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.