The fossils this week celebrate the appearance of an article in the latest issue of Palaios authored by an international team led by my good friend and colleague Michał Zatoń (University of Silesia, Poland). The fossils are strange polka-dotted tubes encrusting Middle Jurassic oncoids and concretions from the Polish Jura — a place I enjoyed visiting last summer with Michał. The fossils were quite mysterious to us, but with the help of our new colleague Yasunori Kano (The University of Tokyo), we think we now have a good idea what they represent. Above you see one of the tubes on a concretion.
The polka dots are actually small, regular divots in the sides of the tubes, as shown above in this view through a scanning electron microscope. It turns out that these concavities are the same size as ooids (rounded carbonate grains) in the depositional environment. In fact, occasional ooids are still in their holes, as shown by the white arrow in the image.
In this cross-section through one of the tubes, each of the exterior holes is lined with a thin layer of carbonate, which is apparently the outer layer of an ooid that was cemented into each space. The tube itself is completely occupied by fine carbonate sediment.
Our hypothesis is that the tubes were formed by some sort of polychaete worm similar to serpulids and sabellids (with which they are associated). The worm may have built a hollow living tube by gluing ooids together and possibly taking advantage of the quick-cementing characteristics of this Jurassic calcite sea. It may have then fed on the surrounding microbial mats that covered the concretion and oncoid surfaces. This hypothesis explains the sessile nature of the tubes, their shape and construction, and their association with thin mineralized layers formed by cyanobacteria.
No polychaetes today are known to build living tubes out of ooids, so these Jurassic forms are thus far unique in the fossil and living record. It was a fun paleontological puzzle to tackle with my friends!
We are proud that our little study was chosen as the cover story for the August 2012 issue of Palaios:
“Unusual tubular fossils associated with microbial crusts from the Middle Jurassic of Poland. Upper left, an exposure of Middle Jurassic (Bathonian) clays at Ogrodzieniec in the Polish Jura; lower left, ESEM pictures of morphology and structure of the Middle Jurassic tubular fossils interpreted as remnants of agglutinated polychaete tubes; lower right, two pictures of tubular fossils encrusting oncoid and concretion; upper right, two pictures of recent agglutinated polychaete tubes from Japan.”
Zatoń, M., Kano, Y., Wilson, M.A. and Filipiak, P. 2012. Unusual tubular fossils associated with microbial crusts from the Middle Jurassic of Poland: agglutinated polychaete worm tubes? Palaios 27: 550-559.
Zatoń, M., Kremer, B., Marynowski, L., Wilson, M.A. and Krawczynski, W. 2012. Middle Jurassic (Bathonian) encrusted oncoids from the Polish Jura, southern Poland. Facies 58: 57–77.