LearnWant to know even more about dinosaurs, the mesozoic era, and modern day living fossils. Read on - Written by Laci Prucinsky, FNA Naturalist. The Timeline: The Mesozoic Era began after the massive Permian-Triassic Extinction Event where 83% of all genera became extinct. This Age of Reptiles is divided into three periods; Triassic, Jurassic, and Cretaceous.
The Triassic was a time of arid inland deserts dominated by fields of seed ferns, giant club and spike mosses, cycads and cycadeoids, and massive conifers. The archosaurs and therapsids roamed these forests. The very first dinosaurs were present but were probably small, bipedal, and definitely predatory. They wouldn’t become dominant until the Jurassic. Lissamphibia, the first frogs, appear in the fossil record here as well as the earliest turtles. Lystrosaurus “shovel lizard” a genus of mammal-like reptiles were the most common land vertebrates in the early Triassic. Scientists speculate as to how this genus survived that “mother of all extinction events”, the Permian-Triassic Extinction Event. With lush jungles of towering conifers; cycads, cycadeoids, and tree ferns in the understory; and ferns and mosses carpeting the ground, the Jurassic was the peak of dinosaur diversity. It’s also the time of aquatic and flying reptiles. Mammals and birds exist at this time but they were small, background players on the stage. Sharks, rays, cephalopods and giant marine crocodiles competed with the ichthyosaurs, pliosaurs, and plesiosaurs in the oceans and seas. The Cretaceous, warm with expanses of open water; saw the shift from gymnosperms or naked seeds, to angiosperms, flowering plants with enclosed seeds. Dinosaurs and the other reptiles began to share center stage with the early monotreme and marsupial mammals, and birds. Mososaurs and plesiousaurs now share the oceans and seas. Then, at 65,000,000 years ago the, Cretaceous-Paleogene Extinction Event occurred. Smaller animals, omnivores, insectivores, scavengers and fresh-water ecosystems survived fairly well. However, the dinosaurs, flying reptiles, and aquatic reptiles did not. The Reptiles What exactly is a dinosaur? How are they different from the flying and aquatic reptile groups? Before we get to dinosaurs we need to know what archosaurs are. Archosaurs, a group of diapsid amniotes, are the most recent common ancestor of birds, dinosaurs, and the crocodilians. They were the dominant land vertebrates after the Permian-Triassic Extinction Event.
Specifically, dinosaurs refer to the clade Dinosauria, which first appeared in the Triassic period approximately 230,000,000 years ago. What exactly is a clade? How does that tell me what a dinosaur is? A clade is a group consisting of a species and all of its descendants; it’s a single branch on a cladogram or tree. So what does that mean for those of us without advanced degrees in taxonomy? Dinosaurs include the ornithischian and saurishcian reptiles: theropods and birds, ankylosaurians, stegosaurians, ceratopsians, ornithopods, and sauropodomorphs. Even more generally, dinosaurs are archosaurs with the limbs held erect beneath the body. The limbs were below the body like posts and not sprawled out to the side like a crocodile.  (Click for larger view) Which leaves us with the flying and aquatic reptiles. The flying reptiles belong to the clade Pterosauria which is split into the suborders Rhamphorhynchoidea and Pterodactyloidea. Pterosaurs had wings very similar to modern day bats; formed by a membrane of skin and muscle stretched from the legs to an elongated fourth finger. The Rhamphorhynchoidea are considered basal, or more primitive, and had long tails and lots of teeth. The Pterodactyloidea were more advanced and had short tails. There’s much debate as to whether all pterosaurs flew, or just some; how they flew, did they actively flap or just launch and glide; and if some were suited to a more terrestrial habit. The aquatic reptiles are a diverse group that doesn’t fit well into a general definition other than they were reptiles that lived mostly or completely in the water. Pliosaurs, plesiousaurs, nothosaurs, and placodonts, are considered sauropterygia or lizard flippers. Ichthyosaurs, mosasaurs, thalatosaurs and more.
Some Numbers: As of 2006 over 500 dinosaur genera have been identified. Genera is the plural for genus, which is the second lowest level in biological classification. Wang, S.C., and Dodson, P. (2006). "Estimating the Diversity of Dinosaurs". Proceedings of the National Academy of Sciences USA 103 (37): 13601–13605. Bibcode 2006PNAS..10313601W. doi:10.1073/pnas.0606028103. PMC 1564218. PMID 16954187. By 2008 1,047 different species had been named. Amos J (2008-09-17). "Will the real dinosaurs stand up?". BBC News. Retrieved 2011-03-23. The tallest dinosaur known from a mostly complete skeleton is also the heaviest, Giraffatitan brancai: [LP2] 39ft tall, 74ft long, 70,000-130,000lbs found in Tanzania. There are potentially larger, and heavier dinosaurs, however they are known from much more fragmentary fossils. Longest complete dinosaur is Diplodocus at 89ft, found in Wyoming. Smallest dinosaur is Anchiornis: 1.1ft and 110g Possibly the largest of the known carnivorous dinosaurs; Spinosaurus aegyptiacus at 47-59ft long and up to 41,800lbs Living Fossils: The term, living fossils, refers to any living species, or group of species, that appear to be the same (or very similar) to species otherwise only known through fossil records. These organisms don’t appear to have changed much since they first appeared in the fossil record.
|
