Some diatom genera are tricky and easily confused with other taxa, especially when examined in light microscope only. But Rutilaria is different: you cannot possibly mistake it for any other diatom. It may look insignificant at first, just another elongate shape among all diatoms. But when you take a closer look at the way the cells of Rutilaria are linked – it becomes intriguing, to say the least.
Rutilaria is probably still around – it’s been reported from some tropical sites – but most of the several dozen known species are exclusively fossil. It has several defining features, but the most important one is also usually the most conspicuous: the priplekton. The word derives from Greek and was first proposed in the 1960s. The periplekton is a specially modified process (i.e., an appendage projecting from the valve) that functions as a linking device. It consists of a tube that may be short or long, and two arms that wrap around the periplekton of the sibling valve. With an artificial pink color, it looks like that:
The periplekton is an engineering miracle. It enables the cells of Rutilaria to slide closer to or away from one another. This effect depends on the length of the periplekton stem: in those species that have short periplekta, there is little room for sliding. But in those taxa that had very long periplekta, like Rutilaria amplectans, the cells could travel back and forth, perhaps adapting the chain to the current? The periplekton also enables some degree of relative rotation between the valves. Although Rutilaria is never abundant in fossil (and possibly living) diatom assemblages, the periplekton must have offered some benefit: given that the oldest known Rutilaria occur in the Late Cretaceous, the strategy of linking cells via periplekta is now some 70 million years long.
Rutilaria was first described and illustrated by Robert Kaye Greville, back in 1863. The original description included the genus as well as several species; as highlighted before, in posts on Strangulonema and other out of the ordinary diatom taxa, you can use some free online resources to get to know Rutilaria better (and Greville himself, too!).
Rutilaria certainly caught Greville’s attention. The style of his formal descriptions is usually very concise. In the case of Rutilaria, he dedicated nearly three pages of print to describing the genus and proposing the species. Greville referred to the periplekton as „nodule”. Also, the illustrations occupy the top row of the published plate:
Using David Williams’ 1988 An Illustrated Catalogue of Type Specimens in the Greville Diatom Herbarium, we can also find out more about the type specimens for Greville’s Rutilaria species – and see the type specimens themselves:
In 1995, Robert Ross of The Natural History Museum published his monumental monograph on Rutilaria. Although it is 26 years old this year, it still is the ultimate resource on this genus, including lots of excellent scanning electron micrographs by Pat Sims, and line drawings made by Robert Ross himself. The monograph was some 40 years in the making, which makes it even an more impressive read. Luckily, you can find it freely available online:
Find Rutilaria intriguing? Here is some further reading you may find useful:
- Check out papers on diatoms (including Rutilaria) vs nanotechnology by Ille Gebeshuber here: https://scholar.google.com/citations?user=je2iOjoAAAAJ&hl=pl&oi=ao
- A 2011 paper including some unexpected Cretaceous Rutilaria – and a discussion on some uncommon diatom linking devices: https://www.researchgate.net/publication/254229211_Rutilariaceae_redefined_A_review_of_fossil_bipolar_diatom_genera_with_centrally_positioned_linking_structures_with_implications_for_the_origin_of_pennate_diatoms
Fossil Diatoms 
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