A giant at the time of the dinosaurs: a former mammal cousin looked like a cross between a rhinoceros and a turtle | Science

Imagine if you came across a rhinoceros with a giant turtle and you fattened the result: you could get something like: Lisowicia bojani, a recently discovered Triassic mammal cousin who had a rhinoceros-shaped body, a turtle-shaped beak, and weighed about 9 tons like an African elephant. Paleontologists say that this surprising creature offers a new vision of the dawn of the dinosaur era. "Who would have thought that there were giant mammal cousins, the size of an elephant, living alongside some of the very first dinosaurs?" marvel Stephen Brusatte, vertebrate paleontologist at the University of Edinburgh.

Researchers had thought that during the recent Triassic, between 240 million and 201 million years ago, the first mammals and their parents "were retreating into the shadows as dinosaurs rose and grew," says Brusatte. "This is the story I told my students at my lectures. But it casts a veil over this simple story, "suggesting that the same evolutionary forces that favored giant dinosaurs also worked on other creatures.

The new fossil, a partial skeleton described online this week in Science, is a former plant eater named dicynodonte; the name means "two dog teeth", in reference to the characteristic defenses of the upper jaw, which resemble oversized canines. Aside from the defenses, the dicynodonts were mostly toothless, with a horny beak like the turtles of modern times. They are part of the large evolutionary group called synapsids, which includes our mammal ancestors, and were among the most abundant and diverse Middle Permian triassic land animals, from 270 million to about 240 million. years. .

Dicynodonts "are the first group of vertebrates to be able to eat plants," says Tomasz Sulej, paleontologist at the Institute of Paleobiology of the Polish Academy of Sciences in Warsaw.

Dicynodonts have evolved into a range of striking shapes: one dug out like modern-day moles, another is the first known vertebrate to live in trees. Some have reached the size of hippos today, which weigh about 1.5 tons. However, the fossil record suggests that the group was declining by the time L. Bojani heavily in view. And even at the peak of the dicynodontes, they were not as numerous as the first dinosaurs.

Sulej, along with Jerzy Dzik, of the Institute of Paleobiology, and Grzegorz Nied, wiedzki, paleontologist of the University of Uppsala in Sweden, discovered the new fossil in a trench. clay formerly mined for brick making in the village of Lisowice, about 100 km northwest of Krakow, in southern Poland. . In 2006, the team learned that someone had found bone fragments on the site. On their first visit, they found fossils within 15 minutes. During 11 years of field work, they excavated over 1000 bones.

They did not immediately recognize the new dicynodonte as such, in part because it's so big, Sulej says. "Our first idea was that it was a sauropod", which were the largest known herbivores of this period, reaching 11 meters long. But skull fragments and limb bones have identified the animal as the largest and most recent dicynodon ever found. The team named it after the village and the comparative anatomist of the 18th century, Ludwig Heinrich Bojanus; they estimate that it was over 4.5 meters long and 2.6 meters high.

Most of the dicynodontes had a posture that seemed awkward for the modern eye: their hind limbs were straight, like those of the mammals of today, but their forelegs were slumped, lizard – style, with a curvature at the elbow. The team suggests that because of the way L. BojaniThe upper part of the skeleton arm is connected to his shoulder, his anterior legs must have been oriented vertically, which gives him a more upright posture than in modern reptiles. This posture, like that of the sauropod dinosaurs and modern mammals, could have helped to support its enormous weight. But others warn that rebuilding a posture without soft tissue can be difficult.

L. BojaniThere were also missing lines in the bones, which in most fossils of dicynodonts mark periods of slowing of bone growth. The animal may have grown exceptionally quickly or has not matured by the time of death. Given the "really amazing" size of the creature, "she probably grew quickly," says paleontologist Jennifer Botha-Brink of the Bloemfontein Palaeosystems Center and the National Museum in South Africa. But she adds that lines indicating a slowdown in growth may have been erased when the bone was remodeled in adulthood, which is happening today among elephants.

The researchers speculated that sauropods have grown to avoid eating. It could have been true for L. Bojanialso, says Sulej. The Lisowice bone bed also contains the remains of a 5 meter long predator – probably a dinosaur – and coprolites (fossilized stool) containing dicynodontes bones.

Researchers will look for more specimens further east in Russia and Ukraine. "There is definitely more to discover," says Niedwiedzki. "How many surprises are still waiting for us in the rocks?"