Saturday, April 16, 2016

Skull Identification


I received a plastic bag with a "gift," a skull with a request for identification.  Some one else might have considered this a threat or at best a bad joke.  Amy Short knew this would be welcomed, a find from her mother's yard.  There were no other clues to its identity, no lower jaw and only one residual tooth.  It had been chewed by other animals in need of calcium.  I was tempted to send it off for an expert opinion from Richard Herman, our local skull guru but decided to take a swing at it.  I got lucky because of several unique features.


It was obviously a herbivore with the absence of canine teeth and the large gap between the incisors and the molars.*  The canine tooth canal was long, 1.5" on a curve suggesting that it was not only a rodent, but one that used its teeth hard enough to require rapid growth.  While the rodentia family is large, including beaver, mice, rats and rabbits, the skull length of 3.5" eliminated a lot of these.

Looking over the choices in Wild Mammals of Missouri (Schwartz) I came across the perfect fit.**  This skull belonged to a woodchuck, a.k.a. groundhog, Marmota monax.  I have been trying to trap the ones that are undermining our century old barn without luck for 3 years, and here was the skull, the ultimate irony.

The key was the length of the skull, but first the rodent identification.  This site from Cornell.edu gives the basic features identifying it as an herbivore by the absent canines and large gap between the incisors and molars.  The long incisor roots suggested unremittingly growing incisors, a characteristic of the Order Rodentia.


Once identified as a rodent in Missouri, the skull length narrowed the field.  It wasn't rounded like a beaver, too large for a rabbit and only a few species are left.  To quote Wild Mammals of Missouri,  "It is easily distinguished from other skulls...by the following characteristics:"
  • Large size
  • Flat skull when seen in profile
  • Postorbital processes projecting at right angles to the length of the skull
  • Depressed area between the postorbital processes 
  • Prominent auditory bullae indicating hearing acuity.


 *For the basics of identifying carnivore, herbivore and omnivore, go to Cornell.edu.

**  Animal Skulls by Mark Elbroch is the ultimate source, very detailed with lots of keys.  Wild Mammals however is a great local resource and has dental guides and measurements that works for almost all of our Missouri skulls.

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