Jurassic sea creatures spent decades as ‘full-time ocean sailors’, crossing the sea on driftwood ‘rafts’, a study claims.
Analysis of these rafts show they could last for as long as 20 years – enough time for the creatures, called crinoids, to grow to maturity, scientists reveal.
The attractive marine animals, which resemble colourful sea lilies, consist of a series of plates connected together in branches with a stem.
For crinoids, rafts became popular locations for colonies, as the structures were high in the water and provided a safe haven to escape predators.
There have been around 6,000 species of crinoids, about 10 per cent of which are alive today, although they don’t tend to float on rafts.
Crinoids fossils dating back to the Jurassic period are common in Yorkshire and around the Dorset coast, including Lyme Regis, part of the Jurassic Coast World Heritage Site.
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Using a giant fossil specimen (bottom left) from Germany, researchers mapped the spatial position of crinoids (bottom right) in one of the largest and best-preserved Early Jurassic floating wood fossils (top)
‘Modern crinoids don’t typically take such journeys, but we’ve since discovered fossilised examples of groups of floating crinoids,’ wrote study author Dr Aaron W. Hunter from the University of Cambridge for The Conversation.
‘However it wasn’t clear whether these were really thriving colonies living on the driftwood or just short-term passengers.
WHAT ARE CRINOIDS?
Crinoids, a distant ancestor of today’s sea lilies.
Crinoids were abundant long ago, when they carpeted the sea floor.
These echinoderms were at their height during the Paleozoic era.
They could be found all over the world, creating forests on the floor of the shallow seas of this time period.
There were so many in places, that thick limestone beds were formed almost entirely from their body parts piled on top of each other.
‘Now my colleagues and I have shown that such rafts could last for as long as 20 years, plenty of time for crinoids to grow to maturity and become full-time ocean sailors.’
Human understanding of crinoids dates back to the 1830s, when English palaeontologist William Buckland – known for the discovery of the Megalosaurus – collected fossils with another pioneering palaeontologist, Mary Anning.
One of their discoveries was the remains of fossilised crinoids, which are close relatives of sea urchins and starfish.
The specimens from Lyme Regis in Dorset, dating back to the Jurassic period more than 180 million years ago, looked like polished brass because they had been fossilised with pyrite, better known as fool’s gold.
Buckland noticed that these crinoid fossils were attached to small pieces of driftwood, which had turned into coal.
‘He hypothesised that the crinoids had been attached to the driftwood while alive, and perhaps for their entire lives, possibly living suspended underneath it,’ Dr Hunter said.
‘Buckland’s idea was initially seen as fantastical and the scientific world remained sceptical, until, that is, the discovery in the 1960s of a truly spectacular group of fossils from Holzmaden, a village not far from Stuttgart, Germany.
‘In among marine reptiles, crocodiles and ammonites, were giant colonies consisting of complete logs covered with hundreds of perfectly preserved crinoids.’
Floating logs that ferried rich communities of sea creatures around the oceans were able to float for decades. The picture shows a reconstruction of marine life from the Jurassic period
In Jurassic times, Holzmaden had been a seabed that was uninhabitable due to low oxygen levels and the crinoids would ‘have clung for life’ to logs as there was no seabed for them to live on.
Scientists have been undecided as to whether the rafts, which feature preserved crinoid colonies, survived long enough for crinoids to grow to maturity, which can take up to 10 years.
Dr Hunter and his team studied the ancient wood rafts, taken from multiple German museums and collections, including Geoscience Centre of the University of Göttingen and the Geological Institute, University of Tübingen.
‘We established that the way to understand how long the colony could have lasted was to develop a “diffusion model”,’ Dr Hunter said.
‘This estimated how long it would take before the log would become saturated with water and fail.’
As the wood in crinoid raft fossils hasn’t been preserved well enough to reveal what species it is from, the raft in the model was represented with a composite of trees known to exist in the Jurassic period, such as conifers, cycads and ginkgo trees.
The analyses revealed that crinoid colonies could have existed for more than 10 years, even up to 20 years, before it started to break, exceeding the life expectancy of modern documented raft systems.
‘There is evidence from museum collections of fragments of wood with entire, fully grown crinoids attached to them that could only have resulted from this kind of collapse,’ Hunter said.
Reconstruction of the crinoid colony based on the a giant fossil specimen from Germany. It shows the crinoids on the right hand side of the long log that makes up the raft community
The crinoids preferred to attach themselves to one end of the log structure, just like a sea captain at the helm.
This pattern resembles that of other modern rafting species such as goose barnacles, which tend to inhabit the area at the back of a raft where there is least resistance.
Amazingly, this could help reveal to researchers the direction of travel of the colony across the ocean.
Other researchers had also proposed that any floating crinoid colony would have grown until the population became too heavy for the wood raft to support it, at which point the log would have sunk to the oxygen-free seafloor where the crinoids would then have become fossilised.
‘However, research on living crinoid populations off the coast of Japan revealed that the animals would be too lightweight, even in large mature colonies, to cause a log to become overburdened and sink,’ Dr Hunter said.
The study has been published in Royal Society Open Science.