Media playback is unsupported on your device Media caption The ten capsules presumably bobbed behind the animal like balloons as it scuttled along the sea floor

A 430 million-year-old sea creature apparently dragged its offspring around on strings like kites - a baffling habit not seen anywhere else in the animal kingdom.

Scientists who discovered the fossil have dubbed it the "kite runner".

Ten capsules tethered to its back appear to contain juvenile progeny, all at different stages of development.

Reported in the journal PNAS, the many-legged, eyeless, 1cm animal is not directly related to any living species.

"There isn't an animal today that it's essentially related to," David Legg, a palaeontologist from the Oxford University Museum of Natural History, told BBC News.

"It's what we refer to as being on a stem lineage. So it belongs to a group that would have evolved and diversified before the modern groups did."

In fact, Dr Legg added, although it clearly had the segmented body and exoskeleton of an arthropod, it was quite a job to figure out precisely where to position the little beast on the evolutionary tree.

"Often you can look at a particular animal and say, that belongs to this group or that group. This one - we had no clue."

Poor parenting?

The fossil was dug up from a site in Herefordshire before being taken to Oxford and computerised. This process involved grinding away the specimen, slice by slice, and photographing each of those sections to assemble a 3D reconstruction.

Then, with this outlandish animal on their screens waggling its legs, long antennae and tethered capsules at them, Dr Legg and his colleagues set about categorising it using "compositional phylogenetics".

Image copyright Briggs et al/PNAS Image caption The fossil - including the capsules, shown here - was ground away and scanned in tiny increments to make a 3D reconstruction

"You take its anatomy, code it into a data set and then run probabilistic methods on it, which will tell you how likely it is that something evolved in a particular way," he explained.

This process suggested that Aquilonifer spinosus ("aquilo" means north wind or kite and the "-fer" suffix means carry) was a mandibulate: it belongs to the same broad group as modern insects, crustaceans and centipedes - but is not a direct ancestor.

"Nothing is known today that attaches the young by threads to its upper surface," said co-author Derek Briggs, from Yale University in the US.

"Modern crustaceans employ a variety of strategies to protect their eggs and embryos from predators - attaching them to the limbs, holding them under the carapace, or enclosing them within a special pouch until they are old enough to be released - but this example is unique.

"It shows that arthropods evolved a variety of brooding strategies beyond those around today - perhaps this strategy was less successful and became extinct."

Image copyright D Briggs/D Siveter/D Siveter/M Sutton/D Legg Image caption The eyeless, 1cm animal is not directly related to any living species

Before settling on the unlikely conclusion that they were tiny, floating, prehistoric prams, the team did consider other explanations for the attached pods; they might have been smaller, parasitic creatures of a different species, for example.

But being tied on to Aquilonifer's tough and rather inedible shell would be a peculiar strategy for a hungry parasite.

It was the variety of shapes seen among the 10 tethered babies that Dr Legg found most convincing.

"We see them develop and begin to resemble the adult form more and more, as they get bigger," he said.

"I'm definitely convinced that that's what they were."

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