Seventy million years ago, they all came to drink in the rapidly drying river: long-necked sauropods, fierce theropods, crocodiles, lizards, and raven-sized birds. They never left. The giant and the tiny were entombed together in the riverbed, forming what is now a spectacular series of mass graves in northwestern Madagascar. Last week, researchers proposed a culprit behind this ancient mystery: harmful algal blooms (HABs), in the very water that had lured the animals.
The remains of such algal blooms “should be more common in the fossil record,” says vertebrate paleontologist Nicholas Pyenson of the Smithsonian Institution in Washington, D.C., who was not part of the study. But he cautions that they are tough to prove.
Bone beds always come with a mystery: Why did so many animals die at once? Floods and volcanoes are sometimes invoked, and for years researchers suspected that drought killed the animals whose fossils accumulated in the Maevarano Formation of Madagascar. Torrential rains punctuating periods of drought might have created turbulent rivers choked with sediment that buried skeletons intact.
One chunk of this formation “is the most fossiliferous package of rock I’ve ever seen,” says Raymond Rogers, a geologist at Macalester College in St. Paul, who has been studying the site for 2 decades. He and his colleagues have so far cataloged nearly 1200 specimens from a single bed a third the size of a tennis court.
Over time, the team grew skeptical of drought as the only explanation. Large and small animals nestle against each other, suggesting that the bodies were buried where they died and that the killer struck all kinds of animals without discrimination. In addition, whatever killed these animals “was fast-acting,” Rogers says, “dropping birds in their tracks.” And it happened again and again, creating multiple layers of bone beds.
Last week, at the annual meeting of the Society of Vertebrate Paleontology here, Rogers noted the arched-back posture of the dead, which suggests neck convulsions; an unusual carbonate crust, similar to those left by algae in other sediments; and the sheer number of dead birds. Taken together, he says, these clues suggest that the killer was “almost certainly harmful algal blooms,” which can develop repeatedly in the same place in late summer.
HABs have been implicated in mass deaths before. In 1878, a Nature paper noted a peculiar hyperextended neck posture—similar to the postures of the Maevarano creatures—in dead livestock near a lake; testing confirmed that the animals had ingested toxic cyanobacteria. And in a 2014 paper in the Proceedings of the Royal Society B, Pyenson and others suggested that toxic algae periodically killed hundreds of whales and other marine animals off the coast of what is now Chile, starting 11 million years ago.
Algae even might be implicated at Germany’s famous Messel Pit, says paleontologist Wighart von Koenigswald of the University of Bonn in Germany, who was not involved in the new study. That series of Eocene mass graves is full of birds and bats, he notes, making one explanation—sudden carbon dioxide degassing from an ancient lake—unlikely. Moreover, the beds include turtles caught in the act of copulation as well as pregnant mares, suggesting that the deaths happened during mating season across different years. years. Toxic algae are “the most plausible explanation,” Von Koenigswald says.
In Madagascar and elsewhere the smoking gun—direct evidence of algae—is still missing, Rogers acknowledges. He plans to hunt for chemical traces or biomarkers of algae in the rocks and fossils. If such evidence is found in Madagascar, says Smithsonian vertebrate paleontologist Kay Behrensmeyer, this “very provocative” idea might help explain other fossil troves. “It opens up a possibility that we probably have not been considering seriously enough.”