Phylogenetic relationships among benthic graptolites.

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Phylogenetic relationships among benthic graptolites

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Since the classic work of Koz ł owski (1949), the phylogenetic relationships among primitive (i.e. non-graptoloid) benthic
orders of the Graptolithina have only rarely been discussed (e.g. Skevington 1963; Obut 1964; Kozłowski 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 evolutionary
lineages leading from a common pterobranch ancestor (Kozłowski 1949, 1962): (1) Crustoidea - Dendroidea - Graptoloidea;
and (2) Tuboidea - Camaroidea. Kozłowski (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,
Kozłowski 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 Kozłowski (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 Kozłowski, 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.

---------Kozłowski (1949) ascribed a specific role to the tuboid genus Idiotubus Kozłowski, 1949, in the Tuboidea -
Camaroidea lineage [Idiotubus is treated here as a junior synonym of Epigraptus Eisenack, 1941 (Mierzejewski 1978; see
also Urbanek 1986). Kozłowski 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
Kozłowski, 1949, p. 109). Epigraptus is especially similar to the camaroid genus Graptocamara Kozłowski, 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 Kozłowski,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 Kozłowski,
1949), and strikingly resembles the collum of the poorly known camaroid species Graptocamara hyperlinguata Kozłowski,
1949. Remarkably, the autothecal apertures of Graptocamara are sometimes occluded, as in numerous representatives of
the Tuboidea and the Camaroidea (cf. Kozłowski 1949).

---------It can be stated with certainty that 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 Kozłowski, Graptocamara Kozłowski, and Tubicamara
Kozłowski. 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 Kozłowski, 1949, Kozlowskitubus Mierzejewski, 1978, and
partly Reticulograptus Wiman, 1901) which possess a more or less developed stolonal system with peridermal envelopes
(Kozłowski 1949, 1963, 1971; Bulman & Rickards 1966; Bulman 1970; Mierzejewski 1978). The significance of these
observations for tuboid evolution remains unclear. Urbanek (1973) pointed out that Kozłowski'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 Kozłowski).

---------The discovery of Camarotubus supports Kozłowski's (1949) view of a close phylogenetic relationship between
tuboid and camaroid graptolites. But paradoxically, the existence of the above mentioned morphological intermediates
does not necessarily confirm Kozłowski's (1949) simple evolutionary pathway from the Tuboidea to the Camaroidea. The
present writer considers the recant data sufficient to cast doubt upon Kozłowski'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 Graptolithina. 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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Dr Piotr Mierzejewski, Count of Calmont
since 2002