The oldest deep-sea Ophiomorpha and Scolicia and associated trace fossils from the Upper Jurassic–Lower Cretaceous deep-water turbidite deposits of SW Bulgaria

Deep-sea trace fossils of the Nereites ichnofacies occur in the uppermost Kimmeridgian–Berriasian deep-water turbidite deposits of the Kostel Formation in SW Bulgaria. The trace fossil assemblage contains Tithonian Ophiomorpha isp. and Ophiomorpha annulata, which are the oldest deep-sea representatives of this ichnogenus. They indicate much older invasion of Ophiomorpha into deep-sea, which was hitherto dated as to mid-Cretaceous. It is not clear whether the invasion failed or Ophiomorpha survived in deep-sea refuge and became widespread after improving oxygenation of the deep seas after the Cenomanian/Turonian anoxic event. Tithonian Scolicia, produced by irregular echinoids, exhibits a similar distribution pattern.