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What kind of bone it this?

I found this dark heavy bone while walking on the beach after the storm on Friday night. What is it? Could it be part of a sea lion? Why is it black and super heavy? 

Benjamin Van Heusden, Santa Barbara - January 14, 2019

Curator Response

Hi Benjamin,

Thanks for sharing your find with us. This looks to be the outer denser bone found along a medium-sized mammal limb bone. My best guess from the image is that it’s a portion of the shaft of a Mule Deer tibia, but to be sure, I’d need to see the bone and compare it to the limb bones of mammals that fit into the size range.

If it’s been buried in mud/sand in the nearshore area between the kelp beds and the shoreline, it’s been in a setting which lacks oxygen. As the organics in the bone decayed in this oxygen-free environment, hydrogen sulfide was generated, which turned the bone black. The bone may change to a tan color after being exposed to oxygen as it dries out.

It probably feels heavy because washing around in the intertidal would wear away the softer matrix of bone, leaving the harder, denser layer of bone behind.

Curator of Vertebrate Zoology Paul W. Collins, M.A.

Benjamin Van Heusden response:

Hello! Thank you for your response. You are exactly right that over the past few weeks it has been getting lighter and lighter!