Early dinosaurs: A phylogenetic study

Abstract

SYNOPSIS Early dinosaur evolution has been the subject of several phylogenetic studies and the position of certain basal forms is currently debated. This is the case for the oldest known members of the group, excavated from the Late Triassic Ischigualastian beds of South America, such as Herrerasaurus, Eoraptor, Pisanosaurus, Saturnalia and Staurikosaurus. A new cladistic analysis of the early dinosaur radiation was performed to assess the relationships among the three major clades (Ornithischia, Sauropodomorpha and Theropoda) and to define the phylogenetic position of the basal members of the group. The most parsimonious hypothesis has Silesaurus opolensis as the sister taxon to a dichotomy including monophyletic Saurischia and Ornithischia. The latter includes Pisanosaurus mertii, and the former all other well-known Triassic dinosaurs. Saurischia is composed of two major monophyletic groups: Herrerasauridae (including Herrerasaurus ischigualastensis and Staurikosaurus pricei) and Eusaurischia (including the theropod and sauropodomorph lineages), while Eoraptor lunensis appears to represent the sister taxon to Eusaurischia. Saturnalia tupiniquim is a stem-taxon to Sauropodomorpha and Guaibasaurus candelariensis might belong to the theropod branch. Some of these hypotheses are, however, not strongly supported. Especially uncertain are the affinities of Silesaurus and Guaibasaurus. The latter can only be safely regarded as a saurischian, while the former might belong to the ornithischian lineage. The dinosaurian affinities of Eoraptor and Herrerasauridae are strongly supported. Yet, the possibility that they (especially Eoraptor) represent basal theropods, rather than basal saurischians, cannot be dismissed. In fact, basal saurischian evolution is still too poorly understood for a definitive hypothesis of relationships to be presented.

The origin and early radiation of dinosaurs

“…The advent of numerical cladistic analyses in the mid 1980s crystallized support for both dinosaur monophyly and the hypothesis that birds evolved from theropod dinosaurs (e.g., Gauthier, 1986;Benton and Clark, 1988;Sereno, 1991a). Today, higher-level phylogenetic analyses continue to find robust support for dinosaur monophyly (e.g., Juul, 1994;Benton, 1999Benton, , 2004Sereno, 1999;Ezcurra, 2006;Langer and Benton, 2006;Irmis et al, 2007a;Brusatte et al, 2008a;Nesbitt et al, 2009b;Brusatte et al, 2010b;Nesbitt et al, 2010), although the exact characters diagnosing the dinosaur group continue to change as new fossils are found and old ideas are reinterpreted.…”

“…Over 50 characters have been cited as dinosaur synapomorphies in both pre-cladistic and cladistic studies (Bakker and Galton, 1974;Benton, 1984;Gauthier, 1986;Benton and Clark, 1988;Novas, 1989;Sereno, 1991a;Novas, 1992;Sereno and Novas, 1994;Novas, 1996;Benton, 1999;Sereno, 1999;Fraser et al, 2002;Langer and Benton, 2006;Irmis et al, 2007a;Nesbitt et al, 2009bNesbitt et al, , 2010Brusatte et al, 2010b). Potential dinosaur characteristics are distributed throughout the body.…”

“…Epipophyses are projections of bone, likely for muscle and ligament attachment, which protrude from the dorsal surfaces of the postzygapophyses of the cervical vertebrae. All basal dinosaurs possess epipophyses (Langer and Benton, 2006), although the size, shape, length, and projection angle of these processes vary considerably (e.g., compare Coelophysis (Colbert, 1989) with the more derived theropod Majungasaurus (O'Connor, 2007)). Basal ornithischians (e.g., Heterodontosaurus) only have epipophyses on the anterior cervical vertebrae, whereas saurischians have epipophyses in nearly all cervical vertebrae (Langer and Benton, 2006).…”

“…All basal dinosaurs possess epipophyses (Langer and Benton, 2006), although the size, shape, length, and projection angle of these processes vary considerably (e.g., compare Coelophysis (Colbert, 1989) with the more derived theropod Majungasaurus (O'Connor, 2007)). Basal ornithischians (e.g., Heterodontosaurus) only have epipophyses on the anterior cervical vertebrae, whereas saurischians have epipophyses in nearly all cervical vertebrae (Langer and Benton, 2006). Epipophyses are not present in the closest relatives of dinosaurs (e.g., Marasuchus, Silesaurus), but are present in some crurotarsans (e.g., Lotosaurus and Revueltosaurus).…”

“…Reanalysis of existing specimens has improved our understanding of character evolution on the lineage leading to Dinosauria (e.g., Sereno and Arcucci, 1994a,b;Langer and Benton, 2006;Brusatte et al, 2010b) and has changed our understanding of the distribution of early dinosaurs in time and space (Parker et al, 2005;Irmis et al, 2007b;Nesbitt et al, 2007). Most recently, quantitative analyses, which take into account this avalanche of new morphological and geological data, have examined in unprecedented detail the macroevolutionary, biogeographical, and paleoecological changes associated with the rise of dinosaurs (e.g., Brusatte et al, 2008a,b;Nesbitt et al, 2009bNesbitt et al, , 2010.…”