Tiny treasures – 100 million year old mammal hairs trapped in amber

Mammal hairs preserved in amber specimen ARC2-A1-3. a – First fragment; b – Line drawing of first fragment; c – Second fragment; d – Line drawing of second fragment; e – Close-up of second fragment to show the cuticular surface.

About 100 million years ago, in a coastal forest located in what is today southwestern France, a small mammal skittered up the trunk of a conifer tree. As it did so it lost a few of its hairs, and this minor event would have been entirely unremarkable if two of those hairs had not settled in some tree sap and, in the course of time, become entombed in a piece of amber which has only recently been discovered. As described by scientists Rumain Vullo, Vincent Girard, Dany Azar, and Didier Neraudeau, this rare specimen gives us a clear look at what the hair of mammals was like during the time when dinosaurs were the dominant vertebrates on land.

Exceptionally-preserved fossil mammals with intact soft tissue remnants have been found before, such as a specimen of the 125 million year old cousin of placental mammals Eomaia, but fur trapped in amber provides a higher level of resolution – it is the original biological material. Likewise, even though hairs trapped in amber have previously been discovered, much of it has come from the past 65 million years, so even though we have every reason to assume that all mammals had hair since the time of their origin during the Mesozoic we don’t know very much about what that hair was like. The new specimen from France, specimen ARC2-A1-3, helps to fill this hole in our understanding.

According to the team of researchers, the two structures preserved within the amber are certainly hairs and not dinosaur “proto-feathers.” The hairs have a scale pattern only seen in the body covering of mammals, but what kind of mammal left these hairs behind? Without an already established sample of fossil mammals with exquisitely-preserved hair samples, it is impossible to tell. In the layer above the one which the amber was recovered the marsupial-relative Arcantiodelphys has been found, while a nearby site of similar age has also yielded a variety of mammal types belonging to Mesozoic mammal groups such as the eutriconodontans, spalacotheroids, dryolestoids, and multituberculates. The hairs could have come from any of these mammals. Nevertheless, the cuticula (or the protective outer layer of a hair formed by overlapping, shingle-like cells) of the hairs is very similar to that of modern mammals, signaling that this feature of mammal hair has been around for a very long time.

Interestingly, though, and contrary to the scenario I posited at the opening of this post, the hairs may not have come from a living mammal. The amber glob containing the hairs also preserved the tough outer shell of a larval fly, and so the researchers hypothesized that the hairs may have come from a dead mammal which was already being used as food source/nursery by flies prior to the amber preservation. If this is correct then it would be a rare look at insect-mammal interactions during the distant past, but since the alternative hypothesis of hairs left by an arboreal mammal cannot be ruled out scientists should take care in drawing inferences from this association.

Vullo, R., Girard, V., Azar, D., & Néraudeau, D. (2010). Mammalian hairs in Early Cretaceous amber Naturwissenschaften DOI: 10.1007/s00114-010-0677-8