Dakosaurus: Giant vacuum cleaner of Early Cretaceous seas

Paleontologists from the UK found out more about the lifestyle of the masters of the Early Cretaceous seas - marine crocodiles from Metriorhynchidae family. It turns out they hunted in the same way as the contemporary killer whale, by tossing a victim in the air and tearing it apart. However, some of them sucked their prey in like giant vacuum cleaners.

The history of marine life of the Mesozoic era looks much more interesting than that of any of their land relatives. In fact, on the continents for nearly the entire period dinosaurs were alone at the top of the food pyramid. But in the sea, there was a real battle for supremacy in the ecosystems.

Apparently, on the border of the Jurassic and Cretaceous periods the "top" layers of aquatic communities were occupied by the crocodiles. However, they did not resemble the modern representatives of this group. The huge reptiles, which are now combined into a family of saltwater crocodiles or Metriorhynchidae, reached six-meters in length, and their skulls exceeded five feet. They had cylindrical bodies and their four limbs transformed into flippers. The tails of saltwater crocodiles ended in fins resembling those of modern sharks.

It is also interesting that the scales of marine crocodiles did not contain protective bony plates. Who were these giants afraid of? After all, there were not predators worse in the ocean in those days. Lightweight skin, oblique shape of the body and limbs transformed into flippers allowed these creatures to develop considerable speed - up to 50 kilometers per hour (and possibly more). Not surprisingly, the paleontologists from the University of Edinburgh (UK) compared these monsters to modern killer whales.

Recently, researchers have thoroughly studied two skulls of reptiles found in England and Germany. The age of both fossils is 150 million years. One of the skulls belonged to a crocodile of the genus Plesiosuchus (130 cm in length), and the other - Dakosaurus (about a meter in length). These creatures lived in the oceans at the time when the waves splashed in place of the British Isles and Diplodocus grazed on the territory of the future North America.

Scientists who studied the structural features of the Plesiosuchus skull found out exactly how this bloodthirsty monster used to hunt. Lead author of the study, Professor Mark Young said that, according to the shape and wear of the teeth of the reptiles, they, like the modern killer whale, first grabbed their prey, then threw it in the air, tore it to pieces and then swallowed. Apparently, most often, other marine reptiles, such as ichthyosaurs, fell victims of these insatiable gluttons.

Smaller Dakosaurus consumed their prey in a more original way. They would open their mouth and suck the prey in. Such a method of hunting is unknown among the modern crocodiles, but the killer whales use it with small prey. Its skull was elongated and was not flattened but rounded. If Dakosaurus quickly opened its mouth, it could create a sharp pressure drop that dragged an unsuspecting victim directly into the stomach of a monster.

Its diet seems to have been somewhat different. "We believe that if Plesiosuchus specialized in other marine reptiles, the Dakosaurus was a generalist. Perhaps it ate fish and everything that floated nearby, including such representative of Metriorhynchidae as relatively small Geosaurus, who seems to have been, like modern barracuda, a scavenger, " said the study co-author Lorna Steele of London's Natural History Museum.

These studies once again support the hypothesis that marine crocodiles were on the top of the ocean ecosystems. As a rule, they preyed on other predators, which is characteristic only for the "kings" of biocenosis. The convergent similarity with killer whales is not accidental - these marine mammals are now also at the top of the marine food chain.

"Our study also says something about the limits of the optimal strategy of the underwater feeding in vertebrates occupying the upper levels of the ecosystem, for example, shear force and occlusion (complete occlusion of the teeth) observed in Dakosaurus that today can be found in smaller killer whales. During the last 10 million years, this mechanism was also observed in numerous fossils of sperm whales. Overall, creatures change over time, but the strategy remains the same. Apparently, it allows completely non-related to each other animals to achieve superiority in the oceans," said Professor Young.

It is hard to disagree with this. The creatures may change, but the winning strategy is usually always the same. This strategy allowed saltwater crocodiles sole control in the Mesozoic seas in the first half of the Cretaceous period. However, around 90 million years ago, their domination came to an end - the bloodthirsty monsters that had long been the terror of marine species, have disappeared, replaced by larger and more agile monsters - such as plesiosaurs and mosasaurs.

In addition, these reptiles likely bred in the same way as their modern relatives - by laying eggs in the sand. Their eggs were covered with hard shell, and there were no viviparous species among them. It turned out that these giants had to crawl out onto land, lay eggs, and then leave as they were helpless when out of the water.

As a result, the offspring of these giants were completely exposed to terrestrial predators that ate eggs and the youngsters. This problem was particularly acute in the middle of Cretaceous, when a host of water birds who were not exactly peaceful has emerged. These warm-blooded creatures, i.e., more active and agile predators, most likely began destroying the offspring of the marine crocodiles so fast that in the early Middle Cretaceous Metriorhynchidae have become quite scarce.

Those who stayed were finished off by the new masters of the seas who later have been driven out by modern sharks and whales. Now the once fearsome rulers of the Mesozoic ocean have nothing but giant fossils left that still evoke the interest of paleontologists and other lovers of antiquity.

Anton Evseev

Pravda.Ru

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