Middle Cambrian Rhabdopleura obuti Durman & Sennikov.

ABSTRACT. 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 branching 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 occuring 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 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.