The Dinosauria, Second Edition / Edition 2 available in Paperback
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By David B. Weishampel, Peter Dodson, Halszka Osmólska
UNIVERSITY OF CALIFORNIA PRESSCopyright © 2004 the Regents of the University of California
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Origin and Relationships of Dinosauria
MICHAEL J. BENTON
Dinosauria is a well-diagnosed clade, and since birds are included within it the group is clearly significant among terrestrial vertebrates. Dinosaurs belong within Archosauria, a broader clade that also includes crocodilians as well as pterosaurs and various basal groups of Triassic age. Over the past thirty years considerable effort has been devoted to disentangling the phylogeny of archosaurs: some relationships have been firmly established, while others have yet to be discovered. This chapter considers the origin of the dinosaurs in terms of phylogeny and the timing of events. The primary evidence comes from a cladistic analysis of the Triassic archosaurs, a paraphyletic group formerly known as thecodontians. That analysis is followed by an account of the evolutionary events that led to the radiation of the most astonishing animals of all time, the dinosaurs themselves.
Previous Cladistic Analyses of Dinosauria
Paleontologists have been interested in the phylogeny of the dinosaurs since their discovery in the 1820s. However, confusion reigned over many aspects of dinosaurian phylogeny until the 1980s, when cladistics began to be applied. The use of cladistic methods led to the solution of many phylogenetic conundrums, but others have continued to defy resolution.
Cladistics as applied to dinosaurs and their close relatives began in the early 1980s with a number of basic studies in which cladograms were compiled by hand from incomplete lists of sometimes poorly delimited characters. In the late 1980s and 1990s data matrices were analyzed by computer algorithms, and evidence was often given of the goodness of fit of particular parts of the tree to the data.
Further developments in cladistic practice have occurred since the publication of the first edition of The Dinosauria in 1990. Some workers have advocated the use of formalized systems for naming and diagnosing taxa. Both before 1990 and since, names have been applied to nodes in cladograms according to the whim of the systematists. Some workers name every node in a cladogram, while others prefer to name only those nodes that are robust (i.e., supported by a great deal of character evidence). In addition, there are differences in the choice of names for clades: some systematists prefer to retain well-known group names that most closely match the traditional understanding of contents and/or diagnostic characters, while others advocate the migration of widely used names to clades subtended by extant groups, the crown-clade concept.
There have also been important developments in the definition and diagnoses of taxa. Until 1990 most cladists equated taxa with their characters (e.g., Aves is diagnosed as those organisms that possess primary flight feathers and wings). Since then, a clear distinction has been proposed between the definitions and diagnoses of clades (de Queiroz and Gauthier 1992, 1994). Clades are diagnosed by characters that evolve at or immediately prior to their origin but do not make individual clades "what they are." Instead, individual clades are named on the basis of their membership. Named taxa are then fixed on the basis of their ancestry.
There are three kinds of definitions of taxon names: apomorphy-based, stem-based, and node-based. An apomorphy-based definition is founded on one or more derived characters (e.g., "Aves consists of all archosaurs with feathered wings used for powered flight"). Apomorphy-based taxa combine definition and diagnosis. Stem- and node-based definitions include ancestors as the basal member of the clade (fig. 1.1). A stembased taxon name refers to a clade that includes all the descendants from a particular cladogenetic event. An example of stem-based definition for Ornithischia is "all dinosaurs more closely related to Triceratops than to Tyrannosaurus." A node-based taxon name is one that is defined by a basal node. For example, a node-based definition for Aves takes the form "Archaeopteryx, Neornithes, their most recent common ancestor, and all descendants." Because we do not know all of the stem taxa in stembased taxa, their diagnoses are not possible. Node-based taxa can be diagnosed on the basis of the derived features that have evolved in or immediately prior to the ancestor of the clade (this practice has been applied to stem-based clades as well). A crown clade is a node-based clade defined solely on the basis of extant forms. For example, the crown group Mammalia is defined as the common ancestor of Ornithorhynchus (platypus) and Homo (humans) and all the descendants of this common ancestor.
The crown-group issue affects the discussion of basal archosaurs and the origin of dinosaurs since certain names have different meanings for different people. Cope (1869a) introduced the name Archosauria for a wide group of extinct and extant reptiles, including anomodonts and rhynchosaurs, and no one has proposed a strict return to his view. However, for most of the twentieth century the name was used to refer to a group that includes modern crocodilians, as well as dinosaurs, pterosaurs, and a variety of Triassic forms, back to the Early Triassic proterosuchids and erythrosuchids, the earliest representative being Archosaurus, from the Late Permian (Tatarian) of Russia (e.g., Romer 1945, 1956, 1966; Hughes 1963; Charig and Reig 1970; Cruickshank 1972; Bonaparte 1982; Carroll 1988; Juul 1994; Gower and Wilkinson 1996; Benton 1997). Archosauria in this sense turns out to include those diapsids that possess an antorbital fenestra—an apomorphy-based taxon.
Gauthier (1986), urging use of the crown-clade concept, defined Archosauria as the clade subtended by living archosaurs (birds and crocodilians), in other words, excluding proterosuchids, erythrosuchids, proterochampsids, and euparkeriids. He argued that this realignment of the name had the advantage that all archosaurs, including fossil forms, would then have predictable soft-part characters (based on extrapolation from living forms). The apomorphy-based Archosauria was then renamed Archosauriformes (Gauthier 1986). Archosauria could also be defined as a stem-based taxon by extending membership down the stem to the next known node, but this approach has not yet been proposed.
Archosauria is treated as an apomorphy-based clade here, with note made to node- or stem-based definitions where relevant. Phylogenetic studies of dinosaurs in their archosaur context have led to the following generally accepted conclusions:
1. Archosauria is monophyletic. This view has been held generally for a long time, although the clade has been regarded as hard to diagnose anatomically (e.g., Romer 1956, 1966) and hence possibly polyphyletic.
2. Archosauria includes a number of basal Triassic forms that are sister groups to Avesuchia sensu Benton (1999), also known as crown group Archosauria sensu Gauthier (1986). The clade consists of two lines, one leading to crocodilians, the other to birds. This split was hinted at by Bonaparte (1975a), Krebs (1976), Cruickshank (1979), and Chatterjee (1982), and it has been confirmed in all subsequent cladistic analyses.
3. The crocodile line, Crurotarsi (Sereno 1991a), consists of Phytosauridae, Ornithosuchidae, Prestosuchidae, Rauisuchidae, Poposauridae, and Crocodylomorpha, but the relationships among those groups are contentious (Gauthier 1986; Benton and Clark 1988; Sereno 1991a; Parrish 1993; Juul 1994; Benton 1999).
4. The bird line, Avemetatarsalia (Benton 1999), consists of Scleromochlus, Pterosauria, and Dinosauromorpha. The South American forms, Lagerpeton and Marasuchus, are close outgroups of Dinosauria (Gauthier 1986; Benton and Clark 1988; Sereno 1991a; Sereno and Arcucci 1993, 1994; Juul 1994; Novas 1996a; Benton 1999). Avemetatarsalia is similar but not equivalent to Ornithosuchia sensu Gauthier (1986) because the former does not include Ornithosuchidae, a clade thought to be included in Crurotarsi (Benton 1999).
5. Dinosauria is monophyletic. Up to 1985, dinosaurs were usually seen as polyphyletic, with as many as three to six ancestors (e.g., Romer 1966; Charig and Reig 1970; Charig 1976a; Krebs 1976; Cruickshank 1979; Thulborn 1980; Chatterjee 1982). Bakker and Galton (1974) and Bonaparte (1976) argued for dinosaurian monophyly before such views became generally accepted (e.g., Gauthier 1986; Benton and Clark 1988; Sereno 1991b, 1997; Sereno and Arcucci 1993, 1994; Juul 1994; Novas 1996a; Benton 1999).
6. Dinosauria includes two clades, Saurischia and Ornithischia. This view has been generally held since Seeley's (1887a) invention of the two names, although he based Saurischia on what would now be seen as a plesiomorphy, namely, the "reptilian" pelvic arrangement. Apomorphies for both clades were given by Gauthier (1986).
7. Saurischia comprises several basal taxa, as well as Theropoda and Sauropodomorpha. These two clades were formerly regarded as having arisen from two or more independent sources among the basal archosaurs (e.g., Charig et al. 1965; Charig 1976b; Cruickshank 1979; Chatterjee 1982), a pattern for which there is no evidence (Gauthier 1986).
8. Ornithischia comprises several basal forms, as well as Thyreophora and Cerapoda (Sereno 1986, 1997, 1998; Weishampel and Witmer 1990a). Thyreophora includes the armored Stegosauria and Ankylosauria, as well as several basal taxa. Cerapoda splits into Marginocephalia (Pachycephalosauria and Ceratopsia) and Ornithopoda (Heterodontosauridae and Euornithopoda).
Archosauria and Its Evolutionary Context
Among living vertebrates, birds and crocodilians are linked as sister groups within the Avesuchia/crown group Archosauria (fig. 1.2). Although seemingly different kinds of animals, these two groups share numerous derived characters of the skull, postcranial skeleton, and soft parts that are absent in other living vertebrates (Gauthier 1986; Benton and Clark 1988). Early molecular studies of the phylogeny of tetrapods were equivocal regarding the nature of Archosauria, and many analyses of protein sequences supported a close pairing of Aves and Mammalia (e.g., Bishop and Friday 1988; Hedges et al. 1990; reviewed in Benton 1990b). More recently, the validity of Archosauria has been accepted based on sequencing of nucleic acids (e.g., Janke and Arnason 1997; Hedges and Poling 1999), although this has been challenged by the suggestion that turtles belong to Archosauria.
Archosauria is included in the larger clade Diapsida (living crocodilians, birds, Sphenodon, lizards, snakes, and their extinct relatives), one of the major clades of tetrapods. The crown group Tetrapoda, four-limbed vertebrates, includes Amphibia and various basal groups, as well as Amniota (Reptilia + Aves + Mammalia). Amniota comprises Anapsida, Diapsida, and Synapsida, diagnosed by the nature of their temporal openings, among other characters (Laurin and Reisz 1995; Benton 1997). The stability of this division of Amniota has been challenged by some morphological and molecular results that suggest that Anapsida should be subsumed within Diapsida: Rieppel and de Braga found that turtles may have been close relatives of lepidosaurs (Sphenodon, lizards and snakes) based on morphological characters (Rieppel and de Braga 1996; de Braga and Rieppel 1997), although their results have been queried (Wilkinson et al. 1997), while complete mitochondrial DNA sequencing suggests that turtles are the sister group to Archosauria (Zardoya and Meyer 1998; Hedges and Poling 1999; Kumazawa and Nishida 1999). These dramatic proposals have not yet been fully tested (reviewed in Rieppel 2000).
Archosauria was established by Cope (1869a) for a broad grouping of amniotes: Crocodylia, Thecodontia, Dinosauria, Anomodontia (i.e., dicynodonts + dinocephalians), and Rhynchocephalia (i.e., sphenodontids + rhynchosaurs). Cope (1869b) then restricted Archosauria to include Dinosauria, Phytosauria, Crocodylia, and Rhynchocephalia, excluding anomodonts. In the 1890s Cope and Baur independently developed the "theory of fenestration," according to which the major lines of amniote evolution could be identified by the numbers of temporal fenestrae, whether none (Anapsida), one (Synapsida), or two (Diapsida), the last two group names introduced by Osborn (1903) to reflect the new phylogenetic ideas.
Osborn (1903) rejected Cope's Archosauria, instead referring archosaur groups to a number of separate sections within Diapsida. This view was followed by many workers until the 1930s, with Broom, von Huene, Haughton, and others referring Triassic archosaurs to Pseudosuchia, Parasuchia, and Protorosauria. Romer (1933) resurrected Archosauria for Thecodontia, Crocodylia, Pterosauria, Saurischia, and Ornithischia but included proterosuchids in Eosuchia within Lepidosauria. Proterosuchids and erythrosuchids were replaced in Archosauria in later works (Romer 1945, 1956, 1966), and this has been the generally accepted viewpoint since (e.g., Hughes 1963; Charig and Reig 1970; Cruickshank 1972; Bonaparte 1982; Carroll 1988; Juul 1994; Gower and Wilkinson 1996; Benton 1997).
Archosauria sensu Benton 1999 is nested within larger clades, the most significant of which are Archosauromorpha and Diapsida. Diapsida comprises some basal taxa, principally Araeoscelidia (Petrolacosaurus, Araeoscelis, and relatives) from the Late Carboniferous and Early Permian, and two major clades, Lepidosauromorpha and Archosauromorpha, which diverged presumably early in the Permian (Benton 1983a, 1984b, 1985; Evans 1984, 1988; Gauthier 1986; Benton and Clark 1988; Laurin 1991; Laurin and Reisz 1995; Dilkes 1997).
Archosauromorpha has been given node- and stem-based definitions. According to the former, Archosauromorpha is defined as the most recent common ancestor of Neornithes (extant birds), Squamata (extant lizards and snakes), and all of the descendants of this common ancestor. Accordingly, the clade comprises Trilophosaurus, Rhynchosauria, Prolacertiformes, and Archosauria, as well as probably thalattosaurs, choristoderans, and drepanosaurids (Benton 1985; Benton and Clark 1988; Laurin 1991; de Braga and Rieppel 1997; Dilkes 1997). Trilophosaurus is an unusual Late Triassic herbivore with no infratemporal openings. Rhynchosaurs are a distinctive clade of Triassic herbivores that had multiple tooth rows and beaklike premaxillary projections. Prolacertiforms are long-necked insectivores and carnivores known from the mid-Permian to the Late Triassic that may comprise an unnatural grouping of diverse archosauromorph taxa (Dilkes 1997). Most cladistic analyses agree that among these archosauromorphs Prolacerta, a prolacertiform, is the sister group of Archosauria and that Rhynchosauria is a more distant outgroup. Other probable archosauromorphs include thalattosaurs, marine Triassic forms, choristoderans, superficially crocodilian-like aquatic animals known from the Triassic to the mid-Tertiary, and drepanosaurids, Late Triassic swimmers.
A stem-based definition for Archosauromorpha (the most recent common ancestor of Prolacerta, Trilophosaurus, Hyperodapedon, and archosaurs and all its descendants) was provided by Laurin (1991:90). However, this definition excludes Protorosaurus, Drepanosauridae, and Tanystropheidae, according to Dilkes's (1997) cladogram. Dilkes (1997:528) gave a revised stembased definition for Archosauromorpha, namely, Protorosaurus and all other saurians that are related more closely to Protorosaurus than to Lepidosauria, but this definition is rejected here since it would refer to a much more restricted grouping according to other cladograms of basal diapsids: it is debated whether "Protorosauria," Rhynchosauria, or Trilophosaurus is the basal archosauromorph taxon (e.g., Benton 1985; Chatterjee 1986a; Evans 1988; Laurin 1991). The present definition can cope with all the competing cladograms and still refers to the same clade contents (Trilophosaurus + Rhynchosauria + "Protorosauria" + Prolacertiformes + Archosauria).
Excerpted from The Dinosauria by David B. Weishampel, Peter Dodson, Halszka Osmólska. Copyright © 2004 the Regents of the University of California. Excerpted by permission of UNIVERSITY OF CALIFORNIA PRESS.
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