Academic journal article The Science Teacher

New Hammerhead Study

Academic journal article The Science Teacher

New Hammerhead Study

Article excerpt

The ancestor of all hammerhead sharks probably appeared abruptly in Earth's oceans about 20 million years ago and was as big as some contemporary hammerheads, according to a new study led by the University of Colorado at Boulder (CU--Boulder).

But once the hammerhead evolved, it underwent divergent evolution in different directions, with some species becoming larger, some smaller, and the distinctive hammerlike head of the fish changing in size and shape, says Andrew Martin, a CU--Boulder professor in the Ecology and Evolutionary Biology Department.

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Sporting wide, flattened heads called cephalofoils with eyeballs at each end, hammerhead sharks are among the most recognizable fish in the world. The bizarre creatures range in length from about 1--6 m (3--18 ft.) and cruise warm waters around the world, Martin says.

In the new study, scientists focused on the DNA of eight species of hammerhead sharks to build family "gene trees" going back thousands to millions of generations. In addition to showing that small hammerheads evolved from a large ancestor, the team showed that the "signature" cephalofoils of hammerheads underwent divergent evolution in different lineages over time, likely due to selective environmental pressures, says Martin.

The team used both mitochondrial DNA passed from mother to offspring and nuclear DNA--which is commonly used in forensic identification--to track gene mutations. The researchers targeted four mitochondrial genes and three nuclear genes, which they amplified and sequenced for the study.

"These techniques allowed us to see the whole organism evolving through time," Martin says. "Our study indicates the big hammerheads probably evolved into smaller hammerheads, and that smaller hammerheads evolved independently twice. …

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