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Eudromaeosaurs Temporal range: Early Cretaceous – Late Cretaceous, Likely Kimmeridgian record
Eudromaeosauria diversity, featuring from top left to lower right: Utahraptor, Deinonychus, Velociraptor and Bambiraptor
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	Saurischia
Clade:	Theropoda
Family:	†Dromaeosauridae
Clade:	†Eudromaeosauria Longrich & Currie, 2009
Type species
†Dromaeosaurus albertensis Matthew & Brown, 1922
Subgroups
†Dromaeosaurinae Matthew & Brown, 1922 †Achillobator †Dakotaraptor? (possible chimera) †Dromaeosaurus †Utahraptor †Yurgovuchia †Saurornitholestinae Longrich & Currie, 2009 †Atrociraptor †Bambiraptor †Saurornitholestes †Velociraptorinae Barsbold, 1983 †Acheroraptor †Adasaurus †Boreonykus? †Dineobellator †Kansaignathus †Kuru †Luanchuanraptor †Linheraptor †Shri †Tsaagan †Velociraptor Eudromaeosaurs of uncertain affinity †Balaur? †Deinonychus †Dromaeosauroides †Itemirus †Nuthetes †Pyroraptor? †Ulughbegsaurus? †Variraptor? †Vectiraptor

Eudromaeosauria ("true dromaeosaurs") is a subgroup of terrestrial dromaeosaurid theropod dinosaurs. They were small to large-sized, feathered hypercarnivores (with diets consisting almost entirely of other terrestrial vertebrates) that flourished in the Cretaceous Period.

Eudromaeosaur fossils are known almost exclusively from the northern hemisphere. They first appeared in the early Cretaceous Period (early Aptian stage, about 124 million years ago) and survived until the end of the Cretaceous (Maastrichtian stage, 66 Ma). The earliest known definitive eudromaeosaur is the probable dromaeosaurine Yurgovuchia, from the Cedar Mountain Formation, dated to 139 million years ago. However, the earlier (143-million-year-old) fossils such as those of Nuthetes destructor and several indeterminate teeth dating to the Kimmeridgian stage may represent eudromaeosaurs.

While other dromaeosaurids filled a variety of specialized ecological niches, mainly those of small predators or larger fish-eating forms, eudromaeosaurs functioned as large-bodied predators of often medium- to large-sized prey. Aside from their generally larger size, eudromaeosaurs are characterized by several features of the foot.

History of study

The Dromaeosaurinae was first erected in 1922 by Matthew and Brown as a part of the "Deinodontidae" (now named Tyrannosauridae). Today, Dromaeosaurinae is defined as a monophyletic group including Dromaeosaurus and all the other dromaeosaurs closer to it than to Velociraptor, Microraptor, Passer and Unenlagia.

Eudromaeosauria was first defined as a node-based clade by Nick Longrich and Philip J. Currie in 2009, as the most inclusive natural group containing Dromaeosaurus, Velociraptor, Deinonychus, and Saurornitholestes, their most recent common ancestor and all of its other descendants. The various "subfamilies" have also been redefined as clades, usually defined as all species closer to Velociraptor, Dromaeosaurus, or Saurornitholestes, respectively.

The subgroups of Eudromaeosauria frequently shift in content based on new analysis, but typically consist of the following groups. For example, the subfamily Velociraptorinae has traditionally included Velociraptor, Deinonychus, and Saurornitholestes, and while the discovery of Tsaagan lent support to this grouping, the inclusion of Saurornitholestes is still uncertain. The Dromaeosaurinae are usually found to consist of medium- to giant-sized species, with generally box-shaped skulls (the other subfamilies generally have narrower snouts). A number of eudromaeosaurs have not been assigned to any particular subfamily, because they are too poorly preserved to be placed confidently in phylogenetic analysis.

Anatomy

Size

The size of three of the largest eudromaeosaurs compared to a human

Most North American and Asian dromaeosaurines from the Late Cretaceous were generally medium to large-sized animals, with an average length between 1.8 metres (5.9 ft); i. e., Dromaeosaurus and Yurgovuchia. However, among the dromaeosaurines were the largest dromaeosaurs ever, with the feathered Dakotaraptor measuring 5.5 metres (18 ft) long, Achillobator 6 metres (20 ft), and Utahraptor up to 7 metres (23 ft).

While most velociraptorines were generally small animals, at least one species may have achieved gigantic sizes comparable to those found among the dromaeosaurines. So far, this unnamed giant velociraptorine is known only from isolated teeth found on the Isle of Wight, England. The teeth belong to an animal the size of dromaeosaurines of the genus Utahraptor, but they appear to belong to a velociraptorine, judging by the shape of the teeth and the anatomy of their serrations.

Skull features

Dromaeosaurines were a group of eudromaeosaurs that can be recognised in having stouter, box-shaped skulls, as opposed to the other subfamilies, which generally have narrower snouts, also, dromaeosaurines are generally more heavily built than the other members of their family, with thick, heavy-set legs, which were designed more for strength, rather than for speed. They differ from velociraptorines, in having a low DSDI ratio; i. e., their teeth have equal-sized serrations, on both the posterior and on the anterior edges. By contrast, velociraptorines often have larger serrations on the posterior side of the tooth, than the anterior, or no serrations on the anterior side at all.

Integument

In 2007 paleontologists studied the ulna of a specimen of Velociraptor and discovered small bumps on the surface, known as quill knobs. The same feature is present in some bird bones, and represents the attachment point for strong secondary wing feathers. This finding provided the first direct evidence that eudromaeosaurs had feathers. Today, it is generally believed that all paravians and oviraptorosaurs (and possibly ornithomimosaurs) had pennaceous wing feathers.

Feet and claw function

Possible modes of predation and combat for eudromaeosaurs

Aside from their generally larger size, eudromaeosaurs are characterized by several features of the foot. First, differences existed in the positions of the grooves that anchored blood vessels and keratin sheathes of the toe claws. In primitive dromaeosaurids like Hesperonychus, these grooves ran parallel to each other on either side of the claw along its length. In eudromaeosaurs, the grooves were asymmetrical, with the inner one split into two distinct grooves and elevated toward the top of the claw, while the single outer groove remained positioned at the midline.

The second distinguishing characteristic of eudromaeosaurs is an expanded and enlarged "heel" on the last bone in the second toe (phalanx), which bore the enlarged, sickle-like toe claw. Finally, the first bone of the second toe also possessed an enlarged expansion at the joint, another adaptation relating to the unusually enlarged claw, and which helped the animal hold the claw high off the ground. Also unlike their more basal relatives, the sickle claw of eudromaeosaurs was sharper and more blade-like. In unenlagiines and microraptorines, the claw is broader at its base.

Classification

Phylogeny

• From Napoli and colleagues (2021)

Paraves	Avialae Deinonychosauria Troodontidae Dromaeosauridae Halszkaraptorinae Unenlagiinae Shanag Microraptoria Eudromaeosauria

Hartman et al., 2018

Paraves Avialae Scansoriopterygidae Euavialae Deinonychosauria Archaeopterygidae Halszkaraptorinae Unenlagiinae Troodontidae Dromaeosauridae Microraptoria Eudromaeosauria

Motta et al., 2020

Paraves Troodontidae Eudromaeosauria Microraptoria Unenlagiidae Avialae Overoraptor Rahonavis

Systematics

According to Turner et al. 2012, technical diagnoses for the first subfamily of eudromaeosaurs, the Dromaeosaurinae, can be established based on the following traits: fully serrated teeth; vertically oriented pubis; pubic boot (or end) projecting anteriorly and posteriorly; the jugal process of the maxilla, in a ventral view to the external antorbital fenestra, is dorsoventrally wide.

When erected by Barsbold in 1983, the second subfamily of eudromaeosaurs — Velociraptorinae, was conceived as a group containing Velociraptor and supposed closely related species. It was not until 1998 that this group was defined as a clade by Paul Sereno. Sereno defined the group as all dromaeosaurids more closely related to Velociraptor than to Dromaeosaurus. While several studies have since recovered a group of dromaeosaurids closely related to Velociraptor, they vary widely regarding which species are actually velociraptorines and which are either more basal or closer to Dromaeosaurus.

Novas and Pol (2005) found a distinct velociraptorine clade close to the traditional view, which included Velociraptor, Deinonychus, and material that was later named Tsaagan. A cladistic analysis conducted by Turner et al. (2012) also supported a traditional, monophyletic of Velociraptorinae. However, some studies found a very different group of dromaeosaurids in velociraptorinae, such as Longrich and Currie (2009), which found Deinonychus to be a non-velociraptorine, non-dromaeosaurine eudromaeosaur, and Saurornitholestes to be a member of a more basal group they named Saurornitholestinae. A larger analysis in 2013 found some traditional velociraptorines, such as Tsaagan, to be more basal than Velociraptor, while others to be more closely related to Dromaeosaurus, making them dromaeosaurines. This study found Balaur, previously found to be a velociraptorine by most analyses, to be an avialan instead.

Saurornitholestinae is the third, and most recently named, subfamily of Eudromaeosauria. The saurornitholestines currently include three monotypic genera: Atrociraptor marshalli, Bambiraptor feinbergi, and Saurornitholestes langstoni. All are medium-sized dromaeosaurs from the Late Cretaceous of western North America. The group was originally recognized by Longrich and Currie as the sister taxon to a clade formed by the Dromaeosaurinae and Velociraptorinae. However, not all phylogenetic analyses recover this group and/or with the same proposed genera.

Paleoecology

Most dromaeosaurines lived in what is now Asia, North America and possibly Denmark during the Cretaceous period, from the Berriasian to the Maastrichtian stages. However, isolated teeth that may belong to African dromaeosaurines have also been discovered in Ethiopia. These teeth date to the Tithonian stage, of the Late Jurassic period.

See also

• Dinosaurs in Jurassic Park
• Timeline of dromaeosaurid research

Gallery

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