Fossil Friday – eggshell fragment

With Easter coming up in a few days, it seems an appropriate time to feature one of the rarer fossils from Diamond Valley Lake, an eggshell fragment.

While bird eggs are largely mineralized (unlike leathery lizard, snake, and turtle eggs), they are of course thin and fragile, as anyone who has dropped a carton of eggs knows. Moreover, if an egg lasts long enough to produce a baby bird, the chick shatters it upon hatching and then tramples the fragments in the nest. All this adds up to fragments of eggs, rather than complete, intact eggs. The fragment shown here is one of the largest recovered from Diamond Valley Lake. The curvature suggests that it may have come from a relatively large egg (chicken-sized, maybe), but there’s a good deal of uncertainty involved in such a prediction. Most eggs are not spherical, but are instead, well, egg-shaped, which means that the curvature is different on different parts of the egg.

Fossils such as eggshells also serve as a reminder of the effects taphonomic bias has on our knowledge of the fossil record. So far, 17 species of birds have been identified from Diamond Valley Lake deposits, although the actual number of bird species was undoubtedly much higher. Those deposits span tens of thousands of years (maybe as much as 200,000). How many eggs would 17 species of birds lay in 100,000 years? Yet, in the deposits we have more mastodon teeth than eggshell fragments. The eggs were mostly broken into tiny, even dust-sized fragments, and the calcium carbonate-rich shells are susceptible to dissolving in acidic groundwater. Between predators, hatching and trampling, and abiotic processes such as dissolution, what must have originally been many millions of eggs was reduced to just a handful of identifiable fragments. Taphonomic processes will selectively remove certain remains, leaving us a fossil record biased in favor of some organisms and against others. This is something to always keep in mind when examining fossil deposits.


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