Amber can be a veritable treasure trove of ancient animals and insects, but it most commonly captures creatures that lived in forests – understandable, given the stuff starts life as tree sap. But now researchers from the Nanjing Institute of Geology and Palaeontology of the Chinese Academy of Sciences (NIGPAS) have found a piece of amber bearing a strange mix of land and sea-dwelling creatures.

Found in northern Myanmar, the amber measures 33 x 9.5 x 29 mm (1.3 x 0.4 x 1.1 in) and weighs just 6.08 g (0.2 oz) – but there's a lot crammed into that tiny space. All up, there are at least 40 individual animals trapped in this amber. Mites are the most common remains, but there are also spiders, millipedes, cockroaches, beetles, flies and wasps. All of these would have been living in and around trees, and are found in amber on a fairly regular basis.

But they were accompanied by some strange bedfellows – sea snails, sea slaters and an ammonite were all found in the piece, too. Marine animals are rare inclusions in amber, for the obvious reason that they don't come into contact with trees all that often.

So how did it happen in this case? The researchers say the most likely story is that the amber was found in a place that was once a coastline, where a shell-littered beach met forest. The flying insects were probably the first victims, getting stuck while the sap was still in the trees on the edge of that forest. The crawling insects were probably next as it flowed down the trunk, and finally onto the beach, where it captured the shells and sea creatures.

A vital clue to this story is the fact that the snail and ammonite shells were empty, and no soft tissue was preserved inside. In the case of the ammonite, the outer layer was broken away, and the entrance to the shell was full of sand. That suggests that the shell had been laying on a beach for a long time before the sap entombed it.

This marks the first time an ammonite has been found in a piece of amber, and the shell is so well preserved the researchers were able to identify the species. Using X-ray micro-computed tomography (micro-CT) scans to take 3D images of the shell's structure, the team determined that the ammonite was a young Puzosia (Bhimaites). From there they were able to confirm an age of about 99 million years.

All kinds of amazing finds have turned up in amber in recent years, including red blood cells, dinosaur feathers, baby snakes, an unknown species of spider with a tail, brand new orders of insects, and even a dinosaur tail, complete with feathers and soft tissue.

The new find was published in the journal PNAS.

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