Leonardo da Vinci's sketch of hexagons could represent Paleodictyon, fossilized burrows of an unknown creature. Credit: Biblioteca Leonardiana

Around 500 years ago, Leonardo da Vinci briefly focused his attention on fossils — and inadvertently triggered a mystery that remains unsolved.

A page of Leonardo's Paris Manuscript I is covered in sketches of marine fossils; among them is a honeycomb-like array of hexagons that palaeontologists think might constitute the first recorded observation of an enigmatic trace fossil called Paleodictyon1. The fossil is thought ny many paleontologists to be an imprint of burrows made by an animal living in loose sediment on the sea floor. Examples of Paleodictyon have been found that date back to the Cambrian period, 542 million to 488 million years ago, and similar structures are still being made on the sea floor today.

But the identity of the animal that generates the hexagons remains elusive. A set of similar, but simpler, fossils could explain why — and one researcher says that they could show that organisms started caring for their young millions of years earlier than thought.

Mark McMenamin, a palaeontologist at Mount Holyoke College in South Hadley, Massachusetts, found the fossils in Nevada and Mexico, in 540-million-year-old limestone from the early Cambrian — the time when complex organisms first started to appear and diversify in the fossil record. They seem to be burrows, each a few tens of micrometres in diameter, forming swarms about 2 centimetres across.

Looking closely, McMenamin noticed that some of the burrow swarms cut through organic pellets 250 to 500 micrometres in diameter — too large to have been generated by whatever made the burrows.

Potential parents

McMenamin thinks that an unknown adult animal deposited the pellets to form a nest around a clutch of its eggs, which failed to fossilize. "The hatchlings then fed on organic matter in the pellets that had been broken down by bacteria," he says. As they ate their way through the nest, the hatchlings left burrows that were preserved in the fossil record.

A Paleodictyon fossil from Austria. Credit: Reprinted from Rona, P. A. et al, Deep Sea Research Part II: Topical Studies in Oceanography, 56, 1700-1712 (2009), with permission from Elsevier

"I noticed that the smaller microburrows tended to cluster in the centre of the swarms," says McMenamin. Towards the edge of the nest the burrows are larger, which he thinks shows that the offspring had begun to grow by the time they reached the perimeter. He discussed the idea at the Geological Society of America annual meeting last week in Charlotte, North Carolina2.

If correct, McMenamin's interpretation would add more than 200 million years to the known record of parenting. "This is a remarkably complex behaviour for the Cambrian," he says.

The hypothesis might also explain why neither Leonardo nor modern researchers have found the animal responsible for the more complicated hexagonal Paleodictyon burrows. Despite their complexity, McMenamin thinks they, too, are burrows produced by hatchlings. This suggests the burrows are occupied relatively briefly, narrowing the odds of the animal dying inside and being fossilized, he says.

More evidence needed

The idea is certainly spectacular, says Gabriela Mangano, who studies Cambrian burrows at the University of Saskatchewan in Saskatoon, Canada. And McMenamin is known for such ideas. Last year he sparked controversy by suggesting that curious linear patterns of vertebra belonging to giant Triassic marine reptiles were evidence of a midden generated by a 30-metre-long cephalopod — despite the absence of any direct fossil evidence for the existence of any such "kraken" (see 'Kraken versus ichthyosaur: let battle commence').

Mirroring reaction to the shell midden hypothesis, Mangano is sceptical of the early nest idea. She says she is "quite adventurous" when it comes to interpreting trace fossils, "but I rely on a detailed morphologic analysis."

Duncan McIlroy, a burrow specialist at the Memorial University of Newfoundland in St John's, explains what would be needed to fully test the nest idea. McMenamin would need to carefully section the rock to build up a 3D picture of the burrows. "I would look for a discrete structure as part of a large semi-permanent burrow system created by the adult," he says.

Could Paleodictyon also be nests? "Sure they could be," says McIlroy. "But it would be very difficult indeed to prove without finding one with associated eggs and juveniles."

That remains a problem: McMenamin says that the burrows are more likely to fossilize than the soft-bodied animals that made them. He plans to keep looking for their remains in his Cambrian specimens, but it might be some time before anyone can confirm his nest hypothesis — let alone discover what animals made them.