Limusaurus
Encyclopedia
Limusaurus is a genus
of toothless herbivorous theropod dinosaur
from the Jurassic
(Oxfordian stage
) Upper Shishugou Formation
in the Junggar Basin of western China
. Limusaurus is also the first definitely known ceratosaur from Eastern Asia, including China. The discovery of Limusaurus also suggests that there may have been a land connection between Asia and several other continents allowing for faunal exchange even though the Turgai Sea
was previously thought to prohibit such a movement.
The type species
is L. inextricabilis. The specific name means "impossible to extricate".
. It is known from two subadult specimens found in close connection; the holotype
, (IVPP
) V 15923, is an almost complete articulated skeleton, and the other, IVPP V 15924, is a nearly completely articulated specimen, only missing the skull. The second specimen is 15% larger than the holotype. A third referred specimen is IVPP V16134. All specimens were young adults when they died, as can be concluded from the extent of bone fusion; from growth lines it was inferred that IVPP V 15924 was in its sixth year when it died.
Limusaurus had a small slender body measuring about 1.7 m in length. It is the first definite ceratosaur from eastern Asia
to be discovered and one of the earliest. Its discovery shows that the Asian dinosaurian fauna was less endemic during the Middle/Late Jurassic period than previously thought and suggests a possible land connection between Asia and other continents during that period.
Limusaurus is a basal
ceratosaur that shares many characteristics with coelophysoids
and tetanurans
. The features present in Limusaurus led to the conclusion that there is a close relationship between the clade
s Ceratosauria
and Tetanurae.
(rhamphotheca), which have been previously reported in non-avian theropods only among the Cretaceous
coelurosaurs. Limusaurus has a long neck, short forelimbs and elongated hindlimbs indicating strong cursorial (running) capabilities. The presence of gastrolith
s in the stomach of both specimens and the toothless beak indicate a herbivorous diet, making it the earliest and most basal
theropod to become adapted to eating plants. The overall aspect of the animal is very similar to that of the Cretaceous
ornithomimid theropods, as well as the Triassic
non-dinosaurian archosaur
Effigia, representing a remarkable case of convergent evolution
among these three distinct groups of archosaur
s.
Limusaurus was a very basal ceratosaur characterized by hands retaining four digits (I–IV), digit I being strongly reduced. It was traditionally thought that the hands of dinosaurs evolved into the wings of birds by the disappearance of the two outward digits (IV and V), in contradiction to embryological studies on birds that showed that the retained digits are the three middle ones (II–III–IV). The hand structure of Limusaurus with its reduced digit I adds more weight to the digit II–III–IV identities for Tetanurae
, among which are birds. Previous to the discovery of Limusaurus, theropods were assumed to have progressively evolved reduced digits on the ulnar side of the manus
. This concept, known as Lateral Digit Reduction (LDR) is in contrast to Bilateral Digit Reduction (BDR), the reduction on digits on both sides of the hand commonly seen in all other tetrapod
groups excluding dinosaurs. However, in Limusaurus, the first digit (Digit I) is strongly reduced, along with other ceratosaurs, suggesting that BDR occurred in their sister group the Tetanurae as well.
Previously, it was thought that digits I–III were retained in tetanurans as a homology
with basal theropods, giving credence to the LDR hypothesis. However, the evidence of BDR in Limusaurus suggests that other non-avian theropods may also have exhibited BDR and the apparent digits I–III in tetanurans may actually be digits II–IV, a previous idea considered by Thulborn and Hamley, but largely ignored in the paleontological community.
Despite the interesting possibilities brought up by Limusaurus, paleontologists are unlikely to stop calling tetanuran digits I, II, and III and switch to calling them II, III, IV. This is because most of their morphological traits resemble those of digits I, II and III of other theropods. It remains possible that bilateral digit reduction occurred in Ceratosauria but not in Tetanurae. The embryology of the bird wing could be explained by a homeotic frameshift of digital identity, as suggested by recent gene expression and experimental data.
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of toothless herbivorous theropod dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
from the Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
(Oxfordian stage
Oxfordian stage
The Oxfordian is, in the ICS' geologic timescale, the earliest age of the Late Jurassic epoch, or the lowest stage of the Upper Jurassic series. It spans the time between 161.2 ± 4 Ma and 155.7 ± 4 Ma...
) Upper Shishugou Formation
Shishugou Formation
The Shishugou Formation is a geological formation in Xinjiang, China, whose strata date back to the Late Jurassic period. Dinosaur remains are among the fossils that have been recovered from the formation....
in the Junggar Basin of western China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
. Limusaurus is also the first definitely known ceratosaur from Eastern Asia, including China. The discovery of Limusaurus also suggests that there may have been a land connection between Asia and several other continents allowing for faunal exchange even though the Turgai Sea
Turgai Sea
The Turgai Sea or Turgai Strait, also known as the West Siberian Sea, was a large shallow body of salt water of the Mesozoic and Cenozoic Eras...
was previously thought to prohibit such a movement.
The type species
Type species
In biological nomenclature, a type species is both a concept and a practical system which is used in the classification and nomenclature of animals and plants. The value of a "type species" lies in the fact that it makes clear what is meant by a particular genus name. A type species is the species...
is L. inextricabilis. The specific name means "impossible to extricate".
Description
The type, and to date the only species, L. inextricabilis, was described in a 2009 paper coauthored by X. Xu, J. M. Clark, J. Mo, J. Choiniere, C. A. Forster, G. M. Erickson, D. W. E. Hone, C. Sullivan, D. A. Eberth, S. Nesbitt, Q. Zhao, R. Hernandez, C.-K Jia, F.-L. Han, and Y. Guo in the journal NatureNature (journal)
Nature, first published on 4 November 1869, is ranked the world's most cited interdisciplinary scientific journal by the Science Edition of the 2010 Journal Citation Reports...
. It is known from two subadult specimens found in close connection; the holotype
Holotype
A holotype is a single physical example of an organism, known to have been used when the species was formally described. It is either the single such physical example or one of several such, but explicitly designated as the holotype...
, (IVPP
Institute of Vertebrate Paleontology and Paleoanthropology
The Institute of Vertebrate Paleontology and Paleoanthropology of China is a prominent research institution and collections repository for fossils, including many dinosaur and pterosaurand cat poo specimens...
) V 15923, is an almost complete articulated skeleton, and the other, IVPP V 15924, is a nearly completely articulated specimen, only missing the skull. The second specimen is 15% larger than the holotype. A third referred specimen is IVPP V16134. All specimens were young adults when they died, as can be concluded from the extent of bone fusion; from growth lines it was inferred that IVPP V 15924 was in its sixth year when it died.
Limusaurus had a small slender body measuring about 1.7 m in length. It is the first definite ceratosaur from eastern Asia
Asia
Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...
to be discovered and one of the earliest. Its discovery shows that the Asian dinosaurian fauna was less endemic during the Middle/Late Jurassic period than previously thought and suggests a possible land connection between Asia and other continents during that period.
Limusaurus is a basal
Basal (phylogenetics)
In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...
ceratosaur that shares many characteristics with coelophysoids
Coelophysoidea
Coelophysoids were common dinosaurs of the Late Triassic and Early Jurassic periods. They were widespread geographically, probably living on all continents. Coelophysoids were all slender, carnivorous forms with a superficial similarity to the coelurosaurs, with which they were formerly...
and tetanurans
Tetanurae
Tetanurae, or "stiff tails", is a clade that includes most theropod dinosaurs, as well as birds. Tetanurans first appear during the early or middle Jurassic Period.-Definition:...
. The features present in Limusaurus led to the conclusion that there is a close relationship between the clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
s Ceratosauria
Ceratosauria
Ceratosaurs are members of a group of theropod dinosaurs defined as all theropods sharing a more recent common ancestry with Ceratosaurus than with birds. There is presently no universally agreed upon listing of species or diagnostic characters of Ceratosauria, though they were less derived...
and Tetanurae.
Paleobiology
Limusaurus shares several cranial features with other ceratosaurs and coelophysids but displays some unique characteristics for the group, such as absence of teeth and the presence of a fully developed beakBeak
The beak, bill or rostrum is an external anatomical structure of birds which is used for eating and for grooming, manipulating objects, killing prey, fighting, probing for food, courtship and feeding young...
(rhamphotheca), which have been previously reported in non-avian theropods only among the Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
coelurosaurs. Limusaurus has a long neck, short forelimbs and elongated hindlimbs indicating strong cursorial (running) capabilities. The presence of gastrolith
Gastrolith
A gastrolith, also called a stomach stone or gizzard stones, is a rock held inside a gastrointestinal tract. Gastroliths are retained in the muscular gizzard and used to grind food in animals lacking suitable grinding teeth. The grain size depends upon the size of the animal and the gastrolith's...
s in the stomach of both specimens and the toothless beak indicate a herbivorous diet, making it the earliest and most basal
Basal (phylogenetics)
In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...
theropod to become adapted to eating plants. The overall aspect of the animal is very similar to that of the Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
ornithomimid theropods, as well as the 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...
non-dinosaurian 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...
Effigia, representing a remarkable case of convergent evolution
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
among these three distinct groups of 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...
s.
Limusaurus was a very basal ceratosaur characterized by hands retaining four digits (I–IV), digit I being strongly reduced. It was traditionally thought that the hands of dinosaurs evolved into the wings of birds by the disappearance of the two outward digits (IV and V), in contradiction to embryological studies on birds that showed that the retained digits are the three middle ones (II–III–IV). The hand structure of Limusaurus with its reduced digit I adds more weight to the digit II–III–IV identities for Tetanurae
Tetanurae
Tetanurae, or "stiff tails", is a clade that includes most theropod dinosaurs, as well as birds. Tetanurans first appear during the early or middle Jurassic Period.-Definition:...
, among which are birds. Previous to the discovery of Limusaurus, theropods were assumed to have progressively evolved reduced digits on the ulnar side of the manus
Manus (zoology)
The manus is the zoological term for the distal portion of the fore limb of an animal. In tetrapods, it is the part of the pentadactyl limb that includes the metacarpals and digits . During evolution, it has taken many forms and served a variety of functions...
. This concept, known as Lateral Digit Reduction (LDR) is in contrast to Bilateral Digit Reduction (BDR), the reduction on digits on both sides of the hand commonly seen in all other tetrapod
Tetrapod
Tetrapods are vertebrate animals having four limbs. Amphibians, reptiles, birds and mammals are all tetrapods; even snakes and other limbless reptiles and amphibians are tetrapods by descent. The earliest tetrapods evolved from the lobe-finned fishes in the Devonian...
groups excluding dinosaurs. However, in Limusaurus, the first digit (Digit I) is strongly reduced, along with other ceratosaurs, suggesting that BDR occurred in their sister group the Tetanurae as well.
Previously, it was thought that digits I–III were retained in tetanurans as a homology
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...
with basal theropods, giving credence to the LDR hypothesis. However, the evidence of BDR in Limusaurus suggests that other non-avian theropods may also have exhibited BDR and the apparent digits I–III in tetanurans may actually be digits II–IV, a previous idea considered by Thulborn and Hamley, but largely ignored in the paleontological community.
Despite the interesting possibilities brought up by Limusaurus, paleontologists are unlikely to stop calling tetanuran digits I, II, and III and switch to calling them II, III, IV. This is because most of their morphological traits resemble those of digits I, II and III of other theropods. It remains possible that bilateral digit reduction occurred in Ceratosauria but not in Tetanurae. The embryology of the bird wing could be explained by a homeotic frameshift of digital identity, as suggested by recent gene expression and experimental data.