Last year, a new and unusual species of rodent was discovered in Laos, called Laonastes aenigmamus, or kha-nyou. Here’s the skull:
It’s notable mainly because of that unusual jaw. It’s hystricomorphous—an obscure term that refers to how the jaw is muscled, in this case with a large masseter muscle (the big chewing muscle you can feel towards the back of your cheek), which inserts into a large hollow or fossa in the jaw—and has a reduced coronoid process (one of the two ‘prongs’ of the jaw), and it’s teeth are bilophodont, referring to a pattern of paired bumps on their surfaces (there are many amazing details about bumps on teeth). Trust the anatomists, it’s a distinctive arrangement.
In the 1970s, an extinct species of rodent, Diatomys shantungensis, was identified from some Chinese fossil remains. A better specimen was found in 2005, and here’s what its skull looks like.
That specimen is about 11 million years old, and it is hystricomorphous, with a reduced coronal process, and bilophodont teeth. It lines up with Laonastes in other anatomical details, too—so what we have here is a rat-squirrel thought to be extinct, back from the dead.
Of course, it wasn’t really extinct, just rare and isolated. This phenomenon is unfortunately called the “Lazarus effect”, but when you get right down to it, it’s not a property of the organism at all, but merely a consequence of our own incomplete knowledge.
The “Lazarus effect” refers to the reappearance of taxa after a lengthy hiatus in the fossil record. The discovery of living examples of taxa that were previously thought to be extinct is a very special case of the Lazarus effect, one that has only rarely been documented among mammals and other vertebrates. With the exception of microbiotheriid marsupials, all fossil mammal taxa that were subsequently found to be alive have been Pleistocene in age and congeneric with their modern counterparts. Uniquely among placental mammals, Laonastes pertains to a clade (Diatomyidae) that was formerly believed to have been extinct for more than 11 million years. Diatomyids join tree shrews, flying lemurs, and tarsiers as examples of ancient and formerly wider-ranging mammalian taxa that are currently living with relictual distributions in southeast Asia.
From a paleontological and phylogenetic perspective, efforts to conserve Laonastes, the sole survivor of a morphologically distinctive family of rodents with deep evolutionary roots in Asia, should be given the highest priority. If it can be preserved, the Paleogene zoo that survives today in southeast Asia can offer invaluable insights regarding past and present biodiversity.
Dawson MR, Marivaux L, Li C-k, Beard KC, Métais (2006) Laonastes and the “Lazarus Effect” in Recent Mammals. Science 311(5766):1456-1458.