Phylogenetic relationships and palaeobiology of a new xenungulate (Mammalia: Eutheria) from the Palaeogene of Argentina

Xenungulates are a peculiar group of bilophodont ungulates, widespread over South America during the Paleocene and early Eocene. Here we describe, illustrate and analyse a new Xenungulata, Rodcania kakan gen. et sp. nov., the first from the lower latitudes of Argentina. It was recovered from the Río Loro Formation (Tucumán Province), for which a Paleocene age was estimated. The material comprises a fragment of the left dentary with an incomplete m2, eroded over the enamel surface, and an almost complete m3. Both teeth share with Xenungulata strong enamel, vertically oriented Hunter–Schreger bands (HSB), and wear facets sloping distally. Rodcania kakan differs from other xenungulates in the m3 trigonid, simplified and mesiodistally short; absence of a paraconid; more oblique protolophid in relation to the mesiodistal axis and a straight cristid obliqua, pointing to the protoconid position; and has a wider and longer talonid. A phylogenetic analysis of 13 species and 36 craniodental characters recovered R. kakan as a basal member of the Carodniidae, supported by 10 synapomorphies in the four most parsimonious trees. The body mass estimation of the Río Loro Fauna and of the rest of the xenungulate taxa revealed that R. kakan is the largest mammal of the Río Loro assemblage, weighing around 165 kg and with a size comparable to Carodnia feruglioi. The increase in body mass showed no clear relation with evolutionary trends in xenungulates. http://zoobank.org/urn:lsid:zoobank.org:pub:FFCB8A36-1CE1-4D5F-90D7-A5D773CF121B

滑距骨目（Xenungulata）是一类独特的双脊齿有蹄类，在古新世至始新世早期广泛分布于南美洲。本文记述、图示并分析了一种新的滑距骨目类群——Rodcania kakan 新属新种（gen. et sp. nov.），这是阿根廷低纬度区域发现的首个滑距骨目物种。该标本采自图库曼省罗罗河组（Río Loro Formation），其地质年代被推测为古新世。标本包含一件带有不完整第二臼齿（m2）的左侧齿骨碎片，牙釉质表面存在侵蚀痕迹，以及一枚近乎完整的第三臼齿（m3）。两枚牙齿均具备滑距骨目的典型特征：坚固的牙釉质、垂直取向的亨特-施雷格纹（Hunter–Schreger bands, HSB），以及向远中倾斜的咬合磨耗面。Rodcania kakan 与其他滑距骨类群的区别特征如下：其m3的下三角座结构简化且近中远中向较短；无下前尖（paraconid）；相对于近中远中轴，下原脊（protolophid）倾斜程度更高；具有一条平直的斜脊（cristid obliqua），指向下原尖位置；且下跟座（talonid）更宽更长。基于13个物种与36个颅齿特征的系统发育分析结果显示，R. kakan 为卡罗尼科（Carodniidae）的基部类群，该结论得到4棵最简约树中10个共有衍征的支持。对罗罗河动物群及其余滑距骨类群的体重估算表明，R. kakan 是罗罗河组合中体型最大的哺乳动物，体重约165千克，体型大小可与Carodnia feruglioi相匹敌。滑距骨类群的体重增长并未呈现明确的演化趋势。http://zoobank.org/urn:lsid:zoobank.org:pub:FFCB8A36-1CE1-4D5F-90D7-A5D773CF121B