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
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
colonies of Bulmanicrusta? sp. encrusting the
surface of a hardground from the Upper Ordovician of
Ohio, U.S.A.
 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 Ă–pik (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
(Ă–pik 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-
Portal of graptolites and pterobranchs sponsored by M.M. plus M. Sztandary - Flags Fahnen