The dog family has its origins in North America and it seems clear that what happened here is that some wolf-relative that happened to look a bit like a fox headed south at some point, where all of its remaining descendants live today. North and South America joined up relatively recently, geologically speaking, so it's likely that the origins of the South American foxes actually lie in the North, already looking somewhat like their modern form before they made the trip.
However, the exact details are still not entirely clear. The best way to get some clarity on the question would be if you could find clear examples of fossils of this group from before they made the crossing. But how to identify such a creature?
One possible place to start is with the modern crab-eating fox (Cerdocyon thous). These animals live across much of South America and they're significant in that genetic analysis seems to show that they represent the earliest diverging branch of the South American foxes that's still alive today. So finding early fossils clearly related to this animal should give us at least some insight into the origins of the group more generally.
When I looked at the modern species, I noted that "the oldest fossil relatives of the living animal actually come from North America, where they lived before the two continents collided". Those animals may represent a separate crossing event from the ancestors of most other South American foxes, but, that far back in time, they would still have been close relatives, so it's on those very fossils that a lot of what we surmise about the origins of the South American foxes rides. So, what are they?
There are, in fact, two fossil species of crab-eating fox known from North America. Cerdocyon texanus lived around 5 million years ago in Texas and New Mexico, but we only know of this from part of a jaw. We have rather more remains of the other species, known from a partial skeleton of about the same age, but significantly, one that was found in Mexico - somewhat closer to the Panamanian land bridge.
The fossil in question was first described (in Spanish) in 1980, when it was identified as belonging to an unknown species closely related to the living crab-eating fox. That unknown species formally received a name in 2009: Cerdocyon avius.
The deposits that the fossil was found in would have formed the coastline of a shallow bay at the time they were laid down, and have also yielded the primitive rabbit Notolagus, which sounds like the sort of thing a fox-like animal might want to eat. But, whether or not that's true, it doesn't tell us much, since the modern crab-eating fox is something of a generalist - as are many other foxes - capable of living in a wide range of environments. However, there is enough of it that we can tell that it closely resembled some of the modern species thought to be relatively primitive, and to represent particularly early branches on the dog family tree.
In fact, in 2010, one researcher pointed out that the fossil had quite a few features in common with the grey fox, a North American species that isn't closely related to the crab-eating fox at all. Now, he and other researchers have come back and taken a much more detailed look at the fossil. The new study found a number of differences between the fossil species and the living one, mainly in the shape of the teeth and jaw. In addition to the grey fox, these are also reminiscent of the living raccoon dog (or tanuki) and of the decidedly extinct Leptocyon, both of which are thought to represent "primitive" branches on the dog family tree.
|Conclusions of the study described in the text|
So, despite what I said four years ago, this animal almost certainly wasn't a crab-eating fox in any meaningful sense. (I mean, it might have eaten the occaisional crab, who knows? But I wouldn't bet on it). But if it isn't that, what is it?
The authors describe it as a vulpine fox, implying a close relationship with red foxes and the like. But the same study also shows that the animals commonly described as "vulpine foxes" don't really belong to a single lineage. And, indeed, this isn't really shocking, since both the grey fox and the raccoon dog have long had an uncertain placement near the base of the canine family tree. (As, for that matter, has the bat-eared fox, which wasn't included in this study).
Instead, the new study finds that the closest relative of what we should now call Ferrocyon is Metalopex, an omnivorous, very fox-like, dog that lived around 10 million years ago from New Mexico and Oregon and is sometimes regarded as the earliest known "fox"... depending, of course, on your definition of "fox". This may well be related, in turn, to the raccoon dog of Asia, although it lived long before the ancestors of that animal would have left the Americas, and it's possible that it just happens to be similarly close to the base of the family tree.
Indeed, while it makes no official statement on the subject, the paper also implies that the other supposed fossil species of crab-eating fox, the one from Texas, is probably even closer to the raccoon dog and should get a new name, too. (Although it is harder to say - we only go on the shape of the teeth in its case).
Either way, it seems unlikely that we now know of any early relatives of the crab-eating fox. The modern species is known from some Pleistocene-aged fossils, by which time it had already reached as far south as Argentina, but where it's ancestors came from is now a mystery once more, and it can't help us with the history of the wider South American foxes.
One step forward, yes, but also one step back. Sometimes science works that way.