A 365-million-year-old shark could swim fast, hunt efficiently and even smell in stereo.
Similar to modern-day hammerhead sharks, the ancient fish had a wide snout and broadly spaced nostrils, allowing for more precise localisation of prey. The discovery of fossils of the animal in the Moroccan Sahara represents the earliest evidence of such sensory specialisation in sharks and other cartilaginous fish – and possibly in all jawed fish, says at the University of Zurich in Switzerland.
The evolutionary history of cartilaginous fish has been notoriously hard to piece together, since they have no bones and it is rare for cartilage to fossilise, says Klug. Thus far, researchers have mostly found flattened fossils, which are difficult to analyse – he likens it to “studying animals that got run over by a heavy truck”.
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Several years ago, Klug and his colleagues discovered that fossils preserved in pyrite or phosphate in former sea basins in Morocco kept their three-dimensional appearance, including the soft tissues. “We even have remains of stomach contents and of the livers and kidneys and so on,” he says.
The researchers found several 3D fossils of ancient sharks – two of which had nearly complete skulls with fully articulated jaws – embedded in a layer of rock from about 365 million years ago. They used CT scans to create digital 3D reconstructions of the animals and compared their specimens with other prehistoric sharks.
The researchers found that their sharks – measuring up to 2.5 metres in length – had prominent ridges on both sides of the tail fins, similar to modern great whites, suggesting they were excellent swimmers, says Klug. They had large eyes and a broad upper jaw ending in a wide nose with nostrils spaced more than half the skull’s width apart.
Widely spaced nostrils help animals determine the direction a smell is coming from, even in murky water. This adaptation would have given the sharks a significant hunting advantage over other predators at a time when underwater species were rapidly evolving and competing for food, says Klug.
The lower jaw, however, was considerably more narrow. “It looks quite funny because [modern] sharks, for example, don’t have this discrepancy between the broad nose and the narrow mouth,” he says.
The researchers named the shark Maghriboselache mohamezanei, for the Arabic word for Morocco and the Greek word for cartilaginous fish, and for Moha Mezane, a French linguist and amateur geologist who found some of the latest specimens. They determined that the animal was closely related to Cladoselache sharks from the same period, found in Ohio – which, at the time, was less than 1000 kilometres away.
These two groups weren’t the ancestors of modern-day sharks, but they represent the earliest species on the branch of the evolutionary tree that led to modern chimaeras, a group of deep-sea cartilaginous fish that includes ghost sharks, ratfishes and holocephalans.
Swiss Journal of Palaeontology
