Eudromaeosauria
Eudromaeosauria is a subgroup of terrestrial dromaeosaurid theropod dinosaurs. They were small to large-sized predators that flourished during the Cretaceous Period. Eudromaeosaur fossils are known almost exclusively from the northern hemisphere.
They first appeared in the early Cretaceous Period and survived until the end of the Cretaceous. The earliest known definitive eudromaeosaur is the probable dromaeosaurine Yurgovuchia, from the Cedar Mountain Formation, dated to 139 million years ago. However, the earlier fossils such as those of Nuthetes 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 specialized piscivores, eudromaeosaurs functioned as hypercarnivores and are suggested to have been predators of medium- to large-sized prey. Aside from their generally larger size, eudromaeosaurs are also characterized by several features of the foot.
History of study
The subfamily Dromaeosaurinae was first erected in 1922 by William Matthew and Barnum Brown as a part of the "Deinodontidae". Today, Dromaeosaurinae has been formally defined as a monophyletic group including Dromaeosaurus and all the other dromaeosaurs closer to it than to Velociraptor, Microraptor, Passer and Unenlagia. This group was also moved to be within its own family, Dromaeosauridae, which was named when it became apparent that Dromaeosaurus was not closely related to tyrannosaurids.Eudromaeosauria itself 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 either Velociraptor, Dromaeosaurus, or Saurornitholestes than to either of the other two.
This group is further subdivided into three subfamilies: Dromaeosaurinae, Velociraptorinae, and Saurornitholestinae. Dromaeosaurines are usually found to consist of medium- to giant-sized species, with generally box-shaped skulls while the other subfamilies generally have narrower snouts. Velociraptorinae has traditionally included the more slender-snouted species which are found primarily in Asia, although this group may also include North American genera like Dineobellator and Deinonychus. Saurornitholestinae, the most recently named subfamily, typically consists of smaller species with shortened 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.
Most eudromaeosaur genera are known from only 1-2 specimens. The major exceptions to this are Deinonychus, Utahraptor, Saurornitholestes, Velociraptor, and Dromaeosaurus, which are each known from multiple reasonably-complete specimens.
Anatomy
Eudromaeosaurs were all bipedal and had relatively long arms in comparison to other theropods, like most other maniraptorans. Their wrists exhibited the typical maniraptoran condition in the semi-lunate carpal, which allowed them to fold their arms against their body in the same way that modern birds fold their wings. However unlike many other groups of coelurosaurs, eudromaeosaurs possessed relatively short metatarsals.Their second toe possessed the archetypal sickle-claw that all known dromaeosaurids bore which was held off the ground so that only the third and fourth toes touched the ground when walking. Eudromaeosaurs also generally possessed long and stiff tails, which are believed to have been used for balance. There is some direct evidence of eudromaeosaurs such as Velociraptor being feathered. Today, it is believed that all eudromaeosaurs were fully-feathered and possessed wings, along with most, if not all, other maniraptorans.
Size
Eudromaeosaurs likely evolved from small ancestors, only around in mass. Later eudromaeosaurs were generally larger than this, with most being less than long and having masses estimated at around.Eudromaeosaurs are also known to have reached relatively large sizes. Among these were the dromaeosaurines Achillobator, at around, and Utahraptor at up to. The largest eudromaeosaurs are estimated to have been more than in mass. At least one velociraptorine taxon 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 appear to have belonged to an animal similar in size to the North American genus Utahraptor, but the morphology of the teeth suggests that the large size may only be homoplastic. Remains from giant eudromaeosaurs are also reported from the Bissekty and Bayan Shireh formations.
Skull and tooth morphology
The main difference in the skull morphology of eudromaeosaur species that has been observed is that those known from Asia have typically narrower skulls than those in North America. This is generally attributed to a phylogenetic difference, but an analysis by Mark Powers and colleagues in 2020 demonstrated that dromaeosaur snouts in general increased in length during the Cretaceous. The reason for this is not fully understood, but it has been suggested that this reflects a change in the preferred prey of dromaeosaurs that existed from the Early Cretaceous to the Late Cretaceous.Eudromaeosaur skulls are also relatively solid in comparison to their primitive coelurosaur ancestors. In particular, the skull pneumaticity of oviraptorosaurs, which share a common ancestor with both birds and eudromaeosaurs is much higher than in any eudromaeosaurs. A 2021 survey of the premaxillae, maxillae, nasals, lacrimals, and jugals of several eudromaeosaurs was conducted in an attempt to reconstruct the ancestral condition of facial pneumaticity for coelurosaurs. The pneumatic elements of all five bones show a marked decline from basal coelurosaurs to derived paravians, with eudromaeosaurs completely lacking pneumatic spaces in their premaxillae. The reason for this evolutionary trend is unclear.
Compared to other clades of theropods, eudromaeosaurs exhibited relatively little variation in the dimensions of their skulls. Some researchers have suggested that this is a result of their relatively conservative ecology. According to this estimation, most eudromaeosaurs are hypercarnivores of prey similar in size or larger than themselves, which imposes constraints on the functionally effective range of skull shapes. In the same analysis, it is suggested that the earliest eudromaeosaurs had skulls more like velociraptorines than dromaeosaurines or saurornitholestines due to the morphological similarity of troodontid skulls.
The teeth of dromaeosaurines differed from those of velociraptorines in having a low DSDI ratio 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.
Feathers and wings
Throughout the 1990s and early 2000s, a variety of fossil discoveries from the Yixian and Jiufotang formations demonstrated that many small microraptorian dromaeosaurids were covered in coats of feathers and possessed fully asymmetrical pennaceous wing feathers. Among such discoveries were the small dromaeosaurs Sinornithosaurus, Microraptor, Changyuraptor, Zhenyuanlong, Wulong, Daurlong, and at least one unnamed taxon.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. In the years since, similar indirect evidence of feathers in true eudromaeosaurs has been found for the genera Dakotaraptor and Dineobellator.
Today, it is generally believed that most, if not all coelurosaurs had a coat of filamentous feathers. Based on the available evidence it is likely that all paravians and oviraptorosaurs had pennaceous wing feathers on their arms.
Feet and claws
The leg proportions of eudromaeosaurs differed considerably from other maniraptorans and also from the closely related microraptorian dromaeosaurids. Most of these taxa possessed short femora with long tibiae and metatarsals, which are generally accepted to have been adaptations for cursoriality. Conversely, eudromaeosaurs had long femora and tibiae but relatively short metatarsals. The exact reasons for these adaptations are not fully understood, but some authors have suggested that this is an adaptation to improve the strength and robustness of the legs for the purpose of using their feet during predation.Aside from their generally larger size when compared to earlier-diverging dromaeosaurids, 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, 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.