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Journal Article

Early Evolution and Higher-Level Phylogeny of Sauropod Dinosaurs

Jeffrey A. Wilson and Paul C. Sereno
Memoir (Society of Vertebrate Paleontology)
Vol. 5, Early Evolution and Higher-Level Phylogeny of Sauropod Dinosaurs (Jun. 15, 1998), pp. 1-68
DOI: 10.2307/3889325
Stable URL: http://www.jstor.org/stable/3889325
Page Count: 79

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Topics: Vertebrae, Spine, Foot bones, Taxa, Hand bones, Dinosaurs, Sacral spine, Phylogeny, We they distinction, Tibia
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Early Evolution and Higher-Level Phylogeny of Sauropod Dinosaurs
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Abstract

Although sauropods played a major role in terrestrial ecosystems during much of the Mesozoic Era, little effort has been directed toward diagnosing Sauropoda and establishing higher-level interrelationships among sauropods. As a consequence, the origin and evolution of major skeletal adaptations in sauropods has remained largely speculative. The cladistic analysis presented here focuses on higher-level relationships among sauropods. Based on 109 characters (32 cranial, 24 axial, 53 appendicular) for 10 sauropod taxa, the most parsimonious arrangement places four genera (Vulcanodon, Shunosaurus, Barapasaurus, and Omeisaurus) as a sequence of sister-taxa to a group of advanced sauropods, defined here as Neosauropoda. Neosauropoda, in turn, is composed of the sister-clades Diplodocoidea and Macronaria; the latter is a new taxon that includes Haplocanthosaurus, Camarasaurus, and Titanosauriformes. Titanosauriformes includes Brachiosauridae and Somphospondyli, a new taxon uniting Euhelopus and Titanosauria. Among macronarians, the position of Haplocanthosaurus is the least stable as a result of the absence of cranial remains. The basic structure of the phylogeny is resilient to various tests and establishes the evolutionary sequence of many functionally significant sauropod adaptations, such as the digitigrade posture of the manus in neosauropods. Other characteristic sauropod adaptations, such as narrow tooth crowns, increases in length and number of cervical vertebrae, and bifid neural spines, are shown to have evolved more than once. As these results underscore, the higher-level phylogeny of sauropods must be based on a broad sampling of character data. The fossil record of sauropods, although relatively limited during the early phase of the radiation (Late Triassic through Early Jurassic), nonetheless indicates that all major clades were established prior to the Late Jurassic, when substantial faunal interchange among major continental regions was still possible. The functional, temporal, and biogeographic implications of the higher-level phylogeny of sauropods are explored. /// Los saurópodos habrián sido un componente importante de las comunidades terrestes durante gran parte del Mesosoico; se ha dirigido poco esfuerzo al estudio de la diagnosis de Sauropoda o a establecer las relaciones entre los distintos subgrupos. Por esto, el origen y la evolución de sus adaptaciones esqueletales han sido especulativos. El análisis cladístico aquí presentado enfoca en las relaciones de alto-nivel de los saurópodos, sobre la base de 109 caracteres (32 cráneanos, 24 axiales y 53 apendiculares) de diéz grupos terminales. Se establece a los cuatro generos (Vulcanodon, Shunosaurus, Barapasaurus y Omeisaurus) como una sucesión de grupos-hermanos a un grupo de saurópodos avanzados, aquí definido como Neosauropoda. Neosauropoda es compuesto por los clados-hermanos Diplodocoidea y Macronaria; el último es un nuevo grupo que incluye Haplocanthosaurus, Camarasaurus y Titanosauriformes. Titanosauriformes incluye Brachiosauridae y Somphospondyli, un nuevo grupo unificando Euhelopus y Titanosauria. La ausencia de restos cráneanos hace menos segura la posición de Haplocanthosaurus entre macronarios. La estructura de la filogenía es confirmada por varios análisis, y pone de manifesto la evolución de adaptaciones importantes, como por ejemplo la mano digitigrada en los neosauropodos. Otras adaptaciones tradicionalemente mencionadas-tales como las coronas angostas, aumentos del largo y numero de vértebras cervicales y las espinas neurales dobles en los vértebras presacras-fueron adquiridas mas de una vez. Esto subraya la importancia de una prueba con amplio muestreo de caracteres para análisis filogenéticos de saurópodos. El registro fósil de los saurópodos, aunque relativamente limitado en los primeros tiempos de su radiación (Triásico tardío hasta Jurásico medio), indica que todos los clados mayores habrián estado establecidos antes del Jurásico tardío, cuando aún era posible el intercambio faunistico entre las mayores regiones continentales. Las implicaciones funcionales, temporales y biogeográficos de la filogenía de alto-nivel de los saurópodos son analizadas.

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