It's intriguing from a number of perspectives. First, it gives lie to the picture most people have of the evolution of animals, that it was some kind of linear progression. It's often seen as a climb up the Great Chain of Being, from something like a jellyfish, to something like a worm, to something like a bug, to fish, amphibians, reptiles, mammals, and finally -- at the top, of course -- is the Pinnacle of Evolution: namely, us.
The truth is (predictably) much more interesting. During the late Precambrian and early Cambrian Periods, in a relatively short amount of time (geologically and paleontologically speaking) all of the ancestors of the major animal groups appeared, as if there was a sudden and drastic push to diversification. At that point there were proto-arthropods, proto-vertebrates, proto-mollusks, and proto-damn-near-everything-else.
Even more fascinating is that there were a number of animal groups around during that time that are of uncertain affinity to the others, and who apparently left no descendants. There's the bizarre Anomalocaris, probably related most closely to early arthropods (its name is Greek for "abnormal shrimp"), with two jointed, spike-lined tentacles and a mouth shaped like a pineapple ring. Opabinia was equipped with no less than five compound eyes and a proboscis like a vacuum-cleaner hose. Most famous is the aptly-named Hallucigenia ("it creates hallucinations"), a worm-like critter with giant eyes, tube-like legs, and a double row of formidable spines down the back.
All three of these are probably branches of the huge group Protostomia, which are still today the most numerous animals on Earth. But there are other fossils from the Ediacaran Assemblage that are even more mysterious, and one of the weirdest ones is the group called rangeomorphs.
They were almost certainly animals, although they were sessile (fixed to the seafloor) via stalks, and had weird frond-like structures of uncertain purpose (but which may have been a mechanism either for oxygen extraction or for filter feeding). So if you were to look at a living one, your initial impression might well be that it was some odd sort of seaweed, and not an animal at all.
A 550-million-year-old fossil of the rangeomorph Charnia masoni, from the Mistaken Point Formation in Newfoundland [Image licensed under the Creative Commons Smith609 at English Wikipedia, Charnia, CC BY 2.5]
If Anomalocaris, Opabinia, and Hallucinogenia are problematic in terms of their evolutionary affinities, the rangeomorphs are complete ciphers. They have no obvious connections to any living animal group, and in some ways more resemble fungi, although that too is speculation. They were apparently quite common during the late Precambrian, so the sea bottom would have been covered with their frilly fronds gently waving in the currents -- but at the moment, exactly what they were is a mystery.
And the mystery just deepened considerably with a discovery that was the subject of a paper last week in Current Biology. The rangeomorphs had another perplexing and unusual feature -- they were connected by thread-like filaments, some of them up to four meters long, that seem to have hooked populations up into a huge network of interlinked individuals.
The purpose of these filaments is unknown, but it could be that the individuals in a network were all clones, and were functioning as a colonial organism a little like modern corals. What it immediately put me in mind of was groves of aspens, which look like bunches of individual trees but are all linked underground by a network of rhizomes -- some of the colonies cover many acres, and one in Colorado is said to be over eighty thousand years old. (This calls into question what we mean by the word "organism;" is each of these trees a separate organism? Is the whole grove a single organism? If so, and you dug a trench down the middle and cut the rhizomes, have you just created two organisms? Like many terms in biology, this word only seems simple until you push on it a little.)
In any case, the rangeomorphs apparently had the world's first social network, but what exactly it was used for we can only speculate at. They were strange animals to say the very least. These sorts of discoveries always make me wonder what the Earth looked like back then -- given how infrequent fossilization is, and how unlikely it is for a rock to remain undamaged through all those millions of years, the chances are that for every one species we have a reasonably good picture of, there are hundreds that we know nothing at all about. The Precambrian water-world of the Ediacaran fauna would have looked a very alien place to our eyes, even though the seeds of all of our modern life-forms -- including ourselves -- were there in those oceans.
Some of those seeds, though, failed to leave behind any progeny, and it seems likely that the rangeomorphs were one of those. Whatever they were, they certainly show no obvious connections to any modern group, animal or otherwise. To me this only increases their fascination -- and with it, the hope that further discoveries may shed some light on this and other groups whose origins are lost in the depths of time.
This week's Skeptophilia book-of-the-week is brand new -- science journalist Lydia Denworth's brilliant and insightful book Friendship: The Evolution, Biology, and Extraordinary Power of Life's Fundamental Bond.
Denworth looks at the evolutionary basis of our ability to form bonds of friendship -- comparing our capacity to that of other social primates, such as a group of monkeys in a sanctuary in Puerto Rico and a tribe of baboons in Kenya. Our need for social bonds other than those of mating and pair-bonding is deep in our brains and in our genes, and the evidence is compelling that the strongest correlate to depression is social isolation.
Friendship examines social bonding not only from the standpoint of observational psychology, but from the perspective of neuroscience. We have neurochemical systems in place -- mediated predominantly by oxytocin, dopamine, and endorphin -- that are specifically devoted to strengthening those bonds.
Denworth's book is both scientifically fascinating and also reassuringly optimistic -- stressing to the reader that we're built to be cooperative. Something that we could all do with a reminder of during these fractious times.
[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]