This discovery, published in the journal Current Biology, was based on measurements of body length and fin dimensions from over 500 living and fossilized shark species.
"The pectoral fins are a critical structure, comparable to our arms," said UCR biology doctoral student and paper first author Phillip Sternes. "What we saw upon review of a massive data set, was that these fins changed shape as sharks expanded their habitat from the bottom to the open ocean."
Longer pectoral fins make shark movements more efficient. "Their fins are comparable to the wings of commercial airplanes, long and narrow, to minimize the amount of energy needed for movement," Sternes explained.
The researchers also noted that open-water sharks became faster than their bottom-dwelling counterparts. "Shark muscle is very sensitive to temperature," said Tim Higham, professor in UCR's Department of Evolution, Ecology, and Organismal Biology and paper co-author.
"The data helped us make a correlation between higher temperatures, tail movement, and swimming speeds," Higham added.
Most living shark species are still bottom dwellers, occupying the benthic zone. These benthic sharks are typically slender, flatter, and medium-sized predators. Only about 13% of modern sharks are fast-swimming open-water predators. Researchers believe oxygen levels near the bottom during the Cretaceous period likely dropped as temperatures increased.
Modern sea surface temperatures average about 68 degrees Fahrenheit. During the Cretaceous, they were much warmer, averaging about 83 degrees. This high heat did not occur overnight, nor did the sharks' evolution.
"We had pretty warm open-sea surface temperatures throughout the era, and then a distinct spike that took place over a one- or two-million-year period," said associate professor at Claremont McKenna College and paper co-author Lars Schmitz.
As global warming prompted evolution in some animal groups, including sharks, it caused the extinction of others. The long-term nature of these changes makes it challenging to predict how sharks or other marine life will respond to current warming trends.
Biologists are observing some sharks, such as tiger and bull sharks, starting to swim farther north. However, it remains unclear whether threatened sharks can adapt and survive the rapidly increasing heat.
"The temperature is going up so fast now, there is nothing in the geologic record I am aware of that we can use for a true comparison," Sternes said.
Research Report:The rise of pelagic sharks and adaptive evolution of pectoral fin morphology during the Cretaceous
Related Links
University of California - Riverside
Explore The Early Earth at TerraDaily.com
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