Insects found on dinosaur feathers are not that kind of parasites

In December 2019, I was in New York City, and while waiting for my husband to recover at the hospital, I came across an article in The Guardian showing a photo of an insect in amber that looked very familiar to me. After reading the title: Dinosaurs had feathers ruffled by parasites, study finds. My first thought was : Very weird, this really looks like a baby scale insect. I should take a look at the original article later.

From The Guardian : https://www.theguardian.com/science/2019/dec/10/dinosaurs-had-feathers-ruffled-by-parasites-study-finds

A day later, I received an email from my former Ph.D advisor, David Grimaldi. He forwarded to me the original article and asked my opinion on the study because the specimens strangely looked a lot like scale insects to him as well.

Gao et al. (2019) described new insects in amber covering fragments of dinosaur feathers and through their interpretation of the morphology, concluded that they were parasites feeding on dinosaur feathers, related to lice. The interpretative drawing shows an insect with chewing mandibles holding onto feathers.

There were important problems about this discovery to me. The identification of the insect was not correct and most importantly, it garnered mainstream media attention. We thought that a quick response was necessary to point out the mistake. A couple of days later, I was back at the American Museum of Natural History after a few years, and was going through Burmese amber pieces for additional scale insect nymphs. In the meantime, we received additional high resolution photos of the described “lice” from the authors, which allowed to confidently confirm that they were scale insects.

Scale insects nymphs are commonly found in amber deposits, and this is not surprising. This developmental stage is the dispersing one, and scale insect nymphs do it so well that they are also coined “crawlers”. Scale insects crawlers can be found everywhere on a plant and the soil as soon as there is an infestation. So given the fact that amber inclusions are made of tree resin falling on the ground and catching insects on its way, crawlers are very likely to end up inside amber.

But why didn’t Gao et al. identify them as such? The main reason is that fossil scale insect nymphs are overlooked and not often described. And this is because the way scale insect systematics was built up. Coccidologists identify and describe extant scale insects using in majority adult female morphology. Also, staining of the insect skin and slide preparation are necessary to provide a description of the setae and secretion pores. By contrast, fossil scale insects are very often found as adult males (which are also mobile and completely different from the females). Although more challenging because slide preparations are not possible, they can still be described because of the exceptional preservation amber allows. Then finally, scale insect nymphs are minute and even though we have descriptions for some of them, it is not easy to examine and describe them at the species level, unless we have also collected adult females. The same situation goes for amber pieces, isolated scale insect juveniles, without an adult stage in the same piece, are usually not described.

After discussing how we would present evidence that the insects in Gao et al. 2019 are indeed scale insects, we drafted a response and submitted it to Nature Communications, 9 days after the original articles was published. It would take over a year of reviewing and communicating with the journal’s editor to finally have it accepted. In the meantime, over the span of one year, Gao et al. 2019 was cited 14 times in other studies published in scientific journals.

Why are they not dinosaur feather parasites?

Figure from Grimaldi and Vea 2021. A-B: previously published photo and drawing of scale insect nymphs. C. Photo of a specimen from the AMNH collection taken by David Grimaldi. D-G: Photos and drawings from Gao et al., 2019

Mouthparts

The major misinterpretation from Gao et al, 2019 is the mouthpart description. The interpretative drawing (Figure below) is not realistic in terms of mouth position. Lice (the group they assigned the fossil to) mouthparts are not positioned ventrally, but are normally extending forward of the head. But on the fossil specimens, the mouthparts position is indeed ventrally, which is characteristic to the mouth location of Sternorrhynchan insects, a subgroup of the Hemiptera order that have sucking mouthparts located very ventrally, usually between the first pair of legs. Sternorrhynchans are strictly feeding on plant sap and include scale insects.

Figure 2 in Gao et al., 2019

The authors also interpreted the mouthparts as chewing type, but none of the specimens examined show clear chewing mandibles. A photo on their publication actually shows external mouthparts that contain a sucking stylet – similar to a straw, as found in scale insects . In one of the specimens, that was drawn by the author, we can see the stylet coiled in the body (See on Figure D for their fossil specimen and A for a comparison with a living scale insect nymph).

Legs

To justify the feeding behavior on dinosaur feathers, Gao et al interpreted the fossil single claw on the insect leg as grasping claws on feathers. However, these claws are not specialised in the way we find in lice species. Scale insects are uniquely characterised with legs carrying a fine single claw (as opposed to two claws in other insects). This claw also bears two long small extensions that are called digitules and they are visible on the photos of one of the specimens they published (on the Figure, see G “setae” label on the leg, and B and C for comparison).

Head thorax abdomen separation

A characteristic of scale insect nymphs is the absence of head-thorax-abdomen separation, a feature that is retained in adult females. The fossils clearly do not show constrictions although the interpretative drawing exaggerated the separation between head thorax and abdomen. Lice do have well separated body parts.

Why were scale insect nymphs on dinosaur feathers?

As mentioned previously, scale insect nymph are common in amber because they were easily captured by resin leaking out of tree trunks. Feathers can also be captured by resin falling on the ground. So it is not surprising to see a co-occurrence of these two in the same amber pieces that is coincidental.

I know, not as sensational as dinosaur parasites.

References

Gao, T., Yin, X., Shih, C. et al. New insects feeding on dinosaur feathers in mid-Cretaceous amber. Nat Commun 10, 5424 (2019). https://doi.org/10.1038/s41467-019-13516-4

Grimaldi, D.A., Vea, I.M. Insects with 100 million-year-old dinosaur feathers are not ectoparasites. Nat Commun 12, 1469 (2021). https://doi.org/10.1038/s41467-021-21751-x

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