Ektopodon stirtoni Pledge, 1986

Pledge, Neville S., 2016, New specimens of ektopodontids (Marsupialia: Ektopodontidae) from South Australia, Memoirs of Museum Victoria 74, pp. 173-187 : 182-185

Ektopodon stirtoni Pledge, 1986 .

( Fig. 5 )

Pledge (1986) named this taxon on the basis of a right dentary with P 3 M 1-3 (SAM P19509) and an isolated RM 1 (P22504). The new material described here adds considerably to knowledge about this species.

New specimens. SAM P23854 , RM 2 with lingual root, collected by J. McNamara, 13 July, 1981 ; SAM P24541 , LM 2 hypolophid, collected by N. Pledge, August, 1983 ; SAM P23989 , RM 3 hypolophid with root, collected by N. Pledge on 5 September, 1982 ; SAM P23988 , LM 4 crown, collected by N. Pledge, 5 September, 1982. All the above were noted (per measurements only) in Pledge (1986). The following have not previously been noted . SAM P29577 , right dentary with M 2-4, collected by D. Williams, 3 July, 1988 ; SAM P30156 , LM 3 , collected by H. Aslin, 1 September, 1989 ; SAM P30175 , LM 3 , found by J. Thurmer, 1 November, 1989 (fig. 4) ; SAM P31637 , left maxilla with M 2, found by J. McNamara, 27 July 1990 ; SAM P31638 , a left M 2 found by B. McHenry, August 1990 ; SAM P33451 , a right M 4 collected by G. Aldridge, October 1991; and SAM P35309 , a left maxilla, with P 3 M 1–3 but missing the canine and M 4, collected by N. Haines, 8 August, 1995 .

Locality. Mammalon Hill ( SAM locality PL7611 ; the Type Locality for the species), northwestern shore of Lake Palankarinna , South Australia .

Age. The Ngama Local Fauna comes from the Mammalon Hill beds (zone D) of the Etadunna Formation. The age of this horizon is uncertain but considered to be Late Oligocene ( Pledge, 1984; Woodburne, 1986; Woodburne et al., 1994).

Revision of specific diagnosis. In addition to the diagnostic features noted by Pledge (1986), these additional specimens of Ektopodon stirtoni demonstrate that this species differs from E. serratus , in greater size, greater length/width ratio of the molars and fewer cusp(id)s. It differs from E. litolophus ( Pledge et al., 1999) in its smaller size, relatively narrower M 1, relatively less regular parastyloph with less uniformly sized cusps, fewer and less uniformly-sized cusps on main lophs with obvious ribs and struts, presence of posterior cingulum. The face is blunter than that of Chunia illuminata (compare figs. 3A and 5B, C).

The original description of E. stirtoni ( Pledge 1986) dealt with the holotype dentary and an M 1 (ibid. plate 3.1B). In the dentary, only the P 3 and M 1 were complete. The present work includes descriptions of the other lower molars (M 2-4) as well as that of P 3, M 2 and M 3. These descriptions incorporate specimens previously noted only in the table of measurements (ibid. table 3.1).

Descriptions. Maxilla. Although the (mostly edentulous) maxilla of Chunia illuminata had been known for some time ( Woodburne and Clemens, 1986b), it did not fully prepare us for the morphology shown by the new specimen of Ektopodon . SAM P35309 is virtually complete, lacking only the thin bone on the medial edge of the palate at the midline suture, the dorsal wing of the maxilla with the nasal contact, the small canine, and the last molar M 4 (figs. 5A-C).

However, part of the jugal which forms the lower border of the orbit and part of the zygomatic arch is also present. This enables us to form a picture of the face of Ektopodon . The orbits had an estimated diameter of up to 15 mm, and were directed forwards and upwards in a wide (est. 50 mm across cheekbones) flat face with a short, narrow muzzle. The lateral face of the maxilla is gently convex, almost flat, from canine to malar process of the zygomatic arch. There is no malar depression or fossa. The maxillo-jugal suture is gently sinuous from just behind the malar to the anterior ‘corner’ of the orbit, with the jugal tapering from less than 3 mm below the orbit and being a uniform 3 mm wide inside the edge of the orbit. The bottom edge of the eye socket is 5.5 mm above the molar occlusal surface. The infraorbital foramen is ovate and situated about 4 mm behind the canine and 3–4 mm above the diastemal crest; it emerges 12 mm posterior in the orbital floor.

The molar occlusal surface shows a slight torsion along its length. The combined length of the cheektooth row, P 3 to M 3 is 23.2 mm. The maximum bone length, from canine alveolus to posterior extremity of the palate, parallel to the molar toothrow, is 37.3 mm. The width of the maxillary palate to the lingual margin of the molars is (2x) 12.7 mm. The maximum palate width measured to the buccal edge of the molars is 22.2 mm. M 4 is represented by three alveoli.

Canine. Not much can be said of this tooth based on its alveolus, apart from its existence and its gingival diameter: maximum 2 mm. The alveolar depth of 5 mm suggests it was neither a large nor particularly functional tooth. It is situated at the extreme corner of the maxilla, next to the premaxillar suture.

P 3 (fig. 5C). Previously, the ektopodontid P 3 was known only from two referred fragments from Lake Tarkarooloo ( Pledge, 1986). These have here been reinterpreted (below) to represent E. tommosi n. sp.

The P 3 of SAM P35309 is somewhat recumbent, with its anterior root extending well into the diastema. However, it is almost transverse to the long axis of the molar-row, and its posterolingual corner is tucked neatly into the angle formed by the ‘protostyloph’ and the protoloph of the first molar. The tooth is fairly typical of the permanent premolars of many diprotodontans: somewhat rectangular with a longitudinal ridge just buccal of the centre-line bearing two major cusps, and a shorter, lower lingual cingular ridge with an anterior expansion that forms a prominent anterolingual corner and is the base for a strong transverse ridge ascending to the anterior cusp. Anterior to this ridge is a slightly weaker and shorter one, midway between the transverse ridge and the trenchant anterior extension of the longitudinal ridge. The longitudinal ridge is crossed by a deep valley that separates the anterior cusp from the slightly lower, double, posterior cusp; the crest from the hind part of this cusp curves around to connect with the posterior end of the lingual cingulum. There are two or three small transverse crenulations in the cingular valley. The longitudinal crest is slightly convex buccally, and is almost perfectly aligned with the anterior transverse crest (the ‘parastyloph’) of M 1.

M 1. This tooth in SAM P35309 is almost identical to the paratype SAM P22504 described by Pledge (1986) .

M 2. In Pledge (1986), the M 2 of Ektopodon tommosi from Lake Tarkarooloo was depicted for purposes of illustrating morphology in Text Fig 3.5 (ibid.) as that of E. stirtoni , the tooth having not then been found at Mammalon Hill. This deficiency has now been rectified with discovery of specimen SAM P23854 – an RM 2 still partly in alveolo in a fragment of maxilla (it lacks only the buccal face of the tooth), SAM P31637 with a complete LM 2 in most of the maxilla, and SAM P35309, an almost complete maxillary dentition (figs. 5A–C).

This bilophodont tooth is wider than long. The protoloph is slightly wider transversely than the metaloph. The tooth’s length is 6.8 mm; its maximum width is 9.5 mm.

The protoloph has eight distinct cusps and an indication of at least one more (as does the metaloph). This is more than in E. tommosi (from the slightly older Tarkarooloo LF). The apices of the protocone and metaconule are equal in height, but the lingual extremity of the base of the former is more acute and thus extends slightly farther in a lingual direction. There is a large, deep groove on the anterior face of the protocone and a slightly weaker posterolingual one that extends into the pocket formed by the short but strong lingual cingulum at the end of the transverse valley. Strong pre- and postprotocristae extend in a slightly buccal direction (in the same way that comparable crests extend from the metaconule) giving a hint of remnant selenodonty. The preprotocrista meets the lingual end of the anterior cingulum. The other precristae do not join or only just contact the precingulum. Cusp 2 is smaller and simpler than either the protocone or cusps 3 or 4. It has a single undivided longitudinal crest and is linked to the protocone by an apical strut and a basal strut from the postcrista. Cusp 3 has a strong pre- and postcrista, with a somewhat weaker parallel set arising lower on the lingual face. It is linked to cusps 2 and 4 by a fine low apical strut. Cusp 4 is similar but the lingual pair of cristae is slightly stronger. Cusps 5 to 8 bear undivided pre- and postcristae which are linked by 2 or 3 subapical struts and several short basal ribs. The extent of the median valley indicates at least one more cusp and possibly two.

In occlusal view, only the metaconule on the metaloph is opposite its counterpart cusp (the protocone) on the protoloph. The metaconule is rounder than the protocone and the two grooves diverge antero- and posterolingually, the anterolingual groove running into the cingular pocket. The postmetaconulecrista runs into a very narrow postcingulum to which the other postcristae are weakly joined. Cusp 2 is large and well-spaced from both the metaconule and cusp 3. Its precrista bifurcates basally, with the new rib extending lingually towards the thickened basal ends of the premetaconulecrista and the postprotocrista. Short struts link it anterobasally and apically to the metaconulecrista and there is a short lingual rib from the postcrista. Cusp 3 is similar to that of the protoloph. Cusp 4 is finer with a bifurcating precrista and a short basal lingual rib from the postcrista. It is linked by an apical strut to cusps 3 and 5. Cusps 5 and 6 are similar with undivided cristae that bear a few short irregular ribs. Cusp 6 joins apically to cusps 5 and 7 with a fine strut. Cusp 7 is irregular, having a fine wavy crista bearing several short ribs or broken struts that link with the remnants of cusp 8.

Of the roots, only the double lingual one supporting the protocone and metaconule is preserved, although its tapered tip is missing. Anterior and posterior transverse roots are represented by their bases which support the buccal ends of the protoloph and metaloph respectively.

SAM P 23854 (fig. 5D) is similar in appearance and construction to the M 2 referred by Pledge (1986) to E. sp. cf. E. stirtoni (= E. tommosi ) from the Tarkarooloo LF. It differs in two obvious respects: (1) the relatively and absolutely greater width of the Mammalon Hill M 2 resulting from (2) at least one extra cusp at the buccal end of each loph. In these features, it appears to be autapomorphic within the genus. In P 31637, by some aberrant occlusal wear or damage, cusps 2–4 on the protoloph and cusp 3 on the metaloph are exceedingly worn, far more so than the other cusps.

M 3. This tooth is represented by two specimens, SAM P 30156 (fig. 5F) and SAM P 30175 (fig. 5E), both slightly damaged by the loss of enamel on the buccal face. The former is the better preserved. M 3 has been described from E. tommosi , e.g. NMV P48753 , at Lake Tarkarooloo, hence a direct comparison is possible with these two additional specimens .

The M 3 of Ektopodon stirtoni is slightly larger than that of E. tommosi and in occlusal outline is less tapered posteriorly since the metaloph is relatively longer and less acutely truncated. The detailed structure of cusp ribs and struts is also less complex. The tooth is 6.7 mm long.

The protoloph is 8.9 mm wide and has eight distinct cusps. It is similar in most respects to that of M 2, differing in that the posterolingual groove of the protocone does not flow into a pocket formed by a lingual cingulum, and in that there are more links joining the crests of cusps 7 and 8. By the same token, the protoloph differs from that of E. tommosi , in having fewer subapical links and struts between the crests of cusps 5 to 8. They are similar in lacking the lingual basin.

The metaloph is 7.6 mm wide with six distinct cusps and a buccal complex. The metaconule is situated somewhat more buccally than the protocone, but not level with cusp 2 of the protoloph. However, the crests of the metaconule do align with those of protoloph cusp 2. Similarly the cristae of metaloph cusps 2 and 3 align with those of protoloph cusps 3 and 4. The pre- and post-cristae of cusp 2 each have an accessory crista that is rather sinuous and arises from further down on the lingual face, parallel to the main crista. This is similar to cusp 3 of the protoloph. Cusp 3 is a smaller version of cusp 2; both have a basal bifurcation of the precrista. Cusp 4 is smaller still, with both pre- and post-cristae bifurcating. The cristae of cusp 5 do not bifurcate, but have several struts and/or ribs on the buccal face alongside the fine apical strut linking the cusp to cusp 6. The cristae of cusp 6 are rather zig-zag because of the several struts linking them to cusp 5 and to the missing buccal face. The putative apical strut linking to cusp 7 is displaced anteriorly and there is a short simple precrista from the apex. Parallel to this is an even shorter precrista (half the length of that of cusp 7), which would arise from the buccal edge of the tooth.

The metaloph thus differs from that of M 2 in ways consistent with the posterior narrowing of the tooth and the molar gradient. It differs from that of E. tommosi in its greater relative and absolute width, and lesser buccal angular truncation, with consequent better development of the postcristae of cusps 5 to 7.

The maxilla of Chunia illuminata (QM F10641) (fig. 3A) suggests from its alveoli that there is a steep molar gradient in that species. The dentary of Ektopodon stirtoni (SAM P19509) indicates that the gradient is less in this species. The new specimens of M 3 described in this paper confirm that it is also less than in E. tommosi , although the alveoli preserved in maxilla P31637 are too damaged to enable tooth gradients to be determined with confidence.

The maxilla SAM P31637 is damaged on all sides. The root of the zygoma is split and the maxilla is broken anteriorly through the alveolus of M 1. The alveolus of M 3 is damaged buccally and that of M 4 is broken across, while the medial edge of the fragment is irregular with no part of the median suture being preserved. While the endocranial surface of the palatal wing of the maxilla is smooth and relatively flat (slightly concave anteriorly), the oral (ventral) surface is noticeable convex anteroposteriorly, rather as it is in Phascolarctos cinereus . This characteristic is not evident in Chunia illuminata where the palate is relatively flat.

Lower dentition. Dentary, SAM P29577 , (figs. 5G, H) is broken off just anterior to the position of M 1, which is missing. It preserves molars M 2-4 in good but more-worn condition than the holotype. The horizontal ramus is torted, and the toothrow lies at an angle of about 30° to this portion of the dentary .

M 2. In the holotype, this tooth is incomplete, having been reconstructed from numerous tiny fragments. Dentary SAM P29577 presents a complete but worn M 2, while SAM P24541 is a slightly worn hypolophid of an M 2 and SAM P31638 a perfect M 2 crown. The following description is based on the last, with additional comments on the others where appropriate .

The M 2 is somewhat rhomboidal in occlusal outline with protolophid and hypolophid having about the same transverse width. In P29577, both protoconid and hypoconid are extremely worn with the enamel breached and the dentine deeply excavated, but only a few of the adjacent cuspids have been breached. In contrast, P31638 is virtually unworn.

The protolophid in P31638 has seven cuspids (eight in P29577) with the protoconid being much larger than the others. Its crest is oblique (at about 80°) to the lingual face. The protoconid in P29577 is so deeply worn that it merges with the second cuspid and the two are difficult to distinguish. In P31638, as in the holotype SAM P19509, cuspid 2 is a single, simple plate closely appressed to the protoconid. On the protoconid, the anterobuccal groove is flanked buccally by a low crest and lingually by a parallel crest that seems (in P29577) to arise from the precristid of cuspid 2. The precristid curves buccally to merge with the remnant of the precingulid. Posteriorly the posterobuccal groove of the protoconid is flanked by a fine cristid arising from the postprotocristid. The postcristid of cuspid 2 has a short buccal rib. The enamel of cuspids 3–6 of P29577 is breached, leaving a thicker and higher ridge on the buccal side. Precristids of cuspids 3–5 are simple but divide slightly as they merge with the precingulid. Their postcristids expand slightly in the base of the transverse valley and each has a minor buccal rib. Cristids of cuspid 6 (and 7 in P29577) are simple. The innermost cuspid (7 of P31638 and 8 of P29577) has a bifid precristid with a basal cuspule (homologous with the larger structure in M 1 of the holotype) developed at the lingual corner of the precingulid.

The hypolophid parallels the protolophid. The hypoconid has deep anterobuccal and posterior grooves separating the bulbous buccal part from the rather sinusoidal cristid. The precristid curves buccally and the postcristid lingually to merge with the postcingulid. All six cuspids of P29577 have breached enamel. Precristid 2 has a weaker buccal rib and an expanded base; postcristid 2 is bifid at its base. On cuspid 3, the precristid is trifid with a weak buccal rib, a stronger lingual rib and the main crest expanded in the bottom of the transverse valley. The postcristid is bifid at its base. Cuspids 4 and 5 are similar, with simple undivided cristids. Cuspid 6 is complex with three parallel anterobuccal cristids which decrease rapidly in size lingually as they are truncated by a low, transverse cristid in the transverse valley. There are two parallel postcristids, the lingual one of which is shorter and cut off by the curving end of the postcingulid. The postcingulid is well developed.

The hypolophid (SAM P24541) has a transverse width (i.e. normal to the lingual face) of 6.4 mm. Characteristically for lower molars of Ektopodon spp. , the lophids are oblique to the tooth row with an acute anterolingual corner. There are seven cuspids, the inner two being combined. The hypoconid is large, its apex just buccal of the mid-line of the tooth. It has a deep anterobuccal groove that swings out basally and an almost longitudinal posterobuccal groove. The cristid obliqua curves buccally and divides basally. The posthypocristid is longitudinal. Cuspid 2 is on the midline of the tooth. Its pre- and postcristids parallel those of the hypoconid and give rise to shorter, basal supplementary cristids on the buccal side. A notched apical strut links the cuspids. Cuspid 3 is more complex with the precristid bifurcating and the postcristid trifurcating. Cuspids 4 and 5 are similar, simple cuspids, linked apically by fine struts. Their cristids do not divide. Cuspids 6–7 are complicated in being almost inseparable but having two diverging pre- and postcristids. The lingual-most precristid is short and notched to produce a basal cusp that extends as a “cingulum” along the transverse valley to the precristid of cuspid 4. All but the penultimate postcristid merge into the postcingulum.

This tooth fragment is similar to that of E. tommosi (NMV P48764) but its ornamentation is less developed. It is smaller than both that specimen and the holotype of E. stirtoni but larger than M

3

of E. stirtoni .

M 3. This tooth is incomplete in the holotype and poorly known in E. tommosi . It is now represented, however, by a complete (but worn) tooth in dentary SAM P29577 and by hypolophid SAM P23989 .

The M 3 resembles that of P29577 except for its lesser degree of wear. Only cuspids 1–3 of the protolophid and cuspids 1–5 of the hypolophid have been breached. There are eight protolophid cuspids and six or seven hypolophid cuspids. The anterolingual cuspule is smaller than in M 2 but slightly larger than in M 4. The tooth is smaller than M 2 and less rhomboid in outline, with the hypolophid narrower than the protolophid.

Hypolophid SAM P23989 is rectilinear and orientated obliquely at about 75–80° to the longitudinal axis of the tooth. The width of the hypolophid is 5.6 mm. There are seven cuspids. The buccal-most cuspid, the hypoconid, is at about one quarter the distance from the buccal end. It has a broad, shallow anterobuccal groove and a shallower posterior groove. The cristid obliqua is longitudinal but swings buccally at the base as it joins a basal strut from the second precristid. The posthypocristid curves lingually and joins the well-developed postcingulum. Cuspid 2 has simple undivided pre- and postcristids; the precristid of cuspid 3 divides halfway; and cuspids 4 and 5 have simple undivided cristae. Cuspid 6 lacks a postcristid but has a short precristid, parallel to that of cuspid 5, that divides basally. Cuspid 7 is displaced anteriorly and gives rise to two slightly diverging postcristids, the lingual one of which merges with the postcingulum. Its precristid is short and apparently joins the last postcristid of the protolophid.

This specimen preserves the posterior root, a long, tapering transverse fang-like structure, 9.8 mm long on the lingual side.

M 4. Three teeth represent M 4. Apart from its high degree of wear, which has reduced the crest angle of the protolophid to 140° or more and almost flattened the hypolophid, SAM P29577 from the dentary (fig. 5H) is virtually identical to the unworn SAM P23988 and P33451 (fig. 5I). Our description of this tooth will be based on the second specimen.

Previously, M 4 was only known from E. tommosi from Lake Tarkarooloo. The new specimens should, therefore, be compared with NMV P48766 ( Pledge, 1986, plate 3.2H), NMV P160517 and SAM P19966. SAM P23988 is less worn than the Tarkarooloo specimens and shows a similar occlusal outline although with more buccal constriction at the transverse valley. The lophids are low and broad, though possibly higher than in E. tommosi . The primary cuspids, the protoconid and hypoconid, stand markedly higher than their subordinate cuspids.

The protolophid is much wider than the hypolophid and its crest is orientated at about 60 o to the lingual face. The anterior edge of the tooth is convex and has a widening precingulid extending from the anterolingual corner, curving backwards, almost to the inner preprotocristid. The protolophid has seven cuspids, as in E. tommosi . The protoconid is situated about one third the distance from buccal end of the lophid. It is relatively high and lacks the obvious buccal grooves present on the protoconids of more anterior teeth. Instead, it has a strong anterobuccal preprotocristid and a subordinate lower cristid parallel to it on the lingual side. A pair of subparallel postprotocristae extends posterolingually to the transverse valley, with the lingual one having two buccally directed ribs.

Cuspid 2 is plate-like. Its precristid almost meets the precingulum at a cuspule but instead swings buccally and parallels it (as a rib) almost reaching the lesser preprotocristid. The postcristid bifurcates basally. Cuspid 3 is also plate-like, its precristid being simple and undivided and its postcristid only thickening at the posterior end. Cuspid 4 is a weaker irregular plate, thick anteriorly, with several minor ribs. It is shorter than cuspid 3 and larger than cuspid 5, which is otherwise similar although posterobuccally curved. Most cuspids are linked by fine, deep-set, apical struts, but on cuspid 6 these are as strong as the pre- and postcristids. The precristid is short and rather bulbous while the postcristid is finer and longer and curves buccally to join distally at the transverse valley the postcristids from cuspids 4 and 5. Cuspid 7 is low and on the extreme edge of the tooth. It is linked to cuspid 6 by a strut stronger than its pre- and postcristids. The precristid is short and merges into the precingulum. The postcristid is little more than a lingual cingulum with several strong basal ribs that increase in size towards the transverse valley. No postcristids actually cross the transverse valley.

The hypolophid is short with only five distinct cusps – fewer than in E. tommosi . The hypoconid is rounder than the protoconid and has a small anterobuccal groove that joins a larger posterobuccal groove on the protoconid to form a shallow pocket. There is no buccal cingulid. The cristid obliqua curves almost in a semicircle to a small basal cusp in the transverse valley, where it joins the precristid from cuspid 2. The posthypocristid curves lingually to join the short postcingulid. Cuspid 2 is a simple cristid with no discernable apex. Posterolingually, it joins the postcingulid at a small cuspule. Cuspid 3 is also a simple cristid bifurcating at the anterior end and not reaching the postcingulid. Cuspid 4 is irregular and links with a transverse cristid in the transverse valley. It is posteriorly short. Cuspid 5 is very low; its cristid is irregular and buccally curved. It converges with but does not meet ribs from the lingual cingulum and posteriorly parallels the postcingulid.

Morphologically SAM P23988, P29577 and P33451 are similar to the M 4 s of E. tommosi but the fine ornamentation of cristids, ribs and struts is simpler. The teeth are also marginally larger.

Remarks. Overall, these newly described teeth of E. stirtoni confirm the distinction of this taxon from E. tommosi from the Tarkarooloo LF and appear to represent a relatively derived species. This is in keeping with the perceived greater age of the Tarkarooloo LF (e.g. Woodburne et al., 1985).

Figures

Figure 3. Chunia spp. teeth: a-d. Chunia illuminata, Woodburne and Clemens, 1986. a. right maxilla. QM F10641, mirror-imaged to show angle of face, Tedford Locality; b. M1 SAM P29081, White Sands Basin; c. SAM P17997, (type) M2 Tedford Locality; d. M3 SAM P33944, Tedford Locality, Lake Palankarinna, Ditjimanka Local Fauna; e. Chunia omega Woodburne and Clemens, 1986, half of M3? (type) SAM P23065, Tom O’s Quarry, Lake Tarkarooloo, Tarkarooloo Local Fauna. Abbreviations: mel, metaconule; pastl, parastyloph; pr, protocone; prl, protoloph; 3, cusp 3; 5, cusp 5.

Figure 5. Ektopodon stirtoni Pledge, 1986, holotype dentary and new material. a-c. new maxilla (SAM P35309): a. lateral; b. dorsal; c. palatal; d. RM2 (P23854); e. LM3 (P30175); f. LM3 (SAM P30156); g. right dentary (SAM P29577), lateral aspect; h. right dentary with M (SAM 2-4 P29577) stereo pair; i, j. holotype right dentary with P, M (SAMP19509);k. LM (P31638);l. RM4 (SAM P33451). Ngama Quarry, Mammalon 3 1-3 3 Hill, Lake Palankarinna; Ngama Local Fauna.