These Jurassic Sea Creatures Spent decades crossing the ocean on rafts. Here’s how

The English city of Lyme Regis is part of the Jurassic Coast World Heritage Site. It was here in the 1830s that William Buckland, better known from the discovery of the first dinosaur, Megalosaurus, collecting fossils with another pioneering paleontologist, Mary Anning.

One of their discoveries was the remains of fossil crinoids, sometimes known as “sea lilies”. Close relatives of seals and starfish, these flower-like animals consist of a series of plates connected in branches by a trunk.

The specimens from Lyme Regis, dating from the Jurassic period more than 180 million years ago, look like polished brass because they are fossilized with pyrite (gold of the fool).

Buckland noticed that these crinoid fossils were attached to small pieces of driftwood that we call lenses that were transformed into coal. He hypothesized that the crinoids were attached to the driftwood while they were alive, and perhaps for their entire lives, possibly living hanging under him.

Modern crinoids usually do not take such trips, but we have since discovered fossil examples of groups of drifting crinoids. However, it was not clear whether these were really thriving colonies living on driftwood or just short-term passengers.

Now my colleagues and I have shown that such rafts can last as long as 20 years, a lot of time for crinoids to grow until maturity and become full-time oceanic sailors.

Buckland’s idea was initially seen as fantastic and the scientific world remained skeptical. Until, that is, the discovery in the 1960s of a truly spectacular group of fossils from Holzmaden, a village not far from Stuttgart, Germany.

file 20200731 14 15j5ylqCrinoid raft fossils have now been found. (R. Haude / Göttingen University)

In between marine reptiles, crocodiles and ammonites were giant colonies consisting of complete logs covered with hundreds of perfectly preserved crinoids.

The German professor Adolf Seilacher and his then student (now professor) Reimund Haude proved to have solved the mystery of Buckland. These floating rafts of crinoids existed then.

This idea was reinforced by evidence that during the Jurassic period, which is now Holzmaden, there had been a sea level rise due to low oxygen levels. The crinoids would have attached their lives to these logs because there was no seabed to live on.

However, not all scientists agree. One of the important questions that was asked was whether these loggers could have survived long enough for the crinoids to grow to adulthood. This can last up to ten years, based on modern growth rates of their living relatives that can still be found at depths of about 200 meters.

A team of scientists from the UK and Japan, led by me, decided to tackle the problem. We were motivated by groundbreaking research into Japanese crinoids by Professor Tatsuo Oji, that were kept alive in the labs at the University of Tokyo.

One of the most important parts of the original theory was that any floating colony of crinoids would have grown until the population became too heavy for the woodcut to support it. The log would have been to the oxygen-free seafloor, where the crinoids would then become fossil.

However, research on living crinoid populations off the coast of Japan found that the animals were too lightweight, even in large adult colonies, to cause a log to overload and sink.

Model breakup

Our research then turned to the wood itself. We found that the way to understand how long the colony could last was to develop a “diffusion model”. This estimated how long it would take before the log would be saturated with water and fail.

The wood in crinoid raft fossils has not been well preserved for us to know what kind it comes from. That we represented it in the model with a composite estimate of trees that we know existed in the Jurassic, such as conifers, cycads, and ginkgo trees.

We found that the floating wood and its crinoid charge could have been at least 15 years old and maybe up to 20 years before the log would start to sink or break. There is evidence from museum collections of fragments of wood with hot, mature crinoids attached to it, which could only have been the result of this type of collapse.

Finally, we used a technique known as spatial point analysis developed by Dr. Emily Mitchell, to plot the spaces between the fossils and find out if the position pattern is ecological, environmental or both. This allowed us to estimate what this crinoid community might look like on the log.

We found that the crinoids did indeed hang under the driftwood, but clustered in the direction of one end of it. Although difficult to observe in the original fossils, the pattern is similar to that of other modern raft species such as goosebumps.

They tend to live in the area at the back of a float where there is the least resistance, which can tell us the direction of travel of the colony across the ocean.

This research has now cast doubt that crinoid raft colonies can exist for many years and can survive to grow to maturity and travel the great distances across the Jurassic Oceans. They are a profound example of similar structures we see in today’s oceans.

These captivating techniques are now being used by a new team to compare living seabed populations with their Jurassic ancestors.

This could reveal how past changes have taken place in marine communities and help scientists understand how such communities can respond to future challenges in an ever-changing world. The conversation

Aaron W Hunter, Science Guide & Tutor, Dept. Earth Sciences, University of Cambridge.

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