 |
 |
| Graptolite Net | Graptolites & Graptoliters | Cephalodiscoidea | Rhabdopleuroidea | Crustoidea | Camaroidea | Tuboidea |
| ________________________________________________________________________________________ GRAPTOLITE NET is edited by Piotr Mierzejewski, the Count of Calmont and Countess Maja A. Korwin-Kossakowska since 2002 |
 |
 |
 |
 |
| The hemichordate Class Graptolithoidea - Piotr MIERZEJEWSKI |
 |
 |
 |
| 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 hemichordate 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 debate" (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, 169-175. 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 Recent pterobranch Cephalodiscus densus. From Schiaparelli, Cataneo-Vietti & Mierzejewski. |
| Related pages: Class Pterobranchea Astogeny in Graptolithoidea |
 |
| 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 Epigraptus. From Urbanek 2004. |
 |
| Cephalodiscus densus Andersson. Recent. SEM stereopair micrograph of a zooid. From Mierzejewski, Kulicki & Schiaparelli. |