Hesperonychus
Encyclopedia
Hesperonychus was a genus
of small, carnivorous
dromaeosaurid
dinosaur
. There is one described species, Hesperonychus elizabethae; the type species
was named in honor of the woman who collected it in 1982. It is known from fossil
s recovered from the Dinosaur Park Formation
of Alberta
, dating to the late Cretaceous
Period (Campanian
stage
) about 75 million years ago).
Hesperonychus is known from one partial pelvic girdle
, holotype
specimen UALVP 48778, collected by Dr. Elizabeth Nicholls in Dinosaur Provincial Park
in 1982. The fossil remained undescribed, however, until Nick Longrich and Phil Currie
published on it in 2009. A number of very small toe bones, including "sickle claws", in the collection of the Royal Tyrrell Museum may also belong to Hesperonychus. Despite their small size, the pubic bones were fused, a characteristic of adult dinosaurs, indicating that the specimen does not represent a juvenile of a known species.
Though known from only partial remains, Longrich and Currie estimated its total length at under one meter and weight at about 1.9 kilograms, making it the smallest known carnivorous dinosaur from North America. The alvarezsaurid
Albertonykus
was smaller, though likely an insectivore
rather than a carnivore.
, a clade
of small dromaeosaurids
previously thought to be restricted to the Early Cretaceous of Asia
. The authors described this find as "remarkable"; the previously youngest known microraptorine was Microraptor
itself from the Aptian
stage of the Early Cretaceous
, so the discovery of Hesperonychus in the Late Cretaceous
Campanian
stage pushed the fossil range of microraptorines forward by 45 million years. While the Late Cretaceous, North American Bambiraptor
had sometimes been classified as a microraptorine, more recent studies (including those by Longrich and Currie) have found that it is more closely related to Saurornitholestes
.
given their closer similarity in size. Nevertheless, Hesperonychus seems to show that microraptorines did not vary much in size, remaining very small relative to other dromaeosaurids throughout their history.
Aside from extending the known range of microraptorines, the discovery of Hesperonychus filled in a gap in the ecology of Late Cretaceous North America. Unlike roughly contemporary environments in Europe and Asia, North America appeared to lack very small carnivorous dinosaurs. In modern ecosystems dominated by endotherm
ic mammals, small animal species outnumber larger ones. Since dinosaurs are also presumed to have been endotherms, the lack of small species and great number of known large species in North America was unusual. Hesperonychus helped to fill that gap, especially since, given the number of fragmentary remains and claws that have been collected (representing at least ten distinct specimens, compared to thirty of the contemporary Saurornitholestes and two of Dromaeosaurus
), it appears to have been a very common feature of the Dinosaur Park Formation environment.
The next smallest carnivore in the environment was the mammal Eodelphis
, which weighed only 600 grams. There does not appear to have been any overlap between the smallest dinosaurs and the largest mammals in ecosystems such as this, which Longrich and Currie explained by hypothesizing that either competition from dinosaurs kept mammals from growing larger (the traditional view), competition from mammals kept the dinosaurs from growing smaller, or both.
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 small, carnivorous
Carnivore
A carnivore meaning 'meat eater' is an organism that derives its energy and nutrient requirements from a diet consisting mainly or exclusively of animal tissue, whether through predation or scavenging...
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'...
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...
. There is one described species, Hesperonychus elizabethae; 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...
was named in honor of the woman who collected it in 1982. It is known from fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s recovered from the Dinosaur Park Formation
Dinosaur Park Formation
The Dinosaur Park Formation is the uppermost member of the Judith River Group, a major geologic unit in southern Alberta. It was laid down over a period of time between about 76.5 and 75 million years ago. The formation is made up of deposits of a high-sinuosity fluvial system, and is capped...
of Alberta
Alberta
Alberta is a province of Canada. It had an estimated population of 3.7 million in 2010 making it the most populous of Canada's three prairie provinces...
, dating to the late 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...
Period (Campanian
Campanian
The Campanian is, in the ICS' geologic timescale, the fifth of six ages of the Late Cretaceous epoch . The Campanian spans the time from 83.5 ± 0.7 Ma to 70.6 ± 0.6 Ma ...
stage
Faunal stage
In chronostratigraphy, a stage is a succession of rock strata laid down in a single age on the geologic timescale, which usually represents millions of years of deposition. A given stage of rock and the corresponding age of time will by convention have the same name, and the same boundaries.Rock...
) about 75 million years ago).
Hesperonychus is known from one partial pelvic girdle
Pelvis
In human anatomy, the pelvis is the lower part of the trunk, between the abdomen and the lower limbs .The pelvis includes several structures:...
, 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...
specimen UALVP 48778, collected by Dr. Elizabeth Nicholls in Dinosaur Provincial Park
Dinosaur Provincial Park
Dinosaur Provincial Park is a UNESCO World Heritage Site located about two and a half hours drive southeast of Calgary, Alberta, Canada or , about a half hour drive, northeast of Brooks....
in 1982. The fossil remained undescribed, however, until Nick Longrich and Phil Currie
Phil Currie
Philip John Currie, AOE is a Canadian palaeontologist and museum curator who helped found the Royal Tyrrell Museum of Palaeontology in Drumheller, Alberta and is now a professor at the University of Alberta in Edmonton...
published on it in 2009. A number of very small toe bones, including "sickle claws", in the collection of the Royal Tyrrell Museum may also belong to Hesperonychus. Despite their small size, the pubic bones were fused, a characteristic of adult dinosaurs, indicating that the specimen does not represent a juvenile of a known species.
Though known from only partial remains, Longrich and Currie estimated its total length at under one meter and weight at about 1.9 kilograms, making it the smallest known carnivorous dinosaur from North America. The alvarezsaurid
Alvarezsauridae
Alvarezsauridae is an enigmatic family of small, long-legged running dinosaurs. Although originally thought to represent the earliest known flightless birds, a consensus of recent work suggests that they are primitive members of the Maniraptora. Other work found them to be the sister group to the...
Albertonykus
Albertonykus
Albertonykus is a genus of alvarezsaurid dinosaur from lower Maastrichtian-age rocks of the Horseshoe Canyon Formation of Alberta, Canada. It is known from forelimb and hindlimb remains from multiple individuals. All but two of the specimens come from a bonebed dominated by Albertosaurus...
was smaller, though likely an insectivore
Insectivore
An insectivore is a type of carnivore with a diet that consists chiefly of insects and similar small creatures. An alternate term is entomophage, which also refers to the human practice of eating insects....
rather than a carnivore.
Classification
A phylogenetic analysis performed by Longrich and Currie found Hesperonychus to be a member of the MicroraptorinaeMicroraptorinae
Microraptorinae is a subfamily of Dromaeosauridae. Microraptorines were among the most primitive dromaeosaurids. They appeared 125 million years ago in China. Many may have been semi-arboreal gliders...
, a 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...
of small dromaeosaurids
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'...
previously thought to be restricted to the Early Cretaceous of 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...
. The authors described this find as "remarkable"; the previously youngest known microraptorine was Microraptor
Microraptor
Microraptor is a genus of small, four-winged dromaeosaurid dinosaurs. Numerous well-preserved fossil specimens have been recovered from Liaoning, China...
itself from the Aptian
Aptian
The Aptian is an age in the geologic timescale or a stage in the stratigraphic column. It is a subdivision of the Early or Lower Cretaceous epoch or series and encompasses the time from 125.0 ± 1.0 Ma to 112.0 ± 1.0 Ma , approximately...
stage of the Early Cretaceous
Early Cretaceous
The Early Cretaceous or the Lower Cretaceous , is the earlier or lower of the two major divisions of the Cretaceous...
, so the discovery of Hesperonychus in the Late Cretaceous
Late Cretaceous
The Late Cretaceous is the younger of two epochs into which the Cretaceous period is divided in the geologic timescale. Rock strata from this epoch form the Upper Cretaceous series...
Campanian
Campanian
The Campanian is, in the ICS' geologic timescale, the fifth of six ages of the Late Cretaceous epoch . The Campanian spans the time from 83.5 ± 0.7 Ma to 70.6 ± 0.6 Ma ...
stage pushed the fossil range of microraptorines forward by 45 million years. While the Late Cretaceous, North American Bambiraptor
Bambiraptor
Bambiraptor is a Late Cretaceous, 75 million year old, bird-like dromaeosaurid theropod dinosaur described by scientists at the University of Kansas, Yale University, and the University of New Orleans....
had sometimes been classified as a microraptorine, more recent studies (including those by Longrich and Currie) have found that it is more closely related to Saurornitholestes
Saurornitholestes
Saurornitholestes is a genus of carnivorous dromaeosaurid theropod dinosaur from the late Cretaceous of Alberta, Montana and New Mexico....
.
Paleobiology
Microraptorines are well known for their small size and, in some cases, ability to fly or glide. Longrich and Currie concluded that it was unlikely for Hesperonychus to exhibit four wings or gliding behavior as in Microraptor, and speculated that it was more likely to be similar to SinornithosaurusSinornithosaurus
Sinornithosaurus is a genus of feathered dromaeosaurid dinosaur from the early Cretaceous Period of the Yixian Formation in what is now China. It was the fifth non–avian feathered dinosaur genus discovered by 1999...
given their closer similarity in size. Nevertheless, Hesperonychus seems to show that microraptorines did not vary much in size, remaining very small relative to other dromaeosaurids throughout their history.
Aside from extending the known range of microraptorines, the discovery of Hesperonychus filled in a gap in the ecology of Late Cretaceous North America. Unlike roughly contemporary environments in Europe and Asia, North America appeared to lack very small carnivorous dinosaurs. In modern ecosystems dominated by endotherm
Endotherm
An endotherm is an organism that produces heat through internal means, such as muscle shivering or increasing its metabolism...
ic mammals, small animal species outnumber larger ones. Since dinosaurs are also presumed to have been endotherms, the lack of small species and great number of known large species in North America was unusual. Hesperonychus helped to fill that gap, especially since, given the number of fragmentary remains and claws that have been collected (representing at least ten distinct specimens, compared to thirty of the contemporary Saurornitholestes and two of Dromaeosaurus
Dromaeosaurus
Dromaeosaurus was a genus of theropod dinosaur which lived during the Late Cretaceous period , sometime between 76.5 and 74.8 million years ago, in the western United States and Alberta, Canada. The name means 'running lizard'....
), it appears to have been a very common feature of the Dinosaur Park Formation environment.
The next smallest carnivore in the environment was the mammal Eodelphis
Eodelphis
Eodelphis meaning "early opossum" is a genus of stagodont marsupials from the Late Cretaceous of North America, with distinctive crushing dentition. Named species include E. browni and the more advanced E. cutleri...
, which weighed only 600 grams. There does not appear to have been any overlap between the smallest dinosaurs and the largest mammals in ecosystems such as this, which Longrich and Currie explained by hypothesizing that either competition from dinosaurs kept mammals from growing larger (the traditional view), competition from mammals kept the dinosaurs from growing smaller, or both.