Benthic Graptolithoidea - Phylogenetic Relationships
Since the classic work of Kozlowski (1949), the phylogenetic relationships among primitive (i.e. non-graptoloid)
benthic orders of graptolites (
Graptolithoidea, i.e. Graptolithina + Pterobranchia; cf. Pterobranchea) have only rarely
been discussed (e.g. Skevington 1963; Obut 1964; Kozlowski 1962, 1966; Bulman 1968; Mierzejewski 1985; Urbanek
1986; Chapman
et al. 1996). Bulman (1968, 1970) accepted the validity of only five diverse orders of sessile graptolites
(
Camaroidea, Crustoidea, Dendroidea, Stolonoidea, and Tuboidea) and considered that their interrelationships to be
unclear. Other graptolite "orders", especially the
Dithecoidea and the Inocaulida, were discussed by Mierzejewski
(1986) who considered the majority of dithecoids (including the family
Chaunograptidae) and inocaulids to be
non-graptolitic (presumably colonial scyphopolyps or hydropolyps, and algae). This view received support, at least in
part, from subsequent redescriptions of some allegedly sessile graptolite genera (e.g. Lo Duca 1990; Mierzejewski
1991).
      Despite the fact that early graptolites are still poorly known, it is possible distinguish two main evolutionaty
lineages leading from a common pterobranch ancestor (Kozlowski 1949, 1962): (1) Crustoidea - Dendroidea -
Graptoloidea; and (2) Tuboidea - Camaroidea. Kozlowski (1938, 1949, 1962, 1966) stressed the importance of budding
patterns and stolonal morphology in diagnosing the sessile graptolite orders. Hence lineage (1) is characterized by
regular triad budding, whereas lineage (2) exhibits diad budding with no regular succession and variably distributed
nodes. However, Kozlowski left open the question of what mode of budding and thecal succession has been adopted
by the common ancestor of all graptolites. There are some indications that the Tuboidea and the Camaroidea
retained the primary pattern inherited from their pterobranch ancestors, presumably related to the
Rhabdopleuroidea (Urbanek 1986). Mierzejewski (1985) supposed that the common ancestor of both lineages must
have been very similar, or even closely related, to the mysterious genus
Maenniligraptus Mierzejewski, 1985, which
exhibits the both diad and triad modes of budding, and represents one of the lowest grades of colonial development.
However, the problem is made more difficult and complex by some aberrant
incertae ordinis graptolites, e.g. genera
Mastigograptus
Ruedemann, 1908, and Micrograptus Eisenack,1974 (see Urbanek 1986). Moreover, Skevington (1963)
has even suggested that the Tuboidea was ancestral to the Denroidea, or
vice versa.
      Until recently, little attention has been paid to the Tuboidea - Camaroidea lineage. These graptolites are poorly
known in terms of their general morphology and astogeny, and their palaeogeographic and stratigraphic distribution.
Their outline morphology was established by Kozlowski (1938, 1949): both are encrusting forms (although some
tuboids have more or less dendroid rhabdosomes), exhibit irregularly diad budding, and possess autothecae which are
strongly differentiated into two portions - a creeping part embedded in a 'thecorhiza' (in tuboids) or 'extracamaral
tissue' (in camaroids), and an erect part, often provided with one or two apertural processes. The
Camaroidea are
distinguished from the Tuboidea mainly by (i) the stronger differentiation of camaroid autothecae into two (camara
and collum); (ii) their lack of bithecae (with the exception of
Bithecocamara Kozlowski, 1949); (iii) the lack of
autothecal dimorphism; and (iv) the common occurence of extracamaral tissue instead of stolothecae. In other
words, morphological changes in this lineage were dominated by a significant reduction in polymorphism, a looser
integration of the colony, and a broadening of the creeping part of each autotheca from the tubular shape of tuboids
to the bulbous camara of most camaroids.
      Kozlowski (1949) ascribed a specific role to the tuboid genus Idiotubus Kozlowski, 1949, in the Tuboidea -
Camaroidea lineage [
Idiotubus is treated here as a junior synonym of Epigraptus Eisenack, 1941 (Mierzejewski 1978;
see also Urbanek 1986). Kozlowski considered
Idiotubus to unite certain characters of both orders and to represent a
truly transient link, comparable with the role played by
Dictyonema Hall, 1851 in the Dendroidea - Graptoloidea
lineage (see Kozlowski, 1949, p. 109).
Epigraptus is especially similar to the camaroid genus Graptocamara Kozlowski,
1949. Both have autothecae which arise singly from the surface of a thecorhiza. Moreover, the autothecal apertures
of
Graptocamara possess a distinct, linguiform process, very similar to that of numerous epigraptid species. The main
difference between the two genera concerns bithecae, which are abundant in the tuboid but absent in the camaroid.
      The encrusting graptolite  Camarotubus graptocamaraeformis Mierzejewski, 2001 seems to represent a stage
morphologically intermediate between the Tuboidea and the Camaroidea. It departs in some respects from typical
representatives of both orders, while simultaneously combining in mosaic fashion some of their key characters. In the
few well-studied tuboid genera, numerous bithecae outnumber autothecae by several times. As a rule, each tuboid
autotheca is surrounded by a few (2-3 or sometimes even 5) bithecae, forming a more or less regular circle. In
contrast, all known camaroid graptolites (with the exception of the enigmatic
Bithecocamara Kozlowski,1949) are
completely devoid of bithecae. In this respect,
Camarotubus is clearly intermediate in position between the two
orders. Its bithecae are few in number distributed irregularly over the surface of the thecorhiza; this is an important
difference from typical tuboids. However, the basal part of an autotheca in
Camarotubus is distinctly tuboid in form,
i.e. creeping and elongated. Yet its erect portion is under-developed, as in the majority of Camaroidea (with the
exception of
Tubicamara Kozlowski, 1949), and strikingly resembles the collum of the poorly known camaroid species
Graptocamara hyperlinguata Kozlowski, 1949. Remarkably, the autothecal apertures of Graptocamara are sometimes
occluded, as in numerous representatives of the Tuboidea and the Camaroidea (cf. Kozlowski 1949).
      It can be stated with certainty that the new Camarotubus intergrades from the Tuboidea to the Camaroidea.
However, while morphologically intermediate, it cannot be considered a truly transient link because of its age, as is
also the case for the tuboid
Epigraptus Eisenack and the camaroids Bithecocamara Kozlowski, Graptocamara
Kozlowski, and
Tubicamara Kozlowski. It is notable that intermediate forms between the two orders are relatively
numerous in comparison with the small total number of tuboid and camaroid genera. These intergradations make it
impossible to define a sharp boundary between both orders. Strikingly, individual genera exhibit different tuboid and
camaroid features. For example,
Epigraptus (=Idiotubus), the typical tuboid graptolite, might playfully to be described
as comprising the camara and apertural apparatus of
Graptocamara, the collum of Tubicamara, and the bithecae of
Bithecocamara (which is also devoid of extracamaral tissue); it certainly lacks the sclerotized stolons of camaroids,
but then some other tuboids do possess this feature. The occurence of different tuboid and camaroid characters
intermediate forms suggests that there were several different evolutionary pathways within the Tuboidea -
Camaroidea lineage, as has been shown for the Dendroidea - Graptoloidea lineage. Moreover, it seems clear that
there was a certain independence of particular rhabdosome elements in the evolution process of the
Tuboidea.
     The presence of well-sclerotized stolons in Camarotubus is of special interest because of the few tuboid genera
(
Tubidendrum Kozlowski, 1949, Kozlowskitubus Mierzejewski, 1978, and partly Reticulograptus Wiman, 1901) which
possess a more or less developed stolonal system with peridermal envelopes (Kozlowski 1949, 1963, 1971; Bulman and
Rickards 1966; Bulman 1970;
Mierzejewski 1978). The significance of these observations for tuboid evolution remains
unclear. Urbanek (1973) pointed out that Kozlowski's (1949) graded series, illustrating successive changes in tuboid
spatial organization, are not necessarily closely relate because of differences in the degree of sclerotization of the
stolon. Later, he suggested that sclerotization of the stolon in the Tuboidea was a gradual process (Urbanek 1986; see
also Bengtson and Urbanek 1986, p. 308). In such a situation,
Epigraptus should be interpreted as distinctly less
advanced than
Camarotubus, in spite of both representing the same morphoecological type and both exhibiting
similar surface features.
Epigraptus may also be interpreted as a morphological intermediate between tuboid
graptolites and some rhabdopleurid pterobranchs (e.g. Middle Cambrian
Rhabdotubus Bengtson et Urbanek, 1986, or
'Rhabdopleura' obuti Durman et Sennikov, 1993), while Camarotubus bridges the Tuboidea and the Camaroidea (and is
especially allied with early Ordovician
Graptocamara Kozlowski).
      The discovery of Camarotubus supports Kozlowski's (1949) view of a close phylogenetic relationships between
tuboid and camaroid graptolites. But paradoxically, the existence of the above mentioned morphological
intermediates does not necessarily confirm Kozlowski's (1949) simple evolutionary pathway from the Tuboidea to the
Camaroidea. The present writer considers the recant data sufficient to cast doubt upon Kozlowski's scheme, and
perhaps reject it altogether. Some Ordovician hemichordates combine, in mosaic fashion, characters of cephalodiscid
pterobranchs and camaroid graptolites (Mierzejewski
1984, 2000 and unpublished results). This may have significant
phylogenetic implications and shed new light on the early evolution of the
Graptolithoidea. The camaroids may, for
example, appear not to be the highly specialized and evolutionarily advanced descendants of the Tuboidea, but even
more primitive forms, closely related to a common ancestor with cephalodiscid-like pterobranchs. These
considerations suggest that
Camarotubus may even has been a primitive tuboid, similar to its immediate camaroid
ancestors. Nevertheless, the problem of the origin of the Camaroidea and their phylogenetic relationships is beyond
the scope of the present paper and will be discussed elsewhere (Mierzejewski,
in preparation).
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Updated: 19.06.2005