Longisquama
From Wikipedia, the free encyclopedia
| Longisquama Fossil range: Early Triassic |
||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
 Illustration of Longisquama insignis
|
||||||||||||||
| Scientific classification | ||||||||||||||
|
||||||||||||||
| Species | ||||||||||||||
|
L. insignis Sharov, 1970 (type) |
Longisquama insignis is an extinct lizard-like reptile known from a poorly preserved and incomplete fossil. It lived during the early Triassic Period, 240 million years ago, in what is now Kyrgyzstan. It is known from a type fossil specimen; slab and counterslab (PIN 2548/4 and PIN 2584/5), and five referred specimens of possible integumentary appendages (PIN 2584/7 through 9). All specimens are in the collection of the Paleontological Institute of the Russian Academy of Sciences in Moscow.
Longisquama has been interpreted differently by different researchers, and is at the center of a large and heavily publicized debate about the origin of birds. To some, Longisquama is the gliding, cold - blooded, protobird; prophesized by Heilmann's hypothetical "Proavis" in 1927, and it proves that birds are not dinosaurs. To others it is a lizard lying in a pile of fern fronds.
Longisquama means "long scales", in reference to long structures that appear to grow from its skin..
Contents |
[edit] Longisquama's 'long scales'
The Longisquama fossil appears to have feather-shaped structures attached to its body. Investigators have interpreted these structures in a variety of different ways. Haubold and Buffetaut (1987) believed that the structures were long, modified scales attached in pairs to the lateral walls of the body, like paired gliding membranes. Unwin and Benton (2001) interpreted them as a single, unpaired, row of modified scales that run along the dorsal midline. Jones et al. (2000) interpreted them as two paired rows of structures that are anatomically very much like feathers, and which are in positions like those of birds' spinal feather tracts. Feather development expert Richard Prum (2001) and also Reisz and Suez (2000) see the structures as anatomically very different from feathers, and think they are elongate, ribbon - like scales. Other observers (Fraser, 2006) believe that the structures are fern fronds which were preserved along with Longisquama and misinterpreted. This last opinion is perhaps reinforced by the fact that several fossils of the structures have been discovered in no association with animal fossils.
Haubold and Buffetaut published a reconstruction of Longisquama with plumes in a pattern akin to gliding lizards like Draco species and Kuehneosaurus, allowing it to glide, or at least parachute. Though this is now thought to be inaccurate, versions of this reconstruction are still often seen on the internet and elsewhere.
[edit] Relationships of Longisquama
The skeletal features of Longisquama are equally difficult to diagnose and, as a result, Longisquama has been placed as a close relative to many different Sauropsid groups. Sharov (1970) determined that it was a "pseudosuchian" (a derived Archosaur) on the basis of two features - a mandibular fenestra and an antorbital fenestra. Jones et al. (2000) see Longisquama as an archosaur, adding to Sharov's two characters a furcula. Olshevsky believes that Longisquama is an archosaur and, moreover, an early dinosaur - a possibility which could actually dispense with almost all of the debate, were it true. Unwin & Benton (2001) didn't think it was possible to diagnose the crucial fenestrae; the holes could just be damage to the fossil. They agreed with Sharov that Longisquama has acrodont teeth and an interclavicle, but instead of a furcula they saw paired clavicles. These features would make Longisquama a Lepidosaur, and that would mean it is not an Archosaur and, thus, not closely related to birds.
[edit] Debate
The debate about Longisquama is one of the most interesting and, certainly, most acrimonious, in all of science. The persistence of this debate raises issues about what are and are not proper methodologies in science, about standards of evidence and credibility, and the inevitable intrusion of emotional investments into human reason. This debate calls into doubt the very objectivity and empiricism of anatomical interpretation. It also shows an uncomfortable relationship between the professional conduct of science and the popular press, where very different standards of evidence are used.
In the consensus view, hundreds of shared anatomical characters support the hypothesis that birds evolved from advanced theropod dinosaurs. Early theropod dinosaurs were probably Endothermic and evolved simple filamentous feathers for insulation, and these feathers later increased in size and complexity and then adapted to aerodynamic uses. This view is increasingly strongly supported by the fossil evidence (see Feathered dinosaurs). Scientists in this camp usually regard Longisquama as a curious Diapsid with specialized scales, ambiguous skeletal features, and no implications to bird evolution.
In the minority view, birds must have evolved from lizard-like, ectothermic animals, which lived in trees and adapted to gliding by developing elongated scales and then pennaceous feathers. They later became Endothermic and used the feathers for insulation. To this group, Longisquama is a perfect fulfillment of their hypothetical predictions, with feathers identical to those of birds and skeletal features in common with birds, and it must therefore be an ancestor of birds.
This basic debate is over thirty years old but there is a new twist. For decades Martin maintained that Maniraptoran dinosaurs were not immediately related to birds (Martin, 1983), and that the similarities between them were just superficial resemblances attributable to homoplasy. But in Martin (2004), he said that he was finally persuaded by Hwang, Norell, Qiang and Keqin (2002) that Maniraptorans are the closest relatives of birds. He now believes that Longisquama evolved into birds, and that some of the birds then became flightless and radiated as the Maniraptora. Thus, in his new view, maniraptorans are not dinosaurs, and the similarities between them are the homoplasies. He credits this hypothesis to Gregory S. Paul, but it is closer to the one been advanced by Czerkas, (2002); Czerkas, 2002; Feduccia, 2005).
Though it is rarely acknowledged, there is one more aspect to this debate. Longisquama could have feathers without challenging the hypothesis that dinosaurs evolved into birds. Instead, it might simply show that feathers evolved far earlier than suspected. If Longisquama is a derived archosaur, perhaps even an Ornithodire or dinosaur, then it might be plausible that it inherited the genes to make feathers or protofeathers from a common ancestor with more advanced dinosaurs.
[edit] Alternate interpretations
Other writers, with idiosyncratic views, have weighed in on Longisquama as well. David Peters, whose methods have been strongly denounced (Bennett, see External Links below), has proposed that a great deal of soft tissue and 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).
[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 266: 125. doi:.
- Fraser, N. (2006). Dawn of The Dinosaurs: Life in the Triassic.. Indiana University Press.
- Haubold, H. & Buffetaut, E. (1987). "Une novelle interprétation de Longisquama insignis, reptile énigmatique du Trias supérieur d'Asie centrale [A new interpretation of Longisquama insignis, an enigmatic reptile from the Upper Triassic of Central Asia]". Comptes Rendus Académie des Sciences du Paris 305: 65–70..
- Jones, T.D., Ruben, J.A., Martin, L.D., Kurochkin, E., Feduccia, A., Maderson, P.F.A., Hillenius, W.J., Geist, N.R., Alifanov, V. (23 June). "Nonavian Feathers in a Late Triassic Archosaur.". Science 288: 2202. doi:.
- 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.
- Prum, R. O./Unwin, D.M., Benton, M.J./Response; Jones, T.D., Ruben, J.A., Maderson, P.F.A., Martin, L.D. (9 March). "Longisquama Fossil and Feather Morphology.". Science 291: 1899 – 1902. doi:.
- Reisz, R.R., Sues, H.-D. (23 November). "The “Feathers” of Longisquama". Nature 408: 428.
- Stokstad E. (23 June). "Feathers, or flight of fancy?". Science 288: 2124–2125. doi:.
- Sharov, A.G. (1970). "A peculiar reptile from the lower Triassic of Fergana". Paleontologiceskij Zurnal: 127–130.
