Photo: The mite and spider pair; Credit: Jason Dunlop

The world's smallest fossil detected by 3D imaging in amber is a miniscule mite, which 49 million years ago decided to hitch a ride on a spider's head. The unusual pair, described in the latest Royal Society Biology Letters, has been entombed in amber ever since.

The mite measures less than than .2 mm long. Here's a closeup view:

Credit: Andrew McNeil

Mites are rare in the fossil record because their small size makes them difficult to study. Using a modern method called phase contrast X-ray computed tomography, Jason Dunlop of Humboldt University Berlin and colleagues successfully scanned the mite and spider in an ancient chunk of Baltic amber. The process enabled them to reconstruct the duo in vivid three-dimensional detail.

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Credit: Andrew McNeil

Here's the mite's underside:

Credit: Andrew McNeil

The mite belongs to a group called the Astigmata, previously almost unknown as fossils.

Dunlop and his team write that members of this group "breed rapidly under ideal circumstances, and in order to facilitate dispersal many of the more basal (i.e. non-parasitic) lineages employ phoresy: attaching themselves to larger arthropod carriers that transport them to hopefully better conditions."

They explain that free-living juveniles handle dispersal in this way.

These young mites "typically adhere to the carrier via a distinct attachment organ or 'sucker plate' composed of various suckers and mechanoreceptors on the underside of the body."

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This mite obviously didn't make it to where he wanted to go. A blop of tree resin dripped or otherwise fell on him and the spider, before fossilizing into amber. The spider is identified as being a member of the family Dysderidae, often commonly known as "cell spiders." Below is a modern member of this family:

Credi: Fritz Geller Grimm

The findings demonstrate that mites and spiders have experienced an intimate, but perhaps not enjoyable (from the spider's perspective), relationship over long geological history. Over this time the mites have evolved specialized adaptations, both in terms of their anatomy and life cycles, to take advantage of the relationship.

The discovery also suggests that Baltic forest conditions made this mite mode of travel necessary and desirable.

There's a lot of fake amber on the market now, but if you have real amber (ways of finding out are summarized here) try looking at it under a magnifying glass. You could be holding anything from prehistoric insects like these to dinosaur feathers.