By Sid Perkins of Nature magazine

An explanation presented this week for a famed and enigmatic jumble of marine reptile fossils has blurred the lines between science and science fiction. The latest proposal suggests that the jumbled remains of these vast marine reptiles were stockpiled by an even more immense creature: a squid-like 'kraken' estimated to be around 30 metres long -- more than twice the size of today's largest known cephalopod.

Since its discovery in 1928 in Nevada, scientists have proposed many explanations for how and why the skeletons of so many large creatures were concentrated in the 228-million-year-old death assemblage, which includes 9 ichthyosaur skeletons around 15 metres long. So far, none of the scenarios match all the evidence, says Mark McMenamin, a palaeontologist at Mount Holyoke College in South Hadley, Massachusetts, who proposed the latest explanation at the Geological Society of America's annual meeting in Minneapolis, Minnesota on 10 October.

Some scientists have suggested that the ichthyosaurs succumbed to a toxic algae bloom and then washed ashore together. Others have contended that the creatures died, like modern-day whales, in a mass stranding. But these theories have several strikes against them, says McMenamin.

Enigmatic evidence

Although some of the ichthyosaur skeletons are relatively intact, others are more broken up and show signs of being exposed to sea water for an extended time before they were entombed by sediments. This disparity strongly suggests that the ichthyosaurs died at different times, not all at once, says McMenamin.

Furthermore, recent analyses by McMenamin suggest that the sediments surrounding the fossils accumulated in waters at least 200 metres deep, not in the shallows -- which makes it very unlikely that the remains ended up in the same place by chance. "It's a very odd configuration of bones," says McMenamin.

And one of the oddest aspects, he notes, is that some of the bones seem to have been rearranged. Some of the isolated vertebrae are preserved in double rows, apparently fitted together like puzzle pieces. One series of vertebrae includes a neck bone and another contains tail vertebrae, indicating that the patterns aren't simply spinal columns that fell apart. This suggested to McMenamin that the bones were moved around by some other creature.

Mythological proportions

Some of the most intelligent creatures in today's oceans, says McMenamin, are cephalopods -- particularly octopuses and squid. In large aquariums, octopuses are known to collect unusual objects and even play with them. McMenamin imagines that during the Triassic period, his proposed kraken would do battle with ichthyosaurs. When it was victorious, the kraken would drag the ichthyosaur's corpse back to its den for a feast.

In McMenamin's purported midden, the kraken would play with the bones of the unfortunate ichthyosaurs, perhaps even arranging them into deliberate patterns. The double lines of disc-shaped vertebrae in the death assemblage closely mimic the arrangement of suckers on a cephalopod's tentacles -- which could make the patterns seen in the ichthyosaur fossils the world's oldest self-portrait, McMenamin suggests.

In their natural environment, octopuses do sometimes grab shiny objects, says James Wood, an expert in cephalopod behaviour and associate director at the Waikiki Aquarium in Hawaii. "They're curious creatures, and they certainly do manipulate their environment," he notes. But while octopuses may pile up rocks outside the mouth of their sea floor dens, for example, they are not known to bring animal remains home. "I've yet to find a vertebrate bone in a cephalopod midden."The idea that an ancient cephalopod would arrange objects into patterns -- even unintentionally -- is highly speculative, says Roger Hanlon, a marine biologist at the Marine Biological laboratory in Woods Hole, Massachusetts. "There's nothing in the scientific literature that suggests that modern-day cephalopods do anything like this," he notes.

It seems the mystery of the ichthyosaur midden is far from solved.

This article is reprinted with permission from Nature magazine. It was first published on October 11, 2011.