More Eocene insects

Most of the Green River Formation (GRF) insect fossils that I have on loan clearly represent either beetles (order Coleoptera) or flies (order Diptera). I’ve already shown a few of the latter (fungus gnat, midge), as well as some that don’t belong to either order (ant, cricket?). Here are a few more that seem identifiable to order, but family-level identification is less certain. Thoughts from the readership would be most welcome.

This fossil shows an aggregation of insects that I believe represent some kind of beetle. Based on shape and size (16.7 mm length) I’m guessing perhaps either a diving beetle (family Dytiscidae) or whirligig beetle (family Gyrinidae). These are both aquatic families, although only the former is among the beetle families recorded from the GRF by Wilson (1978).

IMG_1927_enh_1080x720 IMG_1836_enh_1080x720
There are two insect fossils on this specimen, but the closeup is the one near the center of the rock. It is tiny (3.5 mm in length), and at first I thought it might be a fly (order Diptera). However, dipterist Chris Borkent thinks it might be a small hymenopteran (bee?) because it has what looks to be long multi-segmented antennae. The only bee family recorded for the GRF by Wilson (1978) is Anthophoridae (now included within Apidae), of which this fossil clearly is not a representative. There are six other hymenopteran families recorded in that work, of which Tenthredinidae is the only one that seems plausible. Of course, it could represent a family not recorded by Wilson (1978). Collected along Hwy 139 in Douglas Pass (Garfield Co., Colorado).

Here is a closeup of the other fossil (far right in photo above). This looks to me like a brachyceran fly, and I’ve sent a high resolution version of the image to Chris Borkent to see what he thinks.

IMG_1934_enh_1080x720 IMG_1850_enh_1080x720
The label accompanying this fossil indicates “Mosquito (?),” but to my eye this looks like a true bug (order Hemiptera). It is small—only 5.9 mm in length—and has the gestalt of a plant but (family Miridae) or seed bug (family Lygaeidae). GRF fossils representing the latter but not the former were recorded by Wilson (1978). Also collected along Hwy 139 in Douglas Pass (Garfield Co., Colorado).


Wilson, M. V. H. 1978. Paleogene insect faunas of western North America. Quaestiones Entomologicae 14(1):13–34.

Copyright © Ted C. MacRae 2012

7 thoughts on “More Eocene insects

  1. The first thing that came to mind when I looked at the “aggregation of insects” was that they reminded me of Stratiomyidae larvae (Soldier Flies). The “elytra” may be disassociated parts of other insects, or some other debris.

    The second close-up is not of the right-hand specimen, but rather the “bee”. From what I can see of the right-hand specimen I do get a Dipteran gestalt.

    The bottom specimen is clearly Hemipteran. The very narrow head reminds me of something near Reduviidae.

    • I wondered about whether the “elytron” was actually associated with the insect or not, as the latter does not show normal beetle tagmatization. However, another specimen beneath the insect in the close-up also seems to have an elytron in the same position relative to the body.

      I agree with Diptera (poss. Brachycera) for the right-hand specimen in fossil #2.

      I think you could be right about the Hemipteran being reduvioid rather than lygaeoid due to the narrow head.

      • Regarding the “aggregation of insects” specimen. Could they possibly be Lithohypoderma sp.? The larvae and pupae of the insects are rather large and pretty common within the Green River Formation of Colorado and Wyoming. I am nowhere near an expert but just a thought. Lithohypoderma is currently placed in the Family Oestridae but I believe there is some debate over that.

  2. I think that you have a half-dozen or more botfly larvae in the first photo, but it is hard to be sure. There are areas (some around Douglas Pass) that have tens of thousands of the larvae which is odd, considering that I’ve never seen an adult botfly in the Green River Fm. The bottom specimen is most certainly Hemiptera, but I cannot go beyond the order. I’d like to send you a photo of a nice Mordellidae that I recently collected from the GRF, but don’t know where to send it.?

  3. They match pretty well with Lithohypoderma sp. as suggested by Gary Ratti, who also mentions some doubts as to whether this really is a bot fly. The lack of adults seems to support the doubts, although it could also just be a taphonomic artifact.

    I’d like to see the mordellid photo. Contact me through the “Email Me!” link above and I’ll give you an email address where you can send it.


Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s