Some 565 million years ago, a very curious creature lived deep on the ocean floor. Shaped like an oval with pointy ends, covered in branching structures and lacking an obvious mouth or feeding parts, they were the largest animal alive at the time. Very little is known about the creature Fractofusus, but now researchers from the University of Cambridge have found the earliest example of reproduction in the complex organism.
“The aggregations – or clusters – we found [of Fractofusus] showed they were reproductive and weren’t formed from environmental processes,” explained Dr. Emily Mitchell, who led the study published in Nature, to IFLScience. “Furthermore, we found that the cluster sizes were sufficiently small that they couldn’t have been formed as a result of the Fractofusus releasing seeds or particles into the water column, so we knew that the clusters had to form in a different process. The only process that it could be is using stolons or “runners,” like you find in strawberry plants.”
The researchers analyzed three large bedding plains in Canada, which originally formed part of the ocean floor around Newfoundland and contained thousands of fossilized Fractofusus. They mapped the position of each animal present and, when possible, the size of the creatures. They then ran an analysis of how the fossils related to each other spatially and how their densities varied across the plains.
Because the animals were immobile, it means that they couldn't have just wandered into their current positions, and so their distribution must be related to how they reproduced (in a similar way as with trees). Interestingly, they observed large individual animals surrounded by medium-sized ones, which were in turn encircled by smaller individuals.
What this demonstrated was that the fossils were not randomly distributed but instead formed very distinct clusters. “Small [fossils] cluster around the medium ones, and the medium ones around the large ones,” said Dr. Mitchell. Because they were not randomly spread across the bedding plain, the distribution must have been formed through reproductive processes, argue the researchers.
Amazingly, they can also infer the way in which these early organisms reproduced. Because there was no directionality to the smaller fossils surrounding the larger ones, they think that they must have been formed through runners – thin structures that allow efficient asexual reproduction – with clones of the adult budding off at the ends. This is because if the adults were instead releasing eggs, or free-floating particles into the water column, you would expect them to be taken by the current and then deposited on the substrate in similar directions. But this isn’t what they see, at least not for the smaller fossils.
“When we looked at the random distribution of the larger ones, we found that there was this kind of directionality to them,” says Dr. Mitchell. She suggests that the larger ones might have been formed through “relatively rare release events,” but that once they found a good place to settle, “they reproduce very very quickly. They’re producing these clones of the parents using stolons or runners.”
Adding to our understanding of early life in the oceans, the researchers plan on using this spatial data to further understand how the organisms interacted with each other.
Center image: Dr. Emily Mitchell mapping a section of the surface fossil community on one of the three bedding plains. Credit: EG Mitchell.