A case of Crinoid trench foot?

Article summary by Andrea Brewster, Intern and Exec Member with the Geology Trust

A dark patch of rock on the left marks the Rauchkofel Boden trench, dug in the Cardiola Formation of the Austrian Alps during World War I. Photo courtesy of Annalisa Ferretti, University of Modena and Reggio EmiliaSoldiers digging trenches during the ‘White War’, a series of battles at the border between Austria, Hungary and Italy, fought between 1915 and 1918 in World War I, were probably unaware that their actions were exposing a limestone rock known as the Cardiola Formation that was full of the 450 million year old holdfasts (feet) of juvenile crinoids, distant relatives of today’s sea lilies.

The Cardiola Formation had been investigated in the past, with rock samples containing many coral and trilobite fossils, but it was not until this recent investigation, undertaken by an international team of researchers, that the presence of the juvenile crinoid holdfasts came to light.

A stalked CrinoidMost stalked crinoids today have a free-floating larval stage, that eventually attach to the ocean floor rocks, where they grow to adulthood. As soon as a crinoid dies, it starts to disarticulate as the connective tissues holding the calcite sections of the crinoid together decompose rapidly. As a result of this, it is usual to just find sections of fossilised crinoids. Holdfasts are a rarer find and fossil remains of juvenile crinoids rarer still.

Today’s stalked crinoids are able to release their holdfasts and crawl along the sea floor if they have to, but they don’t do it very often and certainly don’t travel far.

These fossil holdfasts were not attached to rocks, suggesting that they had been attached to some other hard surfaces that had not survived fossilisation, such as gastropods living on the sea floor, hitchhiking far from home before they reached adulthood. 

Unusually the holdfasts had been preserved as minerals of iron oxide so the researchers were able to extract them from their limestone matrix once it had been dissolved, by using magnets.

Holdfasts from very young crinoids that lived 450 million years ago, fossilised in iron oxide. Image courtesy of The Ohio State University. Looking like tiny red stars, these fossils measuring just 1 to 4mm have given scientists new ideas about how a species was able to populate the world’s oceans, surviving millions of years of climate change and still be with us today.

References: news.osu.edu/news/2017/04/03/babycrinoid and dailymail.co.uk/sciencetech/article-4376700/World-War-trenches-Alps-reveal-450m-year-old-fossils.html