When: Late Eocene (the formation is dated from 37 to 34 million years ago)
Where: Inner Mongolia, China
What:Andrewsarchus was named after Roy Chapman Andrews, a paleontologist from the American Museum of Natural History who led several expeditions to China and Mongolia in the early 1900s. It is a large mammal from the late Eocene of China. This is a very cagey introduction, you notice… why? We do not know nearly as much about Andrewsarchus as one might assume from the popular media. All that is known of this animal is a skull (which is fairly poorly preserved) and isolated teeth, there is not even a lower jaw! The skull is enormous, at 2 feet 9 inches (~83 cm) long and almost 2 feet (~56 cm) wide. Andrewsarchus has been reconstructed numerous times, and the vast majority of these reconstructions, besides inventing a body, even get the basic features of its head wrong. Andrewsarchus was not a ‘giant wolf like animal’ as has been depicted before; its eyes were very small and very low set on the skull, its snout ‘pinches in’ from the sides, and the brain-case was relatively very small, to name a few major differences. I have selected two of the better reconstructions to show above.
The problems I mentioned are just in reconstructing the head wrong… as for the body we have no firm idea what it looked like. It could have been a very robust form, or much more gracile than has been depicted. True, the head is massive but there are examples in the fossil record of animals with massive heads and relatively slender bodies. It could have had fully developed hooves and just 2 toes on each foot or it could have had less reduced feet. There is even a suggestion that it might have been an aquatic animal, rather than a terrestrial hunter.
Andrewsarchus is far from the only fossil species we have that is only a skull (or less!), and we can reconstruct the post-cranial anatomy of many of these other scrappy fossils with more confidence than we can Andrewsarchus. That is because the taxonomic position of Andrewsarchus is so uncertain. It was at first grouped with the Mesonychia, an extinct group of uncertain affinity, but with well known anatomy. However, almost from the very start it was removed and placed as an aberrant mesonychid and the latest phylogenetic analysis place it far removed from all mesonychids, within the order Artiodactyla (the even toed ungulates - cows, pigs, hippos, and whales!). Entelodonts fell as the closet relatives of Andrewsarchus but this position is unstable, and it is possible Andrewsarchus is more closely related to whales within Artiodactyla.
This is one fossil I would absolutely love to find a complete skeleton of one day. It is a very intriguing animal, and is fairly well known to the public, but we truly know almost nothing about it!