"Early mammals" refers to the very first kind of animals that were similar to modern-day mammals, but lived a long time ago. They are the ancestors of all the mammals we see today, like cats, dogs, and humans. Full definition
Jurassic fossils show that ecological diversification and the developmental mechanisms behind it occurred in early mammals. (science.sciencemag.org)
The discovery also highlights how little is known about early mammals. (scientificamerican.com)
But during the many millions of years that early mammals spent hiding from the dinosaurs and coming out only at night, they lost two colour receptors. (newscientist.com)