More little tubes - not just tons but tonnes

At the end of my previous post on tubular problematica, I said that I was leaving the subject while I still had my dignity. But then I remembered that I have no dignity. Besides, I remembered a couple more that I really wanted to look at. Neil from Microecos suggested that "Tubular Problematica" was a good name for a band. I beg to differ - a much better one would be "The Coleolus Effect".



Coleolidae: Coleolus and related taxa are found from the latest Proterozoic (McMenamin, 1985) to the early Carboniferous (Yochelson, 1999). As you can see in the picture above (from Yale University), they are small tapering tubes. They were most likely sessile in life with most of the tube projecting above the surface of the sediment (Yochelson & Hlavin, 1985).

Despite their long stratigraphic range and despite seeming to be reasonably common, the affinities of Coleolidae are completely unknown. Yochelson & Goodison (1999) noted that, "The literature on ancient "worm tubes" is scattered and scant, specimens are uncommon, and even a well-preserved one has few diagnostic characters and virtually no aesthetic interest" (ouch!) They have often been compared to Scaphopoda (tusk shells), a recent class of infaunal molluscs with superficially similar tubular shells, and more than one member of the Coleolidae has been initially identified as a scaphopod (Yochelson, 1999; Yochelson & Goodison, 1999). However, the shell structure is inconsistent with a mollusc, and the single known specimen with an intact tip shows that the apex was closed, unlike scaphopods which are open at both ends (Yochelson & Goodison, 1999). Yochelson & Hlavin (1985) considered Coleolus to be an annelid tube, but by 1999 Yochelson was admitting that "Except for formation of a calcareous tube, there is no basis for assignment of the family to Annelida".

Anabaritidae: I've saved the best for last, I can assure you. Anabaritids (shown in a picture from Palaeos) are known from the latest Proterozoic to the earliest Cambrian, and like Cloudina were one of the earliest animals to develop a skeleton (Kouchinsky & Bengtson, 2002). What's really cool about them, though, is what's shown in the cross-section (a) above the side view (b) - anabaritids had triradial symmetry. Triradial symmetry is exceedingly rare in modern taxa, but was found in a small assortment of Ediacaran and Cambrian organisms that have been suggested on this basis to form a grouping known as the Trilobozoa (Fedonkin, 1985). The affinities of the Trilobozoa are uncertain, but most authors interpret them as coelenterate-grade. Most interestingly, Ivantsov & Fedonkin (2002) suggested that the Conulata might have a trilobozoan ancestry. The Conulata (typified by Conularia) were a class of sessile problematica that survived until the Triassic, which would represent a significant increase in time-span for the Trilobozoa. Conularia has a four-fold symmetry, which has led most authors to interpret it as related to the modern Scyophozoa (jellyfish), but the Ediacaran Vendoconularia has a six-fold symmetry which Ivantsov & Fedonkin (2002) compared to the three-fold symmetry of Trilobozoa.

Tribrachidium heraldicum, another member of the Trilobozoa (from Answers.com).





Reconstruction of the possible live appearance of Conularia (from Dry Dredgers).

An alternative to the Trilobozoa interpretation of Anabaritidae was revived by Kouchinsky & Bengtson (2002), who interpreted anabaritids as polychaete worm tubes. This was based on the presence in anabaritids of a chevron-like wall structure, previously unknown except in serpulid polychaetes. However, there is a significant gap in time between the Cambrian anabaritids and the earliest definite serpulids in the Mesozoic. Also, many anabaritid shells preserve internal tooth-like projections that suggest that whatever animal lived in them was fixed in place - if it had been able to move back and forth in the manner of a serpulid worm, it would have probably filleted itself.

REFERENCES

Fedonkin, M. A. 1985. Precambrian metazoans: the problems of preservation, systematics and evolution. Philosophical Transactions of the Royal Society of London Series B - Biological Sciences 311: 27-45.

Ivantsov, A. Y., & M. A. Fedonkin. 2002. Conulariid-like fossil from the Vendian of Russia: A metazoan clade across the Proterozoic/Palaeozoic boundary. Palaeontology 45 (6): 1219-1229.

Kouchinsky, A., & S. Bengtson. 2002. The tube wall of Cambrian anabaritids. Acta Palaeontologica Polonica 47 (3): 431-444.

McMenamin, M. A. S. 1985. Basal Cambrian small shelly fossils from the La Ciénega Formation, northwestern Sonora, Mexico. Journal of Paleontology 59 (6): 1414-1425.

Yochelson, E. L. 1999. Rejection of Carboniferous Quasidentalium Shimansky, 1974, from the phylum Mollusca. Journal of Paleontology 73 (1): 63-65.

Yochelson, E. L., & R. Goodison. 1999. Devonian Dentalium martini Whitfield, 1882, is not a mollusk but a worm. Journal of Paleontology 73 (4): 634-640.

Yochelson, E. L., & W. J. Hlavin, 1985. Coleolus curvatus Kindle ("Vermes") from the Cleveland Member of the Ohio Shale, Late Devonian (Famennian) of Ohio. Journal of Paleontology 59 (5): 1298-1304.