Felines are the strictest carnivores of the thirteen terrestrial families in the order Carnivora, although the three families of marine mammals comprising the superfamily pinnipedia are as carnivorous as the felids. The most familiar felid is the domestic cat, which first became associated with humans about 10,000 years ago, but the family includes all other wild cats including the big cats. The first felids emerged during the Oligocene, about 25 million years ago. In prehistoric times, there was a third subfamily known as Machairodontinae, which included the "saber-toothed cats" such as the well known Smilodon. There were also other superficially cat-like mammals, such as the marsupial sabertooth Thylacosmilus or the Nimravidae, which are not included in Felidae despite superficial similarities.