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| _____________________________________________________________________________________ Graptolite Net edited by Piotr Mierzejewski, the Count of Calmont and Countess Maja Anna Korwin-Kossakowska 2004 |
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| Graptolite Net | Graptolites & Graptoliters | Evolution of Graptolites | Cephalodiscoidea | Rhabdopleuroidea |
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| DORMANCY OR DIAPAUSE IN GRAPTOLITES by PIOTR MIERZEJEWSKI |
| The phenomenon of diapause is poorly understood in graptolites. Only a few authors have dealt with this problem, and they focused almost exclusively on special dormant bodies called graptoblasts. However, thecal occlusion seems to be the most common diapause adaptation among fossil graptolites and extant cephalodiscids. O.M.B. Bulman defined occlusion as a "sealing of thecal aperture by sclerotized film". These structures have been recognized in the Graptoloidea, Dendroidea, Tuboidea, but most often in the Camaroidea. The abundant occlusion of autothecae is a striking morphological feature of some camaroid graptolite colonies. It is commonly believed that the sealing of the autothecae in the Dendroidea and in the Graptoloidea is related to the degeneration, atrophy or necrosis of zooids. On the other hand, R. Kozłowski compared the occluded autothecae of camaroid graptolites to the gonozooids or ovicells of extant cyclostomatous bryozoans. According to him, the camaroid autothecae were occupied by normal active zooids before their occlusion; after sealing their apertures with diaphragms, they degenerated in order to make sppace for their own eggs or embryos. He described and illustrated two types of occlusion in camaroid autothecae, dependent on their shape: (1) autothecae with a collum were occluded by irregular lamellae near the base of the collum, and (2) autothecae devoid of a collum were occluded by diaphragms deposited directly over the apertures. Additional forms of occlusion structures have since been observed (Mierzejewski 2003); for example, in an Ordovician dendroid-like camaroid (Mierzejewski, Maletz and Sudbury in preparation) the distinctly differentiated collum is occluded by a thick, distinctly thimble-shaped "stoppers" inserted in the collum just beneath its aperture (Fig. B).The lack of continuity between the autothecal periderm and the "stoper" is remarkable. On the other hand, autothecae in the Tuboidea (closest relatives of the Camaroidea) are occluded by a thick diaphragm made of cortical tissue which merges directly into the thecal cortex (Fig. C). Autothecal occlusion in the camaroid graptolite Xenotheka klinostoma Eisenack, 1937 differs sharply from other known camaroid and other graptolites: it is made of a unique material, the verrucose fabric, and it spreads across the entire outer surface of the autotheca (Fig. A). |
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| B |
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| Sealing of the thecal aperture (occlusion) in some sessile graptolites. SEM micrographs. A. Xenotheka klinostoma Eisenack, 1937, Llandeilo of Poland. Distal part of autotheca. B. Camaroid gen. et sp. nov. Ordovician erratic boulder. Distal part of autotheca. C. Epigraptus kozlowskii Mierzejewski, 1978. Lower Ordovician of Estonia. Bitheca on the thecorhiza surface. Abbreviations: c, camara; co, collum; o, occlusion; p, apertural processus; t, thecorhiza, v, verruca of verrucose fabric. _____________________ Based on: Mierzejewski, P. 2003. Autothecal morphs and dormancy in the camaroid graptolite Xenotheka. - Acta Palaeontologica Polonica 48 (1): 93-98. |