18 Jul 2017

Why life got so big

About 570 million years ago, large, frond-like creatures suddenly invaded the ocean floors. For over a billion years, the Earth’s oceans were filled with bacteria and microscopic algae, but during the Ediacaran period, from 635 to 541 million years ago, larger multicellular organisms began crowding the seas.

Fossil imprints from the Ediacaran derive from soft-bodied organisms resembling modern-day sea anemones (Cyclomedusa), annelid worms (Dickinsonia) and sea pens (rangeomorphs such as Charnia). Among these bizarre creatures, the rangeomorphs are the most abundant in the fossil record—and also some of the largest.

Artist impression of rengeomorphs (credit: Jennifer Hoyal Cuthill.)

Rangeomorphs were unlike any creature on Earth today. Some were as small as a coin, while others could grow up to 2 meters high. They looked like ferns, with branches spreading out from a central stem, but they likely fed by filtering nutrients from the water, similar to corals. Because rangeomorphs were so different from any known life form, paleontologists still don’t agree whether they were primitive animals related to soft corals, some sort of weird fungus or even a new (now extinct) kingdom of life, the Vendobiota.

These ocean dwellers eventually disappeared after the Cambrian explosion, some 541 million years ago, when fast-moving predators emerged (and probably ate them).

Changes in ocean chemistry

Based on the chemical signature of ancient seawater left on rocks, geochemists think there was a sharp rise in ocean oxygen levels soon after the end of the Gaskiers glaciation, about 580 million years ago. These changes in the ocean chemistry could explain the appearance of larger and more complex marine organisms—more food, bigger bodies. However, even though this may seem quite obvious, it’s actually quite difficult to demonstrate.

Jennifer Hoyal Cuthill and Simon Conway Morris, from the University of Cambridge (UK) and Tokyo Institute of Technology (Japan), used an original approach to tackle this problem.

“We wanted to see whether the increase in body size could point to a rise in oxygen, since the type of growth can tells us whether the animals have nutrients available or not”, says Hoyal Cuthill.

They suspected that Ediacaran organisms were large because they had a ‘nutrient-dependent’ type of growth, rather than an evolutionarily new genetic makeup.

‘Seeing’ extinct creatures grow

Many organisms can’t grow beyond a certain size, regardless of how much they eat. Humans for example, will (unfortunately) just get fatter, not taller, because they are genetically programmed to reach a specific maximum height. But for some organisms nutrient availability can affect body size. This type of nutrient-dependent growth is quite common in invertebrates and plants. Some plants will grow almost indefinitely, as long as there are nutrients (and light) available in the environment.

But how do you measure growth in organisms that lived nearly 600 million years ago?

This is where rangeomorph fossils come in handy.

Hoyal Cuthill and Conway Morris had previously worked with several rangeomorph specimens to study the unusual body plan of these animals. During this research it dawned on them that the rangeomorphs’ complex fractal branching shape, with larger older branches at the bottom and smaller younger branches on top, was the key for testing the nutrient-dependent growth hypothesis.

“It’s like looking back at your childhood photographs and comparing your height through your old photos up to the present day”, says Hoyal Cuthill. “We were inferring the history of growth of a rangeomorph by looking at parts of the structure of different ages”.

The researchers could basically “see” in a single fossil specimen how the animals were growing during their lifetime, by comparing the relative size and shape of younger and older branches.

A unique rangeomorph fossil

Fossil of Charnia (Jennifer Hoyal Cuthill)
The new study focuses on an exquisitely preserved specimen of Avalofractus abaculus, one of the last fossils removed from the Trepassey Formation, in Newfoundland (Canada), before strict restrictions were imposed to protect the site (currently called Mistaken Point Ecological Reserve). Hoyal Cuthill obtained a high-resolution cast from the Royal Ontario Museum and scanned it by CT- microtomography, a technique which uses x-rays to make detailed digital 3D reconstructions.

Two other specimens (Charnia masoni and an undescribed specimen from the South Australian Museum) were also analysed based on digital photographs.

Mathematical and computer models comparing the surface area and the volume of younger and older branches showed that growth gradually slowed down as rangeomorphs got bigger, which is exactly what happens in modern organisms with nutrient-dependent growth.

 “… You’re getting less nutrients as you get larger, so you cannot sustain the same rate of growth, and it slows down”, Hoyal Cuthill explains.

But there was more. Nutrient availability can also affect body shape, which is technically called ecophenotypic plasticity. Hoyal Cuthill and Conway Morris also found that rangeomorphs could rapidly change shape to access higher levels of oxygen in the seawater above them, by growing into a long, tapered shape.

Nutrient-dependent growth provides a mechanism to explain why changes in ocean chemistry caused the appearance of these large organisms in the Ediacaran, some 30 million years before the Cambrian explosion.

Hoyal Cuthill next wants to investigate whether rangeomorphs really are animals, and to which modern groups are they related to.

“Rangeomorphs are quite mysterious and were only relatively recently discovered and identified as Precambrian organisms”, she says. “This is an exciting time and many researchers are looking at the biota of the Ediacaran and finding new fascinating things”.

Reference: Hoyal Cuthill, Jennifer F., and Simon Conway Morris. "Nutrient-dependent growth underpinned the Ediacaran transition to large body size." Nature Ecology and Evolution (2017). DOI: 10.1038/s41559-017-0222-7

This article was published originally as a guest post in the PLOS Paleo Community blog with the title "Why Precambrian life got so big" on the 18-07-2017. You can read it here

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