The Diapsida is divided into two more groups. The Archosauromorpha, and the Lepidosauromorpha. We can tell Archosauromorphs apart from the Lepidosauromorphs by yet more holes in the skull. Archosauromorphs have a fenestra in front of the orbit, called the antorbital fenestra. And a hole in the lower jaw, called the Mandibular Fenestra. Lepidosauromorphs lack these holes. Have another look at the tyrannosaurus skull. We're dinosaurs lepidosauromorphs or archosauromorphs? Dinosaurs were Archosauromorphs. And you can tell, because of the antorbital and mandibular fenestrae. The Lepidosauromorpha includes modern day lizards. Like this kimono dragon, snakes, and the giant, extinct marine lizards called mosasaurs. The Archosauromorpha includes crocodiles, and birds, and the extinct flying reptiles called Tarasaurs. Dinosaurs have a hole in front of the orbit, and on the lower jaw. And so they were Archosauromorphs. The final major division leading to dinosaurs has to do with differences mostly in the ankle bones. Non-dinosaur archosaurs like crocodiles, have an ankle joint in which the two ankle bones articulate with a peg and socket. Dinosaurs and their close relatives have a more hinge like ankle bone, that you can see on this ceratopsian tibia. And that allowed them to have a more upright stance. Although pterosaurs have this hinge like ankle bone, they have so many unique adaptations for flying, but they are not considered part of the dinosaur family tree. We often think of dinosaurs as being the first important prehistoric land animals. But that would be ignoring an incredible diversity of extinct animals. Tetrapods, or animals of four limbs, first crawled on the land more than 350 million years ago. But the first dinosaurs did not evolve until 230 million years ago. What happened in the 120 million years between the first tetrapods and the first dinosaurs? Although we tend to think of dinosaurs as the first amnio-group to diversify and become dominant, it was actually the synapses that diversified and radiated into the terrestrial habitats first. Remember, that the synapses are animals with only one opening behind the eye. And Synapsida includes mammals like us. Dimetrodon was one of the earliest Synapsids, and later Synapsids evolved more mammal like features. Like a more upright posture and specialized ear bones. Some of the Permian Synapsids were small, but some became as large as cows. Some, like Estemmenosuchus had elaborate horns. The Gorgonopsids were the largest carnivores of the Permian, and the dominate herbivores of the late Permian were Dicynodonts. Unusual tusked and beak synapsids. The Cynodonts like Divinia, were the most mammal like of the early Synapsids. The late Permian landscape would have been an alien looking environment. In the last module, we talked about paleo-geography and the super continent, Pangea. By the end of the Permian, all of the world's land masses had collided to form Pangea. The interior of Pangea would have had a highly continental climate, with very extreme temperatures and periods of extreme dryness. These arid landscapes were inhabited by animals that are very unfamiliar to us today. Even though they include some of our closest non-mammalian synapsid relatives. A mass extinction at the very end of the Permian Period wiped out most of the dominant animal groups living at the time. What do you think caused this mass extinction? Check the one answer you think is the most correct. A, volcanoes, B, an asteroid impact, or C, greenhouse gases. Based on our current understanding, the primary cause of this mass extinction, appears to be increased concentrations of greenhouse gases. In particular, carbon dioxide. So C is the correct answer. However, intense vulcanism over thousands of square kilometers in Siberia and possible asteroid impacts could've also played a supporting role. We just don't know for sure. The rain of synapses was cut short by the greatest mass extinction of all time. The end Permian Extinction. The encrustatious extinction event is the most famous, because this is when the dinosaurs died out. But more species died at the end of the Permian. This extinction impacted more than 90% of ocean dwelling animals, and more than 70% of land dwelling ones. At the dawn of the Mesozoic era, in the early Triassic period, life was beginning to recover from the massive end Permian extinction. That extinction event was so severe, that Earth's new ecosystems were only composed of a few species each. One of the hardiest survivors was the herbivorous dicynodont synapsid, lystrosaurus, which was abundant. And the few cynodont synapsids squeaked through as well, which is a good thing for us. It is from these small cynodonts that mammals would evolve. But the synapsid lineage would not become the dominant terrestrial vertebrates for a very long time. As the world's ecosystems continued to recover from the End-Permian extinction, Arcosauromorphs became the dominant terrestrial vertebrates. Do you remember the distinguishing features of archosauromorphs? Check all the answers you think apply. Was it one fenestra in the skull behind the orbit? A single single fenestra in the bones of the lower jaw? Or a fenestra in the skull in front of the orbit? Archosauromorphs, including dinosaurs and their relatives, had two fenestra in their skulls, behind their eyes. As well as fenestra, in front of their orbit and in their lower jaw. So B and C are the correct answers. There're two major lineages of archosauromorphs. You can think of these as the crocodile line and bird line archosaurs based on their living members. In the middle and late Triassic, it was actually the crocodile line archosaurs, that became the dominant terrestrial vertebrates. There were many crocodiles like archosaurs that maybe unfamiliar to most of us because they're often overshadowed by the dinosaurs. There were slender snouted phytosaurs and armored pig snouted aetosaurs. The rauisuchids and prestosuchids looked a lot like crocodiles, but walked with an upright posture. An unusual group of Pseudosuchians, the Poposauroids, including the sailback species and bipedal species that looked a lot like the Therapod dinosaurs. The Triassic period could be described as the age of Pseudosuchians. Although crocodile-like archosaurs would continue to be abundant and diverse throughout the Mesozoic and Cenozoic eras, this is the period in which they truly flourished. In the background of pseudo diversity, the first dinosaurs, and several species of not quite dinosaurs appeared. Footprints from dinosaur-like archosaurs had been found as far back as 250 million years ago, which tells us that the evolution of true dinosaurs began not long after the end Permian extinction. Most of these dinosaur morphs were small and bipedal, and looked a lot like the true dinosaurs. The earliest Dinosauromorphs lacked some of the specializations that characterized Dinosaura proper, such as a hip socket with a hole through it, which we call the perforated acetabulum. The Dinosauromorphs and early Dinosaurus coexisted throughout the late Triassic. But still took second place to the larger and more diverse aetosaurs, phytosaurs, and The earliest true dinosaurs have been found mostly in the middle Triassic of Argentina and New Mexico. Eoraptor, Eodromaeus, Herrerasaurus, and Panphagia were the earliest Saurician dinosaurs. And Pisanosaurus was an early ornothician dinosaur. All of these dinosaurs were relatively small and bipedal, and all except Pisanosaurus were carnivores.