Holotherians
Holotherians is a hypothetical group of mammals that would have existed before the split into the three major extant subclasses: Monotremata (monotremes), Metatheria (marsupials), and Eutheria (placental mammals). The term itself is not widely used in current scientific literature and the concept is largely academic, representing a theoretical ancestral form.
The idea of a holotherian ancestor stems from attempts to reconstruct the evolutionary history of mammals
The study of potential holotherians relies heavily on comparative anatomy, genetics, and the fossil record, though