Dinosauria

Introduction

Dinosaurs were a highly successful group of animals that survived for more than 160 million years. Largely associated with an extinct group of gigantic animals not seen on earth for more than 65 million years—the first published definition of dinosaurs described them as “fearfully great lizards”—science has now linked dinosaurs with modern-day birds. Dinosauria, in fact, refers to an archosaurian subgroup that includes all dinosaurs, archaic avians, and modern-day birds. Dating back to 1842, dinosaurs were properly described as a new order of reptiles, distinguished by the presence of a fused sacrum (a triangular-shaped bone at the base of the spine). Paleontologists now define nonavian dinosaurs as reptile—or birdlike terrestrial animals that lived during the Mesozoic, or the “age of dinosaurs,” and which are characterized by an upright posture, fused sacrum, and, in many species, their massive size.

Fast Facts

Pronunciation: Dinosauria (pronounced DIEN-o-SAWR-ee-a), meaning “terrifying lizard,” a subgroup of archosaurs

Time Period: Late Triassic (235 million years ago) to Current

Size: 6.2 cm to 60 m (2.5 in–200 ft) in length

Weight: 1.6 g to 120 tons (0.06 oz–265,000 lbs)

Diet: Carnivorous, Omnivorous, Insectivorous, and/or Herbivorous

Location: All continents

Lifespan: 100 years or more

Homologous Traits

Although anatomical features of dinosaurs can vary quite significantly between genera, clades, or species, researchers have developed a basic list of many anatomical features that an animal must possess to be truly classified as a dinosaur. Even though crocodilians, pterosaurs, and dinosaurs are grouped together as archosaurs (meaning “ruling lizard”) and all shared a common ancestor, dinosaurs differ in a number of morphological, anatomical, and probably social and behavioral ways.

Perhaps the most defining of all synapomorphies, a trait observed in the first dinosaurs and passed on to all descendants, was an upright or erect posture. Dinosaurs stood with their legs directly underneath their bodies, as is also seen in most mammals, but which is dissimilar to the sprawling stance seen in ancient reptiles. This posture developed due to the evolution of a specialized hip bone and joint, specifically the three bones that make up the pelvis and the opening in which the balled top of the thigh bone sits. Not only is this hip joint used to distinguish dinosaur species from other archosaurs, differences in the orientation of this basic structure are instrumental in classifying dinosaurs into subgroups.

Although fossilized footprints, skin impressions, and feather-like structures provide additional insight and information on dinosaur species and assist paleontologists in defining subgroups of species within Dinosauria, most dinosaur species are defined by their skeletons. A number of significant skeletal modifications are indicative of dinosaur species, such as modified fourth and fifth digits on the hands, only three toes on their feet, a specialized ridge on the humerus and tibia bones, at least three sacral vertebrae, a femur with a ball-like head at one end, and an open hip socket.

Evolutionary Divergences

Dinosaur ancestry dates back more than 230 million years, a time when all land masses were joined as the Pangaea supercontinent. Dinosaurs and the closely related crocodilians and pterosaurs are grouped together as archosaurs. The traits shared by all archosaurs indicate that their common ancestor was likely a small bipedal carnivore with forelimbs shorter than hind limbs, somewhat similar to Lagosuchus. The breakup of the supersized continent, however, saw the rapid diversification and radiation of dinosaurs, including the development of characteristics that separated dinosaurs from other early archosaurs. Dinosaurs are divided into one of two orders based on hip orientation, the Ornithischia (or “bird-hipped” dinosaurs) and the Saurischia (or “lizard-hipped” dinosaurs). Although the oldest known dinosaurs were restricted to southern Pangaea, specifically the rare ornithischians and abundant basal saurischians, by the Late Triassic, dinosaurs such as the saurischian prosauropods and coelophysoids had dispersed across the entire terrestrial landscape. Over many millions of years, the two main dinosaur groups evolved into many diverse species with varied physiologies and anatomical features. The Ornithischia order is comprised of two main groups—the armored, back-plated Thyreophora dinosaurs, which include the well-known ankylosaurs and stegosaurs, and the duck-billed and horned Cerapoda, which include the horned and bone-headed Marginocephalia and the herbivorous duck-billed Ornithopoda dinosaurs.

The Saurischia order is, in turn, comprised of two major groups: large herbivorous sauropods, such as Amphicoelias, Argentinosaurus, Diplodocus, Brachiosaurus,Apatosaurus, and Titanosaurus, and the predominately carnivorous theropods, such as Tyrannosaurus, Allosaurus, Spinosaurus, Giganotosaurus, Oviraptor, and Velociraptor. Also included in Theropoda are modern-day birds, which are, in fact, descended from the lizard-hipped dinosaurs and not the bird-hipped dinosaurs, contrary to what might be guessed based on the names. This is known as convergent evolution, whereby the backward pointing pubis seen in ornithischian dinosaurs and in birds actually evolved separately.

Creatures in This Group

Cladistics is a system that analyzes evolutionary relationships between organisms by grouping them into clades. This system is much more suited to classifying dinosaurs, as it can compensate for the poor fossil record that paleontologists must use. Although many of the terms and groupings used in cladistics are the same used in the Linnaean system, cladistics is considered the more “up-to-date” method of classification and is more often used by paleontologists.

The Dinosauria superorder contains two basic divisions, or orders—Ornithischia and Saurischia. These groups evolved over millions of years into distinct species with varied features and physiologies. Most notably, the two orders are separated by hip orientation, or, more accurately, the evolution of the pelvis—dinosaurs with a birdlike hip structure made make up Ornithischia, while “lizard-hipped” dinosaurs make up Saurischia.

Based on cladistics, Ornithischia is subdivided into the unranked clade Thyreophora (armored herbivores) and the suborder Cerapoda (duck-billed and horned dinosaurs), both of which fall under the unranked Genasauria clade. The horned and armored dinosaurs of Thyreophora include the well-known ankylosaurs and stegosaurs. Ankylosauria is an infraorder of widespread, herbivorous dinosaurs found from the Jurassic to the Cretaceous, and which include Ankylosaurus, Edmontonia, Pinacosaurus, and Euoplocephalus. The herbivorous dinosaurs of the infraorder Stegosauria, found from the Late Jurassic to the Early Cretaceous, are known for their plated backs, and famously include the dinosaur Stegosaurus. Less well known genera from the Stegosauria infraorder include Huayangosaurus, Kentrosaurus, Lexovisaurus, Tuojiangosaurus and the basal genus Scutellosaurus.

The two main subgroups of Cerapoda are the infraorder Ornithopoda (“bird foot”) and the unranked group Marginocephalia (“fringed heads”). The bipedal herbivores of Ornithopoda contain the duck-billed Hadrosauridae family, containing genera such as Hadrosaurus, Edmontosaurus, and Parasaurolophus, common in the late Cretaceous period in Asia and North America; the Iguanodontidae family, which includes various species of Iguanodon from the Late Jurassic to the Cretaceous; Heterodontosaurus, basal genus possibly existing as early as the Late Triassic; and the Hypsilophodontidae, a widespread family of herbivores from the Middle Jurassic to Late Cretaceous. The Marginocephalia include the infraorders Pachycephalosauria, thick-skulled, bipedal dinosaurs such as Stegoceras and Homalocephale of the Pachycephalosauridae family, found in the Late Cretaceous Period in Asia and North America, and Ceratopsia, which include horned and frilled dinosaurs such as Triceratops from the Ceratopsidae family, found in Asia and North America during the Cretaceous, Protoceratops from the Protoceratopsidae family, and Psittacosaurus from the Psittacosauridae family.

The Saurischia order contains all of the carnivorous dinosaurs (the suborder Theropoda, including Tyrannosaurus rex), as well as a lineage of herbivorous dinosaurs, the long-necked Sauropodomorpha suborder. The diverse Theropoda contains many lineages and is subdivided into the Ceratosauria infraorder and Tetanurae, an unranked clade. The Ceratosauria are relatively primitive theropods, including the Ceratosauridae and the Abelisauridae families, while Tetanurae are more advanced theropods such as the Megalosauroidea superfamily and the unranked clade Avetheropoda from which Gasosaurus and the Carnosauria infraorder branch.

The Sauropodomorpha includes the infraorder Sauropoda, which contains the nodes Eusauropoda, made up of larger sauropods with wide rounded snouts, longer necks, and smaller heads like Cetiosaurus; the unranked clade Nesosauropoda, including the Late Jurassic to Cretaceous Diplodocus; the family Camarasauridae, including western United States Camsarasaurus from the Late Jurassic; the unranked clade Titanosauriformes, thought to be the heaviest sauropods branched to the families; Brachiosauridae, including Brachiosaurus found in Africa, the western United States, and Portugal in the Late Jurassic; Titanosauridae, including Titanosaurus from the Late Cretaceous in India; the Lithostrotia (thought by some to be included in the Titanosauriformes clade); and Saltasauridae, including Saltasaurus from Argentina during the Cretaceous.

Ecology

As with all living organisms, the true origin of dinosaur life began billions of years ago when Earth was a seething cauldron of gases and volcanic eruptions. Much later, the evolution of amniotic organisms capable of laying eggs outside the aquatic environment occurred. This seemingly minor development opened up the terrestrial landscape for habitation and is thought to be one of the most important adaptations for the advancement of all vertebrate life; and from this, over many millions of years, evolved perpetually more complex organisms.

All nonavian dinosaur species were egg-laying (oviparous) animals, but not many appear to have exhibited advanced parental care (such as the pre- and post-hatching behavior seen in birds). One probable exception is Oviraptor, which is thought to exhibit brooding behavior, a behavior at odds with most other dinosaur species that likely laid their eggs in covered mounds similar to today's reptiles. Many paleontologists consider the evolution of specialized hip joints as being a major advance for dinosaur species in terms of survival and competition with other early reptiles. This adaptation was instrumental in the development of their upright stance, which gave dinosaurs a competitive edge in terms of locomotion and speed.

One of the enduring questions pondered by researchers is how such massive creatures as dinosaurs managed to coexist in a resource-limited environment. To deal with such intense competition, dinosaurs evolved different dietary habits and filled different ecological niches. Dinosaur species exhibited many different feeding preferences, from carnivores to herbivores and everything in between.

Paleontology News

The story of the dinosaurs is one that is forever evolving. New fossil discoveries continue to shed light on what life was like many millions of years ago, sometimes changing fundamental ideas, sometimes solidifying others. Dinosaur bones have been uncovered for many thousands of years, with various cultures believing them to be monsters, dragons, or even the remains of giant humans. Although the first scientific description of a dinosaur was undertaken in 1824 by geologist William Buckland, the actual term “Dinosauria” was not introduced until 1842 by the English paleontologist Richard Owen, who recognized shared characteristics between the dinosaur fossils thus far discovered, namely Iguanodon, Megalosaurus, and Hylaeosaurus.

Further discoveries, particularly the first dinosaur found in the United States, led to the famous “Bone Wars,” the competition between bitter rivals Edward Drinker Cope and Othniel Charles Marsh to discover dinosaur bones in the West. These two fossil hunters are responsible for the discovery of more than 140 species between them, a sizeable contribution considering that in the entire history of dinosaur discovery on every continent, only about 700 species have so far been described. Dinosaur discoveries in places such as China and South America have prompted a resurgence in dinosaur science and an increased understanding of the evolutionary story of these animals. Of particular interest has been the continued unearthing of feathered dinosaurs in China, providing pivotal evidence linking the ancestry of avian and nonavian dinosaurs.

Bibliography

Books

Fastovsky, David E., and David B. Weishampel. Dinosaurs: A Concise Natural History. New York: Cambridge University Press, 2009.

• Focuses on many aspects of the natural sciences and how they relate to dinosaur biology, evolution, life history, and classification.

--. Evolution and Extinction of the Dinosaurs. New York: Cambridge University Press, 2005.

• Covers complex scientific processes and dinosaur species in an informative and comprehensible way.

Lucas, Spencer George. Dinosaurs: The Textbook. 4th ed. New York: McGraw-Hill, 2004.

• Provides a concise summary of many important aspects of dinosaurs, including their discovery, behavior, biology, and classification.

Parker, Steve. Dinosaurus: The Complete Guide to Dinosaurs. Richmond, ON: Firefly Books, 2009

• Provides useful information on more than 500 dinosaurs, such as discovery and location and characteristics like anatomy and diet.

Weishampel, David B., Peter Dodson, and Halszka Osmólska. Dinosauria. Berkley: University of California Press, 2007.

• Provides resources and scientific papers on dinosaurs, especially looking at the saurischia and ornithisia orders in regards to their evolution, distribution, and ecology.

Journals

Brusatte, Stephen L., et al. “The Origin and Early Radiation of Dinosaurs.” Earth-Science Reviews 101.1–2 (2010): 68–00.

• A discussion of the origin of dinosaurs and how they spread into new habitats.

Langer, Max C., Martin D. Ezcurra, Jonathas S. Bittencourt, and Fernando E. Novas. “The Origin and Early Evolution of Dinosaurs.” Biological Review 85 (2010): 55–110.

• Discusses how dinosaurs originated from fully bipedal ancestors and how their origins help to inform palaeobiology.

Nesbitt, Sterling J. “The Early Evolution of Archosaurs: Relationships and the Origin of Major Clades.” Bulletin of the American Museum of Natural History 352 (2011): 1–292.

• Discusses the origin and evolution of archosaurs, which includes crocodilians, pterosaurs (flying reptiles), and dinosaurs. Includes phylogenetic analysis of approximately 80 taxonomic groups.

Additional Works Used

Books

Martin, Anthony J. Introduction to the Study of Dinosaurs. Malden, MA: Blackwell, 2006.

Norell, Mark, et al. Discovering Dinosaurs: Evolution, Extinction, and the Lessons of Prehistory. Berkeley: University of California Press, 1995.

Journals

Brusatte, S L., R. B. J. Benson, and X. Xu. “The Evolution of Large-bodied Theropod Dinosaurs during the Mesozoic in Asia.”Journal of Iberian Geology 36.2 (2010): 275–96.

—, M. J. Benton, G. T. Lloyd, M. Ruta, S. C. Wang. “Macroevolutionary Patterns in the Evolutionary Radiation of Archosaurs (Tetrapoda: Diapsida).” Earth and Environmental Science Transactions of the Royal Society of Edinburgh 101 (2011): 367–82.

Butler, Richard J., Paul Upchurch, and David B. Norman. “The Phylogeny of the Ornithischian Dinosaurs.” Journal of Systematic Palaeontology 6.1(2008): 1–40.

Jacobsen, Aase Roland. “Feeding Behaviour of Carnivorous Dinosaurs as Determined by Tooth Marks on Dinosaur Bones.” Historical Biology: An International Journal of Paleobiology 13.1 (1998): 17–26.

Nesbitt, S. J. Arizonasaurus and Its Implications for Archosaur Divergence. Proc. R. Soc. Lond. B 207 (2003): S234–37.

Ősi, Attila, Sebastián Apesteguía, and Michał Kowalewski. “Non-avian Theropod Dinosaurs from the Early Late Cretaceous of Central Europe”. Cretaceous Research 31.3 (2010): 304–20.http://www.sciencedirect.com/science/article/pii/S0195667110000029

Sereno, P. C. “Basal Archosaurs: Phylogenetic Relationships and Functional Implications.” Journal of Vertebrate Paleontology 11.S4 (1991): 1–53.

Taylor, Michael. “Sauropod Dinosaur Research: A Historical Review.” Geological Society, London 343 (2010): 361–86.

Varricchio, D. J., et al. “Mud-trapped Herd Captures Evidence of Distinctive Dinosaur Sociality.” Acta Palaeontologica Polonica 53.4 (2008): 567–78.