Earth's Earliest Animals Looked Just Like Plants
Rangeomorphs are like the real-life Kokiri of the Ediacaran period.
Image: Jennifer Hoyal Cuthill
There is a long tradition in fairy tales and fantasy of imagining the world's first sentient creatures as hybrids between plants and animals. Whether it's JRR Tolkien's Ents, George RR Martin's children of the forest, or The Legend of Zelda's Kokiri, the trope of ancient plant people is well-established and thriving.
Well, according to a study published today in Proceedings of the National Academy of Sciences, that imaginative instinct was not far off. The study, led by paleobiologist Jennifer Hoyal Cuthill of Cambridge University, investigated several fossils left by a group of strange organisms known as rangeomorphs.
The first thing to recognize about these creatures is that they are beyond ancient. Rangeomorphs lived in the oceans of the Ediacaran period, a murky era that ran from about 635 to 541 million years ago.
"Rangeomorphs are some of the oldest large organisms," Hoyal Cuthill told me. "[They] seem to have been the most abundant and diverse of these for around 20 million years after they first appear, although rarer fossils thought to be sponges and jellyfish also begin to appear during this time."
"After this, other groups are more widely known, including the full 'Ediacaran biota'," she continued. "From the beginning of the Cambrian Period things step up another notch, many potential competitors and predators appear, and—arguably as a consequence—the rangeomorphs drop out of the fossil record."
But what really sets this group apart as a such a striking, biological one-off is that its members were leaf-like in appearance. "Frondlets" broke off from their main body stems in a fractal pattern similar to that of plants today. But despite this botanical appearance, rangeomorphs were actually one of evolution's first attempts to make animals.
"The geological evidence that rangeomorphs lived in water too deep and dark for photosynthesis is the key point against them being marine algae (plants in the loose sense)," Hoyal Cuthill told me. "Given this, plus their large size and complexity, it seems most likely that they were early animals."
"However, there are other possibilities, including the suggestion by the late Dolf Seilacher that rangeomorphs should be included in a new Kingdom of life," she said.
Indeed, rangeomorphs do seem bizarre enough to merit an entirely new biological category. As Hoyal Cuthill noted, they were among some of the first lifeforms to evolve beyond the microscale. A typical rangeoform measured about ten centimeters tall, but some could grow up to heights of two meters.
The sheer uniqueness of these leafy animals is also what makes them frustrating to study. Normally, paleobiologists can look for clues about extinct animals in their extant relatives, but there is simply nothing like rangeomorphs alive today (though plant-mimicking insects have proved that dressing up as sticks and leaves is always stylish).
To make up for that crucial information gap, Hoyal Cuthill used a variety of rangeomorph fossils to build the first computer reconstruction of the animals. The three-dimensional, simulations include fronds from the deep-marine Avalon Assemblage, and they truly help bring the eerie, ocean world of the rangeomorphs to life.
It's good to see computer reconstructions of these primitive animals now exist, because scientists will need all the intellectual ammo they can get to unravel their mysteries. Given that this group was ancient even by paleontological standards and the fact that they left no living relatives, rangeomorphs took a lot of secrets to the grave with them. And there's nothing more enticing than that.