The Fight Between Tiger Sharks and Dugongs Is Older Than We Think

It’s called the ‘Green Heart’ of Austria with large, mountainous forests throughout the province and inviting vineyards, spas, and castles. Retznei, a former municipality in the district of Leibnitz in the state of Styria, is the last place in the world where one would expect a shark-bitten partial skeleton of an immature sirenian.

Or, maybe not. “Only a few people know that Austria was a tropical paradise several million years ago,” explained researcher Iris Feichtinger of the Natural History Museum Vienna (Geological-Paleontological Department). “Experienced paleontologists frequently find manatees or tiger shark teeth [here]. […] By chance I heard about this peculiar finding, and I had to catch a glimpse of the skeleton. To my great surprise, not only shark teeth were found, but the manatee bones also showed distinct bite marks. This discovery is of special interest because Austria is a landlocked country nowadays. For people previously not interested in paleontology or the geological past, this finding has certainly opened their eyes and awakened their interest in earth history.”

Found in the active quarry ‘Rosenberg Bruch’, operated by the mining company Lafarge-Perlmooser, the sirenian skeletal remains were found by Gerhard Wanzenböck in 2012 and excavated by Martin Gross and Norbert Winkler (both at Universalmuseum Joanneum Graz in Styria) with the permission and assistance of the mining company. The bite mark-bearing ribs and vertebrae of this ancient dugong belong to the extinct Galeocerdo aduncus, which are otherwise rare in the fossil record of the Styrian Basin, possibly due to seasonal migrations. “Most tiger shark teeth are actually found at localities also yielding manatee bones,” Feichtinger said. “Nevertheless, our finding is the first clear evidence of this predator-prey relationship.”

In fact, this is the oldest predator–prey interaction between tiger sharks and dugongs known to date, including multiple teeth marks leading the experts to believe they didn’t just bite the animal but fed upon the Metaxytherium carcass. “It’s a nice feeling to decipher another puzzle piece of our past. Paleontologists are like detectives, working with only a few clues to interpret and unlock secrets of the past. When we have gathered enough clues, pieces can be merged to a large puzzle to reveal the bigger picture – in this case the skeleton with teeth and bite marks,” said Feichtinger. “Many lucky coincidences have come together leading to this unique discovery.”

Modern-day tiger sharks (Galeocerdo cuvier) are common in tropical and sub-tropical waters throughout the world. They are known as opportunistic feeders that prefer to feed on a variety of prey – cephalopods, crustaceans, sharks and rays, bony fishes, reptiles, birds, and mammals – and even undigestible items such as kitchen scraps, tin cans, and clothing! Combined with poor eyesight, their languid lifestyle makes dugongs relatively easy prey for tiger sharks — which is exactly what they take advantage of in Australia.

Here, they defend seagrass ecosystems from overgrazing by dugongs, sea turtles, other species. Marine scientist Dr. Michael Heithaus of Florida International University (FIU) focuses his work on the ecological importance of tiger sharks in the Shark Bay seagrass region. Due to a changing and warming oceanic climate, a 2011 heat wave wiped out the seagrasses which have struggled to bounce back. However, it turns out the tiger sharks are one of the seagrasses’ best allies, by just existing! When they patrol the seagrass beds, the dugongs steer clear in fear of being the shark’s next meal.

According to Feichtinger, Austria was similar to the modern-day Shark Bay coastline off of Western Australia. “About 15 million years ago, Austria was comparable to the habitats of manatee populations living today. Extensive seagrass meadows covered shallow bays, ideal for rearing offspring,” she said. “It is known from Shark Bay that tiger sharks only visit the shallow bays seasonally and becoming potentially dangerous to the manatees. Shark Bay in Australia is a plausible pendant to Austria a few million years ago showing that modern tiger sharks are not on the prowl in the same area the whole year. Unfortunately, we cannot look into the past and the sharks did not leave a diary of their vacation trips, so seasonal migrations of tiger sharks are difficult to document. Nevertheless, compared to the behavior of living tiger sharks, it is likely that they have retained this behavior.”

It’s a fascinating discovery that took years in the making – and may yet reveal even more secrets of this ancient world. The publication is now available online.

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