Rhabdopleura normani Allman, 1869

Mierzejewski, Piotr & Kulicki, Cyprian, 2003, Cortical fibrils and secondary deposits in periderm of the hemichordate Rhabdopleura (Graptolithoidea), Acta Palaeontologica Polonica 48 (1), pp. 99-111 : 106

Rhabdopleura normani Allman, 1869

Fig. 7 .

Thespecimenstudiedisasmallfragmentofcoeneciumconsisting of a branching creeping tube joined to an erect tube ( Fig. 7A ). Its periderm is composed of typical fusellar components, with a very distinct zig−zag suture in the creeping tube, and the classical ring−like segments with irregularly distributedobliquesuturesintheerecttube;theshapeandarrangement of fuselli are very similar to those of Rhabdopleura compacta .Theheightofthefusellivariesaccordingto their position in the coenecium: 32–55 µm for the creeping tube, 15–45 µm for the erect tube. Fusellar collars of the creeping tube are distinctly less prominent than in Rh. compacta ( Fig. 7D ). On the other hand, collars of the erect tube are rather large, with an indistinct band−like structure composed of cortical fibrils ( Fig. 7G ). These cortical fibrils arequiteclearwithadiameterof 270–510nm.Thecondition ofthecoenecium’ssurfacevaries;largeareasofperidermare covered with foreign material, but some parts are rather smooth and reveal details of fine structure. There is no evidenceforsecondarydepositsoneithertheouterorinnersurfaces.Someareaslackthefusellarpellicle,altogetherandrevealthepresenceofextremelydenselypackedfusellarfibrils ( Fig. 5E). The fusellar fibrils seem to be rather short,wavy, and anastomosing with a diameter of 70–105 nm. Broken edgesofperidermrevealfurtherdetails.Asinextinctgraptolites,thebulkofthefibrilsisconcentratedjustbeneaththeinner and outer surfaces of fuselli ( Fig. 7E ). Occasionally, the outer fusellar surface appears to be longitudinally striated ( Fig.7F ),butthissculptureisnothingmorethanwrinklingof the periderm.

Figures

Fig.7. Rhabdopleura normani Allman,1869,Recent(ZPALPb.5/4).SEMmicrographs. A.Fragmentofcoeneciumcomposedofanerecttubeconnected withabranchedcreepingtube. B.Connectionofcreepingtubewitherecttube. C.Erecttube. D.Outersurfaceofcreepingtubedevoidofpellicle. E.Details ofbrokenperidermshowingarrangementoffusellarfibrils. F.Shortverticalfibrilsonperidermoferecttube. G.Peridermofthesameerecttubedevoidof verticalwrinkles.Abbreviations:b,band−likestructurecomposedofcorticalfibrils;c,creepingtube;co,fusellarcollar;e,erecttube;ff,fusellarfibrils;m, foreign material; o, oblique suture; v, wrinkle.