Nimravidae

1. Hoplophoneus 2. Barbourofelis 3. Eusmilus 4. Nimravus 5. Dinictis
Nimravids look like cats, and in all likelihood they probably hunted like cats too, but there are a number of differences. The most clearly definable trait that identifies nimravids requires the preservation of the skull. The precise part of the skull that you need to look at is the auditory bulla, a rounded growth towards the rear and on the underside of the skull that forms part of the ear structure. In true cats the auditory bulla is chambered, split into more than one segment. In nimravids however the auditory bulla is a single chamber with no bony division.
Other nimravid characteristics are harder to spot and usually require an expert eye trained in the differences to identify. In more laymen’s terms, the skulls of nimravids are generally more primitive than true cats, although this should come as no surprise since nimravids appear tens of millions of years before the cats. Nimravid paws are also more like those of canids and adapted more for life on the ground rather than climbing up and through trees. Another additional difference is the proportionately shorter legs of nimravids as well as their more plantigrade postures (walking with the foot bones in contact with the ground) whereas true cats are digitigrade (walking on just their toes).
Nimravids tend to possess enlarged canine teeth, although the extent to which they are enlarged can vary considerably between genera. Some like Hoplophoneus had teeth so large that they were in proportion to later machairodonts like Smilodon. However the canine teeth of nimravids were not as refined as the later machairodonts, with the teeth often slightly thicker. Also while the lower jaws of machairodonts are adapted to accommodate the upper canines, nimravids typically have enlarged flanges of bone that extended down from the lower jaw to help protect the upper canines when the jaws were closed.

Briefly returning to the teeth, the differences in canine size and form between nimravid genera is probably down to different kinds of prey specialisation. The large curved sabre-like teeth of some genera were better suited for work on larger prey where the large teeth could easily sink into a soft spot like the throat where there was less chance for the relatively weak teeth to break against the preys bones. The genera with shorter, rounder canines however probably specialised in hunting smaller prey where tooth contact with bones was more likely to happen. Here the smaller size would reduce the chance of contact with bone while the more conical form reinforced the teeth against breakage on occasions when the teeth did strike bone. In addition the genera that had more heavily serrated teeth may have hunted animals with thicker and/or tougher skin that was more easily sliced thanks to the serration.
Nimravid fossils first appear in the later stages of the Eocene with most known from North America, though Asian fossils are also known from approximately this time. During this period the planet had entered a cooling phase which saw the beginning of a gradual reduction in tropical forests to more open scrub. In these ecosystems nimravids were likely ambush hunters of mammals such as primitive horses like Hyracotherium and oreodonts. As the Oligocene progressed the global cooling and the climate change continued, but the nimravids seem to have thrived during this era and remained one of the main types of predatory mammals across Eurasia and North America.
By the Miocene however the Nimravids seem to have been on the decline. Out of nine established nimravid genera known in 2012, only three of these were known to have Miocene aged fossils. Two of these, Dinictis and Nimravus only seem to have made it as far the Aquitanian, the first stage of the Miocene. The only genus at this time known to have made it as far as the late Miocene (specifically the Tortonian period) is Pogonodon. This decline coincides with the on-going climate change towards grassy plains, something that saw the types of available prey changing, possibly to the point where nimravids could only continue to survive by specialising in their hunting behaviour. Nimravids do not seem to have been the only predator group affected though, with others dominant groups such as the creodonts, mesonychids and entelodonts reducing in the number of genera before disappearing completely in the Miocene.
The one main group of predators that bucked this trend and thrived in the Miocene where the amphicyonids, better known as the ‘bear dogs’. It was probably competition from these and other more advanced predators that forced the remaining nimravids into the side-lines where they had to eek a living in the few locations that could support their method of hunting, until these areas also disappeared and the nimravids with them. So far no nimravid remains are known from any later than the Tortonian period around nine million years ago.

Barbourofelidae

The Barbourofelidae is an extinct family of the mammalian order Carnivora of the suborder Feliformia that lived in North America, Eurasia and Africa during the Miocene epoch (16.9—9.0 Ma) and existed for about 7.9 million years.
Barbourofelidae were previously classified as a subfamily of the extinct Nimravidae, but are now thought to be taxonomically closer to the Felidae than to the Nimravidae. Barbourofelids first appear in the fossil record in the Early Miocene of Africa. By the end of the Early Miocene, a land bridge had opened between Africa and Eurasia, allowing for a faunal exchange between the two continents. Barbourofelids migrated at least three times from Africa to Europe.
Genus Ginsburgsmilus, Prosansanosmilus, Sansanosmilus and Barbourofelis

Ginsburgsmilus is an extinct genus of carnivorous mammal of the family Barbourofelidae that was endemic to Africa during the early Miocene. There is only one known specimen of Ginsburgsmilus napakensis, dated to 20-19 mya.
Prosansanosmilus is an extinct genus of mammalian carnivores of the suborder Feliformia, family Barbourofelidae, which lived in Europe during the Miocene epoch (16.9—16.0 mya), existing for approximately 0.9 million years.
Sansanosmilus is an extinct genus of carnivorous mammal of the family Barbourofelidae endemic to Europe and Asia, which lived during the Miocene, 13.6—11.1 mya, existing for approximately 2.5 million years.
Barbourofelis is an extinct genus of large, mostly carnivorous, mammals of the family Barbourofelidae. The genus was endemic to North America during the Miocene, living from 13.6—5.3 Ma and existing for approximately 8.3 million years. Thought to be lion-sized, it had the longest canines of all barbourofelids. It had a very robust constitution; the largest individuals of B. fricki are thought to have weighed up to 380 kg (829 lbs). They had very prominent flanges on the lower jaws and an unusually shaped skull. The barbourofelids were probably very muscular, resembling a bear-like lion or lion-like bear. Although the nimravid family did evolve into cat-like forms, they left no descendents among modern cats.