More Crunchy Scleritome Goodness

Yep, it's time for another installment on my favourite assemblage of polyphyletic problematica. Two significant new additions have been made to the repertoire of articulated scleritomes:


An assortment of articulated Lepidocoleus (each about an inch long). Take especial note of figure d! From Högström et al. (in press).

Firstly, does anyone remember machaeridians? The animals for which I labelled the discovery of a specimen preserving soft tissue as "the greatest announcement of 2008"? (And now that 2008 has been and gone, I wholeheartedly support that designation.) Well, there's more. Högström et al. (in press) have described a collection of articulated machaeridians from the Devonian Hunsrück Slate in Germany, and among them is a second specimen with soft-tissue remains!

There are a few reasons why this is a very satisfying discovery. Firstly, the specimen supports the annelid affinities proposed for machaeridians by Vinther et al. (2008) when the first soft-tissue specimen was described from the Ordovician Fezouata Formation of Morocco. Secondly, the Hunsrück specimens represent a different family (Lepidocoleidae) from the Fezouata specimen (Plumulitidae), which confirms that the machaeridians do represent a monophyletic grouping, and are not unrelated taxa that have convergently developed similar sclerite morphologies (always a possibility with animals only known from disarticulated sclerites). [I should point out that articulated lepidocoleid scleritomes have been found before, but not preserving any soft tissue.] Whereas plumulitids appear to have had rather loosely articulated sclerites, giving them an ornamental spined appearance, lepidocoleids had a much more tightly-woven, armour-plated scleritome (see the earlier post for a comparative picture).

And thirdly, just as a kind of cherry on the top, one of the other Hunsrück specimens, while it may not have soft-tissue remains, has something else to commend it that some palaeontologists would probably find even more exciting. It's sitting neatly positioned at the end of a well-preserved trail. Trace fossils are often the best evidence you can get for working out the behaviour of extinct animals, but it can be a frustrating exercise because often the best conditions for preserving traces are not very good for preserving the animals that made them, and vice versa. When I was on a palaeontology field trip as an undergrad, I was told that one of the lecturers had a standing offer of a crate of beer for anyone who found a body fossil in association with a trace fossil. Finding a machaeridian in association with a trace fossil, I feel, would have warranted at least two.


Tommotiids. On the left, the articulated Eccentrotheca. On the right, sclerites of Micrina placed to show their suggested life positions. Photo from here.

The second big announcement comes from the Cambrian Arrowie Basin of South Australia - another articulated tommotiid! Last year, I reviewed a new reconstruction of the tommotiid Micrina presented by Holmer et al. (2008). This reconstruction, with two valves on either side of an attached stalk (though do note, it was a reconstruction rather than a description of an articulated specimen, so it's not immune to revision), was intriguing in its resemblance to a basal brachiopod (to which group of Recent animals tommotiids are almost certainly related, sharing a very similar shell microstructure). However, it was in fairly stark contrast to the previously described articulated tommotiid Eccentrotheca, which has its sclerites stacked one above another to form a tubular structure (Skovsted et al., 2008). The new articulated tommotiids described by Skovsted et al. (in press) may just go some way towards bridging the divide.


Paterimicra. On the left, apical (above) and lateral (below) views of the large sclerite S1. On the right, S1 in suggested life position with an S2 sclerite within the triangular notch. Scale bars for this and the next figure = 200 μm. Figures from Skovsted et al. (in press).

Skovsted et al. (in press) have described articulated scleritomes of the tommotiid Paterimitra. Like Eccentrotheca, Paterimitra had more sclerites in its scleritome than the two suggested for Micrina. However, unlike the tubular Eccentrotheca, Paterimitra had the scleritome dominated by a single basal S1 sclerite, shaped a bit like a wonky four-sided pyramid with one side extended out further than the other. On each of these two opposing sides was a deep notch or sinus, with the notch on the steeper side much deeper and with an outwards-pointing flange at the bottom. Inside this deeper notch would sit the smaller, triangular S2 sclerite, which also had an outwards-pointing flange at its bottom end that lined up with the flange of the S1 to form a loose protective tube. There were also a number of smaller, twisted-plate-shaped L sclerites. I have to confess, I'm still trying to work those out to some extent, but as far as I can tell they stacked on one side on the top of the S1 to form some degree of protective covering for the opening of the pyramid.


Lateral view of a partially-articulated Paterimicra specimen with L sclerites fused to the top of the S1 sclerite. It is noteworthy that the available articulated Paterimicra specimens with fused sclerites (this one, which is the only one to retain the L sclerites in place, particularly) show signs of injury or pathology at some point in development. This suggests that sclerite fusion was a pathological response in these individuals, not a normal part of scleritome development, which may partially explain why articulated specimens are so rare. Figure from Skovsted et al. (in press).

Skovsted et al. (in press) suggest a sessile life position for Paterimitra with the S1+S2 pyramid standing point-downwards, attached to the substrate by an organic stalk (like the pedicle of modern sessile brachiopods) passing between the flanges of the sclerites. They suggest Paterimitra may have been derived from an Eccentrotheca-like ancestor by the enlargement of the basal sclerites. Another Eccentrotheca-type lineage may have lost the sclerites entirely to give rise to the modern worm-like phoronids (though note that a few recent authors have suggested, based on soft-body characters shared with linguloids, that phoronids may be derived from within brachiopods). Micrina (in its suggested form) could be derived from a Paterimitra-type animal essentially by the loss of the L-sclerites. After that, it's simply a matter of extending the two remaining sclerites so that the shell is able to fully close (both Micrina and Paterimitra would have been permanently open to some degree, though Holmer et al., 2008, suggested a protective guard of long setae for Micrina), and what you've got is a quite passable basal brachiopod!

REFERENCES

Högström, A. E. S., D. E. G. Briggs & C. Bartels (in press, 2009) A pyritized lepidocoleid machaeridian (Annelida) from the Lower Devonian Hunsrück Slate, Germany. Proceedings of the Royal Society of London Series B - Biological Sciences.

Holmer, L. E., C. B. Skovsted, G. A. Brock, J. L. Valentine & J. R. Paterson. 2008. The Early Cambrian tommotiid Micrina, a sessile bivalved stem group brachiopod. Biology Letters 4 (6): 724-728.

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.

Skovsted, C. B., L. E. Holmer, C. M. Larsson, A. E. S. Högström, G. A. Brock, T. P. Topper, U. Balthasar, S. Petterson Stolk & J. R. Paterson. (in press, 2009). The scleritome of Paterimitra: an Early Cambrian stem group brachiopod from South Australia. Proceedings of the Royal Society of London Series B - Biological Sciences.

Vinther, J., P. Van Roy & D. E. G. Briggs. 2008. Machaeridians are Palaeozoic armoured annelids. Nature 451 (7175): 185-188.