Els dinosaures al segle XXI. 10 al 11 de Febrero del 2005 Cosmocaixa Barcelona Marco filogenético de las aves: historia evolutiva de los terópodos no avianos
PHYLOGENETIC CONTEXT FOR AVES: EVOLUTIONARY HISTORY OF NON-AVIAN THEROPODS
José Ignacio CANUDO
Grupo de Investigación Aragosaurus (http://www.aragosaurus.com/). Paleontología. Universidad de Zaragoza. 50009 Zaragoza [email protected]
In the middle of XIX century Huxley compared the feathered Archaeopteryx to fossil reptiles as well as to living birds: "no two groups of beings can appear to be more entirely dissimilar than reptiles and birds". And then he asked: Are any fossil reptiles more bird-like than living reptiles? The complete skeleton of a Compsognathus was the answer. Huxley observed that its ankle closely resembled the ankles of birds. He concluded that Compsognathus must be "placed among, or close to the Dinosauria; but it is still more bird-like than any of the animals which are ordinarily included in the group". The phylogenetic context for Aves was established, but it was within in the last decades that the consensus about the origin of Aves from a dinosaurian clade has increased.
Theropoda presents some primitive characters associated with a predatory lifestyle, for example labiolingually flattened and serrated teeth. Most of the theropods were carnivores, but some taxa, including birds have adapted to a phytophagous diet. The theropods have been obligate bipeds, with a pelvis and hind limb modified to withstand the entire burden of swift movement. Additional vertebrae are incorporated into the sacrum for a powerful joint between the backbone and pelvis, and even small theropods have a similar powerful pelvis. Moreover the theropods have a joint between the head and neck that gives the head greater mobility. The forelimbs show significant modifications, like extremely reduced size or extraordinary enlargement.
By the Late Triassic, two theropod lineages had arisen: Ceratosauria or "horned reptiles" and Tetanurae or "stiff-tailed theropods". The Ceratosauria are the best-known Triassic and Early Jurassic theropods having a global distribution from their earliest appearance in the fossil record. Ceratosauria has an evident Gondwanan distribution (Neoceratosauria) in the Cretaceous, although fragmentary fossil from Laurasia could be included in this clade.
The history of Tetanurae probably extended into the Triassic, but the oldest tetanurine taxa known is from the late Jurassic. Its snout was more slender than in Ceratosauria. Several lineages lost their teeth independently, or they were modified. Tetanurae are distinctive in having a hand reduced to digits I-III and an ascending process in the astragalus. The cranially enlarged astragalus is visible in young birds as a layer of cartilage. The furcula appeared early in the history of the many non-flying tetanurine, along with more powerful arms and hands, it would appear that the furcula was only secondarily co-adapted into taking a role in the flight of birds. Tetanurae include several basal taxa and two great clades: Spìnosauroidea (with elongated vertebral spines and crocodile-like jaws with specialized teeth) and Avetheropoda.
Within Avetheropoda there are two other large clades: the Carnosauria and the Coelurosauria. Carnosauria are represented by a number of extinct side branches of the gigantic theropods, there are not the ancestors of birds. Descendant of the ancestral coelurosaur form a cluster of evolutionary paths that lead to tyrannosaurids, ornithomimosaurs, maniraptorans and several others. Here again the exact relationships are a little unclear. But researches have appreciation of the fact that each line has characters that distinguish them from other primitive tetanurines like Allosauridae o Spinosauridae. The maniraptoran clade (included Aves) is distinguished from other tetanurines by its skull, arms and tail.