Mammal not a mammal (Score 1)

The mysterious "mammal" is actually a triconodont . They differ from eutherians (placental mammals) which are what we usually think of as "modern" mammals. Aside from toothy traits, these creatures , like marsupials, did not have a closed pelvis. Closing the pelvis changes the reproductive behavior by adding the eutherian trait of estrus. This makes the female mate with every ovulation. Then there is ambulation. Think of a duck. A duck has an open, "U"-shaped pelvis so it walks - - -like a duck. In a cat, which is an eutherian, the pelvis provides a stable circular platform for attachement of legs to the body. Cats can walk fast, in a STRAIGHT line. Possoms waddle. Kangaroos leap, using both legs to propell themselves forward. All of this is to point out that the triconodonts were not agile ambulators. This is the main reason for the demise of the big carnivorous marsupials of Australia and ancient S.America. When the true eutherian arrived, it was able to out-reproduce and out-run the other vertebrates. Think of the destructive eutherian as a rat, eating it's way through extinct species. The first eutherians probably concentrated on the eggs of the other vertebrates and then took-on the pseudo-mammals. Interesting.