Eutherians

Eutherians are another subclass of class Mammalia. The word eutheria comes from the Greek for "true" (Eu) and "wild beast" (Theria). Eutherians are unlike metatherians (marsupials) as they have a true placenta. The oldest eutherian fossil is the Eomaia found in China and dates back to the Cretaceous period. It is though to be small and insectovorous. Mammalian speciation expanded in two periods; 80-90 million years ago and 60-65 million years ago. For the 80-90 million years period the theory for speciation then is the super continent Pangea began to split apart and as the climate change so do niches, leading to some exctinction that would allow small mammals to fill new ones and rise. 60-65 million years ago was the exctinction of the dinosaurs allowing for more environments, niches, and resourses for the early mammals to use. Eutherians all share some distinct characteristics. Eutherians follow a primitive dental formula with slight variation based on species: Both the upper and lower jaw have 3 incisors, 1 canine, 4 premolars, and 3 molars, with a premolar that is clearly different from the molar. Females have a chorloallantoic placenta, vagina cervic uterus and ovaries again with variation based on species and males have a penis bone or baculum.