New Dromaeosaurids (Dinosauria: Theropoda) from the Lower Cretaceous of Utah, and the Evolution of the Dromaeosaurid Tail

Abstract

BackgroundThe Yellow Cat Member of the Cedar Mountain Formation (Early Cretaceous, Barremian? – Aptian) of Utah has yielded a rich theropod fauna, including the coelurosaur Nedcolbertia justinhofmanni, the therizinosauroid Falcarius utahensis, the troodontid Geminiraptor suarezarum, and the dromaeosaurid Utahraptor ostrommaysorum. Recent excavation has uncovered three new dromaeosaurid specimens. One specimen, which we designate the holotype of the new genus and species Yurgovuchia doellingi, is represented by a partial axial skeleton and a partial left pubis. A second specimen consists of a right pubis and a possibly associated radius. The third specimen consists of a tail skeleton that is unique among known Cedar Mountain dromaeosaurids.Methodology/Principal Findings Y. doellingi resembles Utahraptor ostrommaysorum in that its caudal prezygapophyses are elongated but not to the degree present in most dromaeosaurids. The specimen represented by the right pubis exhibits a pronounced pubic tubercle, a velociraptorine trait that is absent in Y. doellingi. The specimen represented by the tail skeleton exhibits the extreme elongation of the caudal prezygapophyses that is typical of most dromaeosaurids. Here we perform a phylogenetic analysis to determine the phylogenetic position of Y. doellingi. Using the resulting phylogeny as a framework, we trace changes in character states of the tail across Coelurosauria to elucidate the evolution of the dromaeosaurid tail.Conclusions/SignificanceThe new specimens add to the known diversity of Dromaeosauridae and to the known diversity within the Yellow Cat paleofauna. Phylogenetic analysis places Y. doellingi in a clade with Utahraptor, Achillobator, and Dromaeosaurus. Character state distribution indicates that the presence of intermediate-length caudal prezygapophyses in that clade is not an evolutionarily precursor to extreme prezygapophyseal elongation but represents a secondary shortening of caudal prezygapophyses. It appears to represent part of a trend within Dromaeosauridae that couples an increase in tail flexibility with increasing size.

Martharaptor greenriverensis, a New Theropod Dinosaur from the Lower Cretaceous of Utah

“…7). Outside Therizinosauroidea the topology of the tree is identical to that found by the study that used the previous draft of the present data matrix [6] and nearly identical to those found by several other recent phylogenetic studies of Coelurosauria [23], [39]–[41]. A decay analysis found low decay indices (Bremer values) of only 1 or 2 for therizinosauroid clades (Fig.…”

New Specimens ofAnchiornis huxleyi(Theropoda: Paraves) from the Late Jurassic of Northeastern China

“…In many previous studies a monophyletic Paraves is recovered as a clade that includes Dromaeosauridae, Troodontidae, and Avialae (Gauthier, 1986;Sereno, 1997, Makovicky andSues, 1998;Xu et al, 1999;Hwang et al, 2002;Makovicky et al, 2003;Xu and Norell, 2004;Makovicky et al, 2005;Novas and Pol, 2005;Norell et al, 2006;Turner et al, 2007Turner et al, , 2012Hu et al, 2009;Senter et al, 2012;Brusatte et al, 2014;Foth et al, 2014;Lü and Brusatte, 2015). Anchiornis huxleyi can be referred to Paraves based on the following derived features: shallow maxilla and premaxilla below the external naris, T-shaped lacrimal; dentary symphyseal region in line with the main part of the buccal margin, chevrons anteriorly bifurcate on the distal part of the tail, nearly symmetric furcula, acromion margin of the scapula with a laterally everted anterior edge, coracoid inflected medially from the scapula, forming an L-shaped scapulocoracoid, humerus longer than the scapula, ilium with the pubic peduncle much larger than the ischiadic peduncle, femur with a posterior trochanter, and the distal end of metatarsal III ginglymoid.…”

“…Within Paraves, the affiliation of Anchiornis is controversial (Xu et al, , 2011Hu et al, 2009;Turner et al, 2012;Senter et al, 2012;Foth et al, 2014). This confusion is caused mainly by the conservative morphology of basal paravians.…”

Shake a Tail Feather: The Evolution of the Theropod Tail into a Stiff Aerodynamic Surface

“…The long tails of many dromaeosaurids appear to have been distally stiffened by elongated prezygapophyses and the elongated processes extending from the anterior portions of the chevron tips [4], [15], [83]. Persons et al .…”

“…Some dromaeosaurids such as Utahraptor (BYU 15465, Table S2 in File S1) appear to have secondarily shortened their prezygapophyses and chevrons, producing a more mobile tail that was suggested to have been a response to the biomechanical demands of a larger body size [83]. Persons et al .…”