Newly discovered ammonites in mid-Cretaceous Burmese amber

Amber is famous for preserving detailed records of terrestrial and soft-bodied organisms that have a low preservation potential elsewhere in the fossil record. Because preservation in amber is biased toward animals such as insects that live in close association with resin-producing trees, inclusions of marine fossils in Mesozoic amber are extremely rare. Here, we report unusual fossil assemblages consisting of ammonites preserved alongside terrestrial arthropods in Burmese amber. Under X-ray microcomputed tomography (CT), we made detailed observations of the preserved septa and sutures in the ammonite samples. On the basis of the suture structure, moderately involute shell, and smooth shell surface without ornamentation, we classified one of the new ammonites within the superfamily Desmoceratoidea. The partial filling of shell chambers with calcite suggests that the ammonites were buried for a short time in the sediment, which started the fossilization process prior to encapsulation in resin. Sand infills within the shell chambers indicate significant weathering and transport within a high-energy environment prior to encapsulation. The arthropod inclusions are represented by terrestrial and possibly aquatic associated forms belonging to Acari (mites); Hemiptera: Cimicomorpha: Vetanthocoridae; and Diptera: Muscomorpha. These insects are surrounded by powdered wood, insect frass, and plant fragments. The combined fossil assemblage seems to suggest that at least some amber-producing trees grew close to the coast, capturing a mixture of forest floor communities and partially fossilized shells redeposited from high-energy coastal waters.