Carodnia karuen, Gelfo & López & Bond, 2024

Carodnia karuen sp. nov.

Figure 6 View FIGURE 6 A-D zoobank.org/ 2705DBC8-B740-4FAD-9859-76DD6357CC5A

Holotype. MLP-PV 90 -II-12-121: right jaw fragment somewhat weathered, with m1 nearly complete and p4 with the basal portion and roots partially preserved.

Etymology. The species name karuen means “old” or “ancient” in the extinct language of the Aónikenk people, who inhabited Patagonia before the formation of the nation-states of Chile and Argentina.

Geographic and stratigraphic provenance. Lower fossil level at Cerro Redondo, east of the Chubut province, Argentina (see Simpson, 1935a). Río Chico Group, Peñas Coloradas Formation, lower Paleocene sensu Clyde et al. (2014). Found

GELFO, LÓPEZ, & BOND: SOUTH AMERICAN XENUNGULATA by J.L. Prado in a Museo de La Plata field trip directed by Rosendo Pascual in 1991.

Diagnosis. Medium-size xenungulate, smaller than Carodnia vieirai and larger than Etayoa bacatensis , and somewhat larger than C. feruglioi . Protolophid of m1 with a strong metaconid and a bit smaller protoconid. Trigonid basin not open lingually but is enclosed by large and well-defined paracristid and metacristid, which are closely pressed against the mesial side of the protolophid in the m1. Precingulid strong and forms a conspicuous shelf as in C. inexpectans but differs from it for being higher and better developed on the lingual side. Hypolophid lower than protolophid, with a strong hypoconid and entoconid. Unlike all other Carodnia species, in which the cristid obliqua is absent in the first molar, C. karuen has a short cristid obliqua extending from the hypoconid up to the base of the distal wall of the trigonid, closer to the lingual side of the protoconid. Also differs from Carodnia feruglioi and C. vieirai for a short entocristid similar to those present in the m3 of C. inexpectans but differing from it in being higher, and almost at the same height as the hypolophid. Broad talonid basin, open lingually.

Description and comparisons. The mesiodistal length of m1 measures 18.21 mm, while the labiolingual dimension is 13.49 mm. The jaw presents two loci preserved, the mesial one with the roots of and part of the enamel of the tooth at the neck is here interpreted as a p4 ( Figure 6 View FIGURE 6 ). The distal one is an m1 with a nearly complete crown structure. The loci assignation rests in the preserved structure of this last tooth, which matches with those present in m1–2 of all Carodnia species. In addition, the roots of the preceding tooth here interpreted as a p4, preserve a distolingual enamel portion of the taloned, which is below the level of the precingulid of the succeeding tooth. This suggests that the more mesial dental piece erupted after the distal one, which agrees with the interpretation of a p4 and m1 for these teeth, following the usual order of eruption in most placental mammals. The lower portion of the jaw is broken, the ramus is robust, high, and at least three times the height of the m1 crown. The lingual side of the dentary is convex, and the labial side is something concave. In contrast to the known jaws of C. feruglioi (UNPSJB-PV 680-1, MPEF-PV 564, and MPEF-PV 8165) there is no foramen in the labial side of the dentary below the mesial root of m1. The p4 is only preserved by a distolingual portion of enamel against the mesial root of the m1, a short piece of dentine at the talonid, and the mesial and distal roots. The roots are mesiodistally compressed and broken on the labial side. The mesial fracture of the jaw exposes the trigonid root of p4, which in frontal view is deep about the high of the preserved dentary.

The m1 is well preserved, with the trigonid somewhat higher, wider, and longer than the talonid. This tooth shows an almost labiolingually oriented bilophodont structure characteristic of Carodniidae . At the mesiolingual side of the trigonid, the enamel is broken and absent from the base of the crown exposing the dentine in the metaconid position. A strong precingulid is sloping down to the labial side and despite being broken, it seems that is not expanded over the labial and lingual side of the tooth. The protolophid like those of the m1 of Carodnia vieirai , is mesially concave in occlusal view, with the cutting plane of the lophid parallel to the one of the hypolophid. The wear facets prld-d and hyld-d are equally developed. The metaconid can be distinguished from the protolophid by a strong enamel edge that bends at the lingual tip of the lophid. The protoconid is not so clearly observed from its occlusal outline. In contrast to C. vieirai or the m 2–3 in C. feruglioi and C. inexpectans where no cristids are developed mesial to the protolophid, C. karuen has a conspicuous paracristid and metacristid. The paracristid abruptly descends mesiolingually from the labial end of the protolophid, and progressively bends lingually, to the mesial base of the trigonid. The short metacristid projects mesiolabially from the metaconid almost reaching the lingual end of the paracristid. Mesially the paracristid and metacristid are close together but their contact, closing the trigonid, is inferred since a portion of enamel is missing. At the mesial base of the tooth and below most of the lower part of the paracristid, there is a remnant of a strong precingulid, which leans from the lingual side of the trigonid to a more basal position in the labial side.

The hypolophid is the principal structure of the talonid and with a similar development to other Carodnia species. In the hypoconid sector of the hypolophid, a smooth cristid obliqua descends up to the base of the distal wall of the trigonid more approximately to the protoconid area. The cristid obliqua is smooth and runs mesiolingually, so the short talonid basin, which is open lingually, is also closed labially by it. The entoconid could be identified in the hypolophid by an enlargement at the lingual end, like the one observed in the metaconid but ovoid in outline rather than circular, and with thinner enamel. A very short entocristid runs mesiolingually from the entoconid. The entocristid delimited a short but high lingual edge of the talonid, which in contrast to the protolophid and the hypolophid lacked any wear. Behind the hypolophid base, a short portion of a strong and flat postcingulid is preserved. Even though the cingulid is far from being complete, it appears to have formed a long ledge or step in the distal part of the molar. The anterior root of m1 is stronger than the posterior one, with both roots being mesiodistally compressed and swollen.

There are certain similarities between C. karuen and in general, with the m1 of Carodnia species, and the first molar of the Pyrotheria Carolozittelia tapiroides Ameghino, 1901 (MACN A-10666) and Carolozittelia cf. C. tapiroides (MACN 17985) described by Kraglievich (1957). Both exhibit a bilophodont pattern with a slightly larger protolophid than the hypolophid (usually described in them as “metalophid” and “entolophid”, respectively), the wear facets prld-d and hypld-d, and the presence of enamel crenulations. However, significant differences can be highlighted, particularly with C. karuen , such as the absence in C. tapiroides of a metacristid resulting in a lingually open trigonid, a more robust paracristid in the region of the protoconid, the absence of an oblique cristid, a hypolophid of nearly equal height to the protolophid, and a greatly expanded labial postcingulid forming a large distal ledge. These differences, plus the structure of the third lobe in the m3 of C. tapiroides , set it apart from the typical xenungulate dental pattern of m3 present in Carodniidae and Etayoidae . However, considering a possible phylogenetic link between Xenungulata and Pyrotheria , as explored by some studies (Gelfo et al., 2008; Muizon et al., 2015), C. tapiroides exhibits the highest number of shared dental features among them.