Convergent evolution occurs when only distantly-related species are under the same selective pressures, and evolve to look alike. A particularly good example of this is the North American flying squirrel (a rodent) and the Australian sugar glider (a marsupial). Put them side-by-side, and they're hard to tell apart, and both have the distinctive kite-like flap of skin between the forelegs and hind legs, allowing them to catch a breeze and glide from tree to tree.
It's important to emphasize that while convergent evolution can result in organisms being similar in appearance or habits, it doesn't ever cause them to fuse into a single species. Flying squirrels and sugar gliders maintain major differences in their genetic make-up, skeleton, dentition, and so on -- so however close the resemblance, they're still two separate species.
Convergence is actually fairly common in the natural world, which is why appearance is such a poor guide to determining who is related to whom. There are only so many solutions to the problems posed by living in a particular environment, so it's inevitable that different lineages will happen on the same ones. Flying, for example, has evolved independently at least four times -- birds, bats, pterodactyloids, and insects. The structure and mechanics is different in each, which is indicative that they were independent innovations.
I was thinking about convergent evolution this morning as I read a paper in the journal Geodiversitas about the discovery of a remarkable fossil in Colombia. It's the best-preserved and most complete skeleton ever found of Anachlysictis gracilis, a Miocene apex predator that belonged to a group called the sparassodontids. (The name comes from the Greek σπαράσσειν, to tear to pieces, and ὀδόντος, tooth -- an indicator of how scary these animals were.)
Here's a photograph of the skeleton:
My guess is that looking at this, you're immediately reminded of the saber-toothed cats such as the famous Smilodon, which also were around during the Miocene Epoch but reached their pinnacle a few million years later, during the Pleistocene. Surprisingly, this parallels my earlier example of the flying squirrel and sugar glider -- the saber-toothed cats were true felids, and thus placental mammals, while Anachlysictus and the other sparassodonts were marsupials. The two species were drawn together by the forces of convergent evolution. If you're a predator, having big nasty pointy teeth is a pretty good adaptation regardless what taxonomic group you belong to.
These striking carnivores were present in South America during what is called the "splendid isolation," prior to the tectonic shift that formed the Isthmus of Panama and allowed for the Pliocene Great Biotic Interchange. South America had developed a unique biota, including not only the sparassodonts but a variety of other marsupial groups, most of which are now extinct. Even the South American placentals didn't do so well, and were outcompeted (or hunted to death) by North American migrants. Not long after the formation of Central America, a great many of the South American groups -- not only the sparassodonts, but the glyptodonts, litopterns, astrapotheres, pyrotheres, and xenungulates -- were gone forever.
The new fossil discovery will allow paleontologists to make some deductions about not only its anatomy, but its behavior. "In a future study we will address all the other bones in its body, which include various sections of the spine, ribs, hip, scapulae -- what we call 'shoulder blades' for humans -- and bones in its legs," said Catalina Suarez, of the Argentine Institute of Nivology, Glaciology and Environmental Sciences, who led the research team. "This will allow us to explore aspects of how it moved, the position in which its neck held its head, whether it was a runner, whether it could climb, whether its hands could hold objects more easily, as many marsupials do when feeding, or whether it was a bit more difficult, as it is for example for a dog or a cat."
It's fascinating to learn more about these long-extinct animals, whose ecological role would be taken over by predatory placental mammals like wolves and the various big cats. Even if they're extinct, their bones still have a story to tell -- of a saber-toothed marsupial who hunted in the forests of Colombia thirteen million years ago.