Chunia illuminata Woodburne & Clemens, 1986
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publication ID |
https://doi.org/ 10.24199/j.mmv.2016.74.15 |
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persistent identifier |
https://treatment.plazi.org/id/03A10712-FFE0-FF81-FCC5-FECDFB4FE2B7 |
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treatment provided by |
Felipe |
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scientific name |
Chunia illuminata Woodburne & Clemens, 1986 |
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Chunia illuminata Woodburne & Clemens, 1986
( Figs. 3A–D View Figure 3 )
Holotype. SAM P17997 View Materials . In their original description of Chunia illuminata, Woodburne & Clemens (1986b) described the RM 2 holotype SAM P17997 View Materials (= RM 3 in their notation following Archer 1978), together with QM F10641 (maxilla fragment with partial LM 2), UCR 15228 (LM
2
) and UCR 15227 (LM
2
).
New Specimens. SAM P29081 View Materials , left M 1 (figs. 3A–D) collected by J. McNamara 9 July 1987; also SAM P33944 View Materials , left M 3, collected by J. Case and J. Clemitson, June 1992, from Tedford Locality .
Locality. White Sands Basin ( SAM locality PL 7719 ), 200 m south of Tedford Locality ( UCMP V5375 , the Type Locality for the species) .
Stratigraphy and Age. Stratigraphically, White Sands Basin is about one metre below the sandy clay layer in Tedford Locality that produced the holotype. This fauna from the Etadunna Formation is considered to be Late Oligocene in age (see above).
Revision of specific diagnosis. In addition to the features noted by Woodburne & Clemens (1986b), including the upper dental formula I?, C 1, P 1-3, M 1-4, the M 1 of Chunia illuminata differs from that tooth in all other ektopodontids in being smaller, having fewer cusps on its lophs and in having a parastyloph that is less loph-like and more cusp-like than that structure in any known species of Ektopodon . The face is ‘longer’ and less obtuse than that of Ektopodon stirtoni .
Description. The M 1 of Chunia illuminata has the same basic outline as that tooth in other species of Ektopodon but differs in several ways noted below. The cusps on the ‘protoloph’ and ‘metaloph’ are all worn apically, so some detail has been obscured.
The parastyloph (“paraloph” of Pledge, 1986) is simpler than that structure in other species, being an oblique blade confluent with the buccal face of the tooth and having three cusps. The minute lingual cusp is offset posteriorly from the end of the loph and gives rise to a pair of postcristae that initially diverge, then converge slightly linguad. A weak precrista descends basally from the point of inflection of the loph. The central cusp is by far the largest of the three but, apart from the loph crest, gives rise only to a short postcrista at right angles to it. However, a strong postcrista arises from near the buccal end of the cusp and continues transversely to almost meet the converging postcristae from the lingual cusp. The buccal cusp is separated from the median cusp by a narrow crevice. There are two postcristae, a buccal one forming part of the “parastyloph” and a lingual one that curves transversely and extends half the width of the loph.
The protoloph has four distinct cusps and a complex structure at the buccal end that could represent either two or three smaller cusps. The lingual cusp, the protocone, is a trigonal pyramid with the precrista being stronger than either the postcrista or the lingual extension of the loph crest. Cusp 2 is the smallest with a short precrista cut off by converging precristae from the protocone and cusp 3. Its postcrista is the simplest (after that of the protocone) and extends to the transverse valley; a weak lingual spur arises about halfway along its length and the distal (posterior) end curves lingually to parallel the transverse valley. Cusps 3 and 4 are basically similar, each being large and having a pair of subparallel precristae and postcristae that are angled slightly linguad. Cusp 3, however, also has a third slightly sinuous precrista, two weak ribs on the buccal face of the outermost precrista and a bifurcation of the buccal postcrista. Cusp 5 shows two strong postcristae reaching the transverse valley and a shorter, bifurcating buccal postcrista. Anteriorly, two sinuous precristae are linked by two or more near-apical struts and a basal strut. The lingual precrista bifurcates just below a strut linking it with the buccal precrista of cusp 4. All cusps (1–5) are linked apically by a fine transverse strut. Cusp 6 is displaced posteriorly and is a small trigonal structure with pre-, post- and transverse cristae.
The transverse valley is deep, trenchant, slightly curved and anteriorly convex. No structures cross it except at the lingual end where there is a notched cingulum. Buccal to this are three weak irregular postcristae at the base of the protocone and four irregular precristae on the metaconule, none of which cross the valley. The lingual cingulum curves and extends up the lingual face of the metaconule (i.e. the posterolingual cusp of diprotodontians previously called the hypocone; see Tedford & Woodburne, 1987).
The metaconule is stronger and more bulbous than the protocone. Besides the irregular basal precristae, there are two short postcristae that form a small pocket on the posterior face. The main precrista is aligned with the main postcrista and with the postcrista of cusp 2 of the protoloph. Cusp 2 of the metaloph is similar in size to cusp 3 of the protoloph. Besides the strut linking it to the metaconule, both of the pre- and postcristae bear several transverse ribs on the lingual face of the lingual cristae and the buccal cristae bifurcate basally. Cusp 3 is similar but with fewer and weaker ribs; only its buccal postcrista bifurcates. However, the apex of the cusp appears to be a triangle of short crests with its base aligned with the buccal cristae. Buccally from here, the structure is unclear. Cusp 4 appears to be a relatively simple structure with a single precrista and a postcrista bearing several weak lingual ribs. Near its apex, however, a short crest leaves posterobuccally to join one coming anterolingually from another, posteriorly offset cusp. A short postcrista arises at the junction of these crests but does not reach the posterior cingulum. This posterior cusp also bears four other radiating crests, two being aligned longitudinally and the others antero- and posterobuccally. Buccal to cusp 4 are three or four weak cusps defined by four simple precristae alternating with three short postcristae that do not meet the cristae from the posterior cusp.At the base of these precristae a low transverse crest parallels the transverse valley. A distinct postcingulum extends from the buccal-most postcrista to the metaconule. Pledge (1986) attempted to equate these buccal structures (in species of Ektopodon ) with the cusp and crest patterns common in diprotodontians but it now appears, even in this relatively plesiomorphic species, that the homology of these cusps in ektopodontids is unclear.
M 3. The new specimen found by Case and Clemitson SAM P33944 View Materials is almost identical to, but the mirror image of, the holotype SAM P17977 View Materials , and paratype SAM P22722 M 2 View Materials s from the same locality. It differs in being slightly longer and narrower, with slightly more prominent equiradial development of crests and struts, and is thus accorded here a more posterior position. This tooth bears some resemblance to the incomplete type specimen of Chunia omega (fig. 3E) from the Tarkarooloo Local Fauna (Woodburne and Clemens 1986) .
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