Is it a mushroom? Is it a jellyfish? No, it's Dendrogramma, a new animal so bizarre in appearance that it has defied attempts to place it anywhere in the vast animal kingdom.
What is agreed is that Dendrogramma belongs somewhere in the very lowest branches of the animal evolutionary tree. But whether it is a stingless jellyfish, a comb jelly or even part of a group of enigmatic organisms that most researchers think went extinct half a billion years ago remains up for debate.
"The specimens of Dendrogramma are very intriguing, and raise nearly as many questions as they answer," says Guy Narbonne at Queens University in Ontario, Canada, who researches those enigmatic organisms, called the Ediacaran biota.
Two species of Dendrogramma have so far been discovered. They were found in 1986 by Jean Just at the Natural History Museum of Denmark in Copenhagen and his colleagues, on the seafloor off south-east Australia at depths of 400 and 1000 metres. The animals were dead by the time they reached the surface, but their bodies were quickly submerged in formaldehyde to keep them intact.
They resemble mushrooms, with a disc on top and a stalk protruding downwards from the centre (see photo). They are very small, the discs averaging 11 millimetres across and the stalks 8 millimetres long for the nine known specimens of Dendrogramma enigmatica. The second species – Dendrogramma discoides – is slightly larger: its disc is 17 mm across and sits on a 4.5 mm-tall stalk.
There are no obvious sex organs in the specimens, no nervous system and no apparent means by which the disc could be flexed to enable movement. Unsurprisingly, then, placing them on a known branch of the animal kingdom has proved a bit tricky. Some things are clear, however. Because they don't have any obvious bilateral symmetry – unlike most animals – the two Dendrogramma species must sit on one of the lowest branches in the animal evolutionary tree, occupied by the few animals that lack this symmetry. This means they probably have affinities with very simple marine animals – but which ones?
"Dendrogramma lack the immediately recognisable characteristics that define the two possible groups we compare them with," says Just.
Dendrogramma's shape is a little like that of a cnidarian – a group containing the jellyfish – but all of the specimens lack the stinging tentacles that are a feature of this group. Alternatively, Dendrogramma might belong within the Ctenophora, or comb jellies – but all of the specimens lack the tentacles and combs of hairs seen in the group.
In fact, the organisms Dendrogramma most closely resemble belong to the Ediacaran fauna. "Their internal system shows a branching pattern similar to some of the Ediacara fossils, as well as lobed features around the mouth opening," says Just.
Dendrogramma's disc-like shape is very reminiscent of three Ediacaran genera – Albumares, Anfesta and Rugoconites. The snag is that all three went extinct half a billion years ago, along with all of the other weird Ediacaran creatures.
Does this mean that Dendrogramma is proof that some Ediacaran creatures have managed to cling on in the remote depths of the oceans for all this time? The comparisons are interesting, says Narbonne, but there might be a simpler explanation. "Dendrogramma has a relatively simple shape and so any similarities with the external shape of Ediacaran taxa could well be coincidental," he says. In other words, there are only so many ways to build a simple animal, and Dendrogramma might have come out looking like the Ediacaran organisms by chance.
But the similarities leave open another possibility: despite their strange appearance, the Ediacaran organisms might actually be related to living animals rather than being a "failed evolutionary experiment".
In fact, other researchers have begun to see similarities between some Ediacaran organisms and living animals.
"Every member of the Ediacara biota was more or less related to an existing branch in the tree of life," says Jim Gehling at the South Australian Museum in Adelaide. The main reason researchers thought otherwise, he says, is because Ediacaran organisms were soft bodied but are often preserved in coarse sand in a way that modern animals are not. "Preserving soft-bodied organisms as impressions in sand, was just too weird to allow that these organisms might have been members [of living animal groups]."
That's not to say that there weren't some truly exotic Ediacarans, including a group of 2-metre-long feather-like organisms called the rangeomorphs. These are still enigmatic, says Gehling, but smaller Ediacaran organisms are probably not.
"It is not too extreme to say that they were the Ediacaran equivalents of small mammals and birds that survived the Cretaceous-Tertiary extinction," he says. They lived in the shadow of the Rangeomorphs, he says, but when these larger organisms went extinct the small Ediacarans may have flourished, evolving into familiar animals.
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