Longisquama
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 Skeletal reconstruction of Longisquama insignis
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Prehistoric
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| Longisquama insignis Sharov, 1970 |
Longisquama ("long scales") was a lizard-like reptile of the early Triassic Period 240 million years ago, which lived in what is now Turkestan. Longisquama had a row of feather-like structures jutting from its back. It was once thought that these plumes were in a pattern akin to gliding lizards like flying dragons and Kuehneosaurus, perhaps even allowing it to glide, or at least parachute. Actually it is a single row of plumes, like a basilisk's dorsal frill. Another set of plumes from the skull may have confused earlier workers. One author has proposed that a great deal of soft-tissue and in fact, impressions of missing bones are scattered throughout the specimen, so that nearly the entire skeleton is preserved despite the specimen appearing to taper off the slab (Peters, 2006).
Because of these and other bird-like traits, Longisquama was at the center of the still-ongoing debate over whether birds evolved directly from dinosaurs. A vast majority of dinosaur and bird researchers, who use cladistic methodology to determine relationships among animals, consider birds to be a specialized branch of dinosaurs that evolved from an animal similar to Velociraptor. However, a few scientists, notably Larry Martin, Alan Feduccia, and Steven Czerkas, contend that birds evolved from an archosauromorph ancestor similar to Longisquama, and that the apparently bird-like dinosaurs such as Velociraptor are in fact birds and not dinosaurs at all (Martin, 2004; Czerkas, 2002; Feduccia, 2005).
Many experts, such as leading feather development researcher Richard O. Prum, say that the feather-like structures on Longisquama are not feathers. Alternative explanations have been skin flaps[1], Draco-like ribs, convergent evolution from scales, parallel evolution, or even fern fronds which simply got fossilized with the animal's skeleton. This last may be supported by the fact that many of the structures have been found without any skeleton attached, though in more than one example they are preserved in the same position. These researchers also contend that the other "bird-like" traits have been misinterpreted as such, citing the fact that they are shared by Longisquamas very un-birdlike relatives, and are adaptations to a tree-dwelling lifestyle also seen in modern arboreal specialists like the chameleon.
George Olshevsky believes that birds did evolve from a Longisquama-like ancestor, but also that Longisquama is itself an early theropod dinosaur [2]. Peters (2000), on the other hand, has posited this evolutionary sequence: Cosesaurus > Sharovipteryx > Longisquama > pterosaurs. Finally, Senter (2004) found Longisquama to be part of a largely arboreal clade of ancient sauropsids, which he named "Avicephala".
[edit] References
- Feduccia, A., Lingham-Soliar, T., and Hinchliffe, J.R. (2005). "Do feathered dinosaurs exist? Testing the hypothesis on neontological and paleontological evidence". Journal of Morphology.
- Martin, L.D. (2004). "A basal archosaurian origin for birds". Acta Zoologica Sinica 50(6): 978-990.
- Peters, D. (2000). "A Redescription of Four Prolacertiform Genera and Implications for Pterosaur Phylogenesis". Rivista Italiana di Paleontologia e Stratigrafia 106(3): 293-336.
- Peters, D. (2002). "A New Model for the Evolution of the Pterosaur Wing – with a twist.". Historical Biology 15: 277-301.
- Peters, D. (2006). "The Other Half of Longisquama". Prehistoric Times 75: 10-11.
- Prum, R.O. (2002). "Are current critiques of the theropod origin of birds science? Rebuttal to Feduccia". The Auk 120(2): 550-561.
