Back to the Scleritome - Tommotiids Revealed!

Disarticulated mitral sclerites of Micrina xiaotanensis. Image from GeoScience World.

Back in January, I brought you Scleritome Week where I looked at a range of fossil organisms that were originally described from bits of disassociated external skeleton. Some of these were still of unknown live appearance, some had turned out once soft-body fossils were discovered to look very different from what anyone had imagined. One such group that I didn't cover (though I did refer to them in passing) was tommotiids. Tommotiids are part of the Cambrian assemblage of scleritome animals that may or may not be related to each other, and may or may not include basal lophotrochozoans (the clade that includes brachiopods, annelids and molluscs). Tommotiids in particular have been suggested to be related to brachiopods, with which they share a similar shell ultrastructure (Holmer et al., 2002). Until recently, no articulated tommotiid specimens had been found, but comparisons of tommotiid sclerites with those of Halkieria and Wiwaxia had led to suggested reconstructions of tommotiids as bilateral armoured slug-like animals. A new paper by Holmer et al. (in press, 2008) suggests a quite different image.

An articulated tommotiid scleritome was recently described by Skovsted et al. (2008), though frustratingly I don't have access to the paper. Far from the imagined bilateral slug, sclerites of the tommotiid Eccentrotheca were joined into an expanding tube-shaped structure. Skovsted et al. inferred that Eccentrotheca was a sessile, vermiform (worm-shaped) filter-feeder. Such an interpretation, they argued, fit well with the potential brachiozoan (brachiopod + phoronid) affinities of tommotiids, though Eccentrotheca may have been more similar in appearance to the worm-like phoronids rather than the brachiopods.

The reconstruction of Holmer et al. (2008) focuses on another tommotiid, Micrina, which is the most brachiopod-like of the tommotiids. Micrina possessed two types of sclerite, the smaller and flatter sellate and the larger, cap-shaped mitral. By comparison with Halkieria, Williams & Holmer (2002) suggested that the two sclerites could have been situated at either end of a slug-shaped animal. However, the revelation from Eccentrotheca that at least some tommotiids might be sessile suggested that this reconstruction should be re-examined.



The new reconstruction of Micrina from Holmer et al. (2008) is shown above in a figure from that paper. Rather than being a slug-like animal, Micrina is reconstructed as a sessile filter-feeder like Eccentrotheca. However, Micrina differs from Eccentrotheca in being cup-shaped rather than vermiform. What it does bear a distinct resemblance to is a basal brachiopod similar to the diagram I used in an earlier post, which are also sessile filter feeders attached to a substrate by a short pedicle. In contrast to brachiopods, the two valves of Micrina would not have been able to form a sealed chamber, but the shell ultrastructure of Micrina does suggest the presence of a fringe of setae that Holmer et al. suggest could have served a protective function. One potential issue with the reconstruction is that mitral valves are generally preserved in much greater numbers than sellate valves when the reconstruction suggests they should be equally abundant, but this may be a preservation artefact resulting from the smaller and lighter construction of the sellate valve.

A sessile reconstruction for tommotiids has interesting implications for the interpretation of other Cambrian scleritome animals. Halkieria was suggested as a stem-brachiopod by Conway Morris & Peel (1995), but this was debated by Vinther & Nielsen (2005) who interpreted Halkieria as closer to molluscs. While the sessile reconstruction of tommotiids does not entirely rule out a halkieriid ancestry for brachiozoans (one could still potentially argue that halkieriids were ancestral to a tommotiid + brachiozoan clade), it does make it significantly less likely. Conway Morris & Peel (1995) suggested that the two large subterminal sclerites at each end of Halkieria could have been brought into apposition to form the two valves of the brachiopod shell, but the sessile tommotiids suggest that the equal-sized valves of brachiopods could have been derived from an unequally-valved ancestor.

They are also interesting by way of analogy with the chancelloriids, those incredibly confusing Cambrian animals whose sclerite structure demands they be lophotrochozoans, but whose sessile habit and radial organisation screams non-bilaterian. While there is no reason to suggest an actual phylogenetic connection between tommotiids and chancelloriids, the presence of a sessile habit in the former, which are almost undeniably lophotrochozoans, suggests that the radial nature of the latter may not be so difficult to resolve with a lophotrochozoan ancestry after all.

REFERENCES

Conway Morris, S., & J. S. Peel. 1995. Articulated halkieriids from the lower Cambrian of North Greenland and their role in early protostome evolution. Philosophical Transactions of the Royal Society of London Series B – Biological Sciences 447: 305-358.

Holmer, L. E., C. B. Skovsted, G. A. Brock, J. L. Valentine & J. R. Paterson (in press, 2008) The Early Cambrian tommotiid Micrina, a sessile bivalved stem group brachiopod. Biology Letters.

Holmer, L. E., C. B. Skovsted & A. Williams. 2002. A stem group brachiopod from the Lower Cambrian: Support for a Micrina (halkieriid) ancestry. Palaeontology 45 (5): 875-882.

Skovsted, C. B., G. A. Brock, J. R. Paterson, L. E. Holmer & G. E. Budd. 2008. The scleritome of Eccentrotheca from the Lower Cambrian of South Australia: lophophorate affinities and implications for tommotiid phylogeny. Geology 36 (2): 171-174.

Vinther, J., & C. Nielsen. 2005. The Early Cambrian Halkieria is a mollusc. Zoologica Scripta 34: 81-89.

Williams, A., & L. E. Holmer. 2002. Shell structure and inferred growth, functions and affinities of the sclerites of the problematic Micrina. Palaeontology 45 (5): 845-873.