Acta Palaeontologica Polonica

An aberrant encrusting graptolite from the Ordovician of Estonia

Piotr Mierzejewski

Acta Palaeontologica Polonica 45 (3), 2000: 239-250

An organic microfossil, Erecticamara maennili gen. et sp. n., superficially similar to some imperfectly preserved chitinozoans, is described as an aberrant camaroid graptolite from the Lower Ordovician Kunda Stage, Aluoja Substage, of the Tallinn area, North Estonia. Its elongated, bottle-shaped or subconical thecae, interpreted as autothecae, are differentiated into a broader proximal part (camara), provided with a convex, rarely flat, bottom, and a narrower distal one (collum), devoid of any kind of apertural processes. The wall of the fossil is made of the fusellar tissue; irregularly distributed oblique sutures of fuselli are not arranged in a zigzag line. A sudden change of fuselli width leading to an appearence of the microfusellar tissue is sometimes observed in the distal part of the tube. The presence of primitive cortex (paracortex? pseudocortex?) is suggested. Robust, elongated vesicles are found inside two autothecae and interpreted as a dormant structure, tentatively compared with cysts of crustoid graptolites or a blastocrypt of graptoblasts. Its upper wall is situated between the camara and collum and looks like a sclerotized diaphragm described in other camaroids. The fossil unites certain characters of cephalodiscid pterobranchs and camaroid graptolites but is not interpreted as truly transient link between these two hemichordate groups.

Key words: Graptolites, Camaroidea, microfossils, ultrastructure, dormancy, Lower Ordovician, Estonia.

Piotr Mierzejewski [], Instytut Paleobiologii PAN, ul. Twarda 51/55, PL-00-818 Warszawa, Poland. Present address: ul. Filtrowa 83 m. 49, PL-02-032 Warszawa, Poland.

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