Longisquama

Taxobox
name = "Longisquama"

---

image_width = 200px
image_caption = Illustration of "Longisquama insignis"
fossil_range = Early Triassic
regnum = Animalia
phylum = Chordata
classis = Sauropsida
subclassis = Diapsida
unranked_ordo = Avicephala?
familia = Longisquamidae
familia_authority = Sharov, 1970
genus = "Longisquama"
genus_authority = Sharov, 1970
subdivision_ranks = Species
subdivision ="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 an ambiguous diapsid preserved on a group of fern fronds and has no bearing on the origin of birds.

"Longisquama" means "long scales", in reference to long structures that appear to grow from its skin.

"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.

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. Sharov's original description also includes an elongate scapula. 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.

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 one view, hundreds of shared anatomical characters support the hypothesis that birds evolved from advanced theropod dinosaurs. Early theropod dinosaurs are hypothesized by some to be endothermic and to have evolved simple filamentous feathers for insulation (though some work has been done to show that these are actually collagen fibers, and to date no analysis has been done of that filamentous material to prove either proto-feather or collagen fiber affinity), and these feathers later increased in size and complexity and then adapted to aerodynamic uses. Scientists in this camp usually regard "Longisquama" as a curious diapsid with specialized scales, ambiguous skeletal features, and no implications to bird evolution.

Conversely, another view is that birds evolved from small, aboreal archosaurs, ectothermic animals, 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" represents not the ancestor of birds, but that it shares many features consistent with a possible proto-bird ancestor.

Both views are supported by the fossil evidence (see Feathered dinosaurs).

This basic debate is over thirty years old but there is a new twist. For decades, many cladistic theorists have maintained that birds originated from a terrestrial theropod dinosaur, despite the majority of the ornithological community, and some vertebrate paleontologists, such as Martin (1983), pointing out the errors and inconsistencies in the characters used to unite the two groups. When Martin first observed the holotype of "Microraptor gui", he began to consider the hypothesis that Maniraptorans are birds, not dinosaurs, and the similarities between terrestrial maniraptorans and dinosaurs can be attributed to terrestriality and ecological impacts on evolution. He credits this hypothesis to Gregory S. Paul, and other hypotheses have been advanced by Czerkas, (2002); [http://www.dinosaur-museum.org/featheredinosaurs/flying_dromaeosaurs.pdf Czerkas, 2002] ; Feduccia, 2005). Longisquama ties into this debate only in the manner that Martin and others hypothesize that birds evolved from a small arboreal archosaur some time in the Triassic. In Martin (2008), it is stated only that "there is nothing in "Longisquama"'s morphology that is inconsistent with a protobird."

Though it is rarely acknowledged, there is one more aspect to this debate. "Longisquama" has been proposed as a possible ancestor to both dinosaurs and pterosaurs as well. The classification of "Longisquama" is highly debated, and even the taxonomic status is often called into question; whether it is an archosaur, an archosauromorph or something else entirely.

The only way to settle this debate is to deepen the exploration into the Triassic and return to the locality from whence "Longisquama" came. Furthermore, Triassic deposits within North America and Asia have been relatively untouched, and also need to be examined.

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).

References

* cite journal
author = Feduccia, A., Lingham-Soliar, T., and Hinchliffe, J.R.
year = 2005
title = Do feathered dinosaurs exist? Testing the hypothesis on neontological and paleontological evidence
journal = Journal of Morphology
doi = 10.1002/jmor.10382
volume = 266
pages = 125
* cite book
author = Fraser, N.
year = 2006
title = Dawn of The Dinosaurs: Life in the Triassic.
publisher = Indiana University Press
city = Bloomington
*cite journal
author = Haubold, H. & Buffetaut, E.
year = 1987
title = 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]
journal = Comptes Rendus Académie des Sciences du Paris
volume = 305
number = serie II
pages = 65–70.
*cite journal
author = 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.
year = 2000
title = Nonavian Feathers in a Late Triassic Archosaur.
journal = Science
volume = 288
number = 5474
pages = 2202
date =23 June
doi = 10.1126/science.288.5474.2202
* cite journal
author = Martin, L.D.
year = 2004
title = A basal archosaurian origin for birds
journal = Acta Zoologica Sinica
volume = 50(6)
pages = 978–990
*cite journal
author = Martin, L. D.
year = 2008
title = Origin of avian flight- a new perspective
journal = Oryctos
volume = 7
pages = 45-54.
* cite journal
author = Peters, D.
year = 2000
title = A Redescription of Four Prolacertiform Genera and Implications for Pterosaur Phylogenesis
journal = Rivista Italiana di Paleontologia e Stratigrafia
volume = 106(3)
pages = 293–336
* cite journal
author = Peters, D.
year = 2002
title = A New Model for the Evolution of the Pterosaur Wing – with a twist.
journal = Historical Biology
volume = 15
pages = 277–301
* cite journal
author = Peters, D.
year = 2006
title = The Other Half of "Longisquama"
journal = Prehistoric Times
volume = 75
pages = 10–11
* 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
*cite journal
author = Prum, R. O./Unwin, D.M., Benton, M.J./Response; Jones, T.D., Ruben, J.A., Maderson, P.F.A., Martin, L.D.
year = 2001
title = "Longisquama" Fossil and Feather Morphology.
journal = Science
volume = 291
number = 5510
pages = 1899 – 1902
date = 9 March
doi = 10.1126/science.291.5510.1899c
*cite journal
author = Reisz, R.R., Sues, H.-D.
year = 2000
title = The “Feathers” of "Longisquama"
journal = Nature
volume = 408
number = 6811
pages = 428
date = 23 November
*cite journal
author = Stokstad E.
year = 2000
title = Feathers, or flight of fancy?
journal = Science
volume = 288
number = 5474
pages = 2124–2125
date = 23 June
doi = 10.1126/science.288.5474.2124
*cite journal
author = Sharov, A.G.
year = 1970
title = A peculiar reptile from the lower Triassic of Fergana
journal = Paleontologiceskij Zurnal
volume =
number = 1
pages = 127–130
date =

External links

* [http://www.fhsu.edu/biology/cbennett/critique.html for serious questions about David Peters' work]
* [http://dml.cmnh.org/2005May/msg00317.html A critique of Martin 2004]
* [http://dml.cmnh.org/2005Oct/msg00222.html A critique of Feduccia et al. 2005]
* [http://www2.ljworld.com/news/2005/nov/13/feathered_reptile_fossil_stirs_evolution_debate/ A newspaper article on newly discovered "Longisquama" plumes]
* [http://www.dinodata.org/index.php?option=com_content&task=view&id=6782&Itemid=67 "Longisquama" at Dinodata]