Hesperonychus
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''Hesperonychus'' (meaning "western claw") is a
genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...
of small paravian
theropod Theropoda (; from ancient Greek , (''therion'') "wild beast"; , (''pous, podos'') "foot"">wiktionary:ποδός"> (''pous, podos'') "foot" is one of the three major groups (clades) of dinosaurs, alongside Ornithischia and Sauropodom ...
dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...
. It may be a
dromaeosaurid Dromaeosauridae () is a family (biology), family of feathered coelurosaurian Theropoda, theropod dinosaurs. They were generally small to medium-sized feathered carnivores that flourished in the Cretaceous period (geology), Period. The name Drom ...
or an avialan. There is one described species, ''Hesperonychus elizabethae''. The
type species In International_Code_of_Zoological_Nomenclature, 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 spe ...
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 preserve ...
s recovered from the Dinosaur Park Formation and possibly from the uppermost strata of the Oldman Formation of
Alberta Alberta is a Provinces and territories of Canada, province in Canada. It is a part of Western Canada and is one of the three Canadian Prairies, prairie provinces. Alberta is bordered by British Columbia to its west, Saskatchewan to its east, t ...
, dating to 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. Campa ...
stage of the
Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the more recent 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 ''cre ...
around .


Description

''Hesperonychus'' is mainly known from one partial
pelvic girdle The hip bone (os coxae, innominate bone, pelvic bone or coxal bone) is a large flat bone, constricted in the center and expanded above and below. In some vertebrates (including humans before puberty) it is composed of three parts: the Ilium (bone) ...
,
holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...
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 discovered from the Dinosaur Park Formation and Oldman Formation, including "sickle claws", in the collection of the Royal Tyrrell Museum have been tentatively referred to as
cf. The abbreviation cf. (short for either Latin or , both meaning 'compare') is generally used in writing to refer the reader to other material to make a comparison with the topic being discussed. However some sources offer differing or even contr ...
''Hesperonychus''. The gracile appearances of these potential toe bones make it unlikely that ''Hesperonychus'' belonged to the
Eudromaeosauria Eudromaeosauria (International Phonetic Alphabet, ; "true dromaeosaurs") is a subgroup of terrestrial Dromaeosauridae, dromaeosaurid theropod dinosaurs. They were small to large-sized predators that flourished during the Cretaceous Period (geolo ...
. 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 partial remains, researchers have estimated that it was a small dinosaur measuring about long and weighing between , making it one of the smallest non-avian dinosaurs known from North America.


Classification

A
phylogenetic In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...
analysis performed by Longrich and Currie found ''Hesperonychus'' to be a member of the Microraptorinae, a
clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...
of small dromaeosaurids previously thought to be restricted to the Early Cretaceous of
Asia Asia ( , ) is the largest continent in the world by both land area and population. It covers an area of more than 44 million square kilometres, about 30% of Earth's total land area and 8% of Earth's total surface area. The continent, which ...
. The authors described this find as "remarkable"; the previously youngest known microraptorine was ''
Microraptor ''Microraptor'' (Greek language, Greek, μικρός, ''mīkros'': "small"; Latin language, Latin, ''raptor'': "one who seizes") is a genus of small, four-winged dromaeosaurid dinosaurs. Numerous well-preserved fossil specimens have been recovere ...
'' itself from the
Aptian The Aptian is an age (geology), age in the geologic timescale or a stage (stratigraphy), stage in the stratigraphic column. It is a subdivision of the Early Cretaceous, Early or Lower Cretaceous epoch (geology), Epoch or series (stratigraphy), S ...
stage of the
Early Cretaceous The Early Cretaceous (geochronology, geochronological name) or the Lower Cretaceous (chronostratigraphy, chronostratigraphic name) is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 143.1 ...
, so the discovery of ''Hesperonychus'' in the
Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the more recent 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 ''cre ...
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. Campa ...
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, other researchers (including Longrich and Currie) have found that it is more closely related to '' Saurornitholestes''. ''Hesperonychus'' was assigned to Microraptoria due to having a spatulate (rounded)
pubic symphysis The pubic symphysis (: symphyses) 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 attache ...
, 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): However, subsequent studies have questioned its identity as a microraptorine, with some researchers excluding the taxon from phylogenetic analyses due to its fragmentary remains and others classifying it in various positions within or outside dromaeosaurids. In 2012, Martyniuk considered ''Hesperonychus'' as a
eudromaeosauria Eudromaeosauria (International Phonetic Alphabet, ; "true dromaeosaurs") is a subgroup of terrestrial Dromaeosauridae, dromaeosaurid theropod dinosaurs. They were small to large-sized predators that flourished during the Cretaceous Period (geolo ...
n. In 2013, microraptorines were considered as averaptorans outside dromaeosaurids. In 2014, Brusatte and colleagues pruned five "wildcard" taxa including the paravians '' Pyroraptor'' and ''Hesperonychus'' from phylogenetic analyses, mainly due to a huge lack of data, indicating that the definitive classifications of these taxa are uncertain. Other studies have also followed this exclusion of ''Hesperonychus''. In 2019, Hartman and colleagues suggested that ''Hesperonychus'' is actually an avialan close to modern
bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...
s like '' Balaur bondoc'' based on phylogenetic analyses, though they disagreed with microraptorines being avialans. In the same year, Rauhut and colleagues considered various genera of theropods including ''Hesperonychus'' as 'problematic taxa' due to their unstable phylogenetic position. In 2020, ''Hesperonychus'' was recovered as a dromaeosaurid and a sister taxon, but not a member, of the polytomic microraptorines.


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'' 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, though it is questionable whether it was indeed a microraptorine or a dromaeosaurid. Whether it was a microraptorine or not, 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 inst ...
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.


See also

* Timeline of dromaeosaurid research * 2009 in paleontology


References

{{Taxonbar, from=Q751087 Paraves Dinosaur genera Campanian dinosaurs Dinosaur Park Formation Fossil taxa described in 2009 Taxa named by Philip J. Currie Taxa named by Nicholas R. Longrich Dinosaurs of Canada