A new study argues that warm-bloodedness first arose in mammal ancestors likely about 233 million years ago in the Late Triassic, 20 million years later than previously thought.
When paleontologist Romain David first laid eyes on a set of penny-size tubes of fossil bone at the National Museum of Natural History in Paris in 2008, he was struck by the unusual shape and varying sizes of these inner ear structures, called semicircular canals.
“It’s quite exciting,” said Jasmina Weimann, a molecular paleontologist at the California Institute of Technology who wasn’t involved with the study. “It is the first time that any relationship between canal shape and temperature has been proposed,” she says, adding that the new method also offers a promising way to trace the evolution of long-extinct animals.and apparently some dinosaursIt allows a more constant body temperature in the face of outside temperature fluctuation.
Warm-blooded animals usually have faster growing bones and sport fur or feathers for insulation, so researchers have studied those features to estimate when mammals’ journey to endothermy started. But those measures aren’t exact proxies and could have first evolved for other reasons. Enter David, now a paleontologist at the Natural History Museum in London, with a new idea about semicircular canals: After doing his Ph.D. on the structures, he noted that mammals have smaller semicircular canals for their body size than other vertebrates. “The whale shark is actually the animal with the largest semicircular canals on Earth, much larger than the canals in whales,” David says. “I thought, ‘Maybe this has to do with body temperature.
However, the study is one piece of a larger unfinished puzzle, cautions Hans Straka, a neurobiologist at the Ludwig Maximilian University of Munich. Endothermy did not just come out of nowhere, he notes, and more fieldwork needs to be done to prove the suddenness of the change. If a new fossil mammaliamorph were to be discovered from a different time period, for instance, the beginning of endothermy would change as well. “Endothermy, it’s not all or nothing,” he says.
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