Parents have all sorts of ways of keeping tabs on their offspring. But an ancient invertebrate found in the rocks of England had a particularly unusual trick: Babies of these spiky aquatic creatures were literally tied to their parents like swarms of tiny kites.
Yale University paleontologist Derek Briggs and his colleagues named the new species Aquilonifer spinosus, or “the spiny kite bearer,” in honour of its pointy appearance and as a reference to Khaled Hosseini’s novel The Kite Runner.
The paleontologists were initially preparing a 3D model of just the adult fossil, an arthropod that swam around what is now the United Kingdom about 430 million years ago. Arthropods are segmented animals with jointed appendages, such as insects, spiders, and crustaceans.
To the team’s surprise, the scans revealed ten little bundles of invertebrate joy, tethered to the larger one by long threads.
“The attached juveniles only emerged as we processed the fossil to generate the reconstruction,” Briggs says.
At first the scientists weren’t sure what these bonus fossils might be, so they considered three likely options: The smaller invertebrates might be parasites, hitchhikers catching a lift, or Aquilonifer babies.
The parasite interpretation doesn’t hold up, the team concludes, because the long threads are attached to the spines of the big Aquilonifer, which would put the smaller animals in “a position far from ideal for this kind of feeding,” says Briggs.
The little arthropods don’t seem to be another species catching a ride, either. The adult probably wouldn’t have tolerated freeloaders, and “it would have removed them, perhaps using the long front appendages,” says Briggs.
A slice of the fossil arthropod shows two of the tethered babies (top left) as well as the adult's head and an antenna-like appendage.
Instead the team thinks the best fit is the more benevolent scenario. The large and small invertebrates share anatomical similarities, as well as literal ties to each other, so the tiny fossils are most likely theAquilonifer’s young, they report this week in the Proceedings of the National Academy of Sciences.
“This is the only occurrence of this kind of brooding behavior known among fossil or living arthropods,” says Briggs. Being tethered to their parent probably served to protect the youngsters, making them less accessible to predators, he adds.
Not that the little Aquilonifer babies were simply drifting along. Briggs and his coauthors propose that the young arthropods could have moved their appendages to give them lift and maneuver to feed on plankton as their parent swam around.
Drawing behaviour out of the fossil record is always difficult, University of California, Riverside paleontologist Nigel Hughes says in an email. But “I think they make a reasoned case, with an imaginative (and possibly even correct!) functional interpretation of how the tethered ‘juveniles’ would sample potential food sources differently from the parent.”
The discovery may also help creepy crawly arthropods get better reputations, he adds.
“Arthropods do generally get bad press”, Hughes says, noting that “unpleasant space aliens in movies often tend to look curiously arthropodial.”
In reality, “arthropod species quite commonly employ strategies that we recognize as more appealing, such as brooding,” he says. Aquilonifer only adds to that picture of conscientious arthropod parents, trailing its young through primordial seas.
By Brian Switek