History:

While it is now known that the nimravids are actually a separate family of cat-like animals that evolved parallel to the true cats, there had been much confusion about placement of the nimravids.  Some archaic sources place Nimravidae with the true sabre-toothed cats as a subfamily of Felidae.  These sources claim that the cat family arose from the nimravids; that Hoplophoneus evolved into Smilodon, and Dinictis evolved into Pseudaelurus.  This is not true.  Nimravids evolved on a separate lineage from the cats; the last common ancestor between them was the miacids.  Proailurus, a descendent of the miacids, evolved into Pseudaelurus, which split into two main groups, one of which became the modern cats, and one became the true sabre-toothed cats: the extinct Machairodontinae subfamily of Felidae.  The nimravids have no relation to these very similar looking animals. Some sources as well place the neofelid group Machairodontinae (true saber toothed cats) within the Nimravidae, although fossil evidence proves that the machairodonts were actually true cats. 

Sabre-toothed feline-like animals evolved separately four times in evolutionary history: in Nimravidae, Machairodontinae subfamily of Felidae, the Hyaenodontidae family from Creodonta, and thylacosmilids (sabre-toothed marsupials), due to a process called convergent evolution, where similar structures evolve in unrelated animals, such as the wings of birds and bats. 

Nimravids are also known as the Paleofelids, or "False Sabre-toothed Cats".  They are called as such because, though they possess many feline-like attributes, they are not true felids.  The family Nimravidae was thought to have diverged from the ancestors of the true cats during the early Eocene, about 55 million years ago.   The last common ancestor between them was the Viverravidae (feline-like) group of miacids. They existed as a group from the late Eocene (36 mya) to the late Miocene (5 mya).  Their diversity reached its peak around 28 mya.

Physical Appearance:

These animals had a most unusual skull structure.  One of the most important structures that sets them apart from Felidae is the auditory bulla.  In the Felidae, the bulla is separated into two chambers by a structure called the septum.  In the nimravids two conditions occur: one having an entirely or partially cartilaginous bulla; the other is the presence of an ossified bulla, with a complete lack of septum.  In some nimravids, a structure similar to the septum occurs, but is structurally different from the septum of the true cats.

Not unique to the nimravids was the presence of a flange on the front of the mandible.  This feature was also present in a few species of machairodonts and  thylacosmilids.    This flange, usually but not always present in the animals with the longest canine teeth, was a bony prominence that projected downward as long as the canine tooth.  This flange was the most prominent in Barbourofelis, and was found to some degree in most genera.  It was absent from Nimravus and Dinaelurus.

Unlike the machairodonts, which had two basic varieties of teeth, the nimravids generally had narrow, pointed canines with varying degrees of length. 

Nimravids possess many of the features of modern cats, including retractable claws and the nictating membrane (or "third eyelid").  Superficially, many species probably looked like cats as well, and many were very muscularly built.  Most species were low-slung, having a heavy-set body set on relatively short legs, and most species had a short tail.  Though the skull shape was rather unique, most species had a somewhat feline-shaped skull, with a shortened rostrum and reduced numbers of premolars and molars.  The genera of Dinictis, Nimravus and Dinaelurus, in particular, had the most cat-like skulls.

Species:

There are three subfamilies of nimravid.  Most nimravid species are relatively rare, and fossils are highly fragmented.  What little is known about some of the more common species is below.

Dinictis was a small nimravid from Late Eocene to early Oligocene (40 mya) in the plains of North America.  They had a sleek body set on short legs, with long tail, and walked plantigrade.  Pictures: Bone Clones | Black Hills Institute 

Eusmilus existed in the late Oligocene (30.5-28.5 mya). It was a dirk-toothed nimravid noted for its long, flattened sabres and very prominent mandibular flange.  Pictures: Bone Clones | Evolutionary Store

Hoplophoneus existed in the late Oligocene (33-30.5).  One of the largest of the nimravids, about the size of a modern jaguar.  Pictures: Bone Clones

Barbourofelis is one of the most recent of the nimravids, existing in the late Miocene (15-6 mya).  It was a lion-sized animal with the longest canines of all the nimravids.   They probably were very muscular, somewhere between a lion and a bear.  They had very prominent flanges on the lower jaws, and an unusual skull shape. Pictures and Info: Paleocraft

Links:

• http://www.boneclones.com/catalog_fossil_cats.htm
• http://www.fmnh.helsinki.fi/users/haaramo/Metazoa/Deuterostoma/Chordata/Synapsida/Eutheria/Carnivora/Aeluroidea/Nimravidae.htm