Superorder Dinosauria

Abstract

The dinosaurs are a distinctive group of reptiles, most closely related to the crocodiles. They have a close, but as yet still problematic relationship with birds. The group comprises two large orders, Saurischia and Ornithischia, originally distinguished on differences in the pelvis.

Keywords: dinosaur, Dinosauria, Saurischia, Ornithischia, Aves, Eoraptor, Herrerasaurus,

Introduction

Traditionally, dinosaurs were considered as reptiles because of the type of openings in their skulls and the structure of their pelvis.

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What is a Dinosaur?

Dinosaurs are a specific group of animals defined by a set of criteria, some of which are esoteric. In contrast to other reptiles, the dinosaur hind limb is swung under the body and moves in a fore-and-aft plane. The pelvis is completely vertical and the hip socket bears the weight of the articulation with the leg at the top of the socket, rather than on its inside as in most other tetrapods.

(See morphology, below.)

Within the group are recognised two great lineages which diverged extremely early in their evolution. These are recognised as orders and named Saurischia and Ornithischia. Saurischians include the carnivorous, bipedal theropods, and herbivorous, quadrupedal sauropods (‘brontosaurs’). Ornithischians were all herbivorous, and include bipedal ornithopods, and quadrupedal horned ceratopsians, armour-plated stegosaurs and ankylosaurs.

The prevailing view today is that birds belong to the saurischian clade, though the evidence is not yet thoroughly conclusive (read more). However, for the purposes of this discussion, the name "dinosaur" refers to the extinct, non-avian animals.

Saurischia

The apomorphies (derived characteristics) defining the Saurischia are a grasping forefoot with strongly offset thumb and second finger the longest, and the long flexible S-shaped neck. The forefoot is distinctive in primitive saurischians, but highly modified in more derived taxa such as sauropods (where they have become elephant-like) and – possibly – birds (where they may have become wings; read more). The neck is generally obvious, but has become highly compressed in carnosaurs.

Ornithischia

The ornithischians are defined by the presence of a process on the pelvis which points down and back along the ischium. the front teeth are small or absent, and are replaced at the front by a horny beak. In primitive forms the teeth are triangular with a series of cusps, though they become more complex in highly derived taxa.

Morphology

The dinosaur morphology is characterised by the absence of a postfrontal bone in the skull, an elongate crest on the humerus for muscles, three or fewer bones in the fourth finger of the hand and one in the fifth finger (if it is present at all), three or more vertebrae in the sacrum, an open hip-socket, ball-like head on the femur, a prominent forward projecting crest on the tibia, and a well-developed sheet of bone joining the astragalus (an ankle bone) to the tibia. Although some of these features may occur in other closely related reptiles (e.g. pterosaurs, Marasuchus), only in the dinosaurs do all of these features occur. (After Carpenter 2002.)

Phylogeny and Evolution

Affinities

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"More recently, dinosaurs are considered to occupy a grey area between traditional reptiles (specifically crocodiles) and traditional birds. This new interpretation is based in part upon the discovery of possible "feathers" on small carnivorous dinosaurs (theropods). Feathers have traditionally been considered an avian character; thus their discovery has added to the growing body of evidence that some dinosaurs were more closely related to birds than they were to other reptiles" (Carpenter 2002).

Fossil Record

Many characteristics of dinosaurs - including their large size and terrestrial habitat - conspire to produce a poor fossil record. The record becomes generally worse as it becomes older, and some intervals are particularly poorly represented. Even during the interval of maximum recorded dinosaur diversity, in the Late Cretaceous, only about forty or so taxa are known worldwide.

Origins

Despite the great differences between advanced ornithischians and saurischians, the primitive members share features demonstrating a common ancestry. This ancestor was probably related to the small, slender-limbed, bipedal dinosauromorphs Marasuchus (= Lagosuchus) and Lagerpeton from the Middle Triassic (~240 Ma) Chañares Formation of La Rioja, Argentina.

Evolution

The dinosaurs underwent rapid evolution, producing novel adaptive characters recognised as suborders, of which there are seven: Theropoda, Sauropodomorpha, Ornithopoda, Stegosauria, Pachycephalosauria, Ankylosauria, and Ceratopsia. Six of these had appeared within the first 50 million years, and the seventh by the last 35 million. None of the suborders went extinct until the end of the Cretaceous.

Initial diversification of the dinosaurs, taxonomically and in size, during the Late Triassic and Early Jurassic, is believed to have been allowed by a fortuitous opening of ecological niches.

"The initial geographical radiation of the dinosaurs was made possible by the continents having previously coalesced into the supercontinent Pangaea. Thus, as new adaptive designs appeared (recognized as suborders, such as Stegosauria), they spread globally very rapidly. Most of the major suborders had evolved by the end of the Early Jurassic (~178 Ma), with the exception of the ceratopsians and pachycephalosaurs. These two groups make their earliest appearance by the end of the Lower Cretaceous (~98 Ma), about the time that Pangaea had broken into two continents, the northern hemisphere Laurasia and the southern hemisphere Gondwana" (Carpenter 2002).

The Origins of Herbivory in Dinosaurs

"The first herbivorous dinosaurs arose from their carnivorous dinosaurian ancestors very early, certainly in the Carnian (Late Triassic, 235-220 Ma). New specimens from Brazil, named Saturnalia, are probably the oldest sauropodomorphs. These are currently under study in Bristol by Max Langer. Slightly younger is Thecodontosaurusfrom Bristol. This was the fourth dinosaur to be named from anywhere in the world, in 1836, and the first dinosaur to be reported from the Triassic. It was widely rumoured that all the specimens of Thecodontosauruswere lost when Bristol City Museum was bombed in 1940. However, some 250 individual bones remain. These show that Thecodontosauruswas a 2.5 m long slender biped. It falls in a basal position in the cladogram, according to a new cladistic study. This new work begins to shed some light on the origin of the sauropodomorphs, the group that later included the giant 50-tonne plant-eaters, like Brachiosaurusand Diplodocus" (Benton 1999).

Extinction

Dinosaurs famously died out at or about the end of the Cretaceous: the debates are endless though some key data are widely regarded as 'facts':

• As a group, dinosaurs were well and truely on the wane prior to the Cretaceous-Tertiary boundary.
• Although there is some evidence that a few populations may have limped into the Tertiary (Zhao et al. 2002, and references therein), dinosaurs are not ecologically significant after the Cretaceous-Tertiary boundary.

Systematics

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Phylum Vertebrata/Craniata

Class Reptilia ???

Superorder Dinosauria

Type: Iguanodon ??? Owen 1841 ???

Order Saurischia

<synonymy>

Type: [Taxon] [Authority]

Original Diagnosis: xxx

Description: xxx

Occurrence: xxx

Discussion: xxx

Summary of suborders etc. ...

Order Ornithischia

<synonymy>

Type: [Taxon] [Authority]

Original Diagnosis: xxx

Description: xxx

Occurrence: xxx

Discussion: xxx

Summary of suborders etc. ...

XING XU, YEN-NIEN CHENG, XIAO-LIN WANG & CHUN-HSIANG CHANG

Oviraptorosaurians are an unusual group of theropod dinosaurs, with highly specialized skulls. Here we report a new oviraptorosaurian, Incisivosaurus gauthieri, gen. et sp. nov., from the lowest part of the Lower Cretaceous Yixian Formation of China. This oviraptorosaurian displays a number of characters closer to more typical theropods, such as a low skull and toothed jaws, thus greatly reducing the morphological gap between oviraptorosaurs and other theropods. Incisivosaurus has a pair of premaxillary teeth resembling rodent incisors and small, lanceolate cheek teeth with large wear facets. These dental features were previously unknown among theropods and suggest a herbivorous diet. The new discovery provides a case of convergent evolution and demonstrates that non-avian theropods were much more diverse ecologically than previously suspected.

Theropoda Marsh, 1881
Maniraptora Gauthier, 1986
Oviraptorosauria Barsbold, 1976
Incisivosaurus gauthieri gen. et sp. nov.

Conclusion

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Further Information

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References

Benton, Mike 1999: The Origins of Herbivory in Dinosaurs. Oral presentation to Palaeontological Association, 43rd Annual Meeting, University of Manchester, 19-22 December 1999.

Carpenter, Kenneth 2002 (in press): Dinosauria (Dinosaurs). In Encyclopedia of Life Sciences. Nature Publishing Group, Macmillan.

Norell, Mark A.; Gaffney, Eugene S.; Dingus, Lowell 1995: Discovering Dinosaurs. Little, Brown and Company, 204 pp.

Xu, Xing; Makovicky, Peter J.; Wang, Xiao-Lin; Norell, Mark A.; You, Hai-Lu ??? date ???: A Ceratopsian Dinosaur from China and the Early Evolution of Ceratopsia. ??? source ???

Zhao, Zi-kui; Mao, Xue-ying; Chai, Zhi-fang; Yang, Gao-chuang; Kong, Ping; Ebihara, Mitsuru; Zhao, Zhen-hua 2002: A possible causal relationship between the extinction of dinosaurs and K/T iridium enrichment in the Nanxiong Basin, South China: evidence from dinosaur eggshells. Palaeogeography, Palaeoclimatology, Palaeoecology 178: 1-17.