If you had to choose the most successful group of animals in the history of life on Earth, the dinosaurs would be among the finalists. For more than 170 million years they dominated the Earth with sizes ranging from tiny creatures a few centimeters long to some of the largest animals to ever walk the Earth. But despite its long evolutionary history, its origin remains a mystery. When did the dinosaurs appear and how were the first?
In Ischigualasto or Valle de la Luna (Argentina), an area rich in fossil deposits, remains of a wide variety of animals have been found dating back to about 230 million years ago, from the Upper Triassic period. It shows that the environment was dominated by the earliest reptiles known as rhynchosaurs (distant relatives of crocodiles, dinosaurs and birds that lived only during the Triassic; they were small but some could be up to two meters long) as well as by cynodonts , a group that began to develop the characteristics that today are exclusive to mammals: warm blood, fur on the body and different types of teeth; In Latin, cynodont means dog’s teeth.
However, this fossil ecosystem reveals to us that there were also a series of species of early dinosaurs , certain small bipedal creatures such as Eodromaeus murphi (with a length of one meter and a weight of 5 kg) and Eoraptor lunensis (also one meter but 10 kg). kg of weight) and larger animals such as Herrerasaurus ischigualastensis that could measure up to 6 meters and weigh 350 kg. For Paul Barrett, an expert in the evolution of dinosaurs at the British Museum, this site in Argentina where the first dinosaurs are being found -and also in others discovered in Brazil- “suggests that they must have had a longer evolutionary history that we still don’t know, and we’re still debating which part of that story is missing.” Now, to talk about the first dinosaurs, we first have to answer a question: what is a dinosaur?
They belong to a group of animals known as archosaurs (dominant reptiles) , which today include birds and crocodiles (some scientists also place turtles in this group). One of the anatomical features that distinguishes them is that they have a fused sacrum of three or more vertebrae (the sacrum is the part of the spinal column that forms the back of the pelvis), and also the lower part of the spine was connected with the sacrum in front and with the tail bones behind. This makes for a much stronger connection between the hips and the spine, “and they did that because they turned some of their vertebrae into a modified structure, the sacrum, which created these additional attachments at their hips,” says Barrett. reptiles have up to two vertebrae fused into a sacrum, but to be a dinosaur, the animal needs at least three.
This feature is so distinctive that the founder of the Natural History Museum in London, Sir Richard Owen, first used it to define them in 1842 after realizing that Iguanodon and Megalosaurus had a sacrum made up of five fused vertebrae. Where did this change come from? It was possibly the result of early dinosaurs walking on their hind legs: “Walking is really important in defining early dinosaurs,” says Barrett. While the legs of crocodiles and lizards stick out from the body at right angles, to become bipedal dinosaurs had to tuck them directly under the body. This involved modifying both the hip joints with the spine and the tops of the thigh bones where they join the hip, as well as the knee and ankle joints.
That the dinosaurs found in Ischigualasto are recognizable as such means that these reptiles appeared much earlier than previously believed. The closest relatives to the dinosaurs have been identified in this century, more specifically in 2010. They are known as the silesaurids , and they lived between the middle and the end of the Triassic. They were quadrupedal animals of intermediate size that looked like long-legged lizards. ” But there is this gap of 10 to 15 million years that separates the common ancestor of silesaurids and dinosaurs from the first true dinosaurs,” adds Barrett.
It is not yet known what fills this gap, but the answer may lie in some fossil remains, dated 240 million years ago, which correspond to an animal the size of a Labrador retriever with a meter and a half tail baptized with the name of Nyasasaurus parringtoni . It was discovered in Tanzania near Lake Nyasa (hence its name) in the 1930s and went unnoticed for decades due to the meager remains: a humerus and six vertebrae. However, and by those coincidences that paleontology has, three were vertebrae from the sacral region, one of the defining characteristics of dinosaurs, which has put paleontologists on the road. The problem is that with so few remains it is very difficult to say with certainty whether or not it was a real dinosaur, but a study published in 2012 confirmed that it had characteristics common to the first dinosaurs and their close relatives : a large number of bone cells and blood vessels – indicative of rapid growth – and a very broad humeral crest, necessary to anchor the muscles of the arm. According to study co-author Sarah Werning of the University of California, Berkeley, “This is a very good example of a transitional fossil ; the bone tissue shows that Nyasasaurus grew as fast as other early dinosaurs, but not as fast as later ones.” .
Two years earlier, in 2010, the fossil remains of Asilisaurus kongwe were discovered in southern Tanzania, whose name says it all: ancestor of the lizard. It is a protodinosaur that lived about 240 million years ago and is more closely related to birds than to crocodiles. To make a comparison, it is to the dinosaurs what the chimpanzee is to the human being, our closest known relative but not a direct ancestor. Asilisaurus lived about 10 million years before the oldest known dinosaurs, had a long tail and weighed up to 30 kilos.
With this and other data, some scientists have ventured to give a brief description of how those first dinosaurs could have been: they had a body length of a couple of meters , they would have been bipedal with small hands capable of grasping things; they were probably carnivores or at most omnivores but never herbivores , and there weren’t too many of them either. Something that has been maintained throughout its history because even when the dinosaurs had already spread over the planet 230 million years ago, they were still few compared to the rest of the fauna.
It wasn’t until after the end-Permian/early-Triassic extinction event—known as the Great Dying for being the greatest in Earth’s history 250 million years ago—that the dinosaurs got their chance . Among the groups that evolved in the wake of the disaster were the first archosaurs, which split into two lineages: one gave rise to the crocodilians ; the other, to pterosaurs (flying reptiles) and dinosaurs .
