Belongs within: Hemichordata.
Contains: Dithecodendridae, Rhabdopleuridae, Wimanicrustidae, Cyclograptidae, Dendroidea.
The Graptolithina, graptolites, are a group of colonial organisms known from the Middle Cambrian to the Carboniferous that produced linear, branched or encrusting colonies.
Further readings from the rocks
Published 6 April 2009
Today, I’m going back to Graptolithina, the graptolites. For those of you who aren’t familiar with graptolites, you can read my previous post on the subject.
As mentioned in that post, it is by now universally accepted that the closest living relatives of the Palaeozoic graptolites are to be found in the Pterobranchia. Pterobranchs are, admittedly, a fairly obscure group in their own right, being minute colonial animals that feed by means of a tentacled lophophore*. Despite their obscurity, though, pterobranchs are not devoid of interest, belonging as they do to the phylum Hemichordata and hence among the closer invertebrate relatives to ourselves. The most basic character uniting graptolites and pterobranchs is that they both have an external covering of chitinous fuselli—their skeleton is constructed in bands, a bit like the bandages wrapping a mummy.
*Yes, you heard me—lophophore.
Despite the two groups usually being treated as separate classes, the distinction between graptolites and pterobranchs is a little vague. Okay, it’s a lot vague. The problem doesn’t lie so much with the graptolites as it does with the pterobranchs. There are three living genera of pterobranchs, each of theme very distinct from each other. In fact, the genera Rhabdopleura and Cephalodiscus are easily more distinct from each other than either is to the graptolites. Rhabdopleura forms a long, linear colony with individual zooids budding off one by one, zooids remaining permanently attached to each other by a stolon, and with a skeleton of only a single banded fusellar layer. Cephalodiscus colonies bud irregularly to form irregular-shaped colonies, zooids do not remain permanently attached to each other but remain in loose association, and the fusellar layer of the skeleton is covered over by an external unbanded cortex. The third genus, Atubaria, has zooids very similar to those of Cephalodiscus, but doesn’t secrete a colonial skeleton at all.
Graptolites combine the regular colony structure and permanent stolon of Rhabdopleura with the external cortex of Cephalodiscus. In Cephalodiscus, cortex is produced by individual zooids crawling out of the colony to plaster cortex on from outside, but it is difficult to imagine how graptolites would have managed this with their permanent stolon. Some authors have suggested that graptolites possessed an extensive evagination of the outer epithelium emerging from the colony openings, partially or entirely converting the exoskeleton into an endoskeleton. To further confuse matters, cortex-like structures have also been identified in fossil rhabdopleurids (Mierzejewski & Kulicki 2001). Overall, it is highly possible that graptolites should really be included within pterobranchs (some authors have used the name “Graptolithoidea” for such a grouping). There are five well-established graptolite orders*. The Graptoloidea are the familiar planktic graptolites, the Dendroidea were upright-branching benthic forms, and the Crustoidea, Tuboidea and Camaroidea were all horizontally-growing encrusting forms. Among these orders, there is a clear division between the Tuboidea and Camaroidea on one hand (in fact, the distinction of these two orders is a little doubtful), and the Crustoidea, Dendroidea and Graptoloidea on the other (Mierzejewski 2001). Colonies of Tuboidea and Camaroidea exhibit diad branching as in modern Rhabdopleura, with zooids branching off the stolon one by one**. The other three orders, in contrast, possess triad branching. Instead of only one zooid branching at a time, two zooids branch alongside each other, a larger autotheca and a smaller bitheca (most graptoloids later showed a secondary reduction or loss of bitheca production). Many different suggestions have been made as to what the distinction between the two zooid forms could have been in life—feeding vs reproductive zooids, males vs females, feeders vs cleaners, etc.—but, of course, there’s really no way of knowing (modern pterobranchs don’t show such inter-zooid specialisations). Many tuboids and camaroids also possessed distinct autothecae and bithecae, but bithecae were distributed irregularly within the colony rather than in regular association with autothecae. A collection of stolon fragments described as early crustoids by Mierzejewski et al. (2005) shows an effectively triad branching pattern but with a slight lag between side-branches, suggesting that the triad pattern could have derived from an ancestral diad pattern by simple shortening of the gap between regular autothecal and bithecal branchings.
*Bulman (1970) recognised six, but the Stolonoidea have not been universally accepted as graptolites. Other authors have recognised further orders such as Dithecoidea, but these have usually been poorly characterised and/or possibly not graptolites.
**Technically, the colony produces one lateral theca at a time. Being soft and squishy, the zooids themselves are not preserved as fossils, but it seems a reasonably safe assumption that each thecal opening housed an individual zooid.
Records of the three encrusting orders are decidedly limited compared to the dendroids and graptoloids, and all three are only known from the Ordovician and Silurian (camaroids are Ordovician only). However, it is debatable to what extent this lower record reliably indicates the encrusting graptolites to have been rarer. The graptolite fossil record is heavily biased towards remains preserved in low-energy environments (Kirk 1979), with the relatively frail graptolite skeleton rapidly being destroyed in high-energy environments. Perhaps the rarity of encrusting forms reflects a higher-energy habitat preference rather than true lack of abundance.
Systematics of Graptolithina
Characters (from Bulman 1970): Colonial organisms secreting a sclerotised exoskeleton with characteristic growth bands (fuselli) and growth lines. Thecae usually arranged in single or double row along branches (stipes) of colony (rhabdosome), rarely in irregular aggregates; thecae may be polymorphic, may be related to internal sclerotised stolon system. Rhabdosomes originating by single bud from initial zooid housed in conical sicula, producing simple, branched or rarely encrusting colonies. Sessile or pelagic.
<==Graptolithina [Camaroidea, Monophyontes, Rhabdopleurida, Stolonoidea, Tuboidea] |--+--RhabdopleuridaeM14 | |--WimanicrustidaeM14 | `--EugraptolithinaM14 | |--CyclograptidaeM14 | `--DendroideaM14 `--CysticamaridaeB70 |--Erecticamara Mierzejewski 2000M14 |--Graptocamara Kozłowski 1949R93 | `--*G. hyperlinguata Kozłowski 1949K48, R93 |--Tubicamara Kozłowski 1949R93 | `--*T. coriacea Kozłowski 1949K48, R93 |--Flexicollicamara Kozłowski 1949R93 | `--*F. bryozoaeformis Kozłowski 1949K48, R93 |--Syringotaenia Obut 1963R93 | `--*S. bystrowi Obut 1963B70, R93 |--Cysticamara Kozłowski 1949M14, R93 | |--*C. accollis Kozłowski 1949K48, R93 | |--C. bicollis Kozłowski 1948K48 | `--C. embryoniformis Kozłowski 1948K48 `--Bithecocamara Kozłowski 1949 [Bithecocamaridae]M14 |--*B. gladiator Kozłowski 1949K48, R93 |--B. misera Kozłowski 1948K48 |--B. platicellata Kozłowski 1948K48 |--B. sinuaticellata Kozłowski 1948K48 |--B. tubicellata Kozłowski 1948K48 `--B. vermicollis Kozłowski 1948K48 Graptolithina incertae sedis: Protistograptus McLearn 1915 (n. d.)B70 Ascograptus Ruedemann 1925B70 `--*A. similis Ruedemann 1925B70 Coelograptus Ruedemann 1947B70 `--*C. problematicus (Spencer 1878) [=Inocaulis problematica]B70 DithecodendridaeM14 Polygonograptus Bouček 1957B70 `--*P. sokolowi (Obut 1953) [=Palaeodictyota sokolowi]B70 Birastrites Geinitz 1866 (n. d.)B70 Buthograptus Hall 1861 (n. d.)B70 Cameragraptus Hundt 1953 (n. d.)M14 Conograptus Ruedemann 1947 (n. d.)B70 Cystoturriculagraptus Hundt 1953 (n. d.)M14 Geminograptus Hundt 1951 (n. d.)B70 Labrumograptus Hundt 1953 (n. d.)M14 Nereitograptus Hundt 1951 (n. d.)B70 Paradimorphograptus Hundt 1951 (n. d.)B70 Phycograptus Gurley 1896 (n. d.)B70 Planktograptus Yakovlev 1933 (n. d.)B70 Protograptus Matthew 1886 (n. d.)B70 Spinosudiplograptus Hundt 1951 (n. d.)M14 Stelechograptus Ruedemann 1947 (n. d.)B70 Strophograptus Ruedemann 1947 (n. d.)M14 Thecocystograptus Hundt 1950 (n. d.)M14 Undograptus Hundt in Nindel 1949 (n. d.)M14 Amansites Brongniart 1849 (n. d.)H62 `--*A. dentatus (Brongniart 1828) (n. d.) [=Fucoides dentatus]H62 Ludensograptus latilobusJT12 Stolonofolliculis Zessin & Puttkamer 1994 [Stolonofolliculidae]MU04 `--*S. signum (Öpik 1930) (n. d.) [=Melanostrophus signum]MU04 Furkagraptus Hundt 1959M14 Halograptus Hundt 1936M14 Hunanodendrum Mu et al. 1974M14 Parademicystograptus Hundt 1950M14 Pleurograptoides Averianow 1931M14 Protabrograptus Ni 1981M14 Sinograptus Shrubsole 1880M14 Undagraptus Hemmann 1951M14
*Type species of generic name indicated
References
[B70] Bulman, O. M. B. 1970. Graptolithina with sections on Enteropneusta and Pterobranchia. In: Teichert, C. (ed.) Treatise on Invertebrate Paleontology pt V 2nd ed. pp. V1–V149. The Geological Society of America, Inc.: Boulder (Colorado), and the University of Kansas: Lawrence (Kansas).
[H62] Häntzschel, W. 1962. Trace fossils and problematica. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt W. Miscellanea: Conodonts, Conoidal Shells of Uncertain Affinities, Worms, Trace Fossils and Problematica pp. W177–W245. Geological Society of America, and University of Kansas Press.
[JT12] Jeppsson, L., J. A. Talent, R. Mawson, A. Andrew, C. Corradini, A. J. Simpson, J. Wigforss-Lange & H. P. Schönlaub. 2012. Late Ludfordian correlations and the Lau Event. In: Talent, J. A. (ed.) Earth and Life: Global biodiversity, extinction intervals and biogeographic perturbations through time pp. 653–675. Springer.
Kirk, N. H. 1979. Thoughts on coloniality in the graptolites. In: Larwood, G., & B. R. Rosen (eds) Biology and Systematics of Colonial Organisms pp. 411–432. Academic Press: London.
[K48] Kozłowski, R. 1948. Les graptolithes et quelques nouveaux groupes d’animaux du Tremadoc de la Pologne. Palaeontologica Polonica 3: i–xii, 1–235.
[M14] Maletz, J. 2014. The classification of the Pterobranchia (Cephalodiscida and Graptolithina). Bulletin of Geosciences 89 (3): 477–540.
Mierzejewski, P. 2001. A new graptolite, intermediate between the Tuboidea and the Camaroidea. Acta Palaeontologica Polonica 46 (3): 367–376.
Mierzejewski, P., & C. Kulicki. 2001. Graptolite-like fibril pattern in the fusellar tissue
of Palaeozoic rhabdopleurid pterobranchs. Acta Palaeontologica Polonica 46 (3): 349–366.
Mierzejewski, P., C. Kulicki & A. Urbanek. 2005. The world’s oldest crustoid graptolites from the upper Tremadocian of Poland. Acta Palaeontologica Polonica 50 (4): 721–724.
[MU04] Mierzejewski, P., & A. Urbanek. 2004. The morphology and fine structure of the Ordovician Cephalodiscus-like genus Melanostrophus. Acta Palaeontologica Polonica 49 (4): 519–528.
[R93] Rickards, R. B. 1993. Graptolithina. In: Benton, M. J. (ed.) The Fossil Record 2 pp. 537–542. Chapman & Hall: London.