Ancient filter-feeder was a “gentle giant”
Extraordinary fossils from the northern tip of Greenland provide a better picture of a dynamic period in early animal life. The fossils date to 520 million years ago, shortly after the simple organic soup of the Precambrian epoch gave way to the explosive biodiversity of the Cambrian era.
The fossil described in Nature this week is a filter feeder, one of the many aquatic animals that earn a living by “raking up” smaller organisms. Modern filter-feeders include everything from copepods and krill to flamingos and baleen whales, which capture dinner with a sieve-like “whalebone” inside the mouth.
It’s a successful strategy — the filter-feeders include the blue whale, the largest animal on Earth.
Although the fossil contains only two feeding appendages for Tamisiocaris borealis, the scientists rue their near miss: While examining their evidence back in the lab, they realized that they had left behind a chunk of rock holding part of T. borealis. “If we had looked at the rock, we would probably have found a complete animal,” groans Jakob Vinther, a lecturer in earth and biological sciences at the University of Bristol, United Kingdom.
It’s not cheap to work “As close to the North Pole as you can get without being on an iceberg,” Vinther says, “but we need to go back.”
Festival of fossils
The discovery site is not just remote but also a prime hunting ground for fossils. “There is a great abundance of fossils that were preserved with soft tissue,” says Vinther. “Usually only the hard parts are preserved” in fossils, but this location includes evidence of skin, cuticle, intestine, even muscle.
The cache includes “immense numbers” of worms, various arthropods, sponges and bony fish, Vinther says. “There are so many species, every time you split a rock, it’s littered with soft bodied organism and arthropods.”
Vinther says the seafloor lacked the burrowing animals that disturb animal structures before fossilization. Periodic inundations of mud created oxygen-free conditions that slowed decay, leaving deposits of hard and soft tissue that readily mineralized.
Although the body was not found, a comparison to related organisms suggested that the full animal would have been 70 centimeters long — a monster at the dawn of complex life.
A different world
The T. borealis specimen lived 520 million years ago, shortly after the “Cambrian explosion” of species began about 542 million years ago. This was a period of dramatic change “If you go back to the Precambrian age, a bit more than 20 million years prior, the world looked completely different,” says Vinther. “The seafloor was covered in microbial mats and the water column was an anoxic slurry of dissolved organic material and bacteria.”
Things changed rapidly with the evolution of many of the major life forms, or phyla, still extant. “As you go into the Cambrian, the water is cleared up, animals are feeding on all the bacteria, there is an explosion of animals that start churning around in the sediment, burrowing,” Vinther says.
Finding a filter-feeder implies that they could find food, such as free-floating animals that fed on plants, Vinther says. “There have to be lot of zooplankton in water, otherwise they cannot filter enough food.”
Let’s get engaged. You play prey!
The find also shows that the animals “are starting to engage with each other as predator and prey,” Vinther says, “and we want to understand how fast this took place, and how this complex ecosystem worked.”
The ancestor of T. borealis “was one of the most fearsome animals in the water,” he adds. “It was sitting on top of the food chain, it had big eyes and cunning in terms of how it chased prey.”
And then evolution happened, and T. borealis evolved into a large but mousy critter that scrounged food while no longer posing a threat to large animals in the sea.
This trajectory of “apex predators moving toward gentle giants” has also taken place more recently, Vinther says. The whale shark, a modern filter-feeder, evolved from a typical, toothy hunter, but the whale shark is “so big, so tranquil. You can go out snorkeling with it, it does not feel you are a threat.”
The same was true, presumably, with T. borealis, Vinther says. “These are gentle giants; they don’t have to be as fast or cunning, they can creep through the water without being threatened by anybody else.”
The ancient sea half a billion years back may be less odd than we assume, Vinther says. “The sea at that time looks completely alien. But they were not completely alien; they were member of the ecosystem and they occupied distinct ecological niches, much like the niches that are occupied today. They had small crustaceans, zooplankton, and at the top, apex predators, and some of these became gentle giants.”
– David J. Tenenbaum