Miacids are thought to have evolved into the modern carnivorous mammals of the order Carnivora. They were small carnivores, superficially
marten-like or
civet-like with long, lithe bodies and long tails. Some species were
arboreal, while others lived on the
ground.[13]
They probably fed on invertebrates,
lizards,
birds, and smaller
mammals like
shrews and
opossums. Their
teeth and
skulls show that the miacids were less developed than modern carnivorans. They had carnivoran-type
carnassials, but lacked fully
ossifiedauditory bullae (rounded protrusions).
Classification
History of classification
Miacidae as traditionally conceived is not a
monophyletic group; it is a paraphyletic array of stem
taxa. Traditionally, Miacidae and
Viverravidae had been classified in a superfamily,
Miacoidea. Today, Carnivora and Miacoidea are grouped together in the crown-clade
Carnivoramorpha, and the Miacoidea are regarded as basal carnivoramorphs. Some species of the genus Miacis are closely related to the order Carnivora, but only the species Miacis australis and Miacis cognitus are a true carnivorans, as they are classified in the family
Amphicyonidae within
Caniformia.
The divergence of carnivorans from miacids is now inferred to have occurred in the middle Eocene (
c. 42 million years ago).[14] Traditionally, the Viverravidae (viverravids) had been thought to be the earliest carnivorans, with fossil records first appearing in the Paleocene of North America about 66 million years ago, but recent cranial
morphology evidence now places them outside the order Carnivora.[15] Later authorities disagreed, and propose that the viverravids arose in North America 66-60 million years ago, spread to Asia then later to Europe, and were the first carnivorans and possessed the first true pair of
carnassial teeth.[16]: p8
It has been proposed that miacids arose in North America and Europe 60-50 million years ago then later spread to Asia.[17] Like the earlier viverravids, they possessed a true pair of carnassial teeth and therefore are related to order Carnivora.[18] They also possessed a full set of cheek teeth, were weasel to small fox sized, and lived in
forests. All modern carnivorans arose from them.[16]: p9
^Kalandadze, N. N. and S. A. Rautian (1992.) "Systema mlekopitayushchikh i istorygeskaya zoogeographei [The system of mammals and historical zoogeography]." Sbornik Trudov Zoologicheskogo Muzeya Moskovskogo Goschdarstvennoro Universiteta 29:44–152.