Spider, insect or something else?

I recently received a batch of fossil insects from a local fossil collector, who is hoping that I and other local entomologists will be able to provide some level of identification beyond just “insect.” All are from the Green River Formation, a lake bed shale deposit dating back to the early to mid-Eocene epoch (45–50 mya). Most major insect orders and families were established and undergoing rapid diversification by this time, and as a result most of the fossils are clearly identifiable at least to order or even family. There is one fossil, however, that has got me stumped. The label that came with the fossil indicates “Spider (?)”, and while at first glance this is the first thing that comes to mind, the more I look at it the more I become convinced that it represents something else. What, however, I do not know.

The fossil is a cast and mold from a split rock, so two views of the fossil are available. I’ve photographed them to try to get a better look at the details and still can’t come to a decision (I’ve even considered a small crustacean or even a plant part). Perhaps somebody who reads this might have an idea?

Colorado: Garfield, Hwy 139, Douglas Pass. Maximum diameter = 22.5 mm.

Colorado: Garfield, Hwy 139, Douglas Pass. Maximum diameter = 22.5 mm.


Mirror half of same fossil.

Copyright © Ted C. MacRae 2012

21 thoughts on “Spider, insect or something else?

  1. It does bear a striking resemblance to some of the inadvertent plant tissue slide mounts my students make during the first day of Berlese extractions… If it is an arthropod, I’m really looking forward to reading about how the determination was made!

  2. Seems pretty small, but otherwise it looks like a hunk of some monocot to me, something like recently germinated Acorus or even grass (if they were around then), although I like Adrian’s idea of a caddisfly case.

  3. It seems we have a consensus on a plant-based origin—I think Geek’s comment about the lack of any obvious segmentation nails it. Intriguing idea by Adrian of a caddisfly case; however, the Green River Formation is derived from ash sediments in a large, shallow, continental sea—if I’m not mistaken I think caddisfly larvae are largely associated with rapidly moving waters.

    • M Winterbourn and Norm Anderson reported that Philanisus plebeius Walker, 1852, is intertidal and deposits its eggs in starfish. There are lots of trichops in standing water and several reported from salt marshes if I remember right.

      • You are correct, although the group to which that family belongs is currently restricted to Australia/New Zealand. Whether more widespread representatives occurred during the Eocene, I don’t know. Perhaps a member of another group associated with a brackish river inlet is a possibility.

  4. Clearly your folks have parsed this out (looked very plant-y to me, as well), but I just wanted to say that this reminds me of, but is harder than, doing a road kill survey. Seeing things that are smashed to bits and trying to figure out what they are. Nice thing about fossils, though, is that they are not gooey. =)

    • Many years ago a friend gave me a book called Flattened Fauna—basically a field guide for identifying road kill. I’ve never actually tried it, as I thought it was a bit of a joke, but apparently there are valid uses for such information.

  5. I agree too, that it looks plant-like. @ macromite: yes, the grasses’ major evolution occurred in the Eocene. I haven’t been able to find any picture of grass fossils from the green river formation. This looks a little more to me like shredded algal/seaweed-like tissue.

    • I also tried to find images of fossil plants from the Green River Formation and was unsuccessful. The thin parts do have a very “monocotish” look to them. At the very least I’m satisfied that it is not an invertebrate animal.


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