Did toxic algae kill these dinosaurs?

The victims come in all shapes and sizes. Long-necked, armour-bearing giants. Sharp-toothed carnivores with knobbly ornaments on their heads. Birds that glided through the ancient air. All of them buried in one spot in the 70-million-year-old rock of Madagascar. And what killed so many charismatic creatures? The answer, one group of palaeontologists has proposed, might be algae.

The skeleton of a predatory dinosaur known as Majungasaurus, found at the Madagascar site along with many other animals, including turtles, birds and plant-eating dinosaurs. Image: Raymond R. Rogers

Earlier this month, at the Society of Vertebrate Palaeontology meeting held in Calgary, Macalester College palaeontologist Raymond Rogers laid out the case. Up until now, drought seemed the most likely culprit behind the creation of the dense bone bed. During this period in Madagascar's prehistory, climatic see-sawing made its mark on the landscape, bringing harsh wet and then dry seasons. The intense seasonal aridity, experts suspected, weakened the dinosaurs and made them more susceptible to injuries, disease and other forms of stress. When the wet season returned, the sediment-laden water buried the dead.

But Rogers and his colleagues came to question this classic explanation: some of the fossilised clues just didn't add up. The remains of the animals in the bone bed – from lizards all the way up to large dinosaurs – seemed to be buried where they died, rather than being transported some distance by moving water. In addition, the repeated nature of the deposit was curious. "At one locality, three are stacked on top of each other," Rogers says. This hints at an event that happened more than once.

What's more, the presence of multiple bird fossils pointed to something unusual at play. "Birds are generally rare in the fossil record," Rogers explains, "[so] to have several dead birds buried together alongside many other animals, big and small, suggested a killing agent capable of knocking animals down in their tracks."

Whatever was killing these creatures was doing so quickly – and the spot where they fell became their final resting place, sediment slowly burying the evidence. That fast-acting culprit may have been toxic algae.

It wasn't necessarily lack of water itself that killed these animals, the researchers suggest. Instead, they propose that a harmful algal bloom might have developed in a drying water source – a phenomenon "increasingly common with warming waters and drying climate," Rogers says. In that scenario, the warm water could have hosted algae that grew out of control, producing an excessive amount of toxins (just as can happen today). For the area's ancient inhabitants, it would have been a deadly lure: as prehistoric animals of all sizes arrived at the shrinking pool to drink, they may have inadvertently slurped up a toxic algal soup that killed them near the spot where they were ultimately buried.

The explanation isn't set in stone yet. As Smithsonian Institution palaeontologist Nicholas Pyenson told Science in response to the new idea, prehistoric harmful algal blooms are difficult to prove. Meanwhile, experts are awaiting the formal publication of the hypothesis to debate the algae explanation. Yet investigations are continuing. "I am presently working with a biogeochemist to further test the ... hypothesis," Rogers says, adding that his "work will focus on trying to identify biomarkers in the associated rocks" that would implicate an algal bloom.

Solving these cold cases is hard work. It's certainly more difficult than a game of Clue. But by looking to the scattering of the bones, and to the rock itself, palaeontologists can start to piece together whether blooms of toxic algae brought down the "terrible lizards".