Longisquama
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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. It is known that a foot and toe set were confused for feather shafts (Peters 2006).
It was once thought that Longisquama was also the earliest known creature with a bird-like furcula, an adaptation modern birds have for flight. Peters (2000) showed that these clavicles (what comprise the furcula in birds) overlap one another anteriorly, as they do in pterosaurs. They frame the sternum, as in pterosaurs. And the interclavicle produces a keel, as in pterosaurs, creating a sternal complex. Such a complex suggests that Longisquama flapped its arms because muscles of adduction are anchored on the sternal complex. Longisquama was quadrupedal and probably arboreal (tree-dwelling), and reached 15 cm (6 in) in length.
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). Peters (2000, 2002) was able to dismiss all prior Longisquama associations with archosauriforms, archosaurs, dinosaurs and birds by demonstrating a better nesting with Macrocnemus through pterosaurs by a comprehensive cladistic analysis that included all prior analyses. No study since then has been able to contradict this hypothesis.
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.
Yet a third position, espoused by scientists like George Olshevsky, could render the question moot. Olshevsky believes that birds did evolve from a Longisquama-like ancestor, but also that Longisquama is itself an early theropod dinosaur [2].
Again, this hypothesis has been bested by the Cosesaurus > Sharovipteryx > Longisquama > pterosaurs cladistic analysis of Peters (2000). Furthermore, the presumed missing posterior half of Longisquama, along with a better understanding of its forelimbs was reported and illustrated by Peters (2006). It turns out that Longisquama was literally a leaping lizard with lemur-like proportions and protowings able to fold in the plane of the wing like those of pterosaurs. The wings originated as yet one more display device on a lizard that was already as exotic as a Brazilian Mardi Gras parader.
Compared to Longisquama, the first pterosaurs had longer forearms, longer wing fingers, shorter legs a shorter torso and a bigger head, all apparently paedomorphic features (features which made pterosaurs more closely resemble a juvenile Longisquama than an adult). Because Sharovipteryx and Longisquama were both bipeds, they were able to do something else with their forelimbs than walk on them. That the first pterosaurs were able to touch the ground while standing erect shows that the argument whether pterosaurs were bipeds or quadrupeds is moot. Evidently wings in pterosaurs originated as display devices on Longisquama, probably used in mating rituals or infant feeding rituals, as baby birds do today. Longisquama was able to flap its proto-wings and glide, but it could not fly. Perhaps flapping increased its ability to leap. If so, a gradual increase in wing size would have incrementally given way to the ability to fly better as the torso and legs shortened concurrently.
[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.
- {{cite journal
| author = Prum, R.O. | year = 2002 | title = Are current critiques of the theropod origin of birds science? Rebuttal to Feduccia | journal = The Auk | volume = 120(2) | pages = 550-561
- 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.
- {{cite journal
| author = Peters, D. | year = 2006 | title = The Other Half of Longisquama | journal = Prehistoric Times | volume = 75 | pages = 10-11
[edit] External links
- A critique of Martin 2004
- A critique of Feduccia et al. 2005
- A newspaper article on newly discovered Longisquama plumes
According to Benton:
According to Peters 2006:Cateogory: Macrocnemia
