Longisquama
Encyclopedia
Longisquama insignis is an extinct lizard
-like reptile
known only from one poorly preserved and incomplete fossil. It lived during the middle or late Triassic Period, 230-225 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 means "long scales"; the specific name insignis refers to its small size. The Longisquama fossil is notable for a number of long structures that appear to grow from its skin. These structures have been interpreted differently by different researchers, and are at the center of a large and heavily publicized debate related to that of the origin of bird
s. To some, Longisquama is the gliding, cold-blooded, protobird predicted by Gerhard 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.
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. They 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.
Other researchers place the scales differently. 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.
Still other observers (Fraser, 2006) believe that the structures are not part of Longisquama at all; they are simply fern
frond
s which were preserved along with the reptile 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.
Other writers, with idiosyncratic views, have weighed in on Longisquama as well. David Peters, whose methods have been strongly denounced, 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).
Sharov (1970) determined that it was a "pseudosuchian" (a "primitive" archosaur
, but as an archosaur a relatively derived reptile) 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 about bird origins, were it true. Unwin & Benton (2001) didn't think it was possible to diagnose the crucial fenestrae; the holes could simply 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 be more typical of a member of Lepidosauromorpha
, meaning that Longisquama is not an archosaur
and thus not closely related to birds. According to a cladistic
study by Phil Senter in 2004, Longisquama would be an even more basal diapsid and a member of Avicephala
, more closely related to Coelurosauravus
.
, and evolved simple filamentous feathers for insulation. 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 real significance to bird evolution.
However, a few scientists prefer the hypothesis that birds evolved from small, arboreal archosaurs like Longisquama. They see these as ectothermic animals that adapted to gliding by developing elongated scales and then pennaceous feathers. The high energy demands of gliding drove the animals to become endothermic and the feathers were then coopted to double as insulation.
. For decades Martin asserted that the anatomical similarities between dromaeosaurid dinosaurs and birds were mere convergences, and not credible evidence of a close relationship.
The discovery of the dromaeosaurid
Microraptor
, a small dinosaur with simple filamentous feathers as well as the long flight feathers characteristic of birds, had major ramifications for the debate about what sort of animal might be the most probable ancestor of birds. Researchers like Padian (1985) had hypothesized a running, terrestrial dinosaur developing feathers and the ability to flap, and Martin and Feduccia were particularly critical of that idea. But Microraptor was interpreted by Xu et al. (2003) as a small, arboreal, gliding dinosaur, thus going beyond the former terrestrial dinosaur/arboreal archosaur opposition. However, in 2004, in response to Microraptor, Martin proposed that Microraptor is a close relative of birds after all but that it, along with all maniraptoran dinosaurs, were not dinosaurs at all. He proposed what he called a compromise hypothesis that all maniraptorans are instead flightless birds, in a scenario where birds evolved from a Longisquama-like basal archosaur
first. Martin credits this hypothesis to Gregory S. Paul
, but Paul is a strong advocate of the relationship of dinosaurs and birds, and believes that maniraptorans are dinosaurs.
Moreover, if Olshevsky is correct and Longisquama is an early dinosaur, then it may be an ancestor of Microraptor, Longisquamas feathers could be homologous
to those of birds, and all three could be dinosaurs.
Lizard
Lizards are a widespread group of squamate reptiles, with nearly 3800 species, ranging across all continents except Antarctica as well as most oceanic island chains...
-like reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...
known only from one poorly preserved and incomplete fossil. It lived during the middle or late Triassic Period, 230-225 million years ago, in what is now Kyrgyzstan
Kyrgyzstan
Kyrgyzstan , officially the Kyrgyz Republic is one of the world's six independent Turkic states . Located in Central Asia, landlocked and mountainous, Kyrgyzstan is bordered by Kazakhstan to the north, Uzbekistan to the west, Tajikistan to the southwest and China to the east...
. 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
Russian Academy of Sciences
The Russian Academy of Sciences consists of the national academy of Russia and a network of scientific research institutes from across the Russian Federation as well as auxiliary scientific and social units like libraries, publishers and hospitals....
in Moscow
Moscow
Moscow is the capital, the most populous city, and the most populous federal subject of Russia. The city is a major political, economic, cultural, scientific, religious, financial, educational, and transportation centre of Russia and the continent...
.
Longisquama means "long scales"; the specific name insignis refers to its small size. The Longisquama fossil is notable for a number of long structures that appear to grow from its skin. These structures have been interpreted differently by different researchers, and are at the center of a large and heavily publicized debate related to that of the origin of bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
s. To some, Longisquama is the gliding, cold-blooded, protobird predicted by Gerhard Heilmann
Gerhard Heilmann
Gerhard Heilmann was a Danish artist and paleontologist who created artistic depictions of Archeopteryx, Proavis and other early bird relatives apart from writing The Origin of Birds, a pioneering and influential account of bird evolution...
's hypothetical "Proavis" in 1927, and it proves that birds are not dinosaurs. To others, it is an ambiguous diapsid
Diapsid
Diapsids are a group of reptiles that developed two holes in each side of their skulls, about 300 million years ago during the late Carboniferous period. Living diapsids are extremely diverse, and include all crocodiles, lizards, snakes, and tuatara...
preserved on a group of fern fronds and has no bearing on the origin of birds.
The 'long scales'
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. They published a reconstruction of Longisquama with plumes in a pattern akin to gliding lizards like Draco species and Kuehneosaurus
Kuehneosaurus
Kuehneosaurus is an extinct genus of Late Triassic reptile from the United Kingdom. Measuring 72 centimetres long , it had "wings" formed from ribs which jutted out from its body by as much as 14.3 cm, connected by a membrane which allowed it to slow its descent when jumping from trees...
, 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.
Other researchers place the scales differently. 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
Richard Prum
Richard O. Prum is William Robertson Coe Professor of Ornithology, and Head Curator of Vertebrate Zoology at the Peabody Museum of Natural History, at Yale University....
(2001) and also Reisz and Suez (2000) see the structures as anatomically very different from feathers, and think they are elongate, ribbon-like scales.
Still other observers (Fraser, 2006) believe that the structures are not part of Longisquama at all; they are simply fern
Fern
A fern is any one of a group of about 12,000 species of plants belonging to the botanical group known as Pteridophyta. Unlike mosses, they have xylem and phloem . They have stems, leaves, and roots like other vascular plants...
frond
Frond
The term frond refers to a large, divided leaf. In both common usage and botanical nomenclature, the leaves of ferns are referred to as fronds and some botanists restrict the term to this group...
s which were preserved along with the reptile 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.
Other writers, with idiosyncratic views, have weighed in on Longisquama as well. David Peters, whose methods have been strongly denounced, 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).
Taxonomy
Like the 'long scales', the skeletal features of Longisquama are equally difficult to diagnose. As a result, Longisquama has been related by scientists to many different Sauropsid groups.Sharov (1970) determined that it was a "pseudosuchian" (a "primitive" archosaur
Archosaur
Archosaurs are a group of diapsid amniotes whose living representatives consist of modern birds and crocodilians. This group also includes all extinct non-avian dinosaurs, many extinct crocodilian relatives, and pterosaurs. Archosauria, the archosaur clade, is a crown group that includes the most...
, but as an archosaur a relatively derived reptile) on the basis of two features - a mandibular fenestra and an antorbital fenestra
Antorbital fenestra
An antorbital fenestra is an opening in the skull, in front of the eye sockets. This skull formation first appeared in archosaurs during the Triassic Period. Living birds today possess antorbital fenestrae, but the feature has been lost in modern crocodilians...
. 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
George Olshevsky
George Olshevsky is a freelance editor, writer, publisher, amateur paleontologist, and mathematician living in San Diego, California.Olshevsky maintains the comprehensive online Dinosaur Genera List...
believes that Longisquama is an archosaur and, moreover, an early dinosaur - a possibility which could actually dispense with almost all of the debate about bird origins, were it true. Unwin & Benton (2001) didn't think it was possible to diagnose the crucial fenestrae; the holes could simply 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 be more typical of a member of Lepidosauromorpha
Lepidosauromorpha
Lepidosauromorpha is a group of reptiles comprising all diapsids closer to lizards than to archosaurs . The only living sub-group is the Lepidosauria: extant lizards, snakes, and tuatara...
, meaning that Longisquama is not an archosaur
Archosaur
Archosaurs are a group of diapsid amniotes whose living representatives consist of modern birds and crocodilians. This group also includes all extinct non-avian dinosaurs, many extinct crocodilian relatives, and pterosaurs. Archosauria, the archosaur clade, is a crown group that includes the most...
and thus not closely related to birds. According to a cladistic
Cladistics
Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...
study by Phil Senter in 2004, Longisquama would be an even more basal diapsid and a member of Avicephala
Avicephala
thumb|right|Artist's reconstruction of [[Coelurosauravus]]Avicephala is a possibly polyphyletic and therefore disused clade of diapsid reptiles that lived during the Late Permian and Triassic periods...
, more closely related to Coelurosauravus
Coelurosauravus
Coelurosauravus is a genus of basal diapsid reptile, with specialized wing-like structures allowing it to glide. These were rod like structures with skin stretched over them; this feature is unique to the genus....
.
Debate over bird origins
The question of what kind of reptile Longisquama is, and what exactly the 'long scales' are, relates to a wider debate about the origin of birds, and whether or not they are descended from dinosaurs.Background
A consensus of paleontologists is persuaded by the hypothesis that birds evolved from advanced theropod dinosaurs. The scenario for this hypothesis is that early theropod dinosaurs were endothermicEndothermic
In thermodynamics, the word endothermic describes a process or reaction in which the system absorbs energy from the surroundings in the form of heat. Its etymology stems from the prefix endo- and the Greek word thermasi,...
, and evolved simple filamentous feathers for insulation. 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 real significance to bird evolution.
However, a few scientists prefer the hypothesis that birds evolved from small, arboreal archosaurs like Longisquama. They see these as ectothermic animals that adapted to gliding by developing elongated scales and then pennaceous feathers. The high energy demands of gliding drove the animals to become endothermic and the feathers were then coopted to double as insulation.
Relationship to feathered dromeosaurs
The basic debate is over thirty years old but both sides draw evidence from recent findings of apparently feathered dinosaursFeathered dinosaurs
The realization that dinosaurs are closely related to birds raised the obvious possibility of feathered dinosaurs. Fossils of Archaeopteryx include well-preserved feathers, but it was not until the early 1990s that clearly non-avialan dinosaur fossils were discovered with preserved feathers...
. For decades Martin asserted that the anatomical similarities between dromaeosaurid dinosaurs and birds were mere convergences, and not credible evidence of a close relationship.
The discovery of the dromaeosaurid
Dromaeosauridae
Dromaeosauridae is a family of bird-like theropod dinosaurs. They were small- to medium-sized feathered carnivores that flourished in the Cretaceous Period. The name Dromaeosauridae means 'running lizards', from Greek dromeus meaning 'runner' and sauros meaning 'lizard'...
Microraptor
Microraptor
Microraptor is a genus of small, four-winged dromaeosaurid dinosaurs. Numerous well-preserved fossil specimens have been recovered from Liaoning, China...
, a small dinosaur with simple filamentous feathers as well as the long flight feathers characteristic of birds, had major ramifications for the debate about what sort of animal might be the most probable ancestor of birds. Researchers like Padian (1985) had hypothesized a running, terrestrial dinosaur developing feathers and the ability to flap, and Martin and Feduccia were particularly critical of that idea. But Microraptor was interpreted by Xu et al. (2003) as a small, arboreal, gliding dinosaur, thus going beyond the former terrestrial dinosaur/arboreal archosaur opposition. However, in 2004, in response to Microraptor, Martin proposed that Microraptor is a close relative of birds after all but that it, along with all maniraptoran dinosaurs, were not dinosaurs at all. He proposed what he called a compromise hypothesis that all maniraptorans are instead flightless birds, in a scenario where birds evolved from a Longisquama-like basal archosaur
Archosaur
Archosaurs are a group of diapsid amniotes whose living representatives consist of modern birds and crocodilians. This group also includes all extinct non-avian dinosaurs, many extinct crocodilian relatives, and pterosaurs. Archosauria, the archosaur clade, is a crown group that includes the most...
first. Martin credits this hypothesis to Gregory S. Paul
Gregory S. Paul
Gregory Scott Paul is a freelance researcher, author and illustrator who works in paleontology, and more recently has examined sociology and theology. He is best known for his work and research on theropod dinosaurs and his detailed illustrations, both live and skeletal...
, but Paul is a strong advocate of the relationship of dinosaurs and birds, and believes that maniraptorans are dinosaurs.
Moreover, if Olshevsky is correct and Longisquama is an early dinosaur, then it may be an ancestor of Microraptor, Longisquamas feathers could be homologous
Homology (biology)
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...
to those of birds, and all three could be dinosaurs.
