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.

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.

Scleritome Week: A Mystery Ending

Welcome to the last post for Scleritome Week. I hope you've enjoyed reading it as much as I've enjoyed writing and researching it, and that it's brought a few surprises (it has for me - I actually expected chancelloriids to be a lot more like sponges, and if you look closely you might notice that I actually referred to them as such in the machaeridian post). This week I've brought you mystery fish, armoured worms, and an animal that one researcher reconstructed looking like one of the Ophanim*. Most of the animals I've covered have become well-known throught the fortunate discoveries of articulated specimens, but I thought I'd end with a little reminder of just what made the scleritome animals so fascinating - the fact that for so many of them we don't yet know what the animal looked like.

*Third choir, comes after the Seraphim and Cherubim.



Eurytholia was described in 2001 (Sutton et al., 2001) for small sclerites the authors described as "hat-like" found from scattered locations in Europe and North America. Eurytholia sclerites are more or less oblong in shape, with a central ridge running parallel with the shorter sides. The figure above from Sutton et al. (2001) shows an assortment of specimens from different angles.

As yet, no articulated specimens showing what the rest of the animal looked like have been found, but the authors were able to make some inferences about it. The sclerites were exterior rather than interior - their microstructure indicates that they were secreted from the underside only, and some specimens show evidence of having been damaged while the animal was alive. They were unlikely to have functioned as teeth due to their unsuitable morphology. They also don't appear properly shaped to have overlapped each other. Due to the relative abundances of sclerites of different sizes, Sutton et al. suggest an "armoured slug" appearance rather like that known for Wiwaxia (reconstruction also from Sutton et al., 2001):



Personally, I can't help thinking it looks like a headless, limbless, tail-less ankylosaurian. But the wonderful thing is that we just don't know if Sutton et al. got it right. Their reconstruction seems plausible enough, but until we find an articulated specimen, who knows what kind of tentacled monstrosity Eurytholia might actually turn out to have been?

REFERENCES

Sutton, M. D., L. E. Holmer & L. Cherns. 2001. Small problematic phosphatic sclerites from the Ordovician of Iapetus. Journal of Paleontology 75 (1): 1-8.

Scleritome Week: The Cactus Animals

They may be a day late, but I promised chancelloriids and here they are!

Chancelloriids are arguably the most frustrating group of animals I'm going to be covering for Scleritome Week. in the cases of machaeridians, Microdictyon and palaeoscolecidans, the identification of articulated specimens revolutionised our understanding of the sclerite-bearing animal. In the case of chancelloriids, despite the availability of a number of well-preserved articulated specimens, we remain very much in the dark. We know what the animals looked like, we have a reasonably good idea of how they were put together, we can infer a lot about their probable life-style. And after all that, we're left with something that just doesn't make a lot of sense.

As you can see in the figure above from Bengtson (2004), chancelloriids were sessile animals, probably filter feeders, with an external covering of star-shaped sclerites. Bengtson (2004) compares their appearance to a cactus, which sounds like a pretty good description to me. Different species varied somewhat in overall shape, from the cylindrical Chancelloria to the more globular Allonnia. Chancelloriids were restricted to the Cambrian, and became extinct by the end of that period (Janussen et al., 2002). Some specimens show a root thickening at the base of the animal that probably served to anchor it in soft sediment.



When first described, chancelloriids were regarded as sponges, with the sclerites compared to sponge spicules. It is true that their overall appearance would have been very sponge-like, but the finer details don't stack up. Sponge spicules are internal structures, secreted by an enveloping layer of sclerocyte cells. In contrast, the chancelloriid sclerites appear to have been at least partially external (the base may have been embedded in the animal's body, with only the spines protruding) and possessed a hollow central cavity that in life probably contained soft tissue (the figure above from Janussen et al., 2002, shows the basal foramina in each individual spine that would have connected the spicule tissue with the rest of the body). Well-preserved specimens from Chengjiang show evidence of a thick epidermis, completely different from the thin and undifferentiated pinacoderm of sponges. On this basis, Janussen et al. (2002) decided that chancelloriids must at least belong to the Epitheliozoa, the clade of all animals except for sponges (Trichoplax plus Eumetazoa). However, as I've recently learnt from Palaeos, one group of sponges, the Homoscleromorpha, does possess a true eumetazoan-like epithelium, though spicules are still of typical sponge construction and nothing like the chancelloriid sclerites.

In 1981, Bengtson and Missarzhevsky suggested an alternative position for chancelloriids as a member of their Coeloscleritophora, along with two order groups of sclerite taxa, the siphonoguchitids and wiwaxiids. The three groups were united by the possession of a hollow sclerite with no evidence of accretionary growth (that is, the sclerite was probably secreted as a unit rather than being added to over the course of the animal's life). Later, Bengtson was to suggest a molluscan affinity for coeloscleritophorans due to similarities in shell secretion.



As far as I know, siphonoguchitids have not yet been found as articulated fossils, but finds from the Burgess Shale mean that the living appearance of wiwaxiids is well-known (picture above from Palaeos). In stark contrast to the sessile chancelloriids, wiwaxiids were mobile animals, a bit like an armoured slug. Authors have differed over whether wiwaxiids were more closely related to annelids or molluscs, but their position somewhere within the trochozoans seems secure. It is a lot more debatable whether the Coeloscleritophora is a monophyletic group, or if the coelosclerite has arisen polyphyletically in unrelated groups. Even before the identification of the wiwaxiid body form, doubts had been cast based on the bilateral nature of individual wiwaxiid and siphonoguchitid sclerites compared to the radial arrangement of chancelloriid sclerites. I can't help asking myself, though, if sea squirts were only known from adult fossils, without any understanding of their development, would any connection be made to other chordates?

REFERENCES

Bengtson, S. 2004. Early skeletal fossils. In Neoproterozoic-Cambrian Biological Revolutions (J. H. Lipps & B. M. Waggoner, eds.) The Paleontological Society Papers 10: 67-77.

Janussen, D., M. Steiner & Zhu M. 2002. New well-preserved scleritomes of Chancelloriidae from the Early Cambrian Yuanshan Formation (Chengjiang, China) and the Middle Cambrian Wheeler Shale (Utah, USA) and paleobiological implications. Journal of Paleontology 76 (4): 596-606.

Scleritome Week: Not just an invert thing

Unfortunately, I can't touch the chancelloriids until tomorrow, but they will be here, I promise.

So far, all the animals I've shown you in relation to Scleritome Week (see here, here and here) have all been definitely in the class of organisms dismissed by the sadly vertebrate-centric as "creepy-crawlies". Nevertheless, the disarticulated scleritome issue is not unique to invertebrates.

The figure at the top of the post (from Valiukevičius & Burrow, 2005) shows scales of Silurian fish of the family Tchunacanthidae. This family was originally described by Karatajute-Talimaa & Smith (2003) as a new order, distinct from all others previously described (while Valiukevičius & Burrow seem a little sceptical of such a high ranking, they do still maintain the family's distinctiveness). The interesting thing for this post is that, so far, tchunacanthids are known only from scales.



Tchunacanthidae are member of the Acanthodii, an extinct class of vertebrates found from the Silurian to the Permian (a couple of examples are shown above in an illustration from here). Acanthodians are sometimes referred as "spiny sharks", a name that probably survives more because it sounds neat than because of its appropriateness for the actual animals. While generally regarded as more closely related to modern bony fishes and tetrapods than actual sharks, acanthodians resembled sharks in having a cartilaginous rather than a bony skeleton. As a result, acanthodian skeletons were rarely fossilised, and usually only the hard mineralised parts survived - teeth, scales and spines. Without the soft tissue holding them together, however, the fossils became disarticulated, just like the sclerites of a scleritome animal. Tchunacanthids are far from being the only family of non-bony fish known only from scattered pieces of armation - articulated specimens (except in those taxa that developed large bony plates) are the exception rather than the rule. Beyond the general features shared by all acanthodians, we are doomed to ignorance about what a living tchunacanthid looked like unless some day we are lucky enough to find one of those rare articulated fossils.

REFERENCES

Karatajute-Talimaa, V., & M. M. Smith. 2003. Early acanthodians from the Lower Silurian of Asia. Transactions of the Royal Society of Edinburgh: Earth Sciences 93: 277-299.

Valiukevičius, J., & J. C. Burrow. 2005. Diversity of tissues in acanthodians with Nostolepis−type histological structure. Acta Palaeontologica Polonica 50 (3): 635-649.

Scleritome Week: Worm Buttons

Unlike the other animals I'll be covering as part of Scleritome Week, the Palaeoscolecida were actually known as entire animals long before their dermal armation was described, but they still meet the Scleritome Week qualifications because said armation was described as isolated problematic fossils before a connection was made between the animal and its armour (Ivantsov & Wrona, 2004). The photo above (from here) shows one of the isolated sclerites, originally described under the name Hadimopanella. Palaeoscolecid sclerites are round and button-like, with a central array of nodules that vary in different species from low and rounded to higher and pointed. Opinions on the nature of these microfossils (to appreciate how small they are, the scale bar on the photo above represents 0.03 mm) varied from some sort of dermal armour to the remains of reproductive cysts (Repetski, 1981). The dermal armour theory, of course, won out when the connection was made between the isolated sclerites and ornamentation on the compressed body fossils almost simultaneously by different authors in 1989 (Ivantsov & Wrona, 2004).



Palaeoscolecidans were a successful group of burrowing worms in the early Palaeozoic, when they were probably even more significant than the annelids (the image above of the holotype of Tabelliscolex hexagonus comes from Han et al., 2007). Originally interpreted as annelids, the segmented appearance is apparently only superficial, and results from alternating bands of larger and smaller plates (Ivantsov & Wrona, 2004). Well-preserved specimens from the Chengjiang Fauna possess an anterior spiny proboscis like that of the modern priapulids, and palaeoscolecidans have most often been regarded as priapozoans*. Other authors have suggested relationships with the modern nematomorphs, or as stem-panarthropods (Han et al., 2007). At the very least, a position within the Ecdysozoa, the clade uniting these three groups, seems well-established.

Tomorrow, I'll move on to chancelloriids.

*Bring on the nomenclatorial quibble. Most authors supporting this affinity have simply referred to palaeoscolecidans as "priapulids". The modern priapulids are a small, well-defined group of worms, while the various Palaeozoic taxa regarded as stem-priapulids show a much higher diversity of body plans (many of them, for instance, were far more elongate than any living priapulid, while no living priapulid possesses a dermal armour like that of palaeoscolecidans). Personally, I'd prefer to only refer to the crown group as priapulids, and use the name Priapozoa to cover the larger group including the stem forms.

REFERENCES

Han, J., J. Liu, Z. Zhang, X. Zhang & D. Shu. 2007. Trunk ornament on the palaeoscolecid worms Cricocosmia and Tabelliscolex from the Early Cambrian Chengjiang deposits of China. Acta Palaeontologica Polonica 52 (2): 423-431.

Ivantsov, A. Y., & R. Wrona. 2004. Articulated palaeoscolecid sclerite arrays from the Lower Cambrian of eastern Siberia. Acta Geologica Polonica 54 (1): 1-22.

Repetski, J. E. 1981. An Ordovician occurrence of Utahphospha Müller & Miller. Journal of Paleontology 55 (2): 395-400.

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.