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- Astogeny in Graptolithoidea -
                                   Astogeny in Graptolithoidea by Adam Urbanek (2004)

A. - C. Early development of a rhabdopleurid pterobranch and a tuboid graptolite
colony. A. Encapsulated larva after metamorphosis (A1) and primary zooid in
Rhabdopleura compacta Hincks (A2). B, C. Comparison of sicular portions in Recent
Rhabdopleura compacta (B) and in Ordovician graptolite Epigraptus Eisenack (C). D.
Sicula (D1) and thecae with underlying stolon system (D2) of an Ordovician dendroid
graptolite Dendrograptus sp. E. Zooidal tubes and internal stolon system in Recent
Rhabdopleura normani Allman. Not to scale.

Source: Urbanek, A. 2004.Morphogenetic gradients in graptolites and bryozoans. - Acta Palaeontologica Polonica 49, 4, 485-504.
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Related pages:
Siculae of tuboid graptolites
Sicula of a crustoid graptolite
Larval 'cocoon' in Cephalodiscus
Early developmental stages  of graptolite colonies can be best interpreted by
comparison with early stages of the colony formation in extant Rhabdopleura. All
graptolite colonies start with a primary zooid placed within a peculiar theca called
sicula. The sicula is composed of prosicula (proximal, initially formed part of sicula,
secretd as one unit) and metasicula (distal part of sicula with normal graptolite
growth lines).