Detection of in situ resinous traces in Jurassic conifers from floras lacking amber
Abstract
Amber deposits are rare in Jurassic successions, occurring in small quantities, whereas Lower Cretaceous strata host many substantial and commonly fossiliferous amber deposits worldwide. Minor amounts of Early Jurassic amber have been reported from Italy, and small amounts of Late Jurassic amber are known from Lebanon, Jordan and Thailand. Other Jurassic amber deposits that require reinvestigation of their age and provenance have also been reported from Denmark and France. Few of these amber deposits contain fossils, the others lack inclusions, suggesting a ‘Jurassic amber gap’ in the fossil record. Here, we surveyed fossil plant collections held at the Swedish Museum of Natural History, Stockholm, for amber and amberproducing plants from Jurassic floras. We focused on the collection of plants from Shaanxi, China and the Middle Jurassic floras of Iran and Afghanistan from the H.-J. Schweitzer collection. Using a hand-held UV light microscope, we scanned the collections for resinous remains as ambers can show autofluorescence with UV light, but found no dispersed amber fragments or droplets. Some researchers refer to fossilized resin within plant tissues under that name to differentiate it from exuded resins preserved as amber, and we follow this convention. We identified a conifer fragment of Elatocladus sp. from China with in situ rods of resin preserved in the leaves and a unique conifer twig (Elatides sp.) from Afghanistan with tiny linear resin traces in leaves that were only visible via autofluorescence with UV light. These resinous traces likely define the former position of resin canals in the leaves, but the resin is not preserved as in situ rods. Instead, it has impregnated the coalified mesophyll, likely during fossilization, to form thin lines (chemical ‘ghosts’ of preserved resin) within the conifer leaf remains.