Article: A new rhabdopleurid hemichordate from the Middle Cambrian of Siberia

Rhabdopleura obuti sp. nov. is described from the late Middle Cambrian Mayan Stage of the Sukhan Depression, Siberia, and is the second rhabdopleurid to be described from the Cambrian. It is probably one of the oldest' living fossils', remaining unchanged for over 520 Ma. This colonial pterobranch consists of stolonal and zooidal tubes. The creeping portion commonly found in most pterobranchs is reduced, with the colony adopting an erect growth habit. The stolonal tubes frequently show dichotomies and contain stolons. It is the earliest record of stolons. The zooidal tubes widen distally and occasionally show branchings to form other zooidal tubes. Fuselli are common and possess a very irregular zig-zag suture. There is no thecal dimorphism. There are several dark bodies occurring within the zooidal tube which have been interpreted as dormant buds and zooidal material. The best preserved zooid occurs in an open-ended tube and is cigar shaped (0-7 by 0-2 mm) and attached proximally to a stolon. This is the earliest record of zooid material. A review of hemichordate zooids in the fossil record is presented. Rhabdopleura obuti is compared with other pterobranchs and graptolites, with the conclusion that it represents an early rhabdopleurid but has characters which represent the start of the graptolite evolutionary story.