EXCEPTIONALLY PRESERVED FIREWORMS FROM THE CRETACEOUS OF LEBANON IDENTIFIED FROM THREE DIMENSIONALLY PRESERVED MYOANATOMY

Muscle tissue has an unequivocal fossil record that dates back as far as the Cambrian, it has featured in few evolutionary studies and is often treated only as a taphonomic curiosity. We describe a new polychaete from the Lagerstätten of Hakel and Hadjoula, northwest Lebanon, in which musculature is phosphatized in exquisite detail. All of the major muscle groups that characterize extant polychaetes are identifiable, including the circular and longitudinal muscles of the body wall, gut muscle and the parapodial muscle complex. To allow meaningful comparison with the phosphatized fossil specimens, extant polychaetes were stained with iodine and visualised using micro-CT. The specimens possess two pairs of dorsal longitudinal muscles, dorsal and ventral circular muscles, and a single pair of ventral longitudinal muscles. While six longitudinal muscle bands are known from other polychaete groups, their presence in combination with circular muscles is unique to Amphinomidae. This is the first time a fossil organism has been diagnosed taxonomically based on myoanatomy alone. These findings demonstrate the capacity of the fossil record to provide zoomorphological myoanatomical detail when the right conditions are met and also the utility of microCT as an effective tool for fast myoanatomical comparison between extant and fossil organisms. The fine preservation of decay prone muscle tissue and poor preservation or total absence of recalcitrant structures such as cuticle and chaetae highlight an important taphonomic paradox, demonstrating that preservation and decay of different tissue types are decoupled during fossilisation.