Propalorchestes sp.

Trusler, Peter W. & Sharp, Alana C., 2016, Description of new cranial material of Propalorchestes (Marsupialia: Palorchestidae) from the Middle Miocene Camfield Beds, Northern Territory, Australia, Memoirs of Museum Victoria 74, pp. 291-324 : 316-321

Propalorchestes sp. cf. P. novaculacephalus

Referred material. QVM 2000GFV406: right dentary fragment, containing M

2-4

(tables 3–4, figs. 10–12).

Locality and horizon. Bin #25 collected from Top Site, Bullock Creek N. T., Camfield Beds middle Miocene.

Description. The short thick ramus is preserved to a similar extent as the Pr. novaculacephalus paratype, NMV P187282: lacks the anterior half of the ventral edge; anterior half of the symphysis; inflected angle; most of the pterygoid fossa; and most of the ascending ramus from a level below the condylar notch. It preserves the following features: a deep semicircular, smoothly concave masseteric fossa with semicircular mandibular masseteric crest that is shallowly concave anteriorly, becoming convex posteriorly in dorsal view; a strong, anteriorly directed coronoid crest on the leading edge of coronoid process level with the anterior root of M 4 in lateral view; a wide intercoronoid sulcus; a small foramen ventral to the interloph valley of M 1 corresponding to the approximate location of the posterior mental foramen of Propalorchestes cf ponticulus, SGM1008 , figured by Murray (1990, fig. 4D); the posterior one thirdof the symphysis is exposed medially by its fracture; a part of the right genial pit; the M 1 and P 3 alveoli are exposed on buccal side; a dorsomedially directed crest or process on the post-alveolar shelf: and a mandibular foramen on the mid-region of the pterygoid fossa. All cheek teeth are damaged on their lingual side, and the M 2 protoloph is broken at its anterobuccal edge ( Figs. 11 , 17 ).

Differs from Pr. cf ponticulus AR1779 ( Murray, 1990, fig. 4A-C) by its slightly smaller symphyseal dimensions and the genial pit being less ventrally positioned.

Differs from Pr. cf ponticulus SGM1008 ( Murray, 1990, figs. 4D–F) by having slightly smaller symphysal dimensions, having a slightly larger mandibular ramus and a genial pit that is not as ventrally positioned (tables 3, 4).

Differs from the Pr. novaculacephalus paratype NMV P187282 by having a posterior mental foramen that is not paired; having a slightly smaller and more gracile ramus with a depth:width ratio at the M 2 of 1.86 (compared to 2.01 for NMVP187282 ); the genial pit more ventrally positioned; a M 1 posterior alveolus that is large and not clearly divided; an intercoronoid sulcus that is wide, but less so than in NMV P187282 (89.5 mm width compared to 143.1 mm); dentition not as worn and proportionally broader molars (as wide, but shorter in length) (tables 2–4; figs. 12, 17) .

Remarks. QMV2000GFV406 is distinctly smaller than the Pr. novaculacephalus paratype NMV P187282; averaging 83.4% of the three mandibular measures and 93.4% of the dentition measures taken for NMVP 187282. With respect to the mandibular specimens tentatively referred to Pr. ponticulus ( Murray, 1990) , QMV2000GFV406 is smaller than SGM1008 (averages 86.5%), but has a slightly thicker ramus (104%); it is

1

smaller than AR1779 (86.0% mandibular depth and 95% symphysis width). The location of the genial pit on the posterior surface of the symphysis in QMV2000GFV406 is intermediate between the mid-position it occupies on NMV P187282 and the ventrally positioned pits on the Riversleigh specimens. SMG1008 has the most ventrally located genial pit from the Riversleigh WHA material. Dental measurements for QMV2000GFV406 are also smaller (average 93.75%) than the Pr. novaculacephalus paratype NMV P187282 and fractionally larger (average 102% for length) than AR11302 and AR1008 from Riversleigh WHA but 114% wider than AR11302. The QMV2000GFV406 dentition is fractionally larger than that of Pitikantia dailyi (tables 2–4; figs. 14, 15).

The general morphology of the new mandible, QMV2000GFV406, is variably comparable to the known Propalorchestes material and although smaller than the Pr. novaculacephalus NMV P 187282 paratype, it is not necessarily more closely aligned to the small Riversleigh WHA Faunal Zones A and B mandibular material tentatively referred to Pr. ponticulus ( Murray, 1990) . As the intercoronoid sulcus of the new specimen is significantly narrower than the Pr. novaculacephalus paratype, (this feature is not preserved for Pr. ponticulus ), and the molar morphology more like Pr. novaculacephalus than Pr. ponticulus , it is only tentatively referred to Pr. novaculacephalus . Mandibular characters were probably highly variable within populations. For example, when Murray et al. (2000a, b) and Black et al. (2010) described and analysed variation in an extensive number of cranial characters for Neohelos and Nimbadon , respectively, they demonstrated that mandibular ramus depth and width, and angle of the ascending ramus were subject to both individual, ontogenetic and sexually dimorphic variation.

Dentition. Lower molars are roundly rectangular and shallowly waisted at the interlophid valley that is equidistant between the anterior and posterior moieties (figs. 12A, 17). The metalophid and hypolophid are slightly crescentic and curved anteriorly with an expanded protoconid and hypoconid positioned slightly anterobuccally on each loph. The anterior moiety possesses a strong precingulid without a “forelink,” but the protoconid is well buttressed anteriorly without connecting to the precingulid. The protoconid is separated from the metaconid by a broad, shallow sulcus that runs to the anterolingual edge of the precingulid cleft. A slight swelling on the precingulid at its midpoint represents a weak paraconid. A moderate “midlink,” or cristid obliqua, lies slightly buccal of the crown midline, strongly projecting from the anterior buttressing of the hypoconid and connecting obliquely with a bulge at the base at the midpoint of the posterior face of the metalophid. There is a faint, rounded buccal cingulid. The lingual cingulid is absent. The postcingulid is prominent and connected to the hypolophid by a postlink immediately buccal of the midline. The postlink is clearly delimited on each side by crescentic grooves leading up from the postcingulid valley. Enamel ornament is weaker than on the upper dentition and mostly present on the lingual side of the interlophid valley.

The wear condition of the QVM2000 GFV406 dentition is moderate and less extensive than the Pr. novaculacephalus NMV P 187282 paratype. The morphology of the pre- and postcingulids is less sharply defined .

The apparent morphological progression of the lower molars is as follows: 1. the length gradient increases slightly from M 2 -M 4 (QMV2000GFV406 and NMV P187282 do not preserve sufficiently complete lophs for width gradients to be ascertained). 2. the “mid-link” or cristid obliqua is positioned buccal to the midline on M 2 and becomes slightly closer to the midline by M 4. 3. the “mid-link” or cristid obliqua becomes wider from M 2 to M 4. 4. the precingulid becomes less sharply defined from M 2 to M 4. 5. the postcingulid and its shelf becomes less prominent from M 2 to M 4 (figs. 12A, 17).

The M 1 of Pr opalorchestes ponticulus has been described in detail by Murray (1990) for the AR1802 holotype and SGM1008. In comparison with the new QVM 2000GFV406 mandible and the Pr. novaculacephalus paratype NMV P187282, the AR1802 postlink is lingually positioned and contrary to the buccal position to be expected from the tooth progression seen in either mandible. The cristid obliqua is buccally positioned to the degree expected. The morphology that remains on SGM1008 shows hypo- and metalophids proportioned as expected of M 1 on the basis of the dental progression seen in the mandibular tooth rows. The link positions on SGM1008 do not correlate with the progression seen on the mandibular specimens or the Pr. ponticulus AR1802 M 1 holotype because the postlink and cristid obliqua are both more medial. Furthermore, AR1802 appears to possess a small double or bisected lingual cingulid.

The M 2 is the smallest and marginally narrowest tooth of the series preserved, but losses and extensive wear, preclude detailed comparisons with the highly worn M 1 morphology of the Pr. ponticulus specimens figured by Murray (1990). The postlink and cristid obliqua are narrow and clearly connect with the wear facets.

The M 3 clearly shows that the metaconid is roundly expanded more than the entoconid, but not to the extent of either the hypoconid or larger protoconid. The latter are more triangular in occlusal outline as a result of the anterior extensions produced by the connection of the cristid obliqua to the hypoconid, and the anterior buttressing of the protoconid.

M 4 is the largest tooth showing a broader morphology of the cristid obliqua, postlink and cingulids; the buccal cingulid is scarcely evident. The metaconid is least expanded and subequal with the entoconid. The cristid obliqua and postlink are close to the longitudinal midline. The referred Pr. ponticulus AR 11302 was described by Murray (1990) as a possibly encrypted, but isolated M 4. It exhibits a closer morphology to the M 4 s of QVM 2000GFV406 and the Pr. novaculacephalus paratype NMV P187282 (figs. 12A, B, 17).

Discussion

The morphology and extent of retraction of the nasal anatomy seen in the QVM 2000GFV 459 specimen is comparable with the degree of retraction seen in P. painei from the Miocene, P. parvus from the Pliocene to Pleistocene and the last surviving palorchestid species, P. azael from the late Pleistocene (fig. 9). Minor details of the morphology vary between each, indicating that Palorchestes continued to develop specialisations of the rostrum in addition to the allometric changes related to evolving larger skull and body size. Detailed morphometric analyses are presently beyond the scope of this description, but with the relative completeness of cranial material now available from Pr. novaculacephalus , P. painei , and P. azael , a more precise relationship between allometry, morphological function and phylogenetic trends might be elucidated.

The vertical elaboration of the face and rostrum of palorchestid skulls represented in chronological sequence, indicates a relationship to an allometry that may be independent of the primary nasal function. For example, the retracted condition of the nasals and dorsal exposure of the nasal cavity to a level immediately anterior to the orientation of the transverse facial plane in P. painei had already been achieved in Propalorchestes . Furthermore, the morphology is very close to the aligned condition of the facial plane, the proximal limit of the nasal aperture, and the orbits found in P. azael . (figs. 3, 5, 6, 9). The progression of increasing vertical height of the skull and face in Palorchestes occurred while maintaining the close relationship between the proximal limit to which the nasal was retracted, the transverse facial plane, and the orbits that are now known to have been present in Propalorchestes .

The relative projection of the rostrum from the transverse facial plane and the deflexion of the rostrum from the frontal plane (measured against the horizontal tooth row) also increased from Propalorchestes to the largest Palorchestes . The proportional changes indicate that as the height of the skull increased, the orbits and retracted nasals were taken with it and together, their relative position continued to move posteriorly. In Propalorchestes the retracted nasals are aligned with the P 3. In Palorchestes painei the alignment had moved slightly to a point between P 3 and M 1, while in P. azael , they align with the posterior moiety of M 3 (fig. 9). In relative terms, the food procuring and processing anatomy has projected anteriorly with respect to the eyes and nasal cavity. The nasals have not retracted posterior to the eyes in P. azael , nor have the orbits maintained their position relative to the tooth row. Allometry and masticatory function may both be influencing the rostrum morphology in this context. Testing the complex factors in this morphological progression for the Palorchestidae will likely remain elusive and would be pivotal on a more complete knowledge of the cranial form of later, more advanced small palorchestids such as P. pickeringi . Due to the apomorphic rostral condition now revealed in Propalorchestes , understanding the factors surrounding its origin will require a more archaic member of the family; one representing a truly intermediate condition, (e.g. intermediate between the rostral form of genera such as Ngapakaldia , Diprotodontinae and that of Propalorchestes novaculacephalus ).

Extensive descriptions of individual Propalorchestes teeth have been given in the literature ( Murray, 1990; Mackness, 1995; Black, 1997a, 2006). Black (2006) remarked that poor preservation and heavy occlusal wear precluded many character comparisons and creates sparse measurement opportunities for meaningful comparisons. The extreme individual variation in dental morphology within extinct Vombatiformes is well known (Black, 2006; Price and Sobbe, 2010). Without a substantial data set, variation within any given population between geographical regions, or over substantial periods of time, is difficult to define for fossil taxa. Regarding extant Vombatiformes, Black et al. (2014) detailed modern koala dental variation and related it to fossil taxa. Sharp and Trusler (2015) provide a brief summary of the morphological variation seen in the wombat genera Vombatus and Lasiorhinus . Following re-examination of Riversleigh Propalorchestes, Black (2006) reassigned two Pr. ponticulus upper molar specimens, previously described from Faunal Zone B deposits (Black, 1997a, b), to Pr. novaculacephalus following study of a new juvenile maxilla fragment with well preserved unworn teeth from a Faunal Zone C deposit. Black (2006) concluded that Propalorchestes range throughout Faunal Zones A–C exhibiting a morphocline with the more plesiomorphic forms in the oldest Faunal Zone A deposits, but essentially places two forms, for which species boundaries are continuing to be redefined, at opposing ends of the late Oligocene to middle Miocene interval. Further evidence of this trend has been presented by Arena et al. (2015) with the inclusion of upper dentition attributed to Propalorchestes ponticulus , that until this time had only been represented by lower dentition. Propalorchestes novaculacephalus is represented by upper and lower dentition from the Bullock Creek LF, but only by upper dentition from Riversleigh Faunal Zone B and C deposits. The diagnosis upon which this had largely depended was based upon NMV P187282, an isolated dentary fragment with M 2-4 and NTMP 862-27, a partial maxilla from Bullock Creek bearing incomplete crowns M 2-3 ( Murray, 1990). Unfortunately, the new material contained in the biochronostratigraphic study of Arena et al. (2015:10) was provided without supporting information, stating that “ Propalorchestes molars are usually heavily worn. Consequently, molar size and shape are the most useful features for differentiating between species.”

Issues concerning the relative states of morphological progression are a major part of the dental literature for palorchestids. For example, Black (2006: 356) stated with respect to the interloph cristae that, “This structure may represent a precursor to a true ‘midlink’…these cristae and enamel bulges do not constitute the true midlink, a synapomorphy that unites species of Palorchestes ( Mackness 1995) to the exclusion of species of Propalorchestes .” Murray (1990: 41) described NTMP 862-27 as “possessing ‘a double midlink-like structure’ formed on either side of a rhomboidal fossa that is positioned buccal of the midline in the interloph valley.”

The difference between cristae and links is a matter of degree, both in terms of evolutionary trends within a population over time and the morphological progression within the dental series, consequent of the ontogenetic inhibitory cascade expression in any individual (Kavanagh et al., 2007; Halliday and Goswami, 2013). The condition and relative wear of the dentition also alters the perception of the morphology and it is clearly seen to influence the descriptions and interpretation of the same features. Like NTMP 682-27, both M 1 s on QVM 2000GFV459 possess a bulge at the midline of the anterior face of the metaloph. This is recorded as a lobe on the wear facet occlusal outline. The Riversleigh WHA Pr. novaculacephalus M 1 s possess incipient bulges (to varying extents) in the same location, but the lesser wear on these specimens precludes strong indication of their bulging form on the occlusal facet outline. Examination of the dentition in all Palorchestes species through a complete range of wear reveals that the bulges, cristae and links are essentially folds of enamel and seldom appear to be fused. This is particularly the case with the midlinks where a highly worn state will produce a characteristic cross in occlusal outline. This is attained when the entire length of the midlink makes occlusal contact and the transverse line (fine cleft) of the cross persists until the ware has entirely breached the enamel. The same pattern and process occurs with the enamel constituting the buccal and lingual sides of the midlink, with the division between them persisting as the sagittal line of the cross until the enamel is breached. The enamel links are just tightly abutted (fig. 13). The morphological expression is one of degree and neither a union or a failure to have reached a union, strictly speaking, occurs as a character state. Swellings on the posterolingual faces of the protocone and metaconule also form buttresses that accentuate the recurvature of the crescentic lophs as occlusal wear advances. These are variably developed and also become cristae in the more derived Palorchestes .

With regard to the evolutionary development of links between cusps or lophs, the Pr. novaculacephalus paratype QMF12429 exhibits misaligned bulges or incipient cristae in the interloph valley. The topographic growth of aligned features by height, length or duplication would easily allow progression to be determined from primitive to advanced states. If major realignments are required to form a connection between features (i.e. to establish a midlink in a derived morphology) the path of development from an unknown origin will be more difficult to determine. Such variability in character states makes it difficult to reconcile the different emphases placed upon them by Murray (1990) and Black (2006) to that of our own. In reviewing Pr. novaculacephalus, Black (2006: 358) concluded “differences between the referred specimens and the paratypes do not warrant specific distinction.” The QMF12429 paratype does not appear to fit within the stated evolutionary trend for the most significant taxonomic character of the palorchestid dentition. The spectrum of variation in the other Propalorchestes material and of the new material described here falls closer to a transitional progression towards the development of a midlink and indeed, a double midlink in Palorchestes , irrespective of where one might delimit taxonomic boundaries. At worst, QMF12429 may simply be aberrant, and at best, may indicate a deviation from the assumed lineage for which it was designated as a paratype.

Similar trends are evident in the lower cheek teeth. The extent that such morphologies become confluent with the tips of the cusps and loph ridges of the crown is always going to be increasingly indeterminable with advancing wear. This makes character conditions difficult to define in a majority of specimens. The definition of fore- and postlinks is less problematic in this respect because the wear is generally less extensive across the more basal regions of the crown and a complete bridge of the form in question with the pre- and postcingulae more easily determined.

Inconsistencies between the Pr. ponticulus published material leads us to reiterate the uncertainties originally stated when the material was described and additional material referred to the taxon. Despite the size discrepancy between the mandibles recovered from the Bullock Creek sites, the dentition is similar and shows a consistent morphological relationship. The morphological similarity between the AR11302M 4, QVM 2000GFV406 andthe Pr.novaculacephalus holotype M 4 s is strong. It is more consistent than the M 1 Pr. ponticulus material from the Riversleigh WHA exhibits between them or would be suggested from the molar morphological progression seen in the mandibular specimens from Bullock Creek.This leads us to tentatively refer AR11302 to Propalorchestes novaculacephalus as the first lower dentition of the species from the Riversleigh Wayne’s Wok Locality, Faunal Zone B (Archer et al., 1989). This would be consistent with Black (2006) who stated that Pr. novaculacephalus (upper dentition) is first recorded in FZ B deposits and becomes more common in the later FZ C, and independently agrees with Arena et al. (2015)

Colbert (2006) regarded the common practice of referring isolated tapiroid dental material to a particular genus or higher taxon without cranial evidence as problematic and a similar issue becomes evident here. The degree of plesiomorphy in the dentition belies the specialization of the facial morphology in Propalorchestes . At the outset, Murray (1986: 209) noted a mosaic of characters in the neurocranial fragment of the holotype, concluding “ Propalorchestes is generally more primitive and simultaneously more specialized than Palorchestes .” He was referring to the only near-complete skulls of the genus available, those of P. painei . Despite this astute observation, taxonomic deliberations have proceeded– inevitably concentrating on isolated teeth. Three studies have added to, and sought to resolve the Oligo-Miocne palorchestid material from the Riversleigh WHA since Murray (1990) first erected Pr. ponticulus (Black 1997a, 2006; Arena et al., 2015). The known issues of dental variation in fossil herbivorous marsupial species ( Murray et al., 2000a; Black and Hand, 2010; Black et al., 2013) apply well to Propalorchestes , testing the assumption that meaningful taxonomic boundaries can be applied to encapsulate apparent trends in small samples.

Figures

Figure 11. Propalorchetes cf novaculacephalus QVM2000GFV406 measured mandibular outline diagram: a, lateral: b, medial: c, occlusal views; scale bar represents 5 cm.

Figure 17. Graphic comparison of QMV2000GFV406 lower right molars in occlusal view with previously published Propalorchetes material Murray (1990): M length of QVM2000GFV406 and Propalorchestes ponticulus AR1802 Murray (1990) scaled to M of NMVP187282; 1 1 AR11302 scaled to M 4 of NMVP187282: SGM1008 size approximated to other M 1 material; series depicted top to bottom in order of increasing M length; left specimens have been reflected to become right specimens for ease of comparison.