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It was a hot day in the early Triassic, 250 million years ago. The riverbed was dry and cracked, with a few puddles of water. The trees and bushes had already lost their leaves for months, the dry season was about to end with an approaching storm. In these conditions, one of the ancestors of all mammals, Thrinaxodon, dug burrows to protect itself from the heat and drought. There, he would spend months sleeping, his metabolism reduced, until conditions improved. But this burrow that was dug to protect Thrinaxodon’s life turned out to be a trap, and became its tomb for ¼ billion years. And strangely, he wasn’t alone. There was a second skeleton inside this fossilized burrow, of an entirely different animal. This unusual companion caught the attention of scientists, becoming famous for being the strangest duo in the fossil record. Today, you will know this story. My name is Abner and welcome to ABC Land. Many fossilized burrows dating from the early Triassic are found in Southern Africa, especially the Karoo region, a vast semi-desert where thousands of fossils have been unearthed. They are around 250 million years old, the time when the planet faced the deepest and most devastating mass extinction of all time: the Permo-Triassic. It was a global catastrophe, which we call “The Great Death” in which about 85% of all life on the planet was eradicated at the end of the Permian. It ended the first great age of animals: the Paleozoic, which saw from the first pulse of animal diversification during the Cambrian, through the colonization of oceans and continents, to the dominance of reptiles and synapsids in the Permian. The survivors turned the Triassic into a great evolutionary testing ground, and unleashed the age of dinosaurs: the Mesozoic. But dinosaurs didn’t dominate right away. They arose 20 million years after the Permo-Triassic extinction, and took another 20 million years to establish themselves as the dominant vertebrates of the continents. Today’s story, then, takes place in the age of dinosaurs, but they were still part of the profoundly distant future, twenty million years ahead. T rex, which lived at the end of the Cretaceous, was still 184 million years in the future. The great death had just taken place, and conditions on Earth were far less than luxurious. The causes that led to this cataclysmic planetary-scale extinction are still very hotly debated, but two things may have harmed the biosphere during this event: An immense volcanic outpouring that may have lasted thousands of years in what is now Siberia and a possible large asteroid impact. Intense climate change has resulted in a condition of global desertification in excruciating heat. To make matters worse, Pangea was at its peak. When supercontinents form, their interiors tend to be very desert-like, as they are so far from any ocean. When supercontinents break up, coastal areas are created, bringing moisture into the landmasses. Adapting to the arid climate, some of the terrestrial vertebrates made holes to escape the extreme heat. Many of these early Triassic holes were collected from the Karoo and are thought to be a direct behavioral response to a changing climate. Burrows make excellent refuges, a good way to avoid predators, and can serve as protection from extreme weather conditions , allowing animals to live in the comfort of their own homes. As much as many of them are found, they are usually empty. In exceptionally rare cases, articulated skeletons of basal mammalian relatives, called therapsids, are found inhabiting these burrows. Some contain individuals lying side by side, resting together in the holes they have created, and sometimes claw marks are also present on the walls of the burrows. It has been suggested that this association between burrow and occupant is an indicator of seasonal numbness . This is a great hypothesis, because it suggests that these individuals were in a state of inactivity called aestivation, similar to hibernation but which happens during particularly dry and hot periods. This can help prevent death from dehydration and starvation for small animals. This behavior would be very pertinent in the challenging conditions of that time. An unusual and unique animal association was hidden inside one of these burrows, collected in the province of KwaZulu-Natal, in 1975. When it was found, only a portion of a skull was exposed, which was identified as belonging to a relatively common therapsid, of the size of a fox, called Thrinaxodon liorhinus. The specimen was broken into two pieces, and more bones were found inside. As nothing extraordinary seemed to be preserved, it was taken to the collection of the University of Wits, in Johannesburg, South Africa, and its secret was only recently revealed, after a new analysis of the fossil. This particular burrow has gone many years without being prepared, a process in which rock is removed to reveal the fossil, in part because the amount of bones makes the process extremely delicate. A decision was then made to examine the specimen without causing any physical harm. He was taken to the other side of the world, to France, where he was subjected to analysis in a synchrotron, a huge particle accelerator laboratory, which obtains incredibly detailed images by accelerating beams of electrons to relativistic speeds. This non-destructive method allowed us to see inside the rock without changing anything from its original condition, preserving information that could not be observed from the outside. Access to this type of equipment has revolutionized the way paleontologists study fossils. These technologies allow us to look inside the rock and determine how complete or important the contained fossil might be. But this time, it was an incredible find! Preserved inside the burrow, side by side with a complete skeleton of Thrinaxodon, is another, similarly sized, complete skeleton of a young amphibian called Broomistega putterilli. And even the amphibian’s skin pattern is still present, a detail that could be lost in physical preparation. As preserved, Thrinaxodon is lying on its stomach, its head turned uncomfortably to the left, as if pushed against the wall of the hole. Broomistega in turn, on its back, exposing its belly, leaning against Thrinaxodon. This association of two very different animals has baffled scientists, and has raised questions about their interactions in life. Interestingly, the amphibian has a series of broken ribs on the right side of its body. It could be assumed that Thrinaxodon attacked Broomistega and dragged it into its lair. The broken ribs, however, show signs of healing, demonstrating that the injury predates the pair’s encounter. In fact, it’s likely the wound was the result of a crush, perhaps he was stepped on weeks before his death. Certainly, this wound affected this animal’s ability to walk, in addition to causing intense pain, particularly during breathing. It literally only hurts when you breathe. An injured amphibian struggling to walk in the heat of the sun is just a meal waiting to be eaten. Based on its skeletal anatomy, we know that Broomistega was adapted for a semi-aquatic lifestyle, but the structure of its limbs suggests that it was incapable of digging. On the contrary, Thrinaxodon’s limbs were adapted for digging, which, together with the findings of other Thrinaxodons inside petrified burrows, suggests that it was indeed the author of the hole. As there is no sign that there was a fight between them, Thrinaxodon was sleeping, stowing, or just happy to have company and tolerated the presence of the intruder who sought refuge. This is a commensal ecological relationship, when Broomistega takes advantage of the conditions created by Thrinaxodon without harming or benefiting it. On the other hand, it is possible that Thrinaxodon was dead inside the burrow, and that Broomistega simply went in to die too. But their exceptional preservation and direct association indicate that they died together. So, following its survival instincts, Broomistega sought protection from the drought by entering the burrow, unaware that this is where it would spend the next 250 million years. A similar behavior was observed in living amphibians, mainly young, that sometimes seek refuge in burrows left by other animals. Most likely Thrinaxodon was stowing when the wounded young Broomistega broke into his lair. Whatever scenario led to this association, both suffered the same fate, when the hole was quickly filled with mud during a storm, preserving both together for eternity. This fossil provides us with a rare window into ecological interactions between very different animals that would never have been considered were it not for the serendipity of this discovery. A piece of one of the most challenging times in the history of life. This video is adapted from an essay from the book, Locked in time, by Dean R Lomax, one of my favorite books in the world. A huge thanks to Lucas Mateus and Heitor de Sá for the incredible original art! If you want to help us produce more and better animations, consider donating any amount to the channel’s art fund, via pix [email protected]. 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