The Long Arm of the Planktivore
The Cambrian oceans hosted a riot of evolutionary novelty. Over a seabead burrowed by penis worms and tread by living pincushions, multi-eyed invertebrates swung their schnozzles after prey and our closest, archaic relatives squirmed through the water. Largest of all were the anomalocaridids – cousins of arthropods that flapped through the water on segmented wings and were equipped with a pair of “great appendages” hanging below a pineapple-ring mouth. Their size and flexible, spiky arms have made them dead ringers for apex predators in the eyes of paleontologists, but new research has cast at least one of these mind-bending invertebrates as a filter-feeder that was only a threat to plankton.
The pioneering planktivore was Tamisiocaris borealis, a relatively new addition to the anomalocaridid family tree named by paleontologists Allison Daley and John Peel in 2010. That description was based on a sole great appendage found in the 520 million year old rock of North Greenland’s Sirius Passet.
Against the slow grind of paleontology publication, however, discoveries in the field can quickly turn up additional parts of organisms that are already on their way to press. Expeditions in 2009 and again in 2011 uncovered additional appendages of Tamisiocaris in an even better state of preservation.
Paleontologist Jakob Vinther, lead author on the new Nature paper that casts the creature as a suspension-feeder, was immediately excited by the finds. “I remember writing on the package with the fossil, ‘Great Appendage!!!!’,” Vinther says, and the delicate details of the new fossils brought up an intriguing possibility that had only been hinted at before. “The fine bristles, which I could see even in the field immediately,” Vinther says, “made me think that this is a filter feeder and I quickly started thinking about the evolution of baleen whales and whale sharks.”
At a glance, the great appendages of Tamisiocaris look to be tipped with long, nasty, backward-pointing spikes perfect for skewering squishy prey. In detail, however, each of the spines has a network of smaller filaments branching off to create what would have been a flexible net in life. This was not an arm for piercing prey, but an appendage for sifting small organisms from the seas.
Through digital reconstructions done by coauthor Martin Stein, Vinther and colleagues replayed how Tamisiocaris must have snagged little morsels. “We could see that upon contraction,” Vinther says, “the appendage would curl up and form a basket, which would concentrate the food particles and in the process of contraction the basket of goodies would be adjacent to the mouth.”
The question is how Tamisiocaris managed to eat those “goodies.” The filtering appendages weren’t able to directly deposit food into the critter’s mouth, and there’s evidence to suggest that anomalocaridids had additional limbs to help feed themselves. Instead, based on recent work by Allison Daley and Jan Bergström, Vinther hypothesizes that “the anomalocarid pineapple mouth was a suction apparatus, like a goldfish mouth.” Since no one has described the mouth of Tamisiocaris just yet, though, discerning the arm-to-mouth feeding method of Tamisiocaris relies on future fossil finds.
Just how a filter-feeder could have evolved from a line of predators is a little clearer.
Even though there has been some debate about just how rapacious classic forms such as Anomalocaris truly were, there were numerous anomalocaridids with spiky great appendages. Some, such as those ofAmplectobelua stephenensis and Stanleycaris hirpex, are the stuff that nightmares are made of. And among this array of anomalocaridids that likely snatched and punctured prey, Tamisiocaris shows how those traits could be tweaked into an entirely different sort of feeding apparatus.
“We see some forms which have slightly longer spines on their appendages,” Vinther says, “perhaps they could have swept up prey in midwater, and by making the spacing of the spines finer, then you would capture things that are smaller.” The netting of Tamisiocaris was a modification of what already existed.
Such transitions have happened multiple times in the history of life. Both filter-feeding sharks and baleen whales evolved from sharp-toothed ancestors, marking alternate routes for planktivores to evolve from predators. And drawing from whales and sharks, artist NocturalSea imagined a speculative version of a filter-feeding anomalocaridid for the All Your Yesterdays art project. “We were all a bit weirded out that someone actually thought up this thing,” Vinther says, as “his reasoning is the same as ours.” A true case of Cambrian convergence.