June 2022 Natural Mystery Answered

Our June Natural Mystery featured a pair of tiny mandibles—and the small size was one of the keys to identifying them. Everyone who submitted an answer correctly identified these tiny mandibles to family. Kirsten Welge, Mike Holtz and Alex went farther and correctly identified the genus these jawbones belonged to.

These are the mandibles of a long-tailed shrew, most likely a Masked Shrew.

Besides the small size, most respondents noted the red enamel as a distinguishing feature of shrew teeth. As Kirsten explains:

“The reddish color of the cusp enamel is due to iron inclusions. While rodents will show orange deposits in the incisors, shrews will show red deposits along their chewing surfaces. When examined in the lab, higher concentrations of iron were found in molar cusps used for grinding or crushing, compared to shearing surfaces – suggesting that the teeth and tooth surfaces that experience the greatest stress show greater iron density.”

Another feature that helps identify these as shrew jaws is the orientation of the incisors. Mike Holtz notes that “Shrews foremost incisors point forward, as opposed to voles or mice that would have a lot more curve to them.” Kirsten adds to this, citing Elbroch, “the upper and lower anterior incisors are greatly enlarged and point forward, and are used like tweezers to capture and hold insect prey.”

Narrowing down the ID beyond the family level gets a little more challenging. As Kirsten notes: "Possible candidates include least shrew (Cryptotis parva), northern water shrew (Sorex palustris), and masked shrew (Sorex cinereus)."

A full list of candidates in the range would also include the Northern Short-tailed Shrew (Blarina brevicauda), Arctic Shrew (Sorex arcticus), and Pygmy Shrew (Sorex hoyi). Northern Shore-tailed Shrew jaws are much larger than this. Distinguishing between Masked, Arctic and Pygmy shew jaws, however, can be quite difficult. Kirsten offers a great analysis of her three candidates:

“Looking at Elbroch's Mammal Skulls, we can compare the shapes of the bony projections at the posterior of the jaw, called the coronoid process (uppermost), condyloid (middle, where the jaw contacts the cranium), and angular (bottom).

The least shrew shows a slightly knobby, squared-off coronoid process, which angles slightly forward towards the toothrow. The condyloid process is also slightly square, and the long, thin angular process is slightly hooked at the tip.

The northern water shrew and masked shrew both show a triangular coronoid process that rises straight up from the end of the toothrow, with a triangular tipped condyloid process and a straight, thin angular process. However, the curve between the posterior base of the coronoid process and the condyloid process is different: the northern water shrew shows a shallow arc, while the masked shrew shows a deeper dip between these features.

While the differences between the northern water shrew and masked shrew are subtle, the features on these two mandibles match the masked shrew."

I agree. I am not sure I could rule out an Arctic Shrew or Pygmy Shrew as a possibility for these tiny jaws, but I do think that a Masked Shrew is the most likely candidate, given how common they are.

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