Welcome to Scleritome Week: The Little Nets

I'm still pretty excited about the articulated machaeridian I wrote about last week. So excited, in fact, that to commemorate that significant discovery I'm declaring this week to be Scleritome Week here at Catalogue of Organisms. Each day this week I'll introduce you to a new Palaeozoic fossil animal of a kind no longer with us today. Not all of them will be scleritome animals in the proper sense of the word, but they will all share a common characteristic - they were all described from disarticulated pieces of dermal armour that gave us little idea of what the entire animal originally looked like. In many cases, more complete specimens have been found that show us the animal's true form. In others, the identity of the original animal remains a near-insoluble mystery.


My first subject is a classic example of just little these isolated elements can sometimes tell us about their source. Microdictyon was first known from tiny roundish to oblong-ish sclerites. The name means "tiny net" and refers to the net-like structure of the sclerites, clearly visible in the example above from here. Differences in appearance between sclerites led to the description of a number of species (Bengtson et al., 1986), but the identity of the animal bearing them was a complete mystery.

It wasn't until the discovery of the Chengjiang Biota in China that a fossil showing the soft anatomy of Microdictyon was discovered, and it's probably fair to say that no-one could have predicted what it looked like (image of fossil and reconstruction from Palaeos):



It turns out that Microdictyon was a lobopod, one of a number of Cambrian marine animals not unlike the modern terrestrial onychophorans*. Paired sclerites sat above each pair of legs. It is most likely that these sclerites served a defensive purpose, but other functions have also been suggested - Jerzy Dzik (2003), a man who has not been above suggesting heterodox interpretations of Cambrian animals in the past (some of which have even turned out to be accurate), suggested that Microdictyon sclerites were similar in structure to trilobite eyes, and might even be homologous. He therefore reconstructed Microdictyon as an elongate animal with a pair of eyes on each segment!

*"Lobopod" is a collective name for tardigrades and onychophorans, which have a soft body with stumpy tubular legs. Some authors have interpreted tardigrades and onychophorans as forming a monophyletic group, but others hold that the lobopod form is the ancestral grade for panarthropods (the clade joining lobopods and arthropods). In the past, the Cambrian lobopods have been interpreted as stem-onychophorans, but beyond the superficial similarities in appearance, it is not unlikely that the shared features are also plesiomorphic for panarthropods in general and a specific relationship to onychophorans should be regarded sceptically (Liu et al., in press).

REFERENCES

Bengtson,S., S. C. Matthews & V. V. Missarzhevsky. 1986. The Cambrian netlike fossil Microdictyon. In Problematic Fossil Taxa (A. Hoffman & M.H. Nitecki, eds.) pp. 97–115. Oxford University Press, New York.

Dzik, J. 2003. Early Cambrian lobopodian sclerites and associated fossils from Kazakhstan. Palaeontology 46 (1): 93-112.

Liu, J., D. Shu, J. Han, Z. Zhang & X. Zhang (in press) Origin, diversification, and relationships of Cambrian lobopods. Gondwana Research.