Is this a real example of the blind leading the blind?
A group of geologists think they have found evidence that eyeless "woodlice", known as trilobites, marched across the ocean floor in prehistoric conga lines.
The animal queues are uncannily reminiscent of the migratory chains that some lobsters form on the seafloor today. But other researchers think it is unlikely that such fragile chains have been preserved.
Many fossil hunters love finding trilobites. These ancient animals, which generally looked a little like woodlice, were a common feature of the oceans between about 520 and 250 million years ago.
Some were tiny, like the 1-2mm-long Acanthopleurella stipulae, while others were large like the 70cm-long Isotelus rex. Some swam like Telephina bicuspis and some burrowed like Agraulos ceticephalus. Some had well-developed eyes like Erbenochile erbeni and some were blind like Trimerocephalus chopini.
It is this last species that Błażej Błażejowski at the Polish Academy of Sciences in Warsaw, Carlton Brett at the University of Cincinnati in Ohio and their colleagues studied. These trilobites are found in huge numbers in 365-million-year-old rocks at Kowala Quarry in central Poland.
But it is not just their abundance that is remarkable: it is the patterns they form. Many of the trilobites are arranged nose-to-tail in queues containing as many as 19 individuals.
The researchers have published their findings in the journal Palaeontology.
Brett says the fossils are some of the strongest evidence yet found that ancient arthropods deliberately came together to form long chains and march across the seafloor together.
"There are reports of queues of older trilobites," he says. "[But] these are the best documented so far."
What's more, Błażejowski and Brett think they can guess what the trilobites were up to.
They realised that the trilobites fell into two size categories. About half of them were relatively small and had nine body segments. The other half were larger and had 10 segments. Such 50:50 ratios are often a sign of sex differences, so they hint that males and females joined the conga lines in roughly equal numbers.
Conceivably, the trilobites were moving to or from a breeding ground.
"We have suggested that these trilobites migrated together into aggregates, molted and mated," says Brett. "The queues or chains may record migration."
In the absence of eyes, the trilobites must have found other ways to stay together in their migratory caravan. Possibly they used chemical signals, or perhaps each animal felt for the individual in front using their antennae.
During these particular migrations, the trilobites evidently met with disaster. Perhaps they were smothered by a pocket of water exceptionally low in oxygen, which suffocated them.
On today's seafloor, some species of lobster form similar sorts of chains. They often migrate further offshore into deeper water in the run-up to winter, perhaps to escape strong winter storms that can churn up sediment in shallower settings.
But exactly why the lobsters choose to migrate in chains, rather than individually, is a bit of an enigma.
"The queues may help reduce drag on individuals," says Brett. "Individuals in chains may also help to protect one another [from predators]."
"Examples indicating some kind of integrative, 'social' behaviour are relatively rare in the fossil record," says Derek Siveter at the University of Oxford in the UK. "This example of Trimerocephalus trilobites very usefully increases our knowledge of such occurrences."
In a study published in 2008, Siveter and his colleagues reported fossil evidence of similar behaviour from 525 million years ago. In this case, peculiar free-swimming arthropods had formed chains as they swam through the seas. Nowadays, strange invertebrates called tunicates form superficially similar-looking chains.
However, not everyone is convinced that the trilobites were really migrating in groups.
It is certainly possible that the Polish trilobite fossils are the remains of ancient seafloor migrations, but the question is far from settled, says Brian Chatterton at the University of Alberta in Edmonton, Canada. "I would prefer a verdict somewhere between unlikely and the Scottish verdict of 'not proven'."
Chatterton has previously studied similar trilobite chains. He thinks it is unlikely that something as fragile as a chain of trilobites trundling across the seafloor would be preserved intact after the trilobites' sudden death. The slightest ocean current would disturb the carcasses and carry some of them away.
He thinks it is more likely that the trilobites dug into the seafloor. In that case, the chains may actually preserve particularly crowded trilobite tunnels that were suddenly flooded with enough oxygen-poor water to kill the animals.
"They would all be pointed in the same direction because they entered the burrow from the down-current end," says Chatterton.
His studies of similar trilobite chains in Morocco identified the occasional individual facing the "wrong" way, which he thinks adds weight to his tunnel idea. "It would make a very confused conga line if some of the members were pointing in the opposite direction," he says.
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