A new combined-evidence phylogeny of Toxodontidae (Mammalia, Notoungulata): exploring the radiation and reconstructing hypothetical ancestors.

Toxodontidae represents a major extinct radiation of South American native ungulates, but persistent polytomies in morphological phylogenies have masked its evolutionary history. To address this, we perform a new combined-evidence phylogenetic analysis. We integrate a revised and dependency-coded matrix of 54 discrete characters with 42 morphogeometric and 5 continuous characters, across a comprehensive sample of 37 taxa. This approach overcomes key logical limitations of previous discrete datasets and quantitatively captures shape variation. The analysis yields a fully resolved and well-supported topology for Toxodontidae, with Adinotherium, Proadinotherium and Nesodon forming a paraphyletic assemblage. We identify five major clades, several of which are strongly supported and formally define them based on their sister-group relationships: Xotodontinae is sister of Altiplaniinae, both are sister to a clade comprising Haplodontheriinae and Mixodontinae + Toxodontinae. Branch length decomposition shows the initial divergence and key internal nodes were dominated by discrete character change, while later radiations were driven by morphogeometric change, indicating diversification through shape reorganization. Ancestral reconstructions reveal a shift from a generalized morphotype to specialized forms with euhypsodonty, incisor hypertrophy and larger dental dimensions. This robust phylogenetic framework resolves the long-standing impasse within Toxodontidae phylogeny and provides a foundation for testing evolutionary and biogeographic hypotheses.