Ektopodon tommosi, Pledge, 2016

Pledge, Neville S., 2016, New specimens of ektopodontids (Marsupialia: Ektopodontidae) from South Australia, Memoirs of Museum Victoria 74, pp. 173-187 : 177-181

Ektopodon tommosi sp. nov.

Ektopodon sp. cf. E. stirtoni Pledge 1986 , pls 3.1D, F, G; 3.2;

3.3D-E (fig. 4).

Holotype. SAM P19962 ( RM 1 ).

Paratypes. NMV P48750-48751 (LM 1) , SAM P19963 ( RM 2 ) , SAM P19950 ( RM 1 ) , NMV P48752 (LM 2) , NMV P48753 (LM 3) , NMV P48757 (LM 4) , NMV P48764 (LM 2) , NMV P48765 ( RM 3 ) , NMV P48766 ( RM

4

).

Referred specimens. NMV P48758 (right lower incisor), NMV P48769 (partial LP 3), SAM P19965 ( RP

3

).

Locality. Tom O’s Quarry, western shore of Lake Tarkarooloo, Callabonna Basin. 31°8.5'S ., 140°6.3'E .

Horizon. A sandy channel deposit within the Namba Formation ( Callen & Tedford, 1976).

Age. Late Oligocene, Tarkarooloo Local Fauna. Biocorrelation suggests this is faunistically equivalent to the Ngapakaldi Local Fauna of the Lake Eyre Basin ( Rich and Rich, 1987), which is a little older than the latest Oligocene Ngama L.F. of Lake Palankarinna, but younger than the Pinpa L.F. from Lake Pinpa.

Diagnosis. The molar teeth are generally 5–10% smaller than comparable elements of E. stirtoni , and the loph(id)s generally have one less cusp. The protostyloph on M 1 is shorter and less loph-like with two cusps, one less than in E. stirtoni . The P 3 is larger than that tooth in E. stirtoni . The mandible is slightly larger than in other species, with a longer diastema.

Etymology. The species name reflects the source of these specimens, Tom O’s Quarry site, (discovered by Cpl. John Thompson – ‘Tom O’ to distinguish him from Tom Rich – of 3 rd RAAME which provided logistic support for Rich’s 1974 expedition; Rich and Archer, 1979), at Lake Tarkarooloo.

Description. See Pledge (1986: 53–60) for a more-complete description. The following is abbreviated and has updated terminology.

P 3. There are still only two fragments known; the incomplete specimen ( NMV 48769) referred to this taxon by Pledge (1986) is correctly ascribed. It agrees morphologically with the P 3 of E. stirtoni (see below) as far as can be compared, but is noticeably larger (length>5.6, width 4.1 vs. length 5.5, width 3.4). The curving longitudinal crest extends from a conspicuous anterior cusp at the anterolingual corner of the tooth. At the midway point, a deep angular saddle divides the crest, separating the anterior cusp from two closely-linked larger posterior cusps. The latter are separated by a deep narrow crevice. The crest ends at the posterolingual corner of the tooth. Its lingual face has a weak basal cingulum with two small cusps.

M 1 (fig. 4A). Only three fragmentary teeth have been found, the holotype, SAM P19962 being one of them. The latter is quite worn and lacks the lingual margin, but compares well with P22504, the M1 of E. stirtoni from Mammalon Hill. However, it differs, apparently, in having one less cusp on each loph and a shorter protostyloph with only two well-developed cusps and the trace of an incipiently-developed third cusp.

M 2 (figs. 4A, C). This molar is broad, roughly trapezoidal, and the largest of the upper molars having only two lophs. The protoloph, with eight cusps, is slightly longer than the metaloph, which has seven, and the crests of the lophs slightly twisted rather than being in the same plane. The crests are also not as sharp as those structures on M 1, nor the transverse valley as deep. On the protoloph, the protocone bears a pair of deep grooves on its anterior and posterior sides, but the resulting ridges do not bifurcate. Similarly, the cristae of the second protoloph cusp do not bifurcate, but those of the 3 rd, 4 th, 5 th and 6 th cusps do (the last only on the posterior side). Similarly on the metaloph, the metaconule has a pair of deep grooves, and the 2 nd, 3 rd, 4 th and 5 th cristae bifurcate, the last only on the anterior side. The posterior groove of the protocone joins with the anterior one on the metaconule to form a deep buccal pocket. All cusps on a loph are joined by a fine, deep-set apical ‘strut’ along the axial plane of the loph. All cristae are cut off by the pre- and post-cingula.

M 3 (figs. 4A, B). This tooth is of similar length to M 2 but is noticeably narrower, and therefore trapezoidal in outline. The protoloph has eight cusps and extends beyond the metaloph at each end. The metaloph probably has seven cusps, but the count is uncertain because of the difficulty in distinguishing between primary cristae and bifurcations. Fine linking struts between cristae increase in number buccally to at least four at the paracone.

M 4 (fig. 4A). This tooth appears to be a stunted version of M 3. It is triangular, and bears only the protoloph which is of low relief and has seven or eight cusps (number uncertain because of the irregular bifurcation of the cristae). The missing metaloph is replaced by the third corner of the triangle: an inchoate network of low crests. The metaconule appears to have merged into the protocone, and that combination presents as a square extension of the protoloph.

Dentaries. Three dentaries are known, one retaining a fragment of the hypolophid of M

3

; the others may be a pair. They are relatively massive, are slightly larger than those of E. stirtoni . Compared with the latter, they have a more convex ventral profile, andalongerdiastema, butthealveolarcheek-toothlength is shorter. A minute alveolus (for a canine?) immediately follows that of the incisor on its dorso-lateral corner.

I 1 (fig. 4D). An isolated right incisor is referred to this position, based on size and its semicircular cross-section to fit the alveolus. It is short, high and somewhat spatulate, but it cannot be proven to relate to this species.

P 3. This tooth is represented by three specimens. It is smaller than the P 3 of E. stirtoni , and more subdued in its features, but otherwise similar. It is ovate in outline, and has on the lingual side a longitudinal cristid that bears four cuspids. An isolated anterior cuspid is followed by a central slightly larger one, then a third even larger one to which is appressed a smaller fourth cuspid. A posterobuccal cingulum forms a small pocket.

M 1 ( Fig. 4E ). SAM P19950 is the only complete specimen of this tooth known. It can be recognised by its distinctive ‘parastylid’. The tooth is roughly rhomboid in outline and wider than long. The ‘parastylid’, at the anterolingual corner of the tooth, is more prominent than that structure in E. stirtoni . The lingual face of the tooth is relatively flat; the buccal ends of the lophids are quite swollen. Lophid crests are sharp and parallel to themselves, but not to the anterior and posterior faces of the tooth. The six protolophid cuspids are not as well-graded as in E. stirtoni . The two most buccal are very closely appressed, while the other four are well-spaced. The same pattern of distribution of cuspids characterizes the hypolophid. The precristids of cuspids 2–4 on the protolophid bifurcate. The same occurs for cuspids 1–3 on the hypolophid.

M 2. This tooth is relatively longer than M 1 and has a more open transverse valley. Each lophid has seven cuspids, the inner-most two being combined. On the protolophid, only the precristid of the protoconid divides, and its lingual branch divides again. On the hypolophid, the hypoconid and cuspid 2 divide on both sides. Although this occurs on cuspid 3, it does so only on the precristid. The precristid of the metaconid has a small notch, as it does in M 1, but it does not develop a ‘parastylid’ at the precingulum.

M 3. This tooth is not represented by a complete specimen. A fragment of hypolophid is preserved in the dentary NMV P48767, Although another M 3, NMV P48765, is more complete, it lacks the protoconid and hypoconid. Based on what is preserved, these specimens show a morphology similar to that of E. stirtoni , with low lophids and a wide transverse valley. On both lophids, the postcristids of cuspids 2 and 3 divide, and on the hypolophid, the precristids also. The lingual part of the crown develops into a network of fine cristids and struts, which differs in the extent of this network development from that in the M

3

of E. stirtoni .

M 4. Despite their superficially different appearances, two specimens, SAM P19966 and NMV P48766 , have been identified as M 4 s. The former is incomplete and slightly more worn than the other; both are from right dentaries. They are roughly triangular as a result of reduction of the entoconid, and are low-crowned with very low, broad lophids. There are six to seven cuspids on each lophid, the number uncertain because of similarities and irregularities of cristids and ribs. The transverse valley is an irregular network of anastomosing cristids and ribs. These teeth are elongate (length:width ratio about 1.14). NMV P48766 fits the alveolus of jaw NMV P48767 .

Figures

Figure 4. Dentition of Ektopodon tommosi nom. nov. a. composite upper dentition, anterior at left: LM1 fragment (NMV P48750), partial RM1 (SAM P19962), LM2 (NMV P48752), RM2 (SAM P19963), LM3 (NMV P48753), LM4 (NMV P48757); b. LM3 (NMV P48753); c. RM2 (SAM P19963); d. RI 1 (NMV P48758); e. RM 1 (SAM P19950. Tom O’s Quarry, Lake Tarkarooloo; Tarkarooloo Local Fauna. Abbreviations: mel, metaconule; pastd, parastylid; pastl, parastyloph; pr, protocone.