Amber

Abstract

The amber fossil record provides a distinctive, 320-million-year-old taphonomic mode documenting gymnosperm, and later, angiosperm, resin-producing taxa. Resins and their subfossil (copal) and fossilized (amber) equivalents are categorized into five classes of terpenoid, phenols, and other compounds, attributed to extant family-level taxa. Copious resin accumulations commencing during the early Cretaceous are explained by two hypotheses: 1) abundant resin production as a byproduct of plant secondary metabolism, and 2) induced and constitutive host defenses for warding off insect pest and pathogen attack through profuse resin production. Forestry research and fossil wood-boring damage support a causal relationship between resin production and pest attack. Five stages characterize taphonomic conversion of resin to amber: 1) Resin flows initially caused by biotic or abiotic plant-host trauma, then resin flowage results from sap pressure, resin viscosity, solar radiation, and fluctuating temperature; 2) entrapment of live and dead organisms, resulting in 3) entombment of organisms; then 4) movement of resin clumps to 5) a deposition site. This fivefold diagenetic process of amberization results in resin→copal→amber transformation from internal biological and chemical processes and external geological forces. Four phases characterize the amber record: a late Paleozoic Phase 1 begins resin production by cordaites and medullosans. A pre-mid-Cretaceous Mesozoic Phase 2 provides increased but still sparse accumulations of gymnosperm amber. Phase 3 begins in the mid-early Cretaceous with prolific amber accumulation likely caused by biotic effects of an associated fauna of sawflies, beetles, and pathogens. Resiniferous angiosperms emerge sporadically during the late Cretaceous, but promote Phase 4 through their Cenozoic expansion. Throughout Phases 3 and 4, the amber record of trophic interactions involves parasites, parasitoids, and perhaps transmission of diseases, such as malaria. Other recorded interactions are herbivory, predation, pollination, phoresy, and mimicry. In addition to litter, amber also captures microhabitats of wood and bark, large sporocarps, dung, carrion, phytotelmata, and resin substrates. These microhabitats are differentially represented; the primary taphonomic bias is size, and then the sedentary vs. wandering life habits of organisms. Organismic abundance from lekking, ant-refuse heaps, and pest outbreaks additionally contribute to bias. Various techniques are used to image and analyze amber, allowing assessment of: 1) ancient proteins; 2) phylogenetic reconstruction; 3) macroevolutionary patterns; and 4) paleobiogeographic distributions. Three major benefits result from study of amber fossil material, in contrast to three different benefits of compression-impression fossils.