial size and striking morphological distinction from those of I. macrourus, E. rufescens and E. kalubu. The largest referred calcaneum (WAM 14.9.16: Supplementary Fig. S6) is 22.2 mm in maximum length. The compact tuber calcis, oblique calcaneo-cuboid facet, and projecting calcaneum-austragalus facet incorporating a short triangular lateral shelf, are all reminiscent of the condition in Microperoryctes39 and P. raffrayana (American Museum of Natural History [AMNH] 151936), but contrast with that of Echymipera39 (Supplementary Fig. S6). Calcanea have not yet been described for any other fossil peramelemorphian.

Phylogeny

Analysis of our morphological dataset (including molecular backbone constraints: Supplementary Fig. S7) produced both poor ingroup resolution and node support for the placement of fossil taxa (Supplementary Figs S8 and S9). We attribute this to missing data and pervasive homoplasy, which was detected during construction of our matrix (see Methods) and subsequently examined via serial pruning of redundant fossils40 and wildcards41 (Supplementary Figs S10–S12). These procedures returned Lemdubuoryctes, together with Bulungu palara, Yarala burchfieldi and species of Galadi as labile stem peramelemorphians. This implies enormously protracted ghost lineages (Fig. 1), but no discrete character states served to unite these taxa as a clade. In fact, the only traits collectively distinguishing basal stem peramelemorphians from more crown-ward bandicoots were the deeply symplesiomorphic20,25 presence of ‘complete’ centrocristae on M1–3 (although this is polymorphic in Galadi speciosus17 and absent in G. amplus18), and an alisphenoid-parietal contact on the lateral wall of the neurocranium (also evident in Madju13). An alternatively derived squamosal-frontal contact is shared by all crown perameloids, and might be diagnostic for this radiation, but was not a major driver of our topologies (see Supplementary Data). Moreover, the monophyly of Yarala21 as a separate sister grouping to Perameloidea was equivocal (Supplementary Table S3), leading us to question the taxonomic utility of Yaraloidea based on these primitive states alone3,20,21.

The other fossil bandicoots Bulungu campbelli, Kutjamarcoot, Crash and cf. Peroryctes tedfordi were variously interpolated amongst crown Perameloidea, with Madju positioned as a sibling lineage. This rendered the genus Bulungu polyphyletic (although constraint trees showed no significant difference: Supplementary Table S3), and the respective classifications of Kutjamarcoot and Crash as either a stem perameloid14 or stem peramelids3,12, were likewise uncertain (Supplementary Figs S11 and S12). Similarly, the earliest dated peroryctine1,4, cf. Peroryctes tedfordi, was only intermittently nested within Peroryctes (Supplementary Fig. S10), a result that compromises existing molecular clock calibrations16. The affinity of Perameles bowensis42, which has also previously been used for dating constraints1,4, was unresolved33 (Supplementary Figs S8 and S9); however, this taxon manifests a posthypocristid-entoconid contact on its lower molars, which is distinctive for both Perameles and the Pleistocene species P. sobbei38 (topologically grouped with living Perameles nasuta and P. gunnii: Supplementary Figs S8 and S9), as well as the enigmatic Miocene–Pliocene genus Ischnodon. Notably, neither Ischnodon nor Liyamayi were recovered as ancestral bilbies3,12,16, and constraint tests on these placements were inconclusive (Supplementary Table S3). Such results corroborate the original taxonomic assessment of Ischnodon, which reported thylacomyid dental similarities but refrained from definitive classification37.

Our assessments of extant bandicoot morphology were consistent with DNA1,4 in returning the xeric-adapted Macrotis lagotis as the most divergent living peramelemorphian (Supplementary Fig. S13). On the other hand, alternate grouping of the extinct arid-zone chaeropodid Chaeropus within Peramelinae (Supplementary Fig. S14) suggests that either extensive dental/osteological convergence3, or incomplete characterization of its scant mitochondrial sequence data43 confound its relationships. Interestingly, inclusion of the extinct desert-adapted thylacomyid, Macrotis leucura, promoted extensive topological degradation (Supplementary Fig. S15). This might be due to its curious ‘peramelid-like’ dental attributes (see matrix scores in Supplementary Data), which could again denote either homoplasy, or the retention of ancestral perameloid states.

Trees generated by the concatenated dataset of morphology + DNA were identical to those produced by DNA alone1, but with amplified support values for weak nodes demonstrating overall signal congruence. This was most notable at the nodes excluding Macrotis lagotis from Peramelidae, and positioning of the Seram bandicoot Rhynchomeles prattorum within Echymipera (Supplementary Fig. S16). Successive deletion of molecular information for major clades44 pinpointed residual morphological conflict over a paraphyletic Peroryctinae + Echymiperinae, and repositioning of Chaeropodidae within Peramelinae (Supplementary Table S4). This concurs with previous studies14,15,16,17,45, which have placed Macrotis and Chaeropus outside of Peramelidae using a molecular backbone, but not with morphology on its own. As expected, the introduction of fossils completely degraded node support (Supplementary Fig. S17), and revealed long-branch effects in the clumped redistribution of taxa (Supplementary Fig. S18). Sequential deletion of highly homoplastic dental-dependant terminals did improve these results (Supplementary Figs S19 and S20), but still failed to yield stable positioning of fossils, perhaps because they integrate insufficient cranial-postcranial skeletal data to accurately discriminate relationships.

Divergence times and ancestral areas

We utilized a DNA dataset with expanded outgroup sampling of diprotodontian, notoryctemorphan and dasyuromorphian taxa to correlate the timeframes and settings for peramelemorphian intra-clade divergences. Alternative fossil constraints (Supplementary Table S5) and Bayesian random local clocks46 (Supplementary Fig. S21) were also implemented to assess possible sources of overestimation16. Despite these tests, our analyses demonstrated an unequivocal origination of the crown bandicoot total-group during the mid-Paleocene around 60 Ma (Table 1; Supplementary Tables S6–S8; Supplementary Figs S22–S30). This corroborates the discovery of possible stem peramelemorphian fossils from the early Eocene3,9,47, but massively predates previous molecular estimates1,4 by up to 40 Ma. In accordance, diversifications amongst chaeropodid, thylacomyid and peramelid family-level clades seem to have commenced in the middle Eocene to Oligocene (~40–30 Ma). These epochs coincide with the tectonic isolation of Australia and instigation of the circum-Antarctic current, which propagated seasonally cool-dry climates and the spread of sclerophyllous vegetation48. Compellingly, our S-DIVA/Bayesian Binary MCMC ancestral area optimisations onto morphological (Supplementary Figs S31 and S32), total evidence (Supplementary Fig. S33), and DNA trees (Supplementary Fig. S34) decisively correlated the basal peramelemorphian split with dispersal into open habitats and an early occupation of xeric ecosystems (Supplementary Tables S9–S12). Unanimous inference of a post early–middle Miocene (after ~20 Ma) rainforest-woodland radiation amongst peroryctines and echymiperines likewise coincides with uplift of the New Guinean landmass and onset “greenhouse” climates49, which propagated higher rainfall and coastal/riparian vegetation50. In contrast, our analyses failed to pinpoint an emergent habitat for peramelines. We attribute this to their rapid expansion into openly vegetated environments1,3,51, compounded by methodological dependence of our probability matrices upon predefined species distribution codes. These are particularly sensitive to highly dispersive organisms, as well as significant area changes through time52. In our case this included the pronounced middle–late Miocene (after ~16 Ma) resurgence of cool-dry climates, and Pliocene predominance of mosaic vegetation, especially incorporating intra-continental grasslands which proliferated across Australia during this interval50.