Huge floating logs carried communities of animals on round-the-world trips during the dinosaur era. The mega-rafts gave marine animals a way to travel without passing through deadly low-oxygen zones.
We have long known of preserved logs up to 14 metres long from the Jurassic period 200 to 145 million years ago. The logs are covered in , an animal related to starfish that has a central body with many long feathery arms. The logs also carry oysters.
Initial studies in the 1960s . However, many palaeontologists believed that the logs could not have stayed afloat and that .
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Now, Aaron Hunter at the University of Cambridge and his colleagues have shown that the logs really could float, probably for years. Hunter had the idea after fishing for crinoids and realising how light they are. Even fully grown adults are 鈥渓ike feathers鈥�, he says.
Instead, the key question was how quickly the wood took on water, which would weigh it down and cause it to rot. Using mathematical models of the flow of water into wood, the team calculated that a log over 10 metres long would stay afloat for at least two years.
If the bark was fairly water-resistant, and was colonised by oysters that provided an additional barrier, such a log could float for 20 years or more. In that time, the crinoids could easily be carried halfway around the planet.
The team also found that the crinoids clustered towards one end and the bottom of the log. This could indicate that the log was moving through the water. 鈥淭he crinoids would have chosen an area of least resistance at the back,鈥� says Hunter.
Similar rafts still exist in modern oceans, but those in the Jurassic were unusually large and long-lived, says Hunter, partly because several marine animals that would today break down floating wood had not yet evolved. These include wood-boring bivalves and shipworms.
The floating rafts were probably important for the crinoids and oysters living on them. During the Jurassic, many seas were shallow and land locked, so much of the water was too low in oxygen for animals, like the Black Sea today. Floating on rafts may have been the only way for the animals to migrate.
Reference:听产颈辞搁虫颈惫,
