December 26th on the Cosmic Calendar: Actual hairy, lactating, bug-eating mammals! Not Synapsids anymore! The earliest mammals lived in the Jurassic and Cretaceous, yes, in the shadow of dinosaurs. The first group of no-doubt-about-it mammals were the Triconodonts (again, the earliest currently known).
Not all of the tricodonts were small though. Repenomamus was a meter long — about the size of a beaver! Big enough to eat small dinosaurs!
You may remember Cosmos, the 1980 PBS series by Carl Sagan. Maybe you also recall Sagan’s “Cosmic Calendar” from the series, where in order to put the immensely vast history of the universe into a comprehensible scale, he mapped it onto a calendar year. In other words, if the entire history of the universe were one year, with the big bang in the first second of midnight on January first, and the present day on the last second of December 31st, New Year’s Eve. A project of mine this year has been to note the major events in the Cosmic Calendar, on the real calendar, on this blog!