The brown moult of a beetle preserved in amber.

The insect moults were preserved in amber, long after the larvae themselves had decayed. Image © CN-IGME CSIC.

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Beetles may have eaten dinosaur feathers and lived in their nests

Beetles fed on the cast-offs of feathered dinosaurs over 100 million years ago.

The insects were frozen in time after being caught in amber, revealing rare evidence of how dinosaurs and insects interacted.

A modern household pest may have once been a dinosaur's best friend.

Beetle larvae were midway through a meal of feathers when they were caught up in sticky sap during the Early Cretaceous. Their remains, preserved in amber from northern Spain, are only the second time that feather-eating behaviour has been found in the fossil record.

The insects appear to be similar to modern skin beetles, or dermestids, which include species such as the carpet beetle that are pests of homes and museums. Over 105 million years ago, the ancestors of these invertebrates fed on the discarded feathers of dinosaurs, possibly helping to keep their nests clean at the same time.

Dr Ricardo Pérez-de la Fuente, one of the lead authors of the study, says, 'While it is unclear whether the feathered theropod dinosaur benefitted from the beetle larvae feeding on its detached feathers, it was most likely unharmed.'

'Our research shows that the larvae did not feed on living plumage and lacked the defensive structures of modern dermestids which can irritate the skin of nest hosts and even kill them.'

The findings of the study were published in the Proceedings of the National Academy of Sciences.

A photograph of tree resin congealing on the bark of a tree.

Amber is made from fossilised tree resin, which can trap unsuspecting invertebrates as well as other biological material. Image © sruilk/Shutterstock.

How does amber form?

The beetles were preserved in amber, a type of fossil that is well-known for preserving the remains of plants and animals.

Amber is made of tree resin, a substance produced by conifers and some other species. It is often produced in response to damage, flowing into wounds before hardening into an impermeable substance.

Any living organism close enough to these trees can become caught in the resin, with amber deposits found to contain a range of life from bacteria to snails, worms and millipedes. As more resin flows around them, they become suspended in it, trapped and unable to escape.

If the resin is not disturbed, then it begins to harden as its chemistry changes. The molecules it contains form long polymers which are very resistant to being broken down, giving it a better chance of surviving being buried and fossilising.

At this point, the material is known as copal, which is still relatively soft. It takes many millions of years for the resin to fossilise fully, during which time most of the soft tissue is likely to have been lost.

This meant that when the researchers found the insect larvae in amber, they were instead looking at moulted exoskeletons. Insect exoskeletons are made of a hard substance known as chitin which preserves much more easily, while the feathers are made of another hard tissue, known as keratin.

The age of the amber means that these feathers must have come from a dinosaur, as the ancestors of modern birds would not appear for another 30 million years or so. Unfortunately, the structure of the feathers isn't enough to identify which type of theropod they came from.

The amber's other contents, however, are much more revealing.

A fragment of amber, with inset images showing a beetle moult and the pigmentation pattern of a dinosaur feather.

The insect moults (inset top) were found closely associated with partly-decayed dinosaur feathers (inset bottom), suggesting the insects could have lived in dinosaur nests. Image © CN-IGME CSIC.

How did dinosaurs and beetles interact?

Inside the amber, the researchers found moults of a variety of shapes and sizes. The largest were up to twice as big as the smallest, and also had different mouthparts.

Though it might seem like they were different species, the researchers found enough similarities between them to suggest they came from the same species. Instead, it's likely that the bigger moults came from older larvae which had more time to develop.

Their characteristics meant that the insects were identified as being dermestid beetles. These animals are one of the few animals able to digest keratin, making them an important part of nutrient recycling in modern and ancient ecosystems.

While it's possible that the feathers were being fed on while they were attached to the host, the researchers think this is very unlikely. Some of the feathers and moults are surrounded by minute fossil faeces, known as coprolites, and have fungi growing in them.

This makes it much more likely that the feathers had fallen off, probably into a nest.

'This is hard evidence that the fossil beetles almost certainly fed on the feathers and that these were detached from its host,' says Dr Enrique Peñalver, the study's first author. 'The beetle larvae lived in accumulated feathers on or close to a resin-producing tree where they fed, defecated and moulted, until a flow of resin serendipitously captured them.'

While the insects likely lived in the dinosaur nests, it's not quite clear what they were doing there. The relative lack of specialised setae hairs on the insect, which can irritate and affect the health of hosts, means that it probably wasn't a parasite, but it's hard to know anything further.

Even in living species, it can be hard to prove the effect of any relationship between invertebrates and their hosts, even in living species.

While it's possible that the beetles and the dinosaurs had a mutually beneficial relationship known as a symbiosis, it's just as likely they were in a commensal one. In this case, the dinosaurs would have felt no impact from having beetles in their nests.

When birds evolved later on, it's thought that some of the dermestids made the jump to eating their feathers instead. Following the extinction of the dinosaurs, these beetles would have eventually given rise to dermestids alive today.