Eudromaeosauria

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. First, there were differences in the positions of the grooves which 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.[4]

The second distinguishing characteristic of eudromaeosaurs is an expanded and enlarged "heel" on the last bone in the second toe (phalange), 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.[4]

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 re-defined as clades, usually defined as all species closer to the groups namesake than to Dromaeosaurus or any namesakes of other sub-clades.[4]

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 is 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 (see section Phylogeny below).[5]

The following classification of the various genera of eudromaeosaurs follows the table provided in Holtz, 2010 unless otherwise noted.[6]

• Eudromaeosauria
  • Subfamily Saurornitholestinae
    • Atrociraptor[7]
    • Bambiraptor?
    • Saurornitholestes
  • Subfamily Dromaeosaurinae
    • Achillobator?
    • Dakotaraptor?[8]
    • Deinonychus?[7]
    • Dromaeosauroides?
    • Dromaeosaurus
    • Itemirus?
    • Utahraptor?
    • Yurgovuchia?[9]
    • Zapsalis?
  • Subfamily Velociraptorinae
    • Acheroraptor?[7]
    • Boreonykus?[10]
    • Dineobellator
    • Linheraptor?
    • Nuthetes?
    • Shri
    • Tsaagan?
    • Velociraptor

• Timeline of dromaeosaurid research