The South American Late Triassic offers the most comprehensive window to the early radiation of dinosaurs. This is enhanced by the discovery of Sacisaurus agudoensis, a new dinosauriform from the Caturrita Formation of Brazil. Various morphological features suggest its close phylogenetic affinity to Silesaurus, and both may be basal ornithischian dinosaurs. Sacisaurus has a pair of elements forming the tip of its lower jaw, hypothesized to be equivalent to the ornithischian predentary. This suggests that during an initial stage of their evolution, those dinosaurs had a paired predentary, which later fused into a single structure. As an originally paired bone, the predentary is comparable to elements that more often form the vertebrate mandible, such as the mentomeckelian bone. Although synapomorphic for ornithischians, the predentary does not seem neomorphic for the group, but primarily homologous to parts of the symphyseal region of the lower jaw of other vertebrates.
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Document Type: Research Article
Museu de Ciências Naturais, Fundação Zoobotânica do Rio Grande do Sul, Porto Alegre, Brazil
Faculdade de Filosofia Ciências e Letras de Riberão Preto, Universidade de São Paulo-USP, Ribeirão Preto, Brazil
Publication date: 01 March 2007
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