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).

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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.

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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

A 50-million-year-old midge

Several of the insect fossils collected from the Green River Formation (45–50 mya) that I am photographing appear to be flies, and specifically members of the “primitive” suborder Nematocera. This is not surprising, as the G.R. Formation of Colorado, Utah, and Wyoming, is composed of shales derived from volcanic ash sediments that were laid down in a system of large, shallow lakes. Most (all?) nematoceran flies are aquatic to some degree in the larval stage, thus the adults are also closely associated with such habitats for mating and egg laying.

Diptera: Chironomidae | USA: Colorado, Garfield, Hwy 139, Douglas Pass

Diptera: Chironomidae | USA: Colorado, Garfield, Hwy 139, Douglas Pass

This particular fossil looked to me a lot like the more elegantly preserved fossil of another fly that I posted a few days ago, which at the time I thought represented a member of the family Mycetophilidae (fungus gnats) or Sciaridae (black-winged fungus gnats). Several knowledgable specialists offered their opinions in comments at this site and at Facebook’s Diptera forum (my thanks to all who offered their opinion), with most settling on Mycetophilidae and Vlad Blagoderov further suggesting subfamily Mycetophilinae. The fossil posted here seemed to me to represent a dorsal view of the same species, but, of course, I’m a coleopterist—so what do I know? Indeed, dipterist Dr. Chris Borkent believes this is actually a species of Chironomidae (common name simply “midges”)—also a nematoceran but differing from Mycetophilidae by their longer front tarsi and longer, relatively narrower wings. Males of the family have thickly plumose (“feathery”) antennae, which are not visible in this specimen and thus suggesting it might be a female. I wouldn’t doubt Chris’ identification  for a second, as he comes from good stock—his father is Art Borkent, a world expert on several families of nematoceran dipterans. Art also agreed after seeing the photo that it looked like a female chironomid midge, so that is what I am going with. Thank you, Chris and Art, for your help in identifying this fossil!

Complete fossil specimen (63 mm x 52 mm maximum each axis).

Copyright © Ted C. MacRae 2012

An elegant Eocene fly


USA: Colorado, Garfield, Hwy 139, Douglas Pass.

Here is one of the more elegantly preserved specimens among the collection of Green River Formation fossil insects that I am photographing. It is obviously a fly (order Diptera), but I don’t agree with the preliminary identification of “Mosquito?” as indicated on its label. Rather, I think it is one of the fungus gnats—also members of the suborder Nematocera and, thus, closely related to mosquitos (family Culicidae), but with distinctly elongate coxae (bases of the legs) and lacking the elongated proboscis that mosquitos use for sucking blood. It’s hard to decide between Mycetophilidae (fungus gnats sensu stricto) or Sciaridae (dark-winged fungus gnats), which differ in whether the eyes meet above the antennae (Sciaridae) or not (Mycetophilidae). However, Borrer & White (1970) mention that species of the former are generally less than 5 mm in length, while the latter range from 5–10 mm. This specimen measures 4.15 mm from the front of the head to the tip of the abdomen, so  maybe that is evidence supporting Sciaridae (although perhaps there were smaller mycetophilids 50 mya than today).

Here is a view of the whole fossil, measuring approximately 50 mm on each side:



Borrer, D. J. & R. W. White. 1970. A Field Guide to the Insects of America North of Mexico. Houghton Miffton Company, Boston, 404 pp.

Copyright © Ted C. MacRae 2012

Beetle, orthop or something else?

I had such helpful participation with my first fossil ID request that I thought I would go to the well again. This one is not so enigmatic as the first—it is clearly an insect, but it’s the only insect fossil among the batch that I haven’t settled on at least an order-level identification. Again, this is one of a set of 20 fossils loaned to me by a local collector for photographs and possible identifications, all coming from the Green River Formation in Colorado and dating back to the early to mid-Eocene (45–50 mya).

USA: Colorado, Rio Blanco Co., Parachute Creek Member. Body length = 11.05 mm.

USA: Colorado, Rio Blanco Co., Parachute Creek Member. Body length = 11.05 mm.

The label for this fossil indicates “Planthopper; Homoptera; Fulgoridae”; however, the short, robust legs and overall gestalt do not look right for either a planthopper or really any of the other hemipteran groups. What I see is an indistinct (mandibulate?) head, a distinct and well-developed pronotum, mes0- and metathoracic segments that are not nearly as heavily sclerotized as the pronotum but also lacking any sign of wings, a distinctly segmented abdomen with 9 or 10 segments, and short robust legs. I’m thinking an apterous/brachypterous coleopteran (Staphylinidae?) or a wingless member of one of the orthopteroid orders (although size alone excludes many of the latter—at more than 11 mm in length it is too large for something like Zoraptera). At first I thought the extension near the apex of the abdomen was a cercus, but I now think this is part of the piece of debris over the abdomen as there is no evidence of a cercus on the left side—another knock against something orthopteroid. Still, the lack of any trace of elytra—however shortened—keeps me from fully endorsing Coleoptera. Okay, so what do you guys think?

Copyright © Ted C. MacRae 2012

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