SciNews has a story about the earliest known mammal.  The early mammal Brasilodon quadrangularis dates to 225 million years ago.  It was found in the Late Triassic outcrops of the Caturrita Formation of Rio Grande do Sul in Brazil and named by J. Bonaparte, A. Martinelli, C. Schultz, and R. Rubert in 2003.  New research published in the journal Anatomy proposes that it is the earliest mammal.  

A tiny animal called Morganucodon is usually considered the first mammal but its oldest fossils, only represented by isolated teeth, date from around 205 million years ago.

Examining the dentitions of Brasilodon quadrangularis, Dr. Martha Richter from the Natural History Museum, London, and colleagues discovered evidence of only one set of replacement teeth — a key feature of mammals known as diphyodonty.

“The first set starts developing during the embryonic stage and a second and last set of teeth develops once the animal is born,” the researchers explained.

“By contrast, reptilian dentitions are different, especially in that replacement is ‘many for one’ (polyphyodonty), in which each tooth site has tooth regeneration many times over the lifetime of a reptile to replace damaged ones.”