Diprotodon, Owen, 1838

Price, Gilbert J., 2008, Taxonomy and palaeobiology of the largest-ever marsupial, Diprotodon Owen, 1838 (Diprotodontidae, Marsupialia), Zoological Journal of the Linnean Society 153 (2), pp. 369-397 : 373-379

DARLING DOWNS DIPROTODON

DESCRIPTION

General morphology of dentary ( Fig. 3 )

Dentary long with slender to deep horizontal ramus, length ~ 500–650 mm; dentary body tapers posteriorly from M 1 to M 4; alveolar border nearly straight to slightly concave; horizontal ramus convex to angular anterior to ascending ramus; diastemal crest short relative to length of dentary; mental foramen anteroinferior to P 3 anterior alveolus; incipient smaller foramen may also be expressed; incisive foramen inferior to incisor alveolus; symphysis elongated, ovoid in outline, posterior portion inferior to M 1 alveolus; vertical ramus half length of total dentary; coronoid process high, blade-like; coronoid notch very deep; condyle high above occlusal tooth line; posterior edge of ascending ramus constricted at neck of condyle; condyle very wide, convex posteriorly, anteriorly inclined, rounded articular surface; digastric fossa wide, elongated, shallow, extending anteriorly inferior to transverse valley of M 3; angular process projects much higher than tooth cheek tooth row; mandibular foramen wide, ovoid, open above tooth crown height.

I 1 ( Fig. 3 )

Single paired lower incisors, chisel-like, slightly forked, procumbent, hypselodont; straight to slightly curved dorsally; occludes with posterior surface of I 1 and horizontal surface of I 2-3; enamel confined to lateral external surface, curving around ventrally to lower mesial quarter, and extended to anterior ventral surface tip; longitudinal groove on mesial side wide and shallow; tooth ovate in cross-section, narrowing towards anterior tip.

P 3 ( Fig. 4A, B )

Bilophodontid; subrectangular to subtriangular in occlusal outline; hypolophid wider than protolophid; protolophid angled anterobuccally; hypolophid angled perpendicular to length of tooth; protolophid higher crowned than hypolophid; metalophid bulbous swelling, positioned on anterobuccal portion of hypoconid, connects to midline of posterior face of protolophid; paralophid crest descends anterolingually to small, pocket-like anterior cingulid; premetacristid weakly expressed, descends from halfway up metaconid; postmetacristid and preentocristid small, variably expressed, meeting at lingual opening of transverse valley, connecting entoconid and metaconid; bucccal cingula on transverse valley weakly developed; postentocristid descends entoconid posterobuccally to form pocket-like posterior cingulid; posterior cingulid well developed with slight thickening of enamel just lingual to midline; posthypocristid descends hypoconid to connect to posterior cingulid, one-third from buccal extremity of cingulid; posterior cingulid sits below height of anterior cingulid of succeeding tooth.

M 1 ( Figs 4A, B , 5A )

Bilophodontid; subrectangular in occlusal outline; hypolophid longer and wider than protolophid; lophids slightly concave anteriorly, angled perpendicular to molar row, transverse valley V-shaped in lingual view, U-shaped in buccal view; hypolophid wider and longer than protolophid; paralophid crest and preprotocrista weakly developed, descends anteriorly from halfway up protoconid, connecting to anterobuccal portion of anterior cingulum; premetacristid weakly developed, descends anteriorly from halfway up metaconid, connecting to anterolingual portion of anterior cingulum; anterior cingulid small, horizontal; metalophid weakly expressed, approximates as a swelling on posterior surface of the midline of the protolophid, descends anterolingually from hypoconid; postmetacristid and preentocristid faint, meet at lingual opening of transverse valley; postprotocristid and prehypocristid absent to weakly expressed, meeting at buccal opening of transverse valley; transverse valley narrow, deep; postentocristid small, descends entoconid to form lingual margin of posterior cingulum; posthypocristid descends hypoconid to form buccal margin of posterior cingulum; posterior cingulum wide, well developed, crosses tooth transversely horizontal to midline then descends buccally to base of hypoconid; slight thickening of posterior cingulid at midline as its connection to the hypolophid becomes more crest-like; posterior cingulid overlaps anterior cingulid of succeeding tooth.

M 2-4 ( Fig. 5A )

Progressively larger than M 1; M 2-4 generally similar in morphology to M 1, however: protolophid and hypolophid more similar in length and width; paralophid crest, preprotocristid and premetacristid rarely expressed; postcristids linking protolophids and hypolophids at lingual and buccal openings of transverse valley more weakly expressed or absent; transverse valleys of M 3-4 more V-shaped in buccal view.

General morphology of cranium ( Fig. 6 )

Long, narrow, ranges from 730 mm to slightly greater than 1000 mm in condylobasal length; premaxilla elongated; rostrum long, tapered anteriorly; diastema long, narrow, deflected slightly anteroventrally relative to cheek tooth row; upper incisors outline trapezoidal in occlusal aspect; buccinator fossa extremely deep; premaxillary septum high; nasals retracted; narial aperture deeper than broad; palate closed, narrowing anteriorly; palatal vacuities short, terminating posterior to cheek tooth row; zygomatic arch elongate, masseteric processes well developed, deep; lateral maxillary fossa very deep; cranial vault domelike in posterior profile, convex in lateral profile; basicranial plane markedly elevated above level of palatal plane; postglenoid process elongated, fused with tympanic process and mastoid–squamosal posteriorly; occiput region broad, angled anteriorly; lambdoid crest short, smoothly arching; occipital condyles very large, 60 mm to> 110 mm in depth; braincase positioned posteriorly, very small, surrounded by air-filled sinuses.

I 1 ( Fig. 7 )

Strap-like, curved ventrally, hypselodont; occlusal notch present on most teeth; I 1 wears on posterior surface, tip extends below occlusal plane of I 2-3; enamel confined to dorsal, lateral and upper one-third of mesial portion; shallow, wide longitudinal groove on dorsal surface; shallow longitudinal groove on mesial surface; wide, shallow groove on ventral, lateral margin; mesial portion deeper than lateral portion.

I 2 ( Fig. 7 )

Left and right I 2 converge at occlusal tip; occlusal surface of crown horizontal, ovoid; anterior angle acute, posterior angle obtuse; curves mesially; narrows towards tip; pointed anterior margin overlaps posterolingual surface of I 1.

I 3 ( Fig. 7 )

Similar in morphology to I 2, but narrower in relation to length; left and right I 3 splayed; crown much more elongate relative to width.

P 3 ( Fig. 4 )

Lophodont; subovoid to subtriangular in occlusal outline; parastyle most anterior cusp lying just buccal to midline forming anterobuccal margin of tooth, variably expressed ranging from slight enamel bulge to large bulbous cusp; protocone wide, transverse, forming lingual portion of tooth, horizontal or slightly posteolingual to paracone; cleft between protocone and paracone shallow to deep; metacone transverse to protocone, posterobuccal to paracone, connected to protocone by protoloph; preparacrista descends anteriorly from paracone to buccal side of parastyle; preprotocrista descends anteriorly from posterior or anterior of protocone to form lingual corner of anterior cingulum; anterior cingulum small, connected either to anterior face of parastyle or connects to preparacrista; secondary anterocrista commonly expressed descending anterior face of paracone to connect to midline of parastyle; postprotocrista and postmetacrista descend posteriorly from apex of protocone and metacone, respectively, forming lingual and buccal corners of posterior cingula; posterior cingula well developed running transversely, curved, lowest at midline; ectoflexus between paracone and metacone variably curved, or with distinct vertical ridges; buccal cingula absent to small, variably expressed.

M 1 ( Figs 4 , 5B )

Bilophodont; subrectangular to subtrapezoidal in occlusal outline; metaloph wider than protoloph; transverse lophs anteriorly convex, angled perpendicular to molar row, V-shaped in buccal and lingual views; preparacrista descends anteriorly from paracone to buccal portion of anterior cingulum; preprotocista descends anteriorly from protocone to form lingual portion of anterior cingulum; anterior cingulum well developed, forming a shallow basin across anterior margin of tooth; incipient forelink variably expressed in basin of anterior cingulum in midline of tooth; postprotocrista descends protocone, consistent with a small lingual cingulum, on lingual opening of transverse valley; postparacrista small to absent; postmetacrista descends posteriorly from high on posterior face of metacone to a small metastyle; metastyle variably expressed, forming posterior buccal corner of tooth; postmetaconulecrista small, descends posterior face of metaconule, forming posterior lingual corner of tooth; posterior cingulum low, running transversely along posterior of tooth, connecting to postmetaconulecrista and metastyle.

M 2 ( Fig. 5B )

Larger than M 1; M 2 generally similar in morphology to M 1, however: protoloph and metaloph more similar in width; postprotocrista and postparacrista variably less developed or absent; lingual cingulum less developed or absent; forelink absent; metastyle not expressed.

M 3-4 ( Fig. 5B )

Progressively larger than M 1-2; generally similar in morphology to M 2, but protoloph wider than metaloph, particularly in M 4 where metaloph is lingually offset; more U-shaped in lingual view; lophs angled more anterobuccally relative to molar row; lingual cingula absent; posterior cingula less developed.

MORPHOMETRIC AND MORPHOLOGICAL COMPARISON

Dentary

Both Diprotodon size classes are abundantly represented in Darling Downs fossil assemblages. To demonstrate those size classes, basic measurements were taken of the depth and width at specific points along the dentary. The morphometric results indicate that there is a bimodal distribution of dentary sizes within Darling Downs Diprotodon ( Fig. 8 ). Such differences reflect large and small form individuals.

There are several observable morphological differences in the shape and form of the dentary of large and small form individuals ( Fig. 9 ). The dentary of mature large-form individuals is generally much longer, deeper and wider than that of similarly aged small-form individuals. Large-form individuals may also develop a very distinctive ‘chin’ where the symphysis is greatly expanded and is much steeper anteriorly ( Fig. 9C–E ). The ‘chin’ apparently developed early in the life of the large form as exhibited in a juvenile eastern Darling Downs individual ( Fig. 9F ). The ventral margin of the horizontal ramus is commonly more concave in the large form than the small form. The posterior portion of the dentary is angular in the large form, and less inclined and rounded in small-form individuals. In the large form, the coronoid notch is deeper and coronoid process taller in relation to the condyle than in the small form. The leading edge of the ascending ramus is inclined slightly posteriorly in the large form but is angled more vertically in the small form. Otherwise, the dentaries are very similar.

Teeth

Although there are clear differences in dentary sizes between both morphs ( Figs 8 , 9 ), those differences are less apparent in analyses of isolated teeth. Therefore, Diprotodon dentaries were scored as to whether they represented the large or small form, and a series of tooth measurements (length, anterior and posterior width) were taken for in situ molars. The morphometric results indicate that there is an obvious difference in tooth size between large- and small-form Darling Downs Diprotodon ( Table 1; Fig. 10 ). The means of most cheek teeth measurements are ~4–17% larger in the large form than the small form ( Table 1).

Over 170 teeth were examined in the Darling Downs Diprotodon assemblage. The molar teeth of Darling Downs Diprotodon are somewhat variable in morphology. Generally, upper molar teeth vary in the degree of development of cingula and stylar cusps. Lower molars also vary in the degree of development of cingulae, and anterior molars vary in the development of the metalophid, which is small, but distinct in some individuals, and reduced in others.

Within Darling Downs Diprotodon , P 3 is one of the more variable teeth in terms of morphology. The upper premolar (P 3) is one of the most important teeth for determining relationships between diprotodontids ( Longman, 1921; Stirton et al., 1967; Hand et al., 1993; Black & Archer, 1997; Murray et al., 2000), and thus its variability is considered here in more detail. The crowns vary in the expression of the cusps, cingula and occlusal outline. The parastyle is the most variable cusp, ranging from being large and bulbous, forming the anterior margin of the tooth, to being small, and confluent with the anterior cingulum. The anterior cingulum (and associated preparacrista and preprotocrista) varies in its development and range around the anterior margin of the P 3. The anterior cingulum may connect either to the buccal or anterior face of the parastyle (especially if the parastyle is large and bulbous), or be confluent and connect to the preparacrista. The Mean (standard deviation, coefficient of variation).

preprotocrista may extend anteriorly from the anterior or posterior portion of the protocone. Where arising from the posterior portion of the protocone, it may form a small, pocket-like lingual cingulum. The ectoflexus between the paracone and metacone is also variable, being smooth, rounded, or well defined and angular in occlusal outline. Buccal cingula may be absent to moderately developed. The cleft between the paracone and protocone may be either deep or shallow. The cleft between the paracone and metacone may also be deep and shallow. Where deep, a characteristic horseshoe-shaped wear pattern is developed on the occlusal surface. Where shallow, the occlusal surface of the tooth may wear into a ring-shaped pattern. P 3 morphology is variable in both large- and small-form individuals, and no single morphology is restricted to either size class.

Lower incisors of large-form Darling Downs Diprotodon tend to be wider and deeper, and less curved than in small-form individuals. For cheek teeth, there are no consistent morphological differences between large- and small-form individuals. However, as demonstrated above, there is an observable morphometric overlap between cheek teeth size of the large- and small-form Diprotodon ( Fig. 10 ).

Figures

Figure 3. Diprotodon dentaries from the Darling Downs. A, lateral aspect of a large-form individual (QMF319). B, lateral aspect of a small-form individual (QMF36129). C, internal aspect of same small-form individual.

Figure 4. Diprotodon P3 and M1. A–B, QMF6633, occlusal and lingual aspects of P3–M1 (Darling Downs). C–D, SAMP36367, occlusal and lingual aspects of P3–M1 (Strezlecki Creek). E–F, QMF11136, occlusal and lingual aspects of P3–M1 (mirrored; Darling Downs).

Figure 5. Diprotodon cheek teeth (M1–4, lacking premolars) from the Darling Downs. A, QMF1517, occlusal aspect of lower right molar row. B, QMF10783, occlusal aspect of upper left molar row.

Figure 6. Diprotodon cranium (small form) in lateral view collected from the Darling Downs (note that the maxilla is broken and anteroventrally displaced and angled slightly vertically).

Figure 7. Diprotodon upper incisors (QMF2485). A, lateral aspect. B, occlusal aspect.

Figure 8. Dimensions of Diprotodon dentaries at position below M3 for the Darling Downs (N = 34), Lake Callabonna (N = 19) and Bacchus Marsh (N = 23) assemblages. Closed markers indicate large-form individuals, open markers indicate small-form individuals.

Figure 9. Line drawings of morphological variation in Diprotodon dentary shape (all specimens from the Darling Downs). A, QMF36129, small form. B, QMF1517, small form. C, QMF319, large form. D, QMF6569, large form. E, QMF10311, large form. F, QMF6633, juvenile large form. Numerals on line drawings denote character variations: 1, anterior portion of ramus gently curved in lateral aspect; 2, anterior portion of ramus expanded and angular in lateral aspect; 3, ventral margin of horizontal ramus straight; 4, ventral margin of horizontal ramus slightly concave; 5, posterior portion of dentary gently curved; 6, posterior portion of dentary angular.

Figure 10. Length versus posterior width for Darling Downs Diprotodon lower molar teeth. A, M1. B, M2. C, M3. D, M4.