DESPITE the fearsome reputation of modern sharks, their ancient relatives never quite matched the predatory skills of their reptile or mammal rivals. A new analysis of their teeth helps explain why.
Until about 65 million years ago, the great ocean-going predators were reptiles that went extinct at the same time as their dinosaur cousins. Amazingly, their ecological niche remained empty for 15 million years, until marine mammals came along. For a long while, it seems, sharks were unable to stay in the hunt.
Chuck Ciampaglio of Wright State University in Celina, Ohio, looked at the shape of the teeth of marine predators from the past 100 million years. He used 10 separate measurements, including cross section, the presence of serrations, the number of cusps and the curvature. Analysis revealed the teeth could be grouped into five designs, with distinct functions.
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Most of the ancient marine reptiles possessed thick, pointed conical teeth able to puncture and hold prey, as well as crush bones. The sharks that lived alongside these reptiles had a variety of tooth types, such as small teeth with low crowns good for clutching, or teeth suited to grinding open hard-shelled prey. But none of the sharks developed conical teeth, even long after the marine reptiles went extinct (Sedimentary Record, vol 3, No 4, p 4).
鈥淣one of the sharks developed conical teeth, even long after the marine reptiles had gone extinct鈥�
Instead, the niche remained vacant until some marine mammals evolved pointed, conical teeth, and others evolved slicing and gouging teeth similar to those of sharks.
Palaeontologists have long recognised similarities between modern marine mammals and some extinct marine reptiles. Dolphins evolved the same body shape as the ichthyosaurs of the dinosaur era, while the first early whale fossil, uncovered in the 19th century, was misnamed Basilosaurus because it was mistaken for a plesiosaur. But this was the first time anyone had done a quantitative analysis of their respective teeth.
