Life in the Ediacaran Sea. Photo from the Smithsonian National Museum of Natural History, by Ryan Somma. CC-BY-SA-2.0
Life in the Ediacaran Sea. Photo from the Smithsonian National Museum of Natural History, by Ryan Somma. CC-BY-SA-2.0

A Lost Way of Making Bodies From Before Skeletons and Shells

The program running on Jennifer Hoyal Cuthill’s computer is deceptively simple. First, it creates a cylinder. As the cylinder grows, it sprouts branches, first to the left and then to the right, always at the same specified angle. Each of the branches then becomes a stem in its own right, producing its own branches according to the same rules.  On they go, branches off branches off branches, four levels deep. The results look like the leaves of ferns, but they’re much more.

They look a lot like an ancient group of creatures called rangeomorphs, which existed in a time before skeletons, shells, legs, mouths, guts, and nervous systems. They were just a few inches long, but in a planet dominated mostly by single-celled creatures, they represented one of the first experiments in building relatively large and complex bodies.

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Fossils of Avalofractus (left) and Charnia (right) next to their virtual counterparts. Credit: Hoyal Cuthill & Conway Morris. 2014.

The rangeomorphs were part of the so-called Ediacaran biota that lived between 540 and 575 million years ago. There’s nothing like them now. They died off completely during the Cambrian Explosion—the event when most of the major living animal groups seemingly exploded into existence. If you condense Earth’s history into a single calendar year, the rangeomorphs would appear on the 15th of November before disappearing on the 18th, during the Cambrian Explosion.

There’s a lot of disagreement about what they were. Most scientists would agree that they were composed of many cells. Some have described them as algae, fungi, lichens, or an entirely different kingdom of life. The most popular guess is that they were early animals, or closely related to us. Whatever the case, they had a very basic body plan with no obvious traces of organs or internal structures.

The best-preserved of the rangeomorph fossils show beautiful patterns that seem to repeat at different scales, with smaller versions of the same shape branching off larger ones. In other words, they look like fractals. Palaeontologists have always described them as such, but more in a metaphorical way than a mathematical one. “It had been suggested that they look a bit fractal-like but that hadn’t been tested,” says Hoyal Cuthill.

She used her skills in computer science to write a program that uses simple rules of branching and growing to churns out a wide variety of body shapes, which look remarkably like actual fossils. The program has just 28 parameters, including the angle of the branches, their curvature, and how quickly the stems grow. Tweak the parameters, and you get a wide rangeomorph zoo. “It’s a relatively simple program but it’s versatile enough to generate a good approximation of the things we observe in the fossil record,” says Hoyal Cuthill.

The program produces three basic shapes: tall fronds like Charnia; flat mats like Fractofusus; and intermediate lettuce-like bundles like Bradgatia. This suggests that the rangeomorphs had partitioned the oceans into three different zones, with different groups specialising at absorbing food from a specific depth. You can see the same trend in modern animals. In the African savannahs, small dik-dik antelopes graze on low-lying leaves, medium-sized impalas eat bigger shrubs, balletic gerenuks rise en pointe to graze from low tree branches, and lofty giraffes reach the most inaccessible leaves.

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Three tiers of rangeomorph body plans. Credit: Hoyal Cuthill & Conway Morris, 2014.

The program also suggests that all of these differently shaped creatures were actually related. That’s always been hard to judge, and some researchers have claimed that the rangeomorphs are really a jumble of completely unrelated organisms. Hoyal Cuthill’s work says otherwise. The fact that many different forms can emerge from small tweaks to the same program “suggests that these things were genuinely related to each other,” she says.

It also gives a basis for classifying fossils as rangeomorphs in the first place. Other scientists proposed that the basic building block of these weird bodies was the ‘frondlet’—a branching, leaf-like shape that could be repeatedly cut-and-pasted into more complex ones. “I think this underestimates just how fractal they are,” says Hoyal Cuthill. Her program suggests that the basic unit is the even simpler hollow cylinder, from which more cylinders branch. “You don’t need to make the whole body out of the frondlets,” she says. “If you have a set of rules for this branching process, you produce the whole organism.” And by extension, any organism whose body could be described in terms of repeated branching cylinders was probably a rangeomorph.

The simulated rangeomorphs also showed that these creatures excelled at packing a lot of body into small spaces. For example, one of them was barely 10 centimetres long, but had a surface area of 58 square metres—roughly the same as a human lung.  “They are incredibly efficient at filling space,” says Hoyal Cuthill. “If I tried to come up with a better arrangement, I don’t know if I could do it.”

This supports the idea that they survived by absorbing carbon and oxygen dissolved in the surrounding seawater. After all, they didn’t seem to have any active body parts. Even stationary filter-feeders like barnacles use sieves to strain food from the water. Corals have stinging cells—tiny harpoons that impale their prey. Even sponges have small hairs that pump water through their inert bodies. But rangeomorphs showed no trace of any of these active feeding structures. They may just have sat there, waiting for stuff to diffuse across their expansive surfaces. Feeding was more like something that happened to them, rather than something they actively did.

But Martin Brasier from the University of Oxford cautions that the “surface areas were seldom maximally exposed”. The branches were often twisted and tightly furled around each other, which contradicts the idea that they were used to absorb dissolved matter from the water. His student Renee Hoekzema is now working on more complex models that will capture these nuances, and “will hopefully help us understand how these beautiful and enigmatic organisms actually functioned

Both groups agree that, for some reason, the rangeomorph body plan didn’t work very well after the Cambrian explosion. Perhaps the new Cambrian animals, with their more complex bodies, were much better at filtering nutrients from the water. Maybe they just ate the rangeomorphs. Either way, the gentle Ediacaran oceans were fine places place for soft-bodied, passive, immobile creatures, but the brutal Cambrian seas were not. The rangeomorphs went extinct, and their beautiful experiment in building fractal bodies was consigned to life’s history books.

Reference: Hoyal Cuthill & Conway Morris. 2014. Fractal branching organizations of Ediacaran rangeomorph fronds reveal a lost Proterozoic body plan. PNAS