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''Hesperonychus'' (meaning "western claw") was a small,
carnivorous A carnivore , or meat-eater (Latin, ''caro'', genitive ''carnis'', meaning meat or "flesh" and ''vorare'' meaning "to devour"), is an animal or plant whose food and energy requirements derive from animal tissues (mainly muscle, fat and other sof ...
dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
. It was a member of the family
Dromaeosauridae Dromaeosauridae () is a family of feathered theropod dinosaurs. They were generally small to medium-sized feathered carnivores that flourished in the Cretaceous Period. The name Dromaeosauridae means 'running lizards', from Greek ('), meaning ...
, along with its larger relatives ''
Deinonychus ''Deinonychus'' ( ; ) is a genus of dromaeosaurid theropod dinosaur with one described species, ''Deinonychus antirrhopus''. This species, which could grow up to long, lived during the early Cretaceous Period, about 115–108 million ye ...
'' and ''
Velociraptor ''Velociraptor'' (; ) is a genus of small dromaeosaurid dinosaur that lived in Asia during the Late Cretaceous epoch, about 75 million to 71 million years ago. Two species are currently recognized, although others have been assigned in the pa ...
''. There is one described species, ''Hesperonychus elizabethae''. The
type species In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specimen ...
was named in honor of Dr. Elizabeth Nicholls of the Royal Tyrrell Museum of Palaeontology who collected it as a student in 1982. It is known from
fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
s recovered from the lowermost strata of the
Dinosaur Park Formation The Dinosaur Park Formation is the uppermost member of the Belly River Group (also known as the Judith River Group), a major geologic unit in southern Alberta. It was deposited during the Campanian stage of the Late Cretaceous, between about 76. ...
of
Alberta Alberta ( ) is one of the thirteen provinces and territories of Canada. It is part of Western Canada and is one of the three prairie provinces. Alberta is bordered by British Columbia to the west, Saskatchewan to the east, the Northwest Ter ...
, dating to the late
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...
(
Campanian The Campanian is the fifth of six ages of the Late Cretaceous Epoch on the geologic timescale of the International Commission on Stratigraphy (ICS). In chronostratigraphy, it is the fifth of six stages in the Upper Cretaceous Series. Campanian s ...
stage Stage or stages may refer to: Acting * Stage (theatre), a space for the performance of theatrical productions * Theatre, a branch of the performing arts, often referred to as "the stage" * ''The Stage'', a weekly British theatre newspaper * Sta ...
) about 76.5 
million years ago The abbreviation Myr, "million years", is a unit of a quantity of (i.e. ) years, or 31.556926 teraseconds. Usage Myr (million years) is in common use in fields such as Earth science and cosmology. Myr is also used with Mya (million years ago). ...
. Though known from partial remains, researchers have estimated that it was a small dinosaur measuring about long and weighing between , making it the smallest carnivorous non-avian dinosaur known from North America.


Description

''Hesperonychus'' is known from one partial
pelvic girdle The pelvis (plural pelves or pelvises) is the lower part of the trunk, between the abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also called bony pelvis, or pelvic skeleton). The p ...
,
holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of several ...
specimen UALVP 48778, collected by Dr. Elizabeth Nicholls in
Dinosaur Provincial Park Dinosaur Provincial Park is a UNESCO World Heritage Site situated a two hour drive east of Calgary, Alberta, Canada; or , about a half-hour drive northeast of Brooks. The park is situated in the Red Deer River valley, which is noted for its strik ...
in 1982. The fossil remained undescribed, however, until Nick Longrich and
Phil Currie Philip John Currie (born March 13, 1949) 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. In the ...
published on it in 2009. A number of very small toe bones, including "sickle claws", in the collection of the
Royal Tyrrell Museum The Royal Tyrrell Museum of Palaeontology (RTMP, and often referred to as the Royal Tyrrell Museum) is a palaeontology museum and research facility in Drumheller, Alberta, Canada. The museum was named in honour of Joseph Burr Tyrrell, and is situ ...
also belong to ''Hesperonychus''. The gracile appearance of these toe bones makes it unlikely that they belonged to a member of
Eudromaeosauria Eudromaeosauria ("true dromaeosaurs") is a subgroup of terrestrial dromaeosaurid theropod dinosaurs. They were relatively small to medium-sized, feathered hypercarnivores (with diets consisting almost entirely of other terrestrial vertebrates) ...
. 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.


Classification

A
phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...
analysis performed by Longrich and Currie found ''Hesperonychus'' to be a member of the
Microraptorinae Microraptoria (Greek, μίκρος, ''mīkros'': "small"; Latin, ''raptor'': "one who seizes") is a clade of basal dromaeosaurid theropod dinosaurs. The first microraptorians appeared 125 million years ago in China. Many are known for long feather ...
,Canadian dig yields tiny dinosaur
a 16 March 2009 article from
BBC News BBC News is an operational business division of the British Broadcasting Corporation (BBC) responsible for the gathering and broadcasting of news and current affairs in the UK and around the world. The department is the world's largest broadca ...
a
clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...
of small
dromaeosaurids Dromaeosauridae () is a family of feathered theropod dinosaurs. They were generally small to medium-sized feathered carnivores that flourished in the Cretaceous Period. The name Dromaeosauridae means 'running lizards', from Greek ('), meaning ...
previously thought to be restricted to the Early Cretaceous of
Asia Asia (, ) is one of the world's most notable geographical regions, which is either considered a continent in its own right or a subcontinent of Eurasia, which shares the continental landmass of Afro-Eurasia with Africa. Asia covers an area ...
. The authors described this find as "remarkable"; the previously youngest known microraptorine was ''
Microraptor ''Microraptor'' (Greek, μικρός, ''mīkros'': "small"; Latin, ''raptor'': "one who seizes") is a genus of small, four-winged dromaeosaurid dinosaurs. Numerous well-preserved fossil specimens have been recovered from Liaoning, China. They ...
'' itself from the
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 121.4 ± 1.0 Ma to 113.0 ± 1.0 Ma (million years ago), a ...
stage of the
Early Cretaceous The Early Cretaceous ( geochronological name) or the Lower Cretaceous (chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 145  Ma to 100.5 Ma. Geology Pro ...
, so the discovery of ''Hesperonychus'' in the
Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', the ...
Campanian The Campanian is the fifth of six ages of the Late Cretaceous Epoch on the geologic timescale of the International Commission on Stratigraphy (ICS). In chronostratigraphy, it is the fifth of six stages in the Upper Cretaceous Series. Campanian s ...
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 ''Saurornitholestes'' ("lizard-bird thief") is a genus of carnivorous dromaeosaurid theropod dinosaur from the late Cretaceous of Canada (Alberta) and the United States (Montana, New Mexico, Alabama, North Carolina, and South Carolina). Two spec ...
''. ''Hesperonychus'' was assigned to
Microraptoria Microraptoria (Greek, μίκρος, ''mīkros'': "small"; Latin, ''raptor'': "one who seizes") is a clade of basal dromaeosaurid theropod dinosaurs. The first microraptorians appeared 125 million years ago in China. Many are known for long feather ...
due to having a spatulate (rounded)
pubic symphysis The pubic symphysis is a secondary cartilaginous joint between the left and right superior rami of the pubis of the hip bones. It is in front of and below the urinary bladder. In males, the suspensory ligament of the penis attaches to the pubic ...
, a strong posterior curvature of the distal shaft of the pubis, and lateral tubercules on the pubes (which are expanded into 'wing-like' structures in the case of ''Hesperonychus''). Cladogram (2012):


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 ''
Sinornithosaurus ''Sinornithosaurus'' (derived from a combination of Latin and Greek, meaning 'Chinese bird-lizard') is a genus of feathered dromaeosaurid dinosaur from the early Cretaceous Period (late Barremian) of the Yixian Formation in what is now China. ...
'' 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 An endotherm (from Greek ἔνδον ''endon'' "within" and θέρμη ''thermē'' "heat") is an organism that maintains its body at a metabolically favorable temperature, largely by the use of heat released by its internal bodily functions inste ...
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'' (, "running lizard") is a genus of dromaeosaurid theropod dinosaur which lived during the Late Cretaceous period (middle late Campanian and Maastrichtian), sometime between 80 and 69 million years ago, in Alberta, Canada and the ...
''), 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'', from eo- plus /nowiki>''delphis''.html" ;"title="/nowiki>/nowiki>''delphis''">/nowiki>/nowiki>''delphis'', thus meaning "very early opossum", is a genus of stagodont metatherians from the Late Cretaceous of North America, with ...
'', 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.


See also

*
Timeline of dromaeosaurid research This timeline of dromaeosaurid research is a chronological listing of events in the history of paleontology focused on the dromaeosaurids, a group of sickle-clawed, bird-like theropod dinosaurs including animals like ''Velociraptor''. Since the ...
* 2009 in paleontology


References

{{Taxonbar, from=Q751087 Late Cretaceous dinosaurs of North America Microraptorians Fossil taxa described in 2009 Taxa named by Philip J. Currie Dinosaur Park fauna Oldman fauna Paleontology in Alberta Campanian genus first appearances Campanian genus extinctions