Scientists at Memorial University may have solved a mystery at the province’s newest UNESCO World Heritage Site, Mistaken Point.
Namely, what were the giant Ediacaran fossils found on the southern tip of Newfoundland’s Avalon Peninsula, and why are they so large?
For many years, the 570-million-year-old fossils at Mistaken Point, named Fractofusus misrai after the Memorial University graduate student who first discovered them, have been at the center of debates regarding the earliest macroscopic fossils of the Ediacaran biota.
The fractal-like, or “broken” fossils of Mistaken Point have been interpreted as many things: giant single-celled organisms, fungi, relatives of the jellyfish, and even a completely extinct experiment at life that was neither plant nor animal.
Dating of the fossil layers shows that they are among the oldest fossils anywhere in the world. But what is even more remarkable is that these first fossils are enormous, up to one metre in length, and dating nearly 30 million years prior to the first evolutionary evidence of comparatively tiny worms and trilobites.
In a new paper recently published by London’s The Geological Society, Dr. Suzanne Dufour, Department of Biology, and Prof. Duncan McIlroy, Department of Earth Sciences, have taken a novel approach to interpreting the Mistaken Point fossils. Instead of comparing the fossils to modern organisms, the researchers considered the probable mode of life and biological challenges likely to have been experienced by these earliest animals.
“I have always been puzzled by the fact that the majority of the organisms in the early Ediacaran biota were incapable of locomotion,” said Prof. McIlroy. “While working on a different experiment, I made the observation that underneath anything organic rich on the seafloor, bacteria in the sediment produces large volumes of hydrogen sulfide, which is toxic to animals. So, how could the Ediacaran survive if they could not move?
“In recent years, it has become increasingly well known that many animals have sulfide-fuelled bacterial symbionts in their oxygen-rich tissues, such as gills,” he added. “This is an arrangement in which the bacteria use both oxygen and the toxic hydrogen sulfide as an energy source, thereby detoxifying the waters around the host animal and also providing the host with nutrients.”
Dr. Dufour is a specialist in such symbioses, particularly in modern clams with sulfur-oxidizing symbionts. She says that Fractofusus had a very high surface area on its lower surface, which is uncommon for an animal that could not move.
“The possibility that the organism had bacterial symbionts to harvest energy from the sediment, and to detoxify its lower surface, seems logical.”
“Fractal-like growth is usually intended to maximize surface area, so it naturally follows that Fractofusus was not trying to protect itself from sulfide buildup, it actually seemed to want to experience it,” she said. “The possibility that the organism had bacterial symbionts to harvest energy from the sediment, and to detoxify its lower surface, seems logical.”
Drs. Dufour and McIlroy consider that the organism either transported oxygen below its body by moving water using whip-like appendages called cilia on their outermost cells, or through its body via the acellular mesoglea. Prof. McIlroy explains that a modern example of mesoglea is the jelly portion of jellyfish, which might also explain how Fractofusus could grow so large.
Since Fractofusus might then consist of a thin layer of cells around the mesoglea, it could become as large as some of the largest jellyfish, which can reach two metres in diameter, because even a large organism would consist of relatively few cells.
Tree of life
So where does Fractofusus and its relatives belong in the tree of life? The researchers have compiled a list of the essential characters an animal would need to have evolved in order to live in this way: one external layer of cells, mesoglea, cilia and symbionts.
“These characters are similar to those of the Placozoa, simple jelly-like creatures that wander the seafloor using cilia on their lower surface, periodically stopping to feed on microbial mats using enzymes exuded from their lower surface, absorbing the resultant soupy water, and moving on once sulfide levels become too high,” said Prof. McIlroy.
The pair propose that Fractofusus and its relatives had a grade of organization more primitive than that of the wandering Placozoa, and were, in fact, simpler than any modern animals we yet know of.