Description and comparison of Naraboryctes philcreaseri with species of Notoryctes
In the description below, case denotes upper (e.g. M2) and lower (e.g. m2) teeth, and figures refer to those in the primary text.
(a) Upper dentition
Premaxilla (QM F51322) has I1-4 within alveolar process. Part of an alveolus for probable I5 preserved at posterior margin. Although possible this alveolus alternatively represents C1 positioned at the maxilla/premaxillary boundary, an alveolus for C1 appears to be completely within the maxilla in other specimens (e.g. QM F23716; figure 2l). I1-4 relatively unspecialised, although I1-3 show flattened cutting surfaces. In contrast, upper incisors of N. typhlops highly reduced with little capacity to cut. I4 in N. philcreaseri is smallest incisor and peg-shaped. I5 unknown but fragmentary alveolus noted above suggests it was marginally larger than I4. No upper canine preserved. Alveolus for this tooth round rather than oval with no diastema between it and P1. The c1 has no wear facets attributable to occlusion with C1 suggesting latter was small and possibly non-caniniform. QM F23716 retains P1-3. P1 minute and probably non-functional. Larger P2 and P3 highly compressed with single anteriorly-oriented crest descending from large central cusp to join anterior terminal cusp. P3 abuts anterior cingulum of M1 (although this conjunction not apparent in figure 2l because of damage during preparation). Posterior width of dP3 (QM F51323) enormous relative to anterior width. Buccal length longest overall dimension. The dP3 protocone relatively small and stylar shelf greatly reduced with stylar cusp D the only stylar cusp present. Stylar cusp B position occupied by paracone which is buccal to metacone. Very long postmetacrista but no centrocrista or preparacrista. Anterior cingulum large and triangular. Ectoflexus very small.
Presence of M1-4 can be inferred from isolated maxillae (QM F54502) but only M1 in situ (QM F23716). Referral of isolated molars to other dental loci based on inferred meristic gradients. In this way, M1-3 have been identified but M4 remains unknown. M1-3 triangular with variable degree of anteroposterior compression depending on locus with M1 showing least and M3 most anteroposterior compression. Posterior width the largest, and buccal length smallest, dimensions in M1-3. All upper molars have metacone, paracone, protocone. Metacone consistently larger than protocone which in turn is larger than paracone. Preprotocrista longer than postprotocrista and in M1 is subhorizontal (parallel to occlusal plane) prior to reaching base of paracone. Postprotocrista descends almost vertically from protocone. Metacone and paracone very close to each another and joined by short, straight centrocrista. Curved postmetacrista longest crest and terminates in small cusp (possibly stylar cusp E) at metastylar corner; second cusp, referred to here as ‘medial postmetacrista cusp’, present approximately half-way along postmetacrista. Both cusps visible on little-worn specimens of M1-3 but often indistinguishable on worn teeth. Preparacrista, which is shorter on M1 than on M2-3, poorly preserved in specimens with moderate degrees of wear. It runs close to anterior margin of stylar cusp B on M1 but reaches parastylar corner (possibly stylar cusp A) on M3. Neither para- or metaconules evident on any tooth. Stylar cusp B marginally larger than stylar cusp D and as tall as paracone on M1. Stylar cusp B and stylar cusp D separated by distinct ectoflexus which is generally smallest on M1 and largest and triangular on M3. Trace of stylar cusp C present in little-worn specimens as bulge on posterior slope of stylar cusp B. Anterior cingulum, present on all known molars and generally triangular but variable in width. Meristic gradients from M1 to M3: paracone increases in size relative to metacone; postmetacrista decreases in length anteriorly; the degree of anteroposterior compression of tooth crown increases.
(b) Lower dentition
Dentaries indicate lower dental formula of i1-3 c1 p1-3 m1-4. Compared with N. typlops, incisors of N. philcreaseri taller, more closely-spaced and anteriorly inclined in jaw. Crowns flared, concave lingually, convex bucally, and terminate in sharp, slightly curved ridges. The i1 is smaller than i2. Of three lower incisors, i3 the smallest. The c1 the smallest tooth in lower dentition and simple in overall morphology with short crest running from blunt anterior tip. On all lower premolars, single lingual blade descends posteriorly from tall protoconid to meet small posterior cusp. Buccal to this blade, crowns concave. In terms of height, width and length, p3 > p2 > p1. Minute peg-like tooth between p1 and p2 in QM F23717 almost certainly supernumerary tooth. In all four lower molars, trigonid both longer (measured on lingual margin) and wider (measured either perpendicular to lingual margin or along anterior and posterior margins) than talonid. Trigonid comprises bucally-situated protoconid, paraconid at anterolingual and metaconid at posterolingual corner. Protoconid tallest cusp. Metaconid taller than paraconid on m1-3 but metaconid and paraconid subequal on m4. Trigonid cuspids sharp in unworn specimens. Paracristid and metacristid subequal in length in m1 and m4 but paracristid marginally longer on m2 and m3. Well-developed ‘carnassial notch’, forming angle of roughly 90 degrees, present in both paracristid and metacristid. Trigonid basin deep and narrow, lingually-open and deepest between metaconid and paraconid. Anterior cingulid diminutive in m1 and medially situated along anterior margin of tooth. Moving backwards along tooth row, anterior cingulid progressively wider, occupies increasingly longer portion of anterior face and increasingly curved upwards at anterior edge reaching most extreme development in m4. Anterior cingulid widest lingually and descends and narrows towards buccal side of tooth. Figure 3a-c illustrates gradient of development along tooth row. Within any tooth position, degree of development of anterior cingulid variable between specimens. Anterior cingulid of m2 and m3 occasionally has shallow notch, possibly the result of wear.
In m1-3, distinct hypoconid, hypoconulid and entoconid present with hypoconid largest and entoconid smallest cuspid. Number and height of talonid cusps of m4 variable, although three cuspids usually present. Hypoconulid invariably closer to (‘twinned with’) entoconid than to hypoconulid, but not immediately posterior to entoconid as it is in peramelemorphians. Hypoconid slightly compressed anteroposteriorly. Minute cusp occasionally present low on posterior margin of tooth between hypoconulid and hypoconid, and presumably neomorphic. Talonid basin flat-based and open. Cristid obliqua short, contacts trigonid buccal to ‘carnassial notch’ of metacristid. No posterior cingulid. Small cuspid occasionally present low on buccal margin of tooth in depression (hypoflexid) between trigonid and talonid. Meristic gradients: anterior width (crown width measured along paracristid), overall lingual length and length of trigonid (measured along lingual margin) increase from m1 to m3 and then decrease on m4; posterior width (measured along the posthypocristid of talonid) and talonid length subequal on m1 to m3 and smaller on m4; protoconid increases in height from m1 to m3 and reduces again in m4; paraconid increases in height relative to metaconid from m1 to m4; anterior cingulid broadens, lengthens and increases in curvature from m1 to m4.
(c) Cranial material
Left partial premaxilla (QM F51322; figure 3k) referable to N. philcreaseri apparently unbroken at anterior border, which forms nasal opening and anterior suture with right premaxilla. QM F51322 indicates that the intact external nasal aperture in N. philcreaseri was ovoid when viewed anteriorly, unlike square aperture in N. typhlops. When viewed laterally, anterior margin of external nasal aperture reclined posterodorsally in QM F51322; in N. typhlops anterodorsal part of facial process of premaxilla and rostral ends of nasal bones extend further anteriorly, overhanging external nasal aperture, and curve downwards, providing protection to nose during digging. Facial process of the premaxilla laterally convex in QM F51322; in N. typhlops this area of premaxilla is flat. Major differences in the mandible of N. philcreaseri (QM F23717, QM F23719, QM F51328; figure 3a-f) relative to mandible of N. typhlops: ascending ramus taller and forms comparatively sharper angle to main body of dentary; masseteric fossa deeper with wider angle between anterior and posterior borders; posterior border of masseteric fossa lacks anteriorly-oriented process found in N. typhlops.
Humerus (QM F54559; figure 4b,e) very similar to that of species of Notoryctes but differing as follows. Shaft is oval rather than triangular in cross section with the greatest diameter mediolateral and lacking concavity of outer posterior surface. Delto-pectoral tuberosity prominent but less well developed; it is equally extensive laterally but not expanded distally and proximal surface is concave. In Notoryctes spp. the tuberosity forms long, distally convex shelf on proximal half of humerus. A supracondyloid foramen, which is absent in Notoryctes spp., is present and well developed. Olecranon fossa, which receives anconeal process of ulna during extension of elbow joint, occurs as either a single, wide centrally-positioned depression or a deep medial fossa connected to shallow lateral depression; in Notoryctes spp. lateral fossa deeper than medial and the two separated by medioposterior part of trochlear process. Capitulum and trochlea continuous on anterior surface. Trochlea is simple, oval shape without the anterior or posterior expansion seen in Notoryctes species. Capitulum is oval and pronounced but not as expanded on lateroanterior margin. Condylar area (measured from medial epicondyle to lateral epicondyle) less well developed. The head (proximal articulation of humerus with scapula) smaller but greater tuberosity slightly larger.
Ulna (QM F54559; figure 4h,k) referable to Naraboryctes philcreaseri clearly very similar to ulnae of species of Notoryctes but differs in several features including the following. Although olecranon process approximately one third of entire length of limb (a n exceptionally enlarged state unique among marsupials to notoryctids) nevertheless shorter than that process in Notoryctes and not curved as sharply medially. Ulna of N. philcreaseri also less flattened anteroposteriorly. Crest extends from anconeal process proximally on anterior surface to separate and define medial and lateral facets. Also unlike Notoryctes, there is a notch just medial to distal tip. Small ridge occurs on a large medial section of the dorsal aspect of the ulna.
Although overall proportions of humerus and ulna basically similar in species of Notoryctes and Naraboryctes, in all ways in which they differ, Notoryctes spp. depart further from proportions exhibited by more plesiomorphic australidelphians such as peramelemorphians and dasyuromorphians.
Table 1. Measurements (in mm) of Naraboryctes philcreaseri gen. et sp.nov. Abbreviations: #holotype; *alveolar row length; ^M1 dimensions.