The moss Macromitrium richardii (Orthotrichaceae) with sporophyte and calyptra enclosed in Hymenaea resin from the Dominican Republic

ominican amber is an important source for Early Miocene bryophytes. We report the moss Macromitrium richardii Schwägr., an extant representative of the Orthotrichaceae, from the Dominican amber collection of the American Museum of Natural History. This species is currently a widespread Neotropical epiphyte. The specimen includes several gametophytes and sporophytes, and represents the first fossil record of Orthotrichaceae. Alongside the Macromitrium shoots we observed several fragments of the liverworts Cheilolejeunea antiqua and Frullania sp. The unusual thermal behavior of the resin sample initially led to doubts about the Miocene age of the specimen, but chemical analyses of the Hymenaea resin provides evidence that the specimen represents a highly oxidized sample of Miocene Dominican amber rather than an artificially thermally-treated subfossil resin (copal). Our inclusion demonstrates the exceptional preservation potential of tree resin, but our observations also suggest that provenance (including any possibility that a modern resin has been thermally treated to make it appear older) should be scrutinized when single pieces with atypical thermal behavior and exceptionally well-preserved extant morphotypes come to light