Harmful algal blooms may have killed this carnivorous theropod dinosaur, discovered by researchers excavating a series of 70-million-year-old bone beds in northwestern Madagascar. Photo: Andrew Farke

Carolyn Gramling writes in ScienceMag that seventy million years ago animals of all sizes came to drink in a rapidly drying river in northwestern Madagascar, and never left. Tiny birds and mighty dinosaurs were entombed together in the riverbed, forming what is now a spectacular series of mass graves.

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.

Last week at the annual meeting of the Society of Vertebrate Paleontology, Raymond Rogers, a geologist at Macalester College in St. Paul, MN, 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 Madagascar 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.

In Madagascar and elsewhere the smoking gun — direct evidence of algae — is still missing, Dr. 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.”

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