| Graptoblasts and Paleoecology of the Crustoidea, primitive sessile graptolites by Piotr Mierzejewski ____________________________________________________________________ |
| As is well known, there is a very extensive literature on the paleoecology of the Graptolithoidea. However, paleoecologic knowledge of crustoid graptolites is still insufficient. The main reason for this situation is that they are extremely rare fossils and their known record is often limited to tiny fragments of isolated autothecae (Kozłowski, 1962; Mierzejewski 1977, 1984, 1985, 1988). Only in exceptional cases, one can find small fragments of colonies consisiting of uniserial or biserial chains of autothecae. The nature of the fossil record is easily explained, because most fossils were obtained by chemical methods. The morphology of crustoid autothecae suggests that they were attached to the substrate by their lower, structureless surfaces and formed incrusting colonies, resembling those of recent Rhabdopleura (Kozłowski 1962, 1966). Combining ecological information about mode of life and occurence of living rhabdopleurids with the striking similarity in morphology between the Crustoidea and the Rhabdopleurida, one may suppose that crustoid graptolites built colonies encrusting a firm substrate at moderate depth in relatively cold water. It is remarkable that chemically isolated crustoids were usually derived from marly limestones yielding tabulates, bryozoans, brachiopods, annelids, molluscs, pterobranchs and various benthic graptolites. |
| Unexpectedly, graptoblasts, in spite of their rarity, have begun to play an important role in discussions on paleoecology of the Crustoidea. First, Urbanek (1983) assumed that the presence of graptoblasts in crustoids was an adaptation allowing the species to survive adverse conditions when the rest of colony desintegrated. This point of view was fully supported by the important observations of Mitchell et al. (1993) on in situ colonies of Bulmanicrusta? sp. encrusting the surface of a hardground from the Upper Ordovician of Ohio, U.S.A. |
| Graptolite Net I Graptolites & Graptoliters I Rhabdopleuroidea | Cephalodiscoidea | Crustoidea | Camaroidea | Tuboidea |
 |
| These authors were of the opinion that crustoids routinely produced graptoblasts, which presumably should treated as a normal part of crustoid ecology. According to them, the Crustoidea 'were probably ecological opportunists of very low to low spatial persistence, and were adapted to inhabiting ephemeral or disturbing prone habitats in a shallow-water, low-latitude environment' (Mitchell et al. 1993: pp. 1014-1015). It is worth mentioning that Professor C.E. Mitchell in his review of this paper wrote as follows: 'The reproductive role of graptoblasts (...) seems to me entirely consistent with our view of these organisms as pioneering encrusters. The ability to produce mobile asexual larvae quickly following the return of favourable conditions would be of clear benefit to an opportunist colonizer of hard substrates.' |
| Silurian graptoblast (graptoblastoides stage) from Anticosti Island (Beescie Formation, locality A-5C). |
| It is interesting to note here some entirely different observations made by myself (Mierzejewski, unpublished). In 1985, I had the opportunity to study several in situ graptolite colonies encrusting surfaces of different calcareous fossils, namely nautiloids and echinoderms, originating from Ordovician deposits of Estonia. These forms, now housed at the Institute of Geology, Technical University of Tallinn (Estonia), were previously described Öpik (1928, 1930) and Obut (1960, 1964) as dendroids and dithecoids. Kozłowski (1962 and personal communication 1975) regarded them as members of the Crustoidea; Bulman (1970) allocated them in the crustoid family Hormograptidae. When studying these graptolites, I found that the majority of colonies were provided with distinct terminal stolothecae, but there were no traces of graptoblasts. These colonies were very similar in shape to that of Mitchell et al. (1993). Therefore, one may suppose that graptoblasts may be not constant constituents of crustoid colonies, at least in the Hormograptidae. Moreover, the fact that two colonies of graptolites under discussion, assigned to Hormograptus ramulus (Öpik 1928), were found on the inner surface of a nautiloid shell shed new light on the crustoid mode of life. Thus, it is now clear that not all members of the Crustoidea were inhabitants of disturbance-prone envoronments, as it was suggested by Mitchell et al. (1993), because some of them inhabited cryptic environments, such as concave molluscs shells (as do Recent forms of Rhabdopleura, see Stebbing 1970). Presumably, there were any number of other suitable host shells for crustoids, e.g., empty skeletons of echinoderms, moults of arthropods, shells of brachiopods, and holes in rocks made by various borers. Such habitat was very favorable for colony survival and avoiding adverse conditions. It seems possible that crustoids inhabiting cryptic environments did not produce graptoblasts at all. Based on: Mierzejewski, P. 2000. On the nature and development of graptoblasts. - Acta Palaeontologica Polonica 45, 3, 227-238. |
 |
 |
 |
 |
 |
 |
 |
 |
| GRAPTOLITE NET is edited by Piotr Mierzejewski, the Count of Calmont and Countess Maja Korwin-Kossakowska since 2002 |
| Portal of graptolites and pterobranchs sponsored by M.M. plus M. Sztandary - Flags Fahnen |