| Description |
Late Cretaceous ankylosaurids from western North America (Laramidia) are a clade of armored, ornithischian dinosaurs of which fossils are generally rare and known taxonomic diversity is low. Previous phylogenetic studies enhanced the understanding of evolutionary relationships amongst the Ankylosauridae, but their evolutionary tempo and mode, as well as their biogeographic relationships, have been largely ignored and remain poorly understood. Two new ankylosaurid taxa (UMNH VP 21000 and UMNH VP 20202) from the upper Campanian Kaiparowits Formation of southern Utah contribute significantly to elucidating the phylogenetic and biogeographic history of the clade. The two newly described taxa were utilized in a phylogenetic revision of the clade Ankylosauridae. Resulting phylogenies were ‘smoothly' and ‘strictly' temporally calibrated and used to carry out a quantitative biogeographic analysis, estimating ancestral range and dispersal events for Asian and Laramidian ankylosaurid dinosaurs. Geochronologic, biostratigraphic, and sequence stratigraphic data (Cretaceous sea level curves) for North America and Asia allowed me to determine whether the inferred timing of diversification of Laramidian ankylosaurids and their dispersal from Asia, as reconstructed from the biogeographic analysis, were consistent with the timing of sea level changes. I conclude that UMNH VP 21000 represents a new species of Anodontosaurus and forms a clade with its chronostratigraphically younger sister taxon A. lambei, suggesting a Laramidian origin that excludes a close evolutionary relationship with Asian taxa. Specimen UMNH VP 20202 represents a new genus and species of ankylosaurid, forming a clade with its southern Laramidian sister taxon Nodocephalosaurus kirtlandensis, and is deeply nested with other Asian taxa, supporting an Asian origin for the clade. Laramidian ankylosaurids derived from multiple Asian lineages during two intercontinental dispersal events as a result of falling stage sea levels within the lower and upper Campanian. Initial dispersal occurred around 83 Ma, followed shortly by cladogenesis-driven vicariance around 82 Ma during low stand sea levels, resulting in a Laramidian lineage ancestral to the clade UMNH VP 20202 + Nodocephalosaurus kirtlandensis. A second dispersal event occurred around 77 Ma, resulting in a Laramidian lineage ancestral to the clade UMNH VP 21000 + Anodontosaurus lambei, and all other western North American ankylosaurids. |
