The Writing in the Rocks

Okay, we'll see how long this one lasts - I'm going to attempt to add another feature to this site. Like anyone working in the field of science (and doubtless any other academic discipline), I've accumulated quite a pile of references of one form or another. In fact, my EndNote library has a little shy of 6000 entries, most of which lurk in a pair of ominous-looking filing cabinets sitting in the back of the office at home that Jack has given up trying to ask me to bring some order to. So the idea is that each week I'll select one of the entries in my EndNote more or less at random to read over and review. It may be a paper of great significance, it may be something completely trivial. It may be of interest to many, it may be of interest to almost none. All I know is that it will hopefully give me an opportunity to actually read some of the things I've probably taken a copy of at some point ande flicked through briefly before filing it away never to be seen again. So, on to this week's review, of the article that was my 1000th entry into EndNote:



Zhang, Y.-D., & A. C. Lenz. 1997. Uppermost Wenlock and Ludlow graptolites from southern Yunnan, China. Canadian Journal of Earth Sciences 34: 1220-1238.

I'm probably showing unbelievable levels of geekiness in saying so, but I have a certain degree of affection for graptolites. They were one of first examples I became aware of as a lad of a completely extinct lineage of fossil organisms of uncertain relationships to anything alive today. Graptolites were an abundant group of colonial animals in the Palaeozoic. The name Graptolithus can be translated from Latin as "rock with writing", and was originally coined by Linnaeus for what he thought were mineralisations of inorganic origin (Linnaeus' original classification covered minerals as well as plants and animals).

The most speciose lineage of graptolites, the Graptoloidea, was planktonic (the picture above comes from here, and shows a reconstruction of the basal planktonic graptolite Rhabdinopora), but other orders of graptolites were sessile and benthic. Most modern authors agree that the graptolites were closely related to the pterobranchs, a small group of modern colonial animals (both in the sense of not including many species, and in being small in size). The story of how the relationship between graptolites and pterobranchs came to be recognised is an interesting one in its own right - critical well-preserved specimens of early sessile graptolites were described by Roman Kozłowski from the Holy Cross Mountains in Poland (Kozłowski, 1949), but the results of his research almost never saw the light of day due to the minor inteference of the Second World War.

As I've already indicated, the earliest graptolites were benthic, and one of the most successful benthic orders were the Dendroidea, so-called because of their multi-branching tree-like structure. The Graptoloidea were derived from dendroid-type ancestors, and basal graptoloids such as Rhabdinopora can essentially be described as a dendroid detached from the bottom and hung upside down. Many popular books will then go on, as I am about to, to describe the subsequent history of graptoloids in a misleadingly linear way. However, the dendroids did not disappear with the rise of the graptoloids, though they never achieved the diversity of the latter. In fact, the dendroids survived long after the graptoloids had fallen by the wayside - dendroids were still alive and well in the early Carboniferous, while graptoloids never made it past the Devonian.



Nevertheless, as time went by the early multi-branched planktonic forms gave way to descendents with a far simpler organisation. Four-branched taxa gave rise to two-branched taxa, which in turn gave way to the linear monograptids (the illustration at left comes from the University of Oslo, and shows the single-rowed Spirograptus turriculatus). Another group of graptoloids, the retiolitids, took a different approach to lightening the colony structure, and reduced the colonial wall to a minimalist net-like framework (Kozłowska-Dawidziuk, 2004 - I'd recommend taking a look at this article, especially if, like me, you've had some difficulty in imaging retiolitids as live animals).

Graptoloids are a very useful group of organisms for biostratigraphy, combining the ideal features of wide distribution of individual species with relatively rapid species turnover. Biostratigraphy is the main focus of Zhang & Lenz's (1997) paper. The Ludlow epoch was in the later Silurian (see the Palaeos page) and the fauna described by Zhang & Lenz consists entirely of monograptids and retiolitids, the only graptoloid groups to survive the end of the preceeding Wenlock epoch. There's not much to say about this paper - it's a fairly standard example of a faunal survey, describing the graptolites found in the Shuiqingliangzi section in southern China. No new taxa were described, though detailed redescriptions were given of a number of taxa.

REFERENCES

Kozłowska-Dawidziuk, A. 2004. Evolution of retiolitid graptolites - a synopsis. Acta Palaeontologica Polonica 49 (4): 505-518.

Kozłowski, R. 1949. Les graptolithes et quelques nouveaux groupes d’animaux du Tremadoc de la Pologne. Palaeontologica Polonica 3: 1-235.