Cosesaurus
Encyclopedia
Cosesaurus is a genus of prolacertiform
archosauromorph reptile. It is known from a single, hand-sized fossil from the middle Triassic
period of Spain
. The fossil is a perfect impression of a largely articulated and complete specimen preserving soft parts, including an adhering jellyfish
, but no bones remain in these impressions.
In 1977, Ellenberger proposed that Cosesaurus was an ancestor of modern bird
s. That publication followed the description of the bird-like theropod, Deinonychus
, but it appeared long before the theropod ancestry of birds had been widely accepted. In his large and highly detailed treatise, Ellenberger interpreted the following traits in the fossil: a strap-like scapula
, a furcula
(wishbone), a keeled sternum, beak-like jaws, a retroverted pubis
and tail feather
s. Some of these interpretations have not been supported by subsequent research. Padian and Chiappe (1998) regarded Cosesaurus as a member of the archosauromorph
clade Prolacertiformes
.
Peters (2000) found that Cosesaurus nested with Sharovipteryx
, Longisquama
and pterosaur
s in a clade, the Fenestrasauria, though at least his inclusion of pterosaurs in this group has not been supported by any other researchers, who criticized his methods as flawed. Like Ellenberger (1977), Peters found an antorbital fenestra
without a fossa
was a shared trait of these taxa, convergent with the antorbital fenestra with fossa found in archosaurs and archosauriformes
. Later, Peters (2009) described a small pteroid in Cosesaurus. This was reduced from the fenestrated-type of coracoid found in most lizards and the prehistoric taxon Huehuecuetzpalli, by simply increasing the size of those fenestrations. While a disc-like coracoid is suitable quadrupedal locomotion (all lizards have such a coracoid) stem-like coracoids are better for flapping, as in birds and pterosaurs. This would require at least occasional bipedal locomotion, as in certain lizards. Supporting that hypothesis, the feet of Cosesaurus have been matched to the sometimes bipedal and narrow gauge Triassic Rotodactylus tracks, which are distinguished by the impression of digit 5 far behind the digitigrade impressions of the other toes. Cosesaurus also had four sacral vertebrae, two more than in any lizard. These may have helped support a bipedal configuration by adding strength at the fulcrum located at the hip joint.
Prolacertiformes
Prolacertiformes were an order of archosauromorph reptiles that lived during the Permian and Triassic Periods...
archosauromorph reptile. It is known from a single, hand-sized fossil from the middle Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
period of Spain
Spain
Spain , officially the Kingdom of Spain languages]] under the European Charter for Regional or Minority Languages. In each of these, Spain's official name is as follows:;;;;;;), is a country and member state of the European Union located in southwestern Europe on the Iberian Peninsula...
. The fossil is a perfect impression of a largely articulated and complete specimen preserving soft parts, including an adhering jellyfish
Jellyfish
Jellyfish are free-swimming members of the phylum Cnidaria. Medusa is another word for jellyfish, and refers to any free-swimming jellyfish stages in the phylum Cnidaria...
, but no bones remain in these impressions.
In 1977, Ellenberger proposed that Cosesaurus was an ancestor of modern 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. That publication followed the description of the bird-like theropod, Deinonychus
Deinonychus
Deinonychus was a genus of carnivorous dromaeosaurid dinosaur. There is one described species, Deinonychus antirrhopus. This 3.4 meter long dinosaur lived during the early Cretaceous Period, about 115–108 million years ago . Fossils have been recovered from the U.S...
, but it appeared long before the theropod ancestry of birds had been widely accepted. In his large and highly detailed treatise, Ellenberger interpreted the following traits in the fossil: a strap-like scapula
Scapula
In anatomy, the scapula , omo, or shoulder blade, is the bone that connects the humerus with the clavicle ....
, a furcula
Furcula
The ' is a forked bone found in birds, formed by the fusion of the two clavicles. In birds, its function is the strengthening of the thoracic skeleton to withstand the rigors of flight....
(wishbone), a keeled sternum, beak-like jaws, a retroverted pubis
Pubis
Pubis may refer to:* Pubis * Mons pubis, a padding of fat that protects the pubis bone...
and tail feather
Feather
Feathers are one of the epidermal growths that form the distinctive outer covering, or plumage, on birds and some non-avian theropod dinosaurs. They are considered the most complex integumentary structures found in vertebrates, and indeed a premier example of a complex evolutionary novelty. They...
s. Some of these interpretations have not been supported by subsequent research. Padian and Chiappe (1998) regarded Cosesaurus as a member of the archosauromorph
Archosauromorpha
Archosauromorpha is an infraclass of diapsid reptiles that first appeared during the late Permian and became more common during the Triassic. Included in this infraclass are the groups Rhynchosauria, Trilophosauridae, Prolacertiformes, Archosauriformes, and, tentatively, Choristodera...
clade Prolacertiformes
Prolacertiformes
Prolacertiformes were an order of archosauromorph reptiles that lived during the Permian and Triassic Periods...
.
Peters (2000) found that Cosesaurus nested with Sharovipteryx
Sharovipteryx
Sharovipteryx , was an early gliding reptile, from the middle-late Triassic period . Fossils have been found from the Madygen Formation of Kyrgyzstan along with the unusual reptile Longisquama...
, Longisquama
Longisquama
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...
and pterosaur
Pterosaur
Pterosaurs were flying reptiles of the clade or order Pterosauria. They existed from the late Triassic to the end of the Cretaceous Period . Pterosaurs are the earliest vertebrates known to have evolved powered flight...
s in a clade, the Fenestrasauria, though at least his inclusion of pterosaurs in this group has not been supported by any other researchers, who criticized his methods as flawed. Like Ellenberger (1977), Peters found 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...
without a fossa
Fossa
Fossa may refer to:*Fossa dei Leoni, an Italian football supporters association of Milan*La Fossa, an Italian rap group*Formaggio di fossa, a sheep's milk cheese from Sogliano al Rubicone, Italy*Cacio di Fossa, a hard, sharp sheep's milk Italian cheese...
was a shared trait of these taxa, convergent with the antorbital fenestra with fossa found in archosaurs and archosauriformes
Archosauriformes
Archosauriformes is a clade of diapsid reptiles that developed from archosauromorph ancestors some time in the Late Permian...
. Later, Peters (2009) described a small pteroid in Cosesaurus. This was reduced from the fenestrated-type of coracoid found in most lizards and the prehistoric taxon Huehuecuetzpalli, by simply increasing the size of those fenestrations. While a disc-like coracoid is suitable quadrupedal locomotion (all lizards have such a coracoid) stem-like coracoids are better for flapping, as in birds and pterosaurs. This would require at least occasional bipedal locomotion, as in certain lizards. Supporting that hypothesis, the feet of Cosesaurus have been matched to the sometimes bipedal and narrow gauge Triassic Rotodactylus tracks, which are distinguished by the impression of digit 5 far behind the digitigrade impressions of the other toes. Cosesaurus also had four sacral vertebrae, two more than in any lizard. These may have helped support a bipedal configuration by adding strength at the fulcrum located at the hip joint.