is edited by
Piotr Mierzejewski, the Count of Calmont and Countess Maja A. Korwin-Kossakowska
since 2002
The hemichordate Class Graptolithoidea
Contrary to the prevailing consensus among paleontologists that the
graptolites are an extinct group, Professor Peter N. Dilly (London), an
authority on pterobranchs, claimed that the Recent pterobranch
Cephalodiscus graptolitoides Dilly, 1993 is probably
classifiable as a living graptolite. However, it is important to note
that in the past, a similar conclusion was made by an eminent Russian
zoologist Professor Vladimir N. Beklemishev (1951, 1970), author of
the famous "
Principles of Comparative Anatomy of Invertebrates",
translated into several languages. He erected the class
Graptolithoidea to embrace two diverse taxa: the extinct class
Graptolithina (with the orders
Stolonoidea, Camaroidea, Tuboidea,
Dendroidea, and Graptoloidea), and the extant class Pterobranchia
(with the orders
Rhabdopleuroidea and Cephalodiscoidea).
    The most important common feature of the Graptolithoidea (with
the exception of enigmatic living
Atubaria Sato, 1936) is the presence
of an organic sclerotized exoskeleton composed of growth bands
called fusellar increments or simply fuselli. As stated by Professor  
Roman Kozłowski (1947, 1949, 1966), fusellar tissue is not known in
any other fossil or living animal (for summary of ultrastructural and
biochemical study of this tissue see
Mierzejewski & Kulicki 2001).
The presence of such a peculiar structural element makes the
Graptolithoidea a very coherent group. It is remarkable that
morphological differences between some living and fossil
Graptolithoidea are less pronounced than the differences between
the sessile and plaktic orders within this class, as was stated by
Professor Adam Urbanek (Warsaw) in his important paper entitled
The enigma of graptolite ancestry: Lesson from phylogenetic
" (Urbanek 1986).

    As it was pointed by Urbanek (1986), Beklemishev (1951, 1970)
erected the class Graptolithoidea  anticipating the results of future
submicroscipic and molecular investigations and eliminated the
actualistic bias in the recognition of the Graptolithina and
Pterobranchia as taxa of the same rank. Recent morphological and
ultrastructural investigations by
Urbanek & Dilly (2000) and
Mierzejewski & Kulicki (2001) on the stolon system and periderm of
the fossil and living Rhabdopleuroidea seem to support decisively
Beklemishev's concept of the class Graptolithoidea. Now the names
Pterobranchia and pterobranchsa are retained as common, non-
taxonomic names given to cephalodiscids and rhabdopleurids, i.e.,
extant orders of the Graptolithoidea.

Mierzejewski and Urbanek (2004) write that some Paleozoic
representatives of the Graptolithoidea represent "an unusual
combination of features of extant cephalodiscids and extinct
graptolite hemichordates. The overall morphology of the zooidal tube
is strongly reminiscent of cephalodiscids of the Recent subgenus
Cephalodiscus (Orthoecus) Andersson, 1907. [...] On the other hand, a
heavy cortical-like coating, both external and internal, points to a
graptolite assignement of the forms in question."

"However, at the present stage of knowledge it seems safe to
conclude that hemichordates - like many other invertebrate groups -
passed through a phase of radiation in the Early Cambrian. Some of
these clades resulting from this radiation, became fast established
and long lasting, providing a model for understanding the structural
plan of fossil hemichordates. Some minor clades were more transient,
and represent a non-conventional combination of features.
Melanostrophus and Pterobranchites (for the fine structure of the
later, see
Mierzejewski 1984) are among such early representatives
of pterobranch-like clades, which had an ability to produce thick
cortical deposit made of cortical fibrils. Therefore, in the light of
recent data, the old belief that the cortex is a novelty
(synapomorphy) of the "true" graptolites, acquired  relatively late in
the morphological evolution, is no longer  tenable. Some of fossil taxa
otherwise representing a pterobranch level of morphology, were able
to form cortex. Moreover, as indicated by observations made by
Mierzejewski & Kulicki (
2001, 2003), both fossil and extant
rhabdopleurids were able to lay down cortical fibrils. Cortex could in
this way appear prior to the attaiment of the graptolite structural
grade, being expressed in different clades to different degrees."

    Urbanek (1994) wrote that 'it is little likely that the graptolite
skeleton could generally be derived from the skeletons of
Cephalodiscus.' But the fact remains, that there is one striking
similarity between some colonies of cephalodiscids and camaroids -
skeletal structures housing zooids, i.e. zooidal tubes of some
Cephalodiscus (Orthoecus) species and thecae of the Camaroidea are
embedded in a common tissue, named the
common coenecial substance
and the
extracamaral tissue respectively. Of course, one could arrive
at the conclusion that this similarity and the combining of
cephalodiscid and camaroid features in
Erecticamara may not be
significant at all. However, these fact may also suggest that
encrusting graptolites (
Camaroidea, Crustoidea, Tuboidea) did not not
emerge monophyletically from the common ancestor.

Based on:

Mierzejewski, P. 2000. An aberrant encrusting graptolite from the
Ordovician of Estonia
. - Acta Palaeontologica Polonica 45, 3, 239-250.

Mierzejewski, P. & Kulicki, C. 2002. Discovery of Pterobranchia
(Graptolithoidea) in the Permian
. - Acta Palaeontologica Polonica 47, 1,

Mierzejewski, P. & Urbanek, A. 2004. The morphology and fine
structure of the Ordovician
Cephalodiscus-like genus Melanostrophus. -
Acta Palaeontologica Polonica 49, 4, 519-528.
Colony of the Ordovician pterobranch
Melanostrophus fokini.
From Zessin & v. Puttkamer.
Zoidal tube of the Jurassic pterobranch
Rhabdopleura kozlowskii
From Mierzejewski & Kulicki
Comparison of sicular portions in Recent
Rhabdopleura compacta and in Ordovician
tuboid graptolite
From Urbanek 2004.
Cephalodiscus densus Andersson. Recent.
SEM stereopair micrograph of a zooid.
From Mierzejewski, Kulicki & Schiaparelli.