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.
Bulman, O. M. B. 1970. Graptolithina with sections of Enteropneusta and Pterobranchia. In Treatise on Invertebrate Paleontology Part V, 2nd ed. (C. Teichert, ed.) pp. V1-V149. The Geological Society of America, Inc.: Boulder (Colorado), and the University of Kansas: Lawrence (Kansas).
Kirk, N. H. 1979. Thoughts on coloniality in the graptolites. In Biology and Systematics of Colonial Organisms (G. Larwood & B. R. Rosen, eds.) pp. 411-432. Academic Press: London.
Kozłowski, R. 1949 ("1948"). Les graptolithes et quelques nouveaux groupes d’animaux du Tremadoc de la Pologne. Palaeontologica Polonica 3: I-XII, 1-235.
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.