The phylogeny of Macraucheniidae (Mammalia, Panperissodactyla, Litopterna) at the genus level

Macraucheniidae is one of the most diverse families of South American native ungulates. In the last decade, Macraucheniidae systematics has been the focus of several studies with the aim of enhancing understanding of the diversity and phylogenetic relationships of the group. This study proposes a new phylogenetic hypothesis for the Macraucheniidae based on new characters and new outgroups. We included eight non-macraucheniid taxa as outgroups (three Didolodontidae, one Protolipternidae, two Proterotheriidae, and two Adianthidae); additionally, we included the 16 genera that are currently accepted as belonging to Macraucheniidae based on other cladistic studies. The matrix used in the cladistic analysis comprised 24 taxa and 71 characters. The genus Polymorphis was recovered as the sister taxon of a clade composed of Proterotheriidae + Adianthidae + Macraucheniidae, not as a macraucheniid. Disregarding Polymorphis, all other genera attributed to the Macraucheniidae were recovered in a monophyletic group. We found support for two macraucheniid clades, one formed by Cramaucheniinae (including Theosodon) and the other by all Macraucheniinae with the inclusion of Llullataruca. The late Pliocene Windhausenia was recovered as the sister group of ‘Quaternary macraucheniids’, with Xenorhinotherium as the sister group of Macraucheniopsis + Macrauchenia. Based on this topology, Polymorphis must be considered as family incertae sedis. Our findings have significant implications for understanding the origin and diversification of Macraucheniidae, altering the temporal range of the family to the late Oligocene–Late Pleistocene/Holocene. Additionally, Llullataruca becomes the oldest recorded member of the Macraucheniinae. The evolution of the nasal retraction in the iconic species Macrauchenia patachonica has consistently been interpreted through the lens of the phyletic gradualism model, with Theosodon as the intermediate form between species with totally reduced or absent nasals, and those basal forms with well-developed nasals and terminal nostrils. However, the monophyly of Cramaucheniinae suggests a rapid morphological shift among Macraucheniinae, which fits with a punctuated equilibrium evolutionary model.