The Mesozoic Era began with the Triassic Period over 250 million years ago. There weren't many dinosaurs around at all during the early part of the Triassic. However by the late Triassic many small bipedal dinosaurs were spreading out all over the planet. These species represented the earliest members of the Sertian Theropods and sauropods as well as primitive Ornithischian dinosaurs. Early dinosaurs are often poorly preserved in the fossil record. Like at the Danek Bonebed they are typically preserved in a mud stone, which tends to squish and distort the skeletons. Very similar to this Herrerasaurus skull I have here. Now just to orient you I'm going to point out a couple features. Here's the nose, the orbit, the upper jaw and the lower jaw, but I have a better reconstruction to show you. Here's the reconstructed skull of Herrerasaurus, and you can see that it is a diapsid based on the skull openings. Now the relationship of early dinosaurs is often confusing because we don't know which dinosaur groups they're more closely related to. The current theory is that Herrerasaurus is an early theropod. Another skull I have here of Plateosaurus is something called a Prosauropod, or just a very early long necked dinosaur, and finally the relationship of another early dinosaurs called eoraptor is currently unresolved. During the Triassic as dinosaurs were evolving to fill large bodied ecological niches on land the other amniote groups were evolving to fill the sea and the air. Here's a skeleton of an animal called an Ichthyosaur. The name Ichthyosaur literally means fish lizard, but Ichthyosaurs are not lizards, and they certainly aren't fish, and they aren't dinosaurs either. Even so the name Ichthyosaur still seems fitting, because they are a group of reptiles that took on a fish like life style and evolved a very fishy body form. The ancestors of Ichthyosaurs were fully terrestrial reptiles, but just like the ancestors of modern whales, dolphins, seals, and sea turtles, the group found success by making an evolutionary return to water. To adapt to an aquatic lifestyle, Ichthyosaurs evolved paddle like front and hind limbs as well as a fin tail. They also had a sharp leg dorsal fin. The long snouts of most Ichthyosaurs resemble those of dolphins and they are filled with conical teeth. Good equipment for pi severus diet. Despite their many fish like adaptations Ichthyosaurs never evolved gills and needed to come to the surface in order to fill their lungs with air. Late in the Triassic period Ichthyosaurs were joined in the seas by another group of reptiles that also evolved an aquatic lifestyle. The Plesiosaurs. This one you can see here. You're looking at its underbelly. Now most Plesiosaurs had large chests and torsos, broad paddle shaped limbs, and relatively short tails. In front of their shoulders Plesiosaurs varied tremendously. Some had short necks and huge jaws, and others like the famous Elasmosaurus had elongated serpentine necks with small heads. Like the seaways of the dinosaurs age the sky was a busy place. Insects had evolved flight at least as far back as the Carboniferous Period. During the Triassic Period a group of animals branched off from the archosaur family tree at roughly the same time that dinosaurs did. We call this group the Pterosaurs. The Pterosaurs were the first vertebrates able to fly. Unlike birds, which have wings made of feathers, and bats that have wings made from skin stretch between fingers Pterosaurs have membranous wings that are supported by a single extremely elongate finger. Early Pterosaurs belonged to a group called Rhamphorhynchoids, which were common in the late Triassic and throughout the upcoming Jurassic period. Rhamphorhynchoids are small Pterosaurs with the largest forms approaching the size of a modern raven or hawk. Long tails characterized the Rhamphorhynchoids, and some Rhamphorhynchoid tails terminated in flaps of skin called tail veins. The function of these tail veins is unclear, but it is hypothesized that they acted as in flight rudders or stabilizers, and might have been used in courtship displays. The Jurassic Period began about 200 million years ago. Giant Sauropods were the dominant herbivores on the planet, and they were represented by several diverse lineages including the long Diplodocids and the tall Brachiosaurs. Stegosaurus like this specimen here and small anthropoids were also present. Characteristic theropod dinosaurs of this time include predators like Allosaurus, and the nose horn Ceratosaurus. The first bird Archaeopteryx evolved in the late Jurassic period. So what was happening in the sky during the Jurassic period? First of all this was when the bird line of Theropods evolved. It was also at this time that the Rhamphorhynchoid Pterosaurs from the late Triassic gave rise to a new Pterosaur group. The Pterodactyloids. Pterodactyloids differed from Rhamphorhynchoids in the morphology of their tails, which were short. Also, the carpals in their wrists were elongate and made a greater contribution to the length of the wing. There were many species of small pterodactyloids. Some smaller that a robin, but some species had wingspans of over ten meters making them the largest animals ever to fly. This is the skull and partial skeleton of the Pterodactyloid Pterosaur, Pteranodon. It shows off the most striking difference between Rhamphorhynchoids and many Pterodactyloids. Bony head crests. Although several species of Rhamphorhynchoids did have small crests made of soft tissues, large bony head crests were common among Pterodactyloids. After the Jurassic period came the last period of the Mesozoic Era. The Cretaceous, which began about 145 million years ago. In the early Cretaceous Stegosaurs became more rare as their ecological niche was replaced by the armored Ankylosaurs. The giant Sauropods began their decline and were not seen across the same ranges that they had in Jurassic times. The thumb spiked Iguanadons were more common. Spinosaur Theropods became more abundant as did the giant Carcharodontosaurus Theropods that rivaled the later Tyrannosaurs in size. Primitive birds with better flying abilities than Archaeopteryx became more abundant. These included species like Confuciusornis and Sapeornis. By the late Cretaceous, the Iguanodons had been replaced by the duck billed Hadrosaurids like this Prosaurolophus here. Ceratopsians and Pachycephalosaurus appear for the first time, and some Ankylosaurus have evolved tail clubs. The Sahelanthorus Theropods, including the Ovoraptors, Ornitomimids, Therizinosaur, Allosaurids, and Tyranosaurids became more abundant in the Northern Hemisphere. While in the Southern Hemisphere the Abelisaurus Theropods were the dominant predators. The dominant herbivores in the Southern Hemisphere are a single lineage of the once diverse Sauropods. The round bodied Titanosaurs. Birds continued to evolve into a variety of lineages that are now extinct, including the Enantiornithes, or opposite birds, and Hesperornis, which were loon like diving birds with teeth. Some species of birds such as shore birds that are still alive today also evolved during the late Cretaceous. It was during the Cretaceous period that a third major reptilian group began patrolling Mesozoic waters joining the Ichthyosaurs and Plesiosaurs. Mosasaurs like this Clidastes were reptilian relatives of modern monitor lizards and snakes. Like Ichthyosaurus and Plesiosaurus, Mosasaurus had tail fins and limbs modified into paddles, but the bodies and tails of Mosasaurus were more elongate. Most Ichthyosaurus, Plesiosaurus, and Mosasaurus are the right size to have preyed on small and medium size fish, but some like the giant Pliosaur Plesiosaurs were true sea monsters with huge jaws and bodies over 18 meters long. These aquatic giants seem adapted for deep sea, big game hunting, and they likely ate large fish and other marine reptiles. Let's take a minute to review a few things we know about the distant aquatic and avian cousins of the dinosaurs. Pterosaurs and the various groups of Mesozoic marine reptiles are often mistaken for dinosaurs. Ichthyosaurs, Plesiosaurs, and Mosasaurs are all more closely related to modern reptiles like lizards and snakes. Terrasaurus were close relatives of dinosaurs, but fall outside the group. Here is one characteristic that distinguishes them. Looking at this lower jaw of a Tyrannosaurus you can see a large opening called the mandibular fenestra. Now, look at the jar of the Pterosaur Anhanguera, and you can see that as in all Pterosaurs the mandibular fenestra has been lost. Here are a number of dinosaurs. Select the dinosaur that would have been alive at the end of the Cretaceous Period. Multiple answers may be correct, so check all that apply. A, Stegosaurus. B, Tyrannosaurus. C, Triceratops. D, Pachycephalosuarus, or E, Giraffatitan. Stegosaurus, Giraffatitan, and their relatives were long gone by the end of Cretaceous. Tyrannosaurus, Triceratops, and Pachycephalosaurus, however were all alive at the end of the Mesozoic Era, so answers B, C, and, D are correct. As we end this module on geologic time here's another chance to interact with the geologic time scale. We hope that it's given you an opportunity to appreciate not only the age of the Earth, but also the place of dinosaurs within that long history. From when they first appeared until the time they went extinct non-avian dinosaurs were around for almost 180 million years. Any one particular species of dinosaur only lasted a few million years before it disappears from the fossil record. Each of these species would have either gone extinct or evolved into other species. Determining of two species of dinosaurs lived at the exact same time is tricky business, but as new fossils are discovered our understanding of dinosaurs species, and their place within the great expanse of time know as the Mesozoic Era becomes more complete.
