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 is the super continent Pangea began to split apart and as the climate change so do niches. This may have led to some exctinctions that allowed small mammals to fill new niches and rise. 60-65 million years ago was the exctinction of the dinosaurs, which allowed for more environments, niches, and resourses for the early mammals to use.
Eutherians are a 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 and give birth to more precocial young. The oldest eutherian fossil found so far is the Eomaia. It was found in China (Paleartic zoogeographical zone) and dates back to the Cretaceous period. It is though to be a small and insectivorous early mammal.
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, a vagina (some do have a cloaca), a cervic, a uterus, and ovaries (again with variation based on species) and males have a penis bone or baculum.
Comments
Suggestion
Your first paragraph has a lot of information. Specifically, the second sentence would be better if it was split into three sentences, and each point elaborated upon. This avoids cramming too much information into one sentence and writing run-on sentences. It also provides you the opportunity to provide more background information.
Suggestions
In the first sentence, use a colon instead of a semicolon. In the second sentence, I don't think you need to describe the speciation with "the theory for speciation is," just start describing it!
Comment
I feel as though there is a lot of broad information and not a lot of details. Try to put more background info for some of these facts.