Argentodites coloniensis, Kielan-Jaworowska & Ortiz-Jaureguizar & Vieytes & Pascual & Goin, 2007

Kielan-Jaworowska, Zofia, Ortiz-Jaureguizar, Edgardo, Vieytes, Carolina, Pascual, Rosendo & Goin, Francisco J., 2007, First? cimolodontan multituberculate mammal from South America, Acta Palaeontologica Polonica 52 (2), pp. 257-262 : 257-260

Argentodites coloniensis sp. nov.

Derivation of the name: After the Upper Cretaceous La Colonia Formation, in which the holotype specimen was found .

Holotype and the only known specimen: MPEF 604 , nearly complete?left p4, without roots.

Type horizon and locality: Middle part of the La Colonia Formation (Upper Cretaceous:?Campanian or Maastrichtian), Arroyo Mirasol Chico Bajada Moreno area, Chubut Province, Patagonia, Argentina ( Fig. 1 ).

Measurements.—The maximum length of the tooth, measured on the lingual side, is 4.15 mm, maximum height on the same side measured above the ventral, medial incurvature of the enamel between the bases of the roots is 2.10 mm, and maximum width in occlusal view (measured in the anterior half of the tooth) is 1.35 mm.

Diagnosis.—The strongly convex anterior margin of the tooth and the normal prismatic enamel with average prism diameter of 3.8 ̊m suggest affinities of Argentodites coloniensis to Cimolodonta. Shares presence of the normal prismatic enamel with members of the Ptilodontoidea, but differs from Laurasian Cimolodonta (including Ptilodontoidea) in having a relatively long and straight posterior margin, and nearly straight dorsal margin (if placed in presumed life position), both features characteristic of some “plagiaulacidans” (i.e., Paulchoffatiidae , Plagiaulacidae , and Allodontidae ). Differs from Laurasian cimolodontans with large p4s in having labial side built as nearly a mirror image of the lingual side. Differs from Paracimexomys (the only member of the Paracimexomys group in which the enamel microstructure is known) in having normal prismatic, rather than gigantoprismatic enamel.

Life position of the?left p4.—The posterior lower premolar (p4) in “plagiaulacidans” (with exception of Arginbaataridae ) is subrectangular, with long anterior and posterior margins directed roughly vertically; in most cimolodontans, in contrast, the p4 is arcuate with convex anterior and posterior margins ( Kielan−Jaworowska et al. 2004; Hahn and Hahn 2006a). Argentodites is morphologically intermediate between these two patterns, as it has a long, straight posterior margin, but a strongly convex anterior margin. If one places the studied p4 with its posterior margin only slightly turned counterclockwise from a completely vertical position ( Fig. 2B ) the upper margin of the tooth will appear nearly straight, as characteristic of “Plagiaulacida”. We believe that this might have been the life position of the studied p4, as in such a position the middle margins of the roots are symmetrically arranged.

In Fig. 2C , the same tooth is figured from the other side, with straight posterior margin placed obliquely, toward the middle of the tooth. However, in no cimolodontan p4 is the posterior margin as long as in Argentodites . In Fig. 2C the dorsal margin is convex upward, as in cimolodontans with large p4s. We believe that the life position in Fig. 2B is more probable than the one in Fig. 2C .

As the lingual side of Argentodites p4 is nearly a mirror image of the labial side, it is difficult to demonstrate unequivocally whether the studied tooth is the right or the left one. We tentatively believe that the side illustrated in Fig. 2B is labial, as it is less convex than the opposite side ( Fig. 2C ), as characteristic of cimolodontans with large p4s. In the upper right corner of the labial side ( Fig. 2B ) is the postero−labial ridge (or cusp). However, on the lingual side of the tooth ( Fig. 2C ) there is also a ridge, almost identical to that which occurs on the labial side ( Fig. 2B ). We designate the ridge seen in Fig. 2C the postero−lingual ridge. In spite of this ambiguity and unknown function of the postero−lingual ridge, we identify the more convex side of the tooth as the lingual, and the tooth as the?left p4.

Description.—The crown of the tooth ( Fig. 2 ) is almost complete, covered with well preserved enamel, which is apparently missing from a large area of the ventral part of the lingual side and from the small antero−ventral strip over the base of the anterior root, on the labial side. The preserved part of the crown shows that the anterior root was more robust (wider) than the posterior one, as is typical for cimolodontan (but not “plagiaulacidan”) p4s. The enamel−covered anterior triangular lobe, which in most multituberculates extends over the base of the anterior root on the labial side, is not recognizable, as the root has not been preserved and the crown of the tooth is broken dorsal to it. The crown is also broken over the posterior root.

At the presumed life position of the tooth ( Fig. 2B ), the dorsal margin is nearly straight; the posterior margin is long and straight, directed downward (and insignificantly medially). At this position the anterior margin is strongly convex anteriorly. The dorsal margin is well preserved, covered with eight apical cusps (serrations), the first seven of which are sharp, indicating a young age for the individual; only the eighth cusp bears insignificant wear. The first cusp is situated at about one third of the tooth length from the front. All the cusps on both sides are provided with ridges, which are more prominent on the labial than on the lingual side. The first seven labial ridges connect to their lingual counterparts at the very edge of the crown. The labial and lingual 8 th ridges do not reach the 8 th serration and are not connected at the top. The ridges on labial and lingual sides are equally prominent and of comparable length. The apparent differences in prominence of the ridges and their length seen in Figs. 2B and 2C 2 are caused by different lighting. The first labial (and lingual) ridge extends parallel to the convex anterior margin of the tooth. On the labial side to the rear of the last cusp, there is a very short postero−labial ridge, which reaches the posterior margin of the tooth, below the 8 th serration. An identical ridge (postero−lingual) is present on the lingual side of the tooth ( Fig. 2C ), a feature which does not occur in Laurasian cimolodontans.

Below the postero−lingual ridge there is a tiny ridge, concave upward, hardly discernible ( Fig. 2C 1 ). We have not discerned such a ridge on the labial side. At the bottom of the preserved lingual side of the crown, there is a structure which could be a poorly preserved row of tiny?cusps ( Fig. 2C 1 ), but which we regard as an artifact.

Figures

Fig. 1. Position of South America and adjacent lands, 85–63 million years ago, showing location of the formations that yielded Late Cretaceous multituberculates. Black arrows indicate migrations of vertebrates between North and South Americas. Note the division of South America by sea into South Gondwanan Province (light grey) and North Gondwana Province (square hatching). Modified after Pascual (2006).

Fig. 2. SEM micrographs of?left p4 of Argentodites coloniensis gen. et sp. nov., MPEF 604 (Upper Cretaceous La Colonia Formation, Chubut Province, Argentina) in different views. A. Dorsal view. Stereo−photograph (A1) and explanatory drawing of the same (A2). B. Labial view of the same, figured in a presumable life position, with posterior margin arranged sub−vertically. C. Lingual view of the same, with posterior margin turned counterclockwise from presumable life position. Explanatory drawing (C1) and stereo−photograph (C2). D. Postero−dorsal view of the same. Stereo−photograph (D1), and explanatory drawing (D2).