Interrelationships of Mesozoic mammals

Abstract

Polyphyletic or monophyletic origins of mammals have been subjects of considerable controversy for a century. Late Triassic—Early Jurassic mammals are more diverse than previously thought. The stumbling block in establishing mono‐ or polyphyletic origins of mammals is the uncertain position of the Haramiyidae (the oldest Multituberculata, and possibly the oldest mammals), known only from isolated teeth. Triconodonta and Theria probably shared an unknown common ancestor, while Monotremata possibly branched from early Eupantotheria at the end of the Early Jurassic, before the Dryolestoidea did, from the forms in which the cochlea started to coil and small cerebellar hemispheres developed. Aegialodontia gave rise to Metatheria but not to Eutheria. Deltatheroida belongs to Metatheria. A group of Cretaceous therian mammals with tribosphenic molars informally classified as “Tribotheres”; cannot be assigned either to Metatheria or Eutheria.