Linheraptor exquisitus

Xu, Xing, Choiniere, Jonah N., Pittman, Michael, Tan, Qingwei, Xiao, Dong, Li, Zhiquan, Tan, Lin, Clark, James M., Norell, Mark A., Hone, David W. E. & Sullivan, Corwin, 2010, A new dromaeosaurid (Dinosauria: Theropoda) from the Upper Cretaceous Wulansuhai Formation of Inner Mongolia, China, Zootaxa 2403, pp. 1-9 : 2-7

Linheraptor exquisitus gen. et sp. nov.

Etymology. The generic name refers to the animal’s status as a predatory dinosaur (‘raptor’) from Linhe, Nei Mongol, China (area of origin); the specific name refers to the exceptional preservation of the holotype specimen.

Holotype. IVPP V 16923 , an articulated, nearly complete skeleton.

Locality and Horizon. Bayan Mandahu, “The Gate” locality, Wulansuhai Formation, Campanian, Upper Cretaceous ( Jerzykiewicz et al. 1993).

Diagnosis. Dromaeosaurid that can be distinguished from other known dromaeosaurid taxa by the presence of the following autapomorphies: greatly enlarged maxillary fenestra sub-equal in size to external naris; several large foramina on lateral surface of jugal. Differs from other known dromaeosaurids except Tsaagan in the following features: large and anteriorly located maxillary fenestra; lacrimal lacking lateral flange over descending process and with relatively broad medial lamina; sharp angle between anterior and ascending processes of quadratojugal; contact between jugal and squamosal that excludes postorbital from infratemporal fenestra. Differs from Tsaagan in the following features: absence of osseous inner wall partly blocking antorbital fenestra; sharply rimmed ventral margin of antorbital fossa; considerably smaller angle between frontal and jugal processes of postorbital; anteroventrally curved postorbital process of squamosal; considerably shorter quadratojugal process of squamosal; dorsoventrally shorter lateral flange of quadrate; less curved and less posteriorly inclined quadrate shaft; paroccipital process more laterally oriented; angular more extended posteriorly towards glenoid fossa; considerably deeper posterior end of mandible such that glenoid fossa is approximately level with tooth row; pneumatic foramen present on axis vertebra.

Description and comparison. The holotype specimen is probably an adult individual as indicated by the complete closure of the neurocentral sutures of all preserved vertebrae and the fusion of the tibiotarsus. Measuring approximately 1.8m in total length, Linheraptor is a relatively small theropod ( Table 1), but is similar in size to other Asian Late Cretaceous dromaeosaurids ( Norell & Makovicky 1997, 1999; Xu 2002; Norell et al. 2006; Turner et al. 2007a, 2007b).

The skull as preserved is shallower than that of Tsaagan , which has a taller maxilla ( Norell et al. 2006). However, preserved skulls of the dromaeosaurid taxon Velociraptor mongoliensis show considerable variation in their proportions due to preservational deformation. Thus, caution is warranted when differentiating dromaeosaurid taxa based on skull proportions alone. For comparison, the ratio of snout length to snout depth at the level of the anterior border of the antorbital fossa is about 0.2 in Linheraptor and 0.3 in Tsaagan . The skull is only slightly longer than the femur ( Table 1), whereas derived dromaeosaurids such as Deinonychus appear to have proportionally longer skulls ( Ostrom 1969). The cranial openings are similar in size, shape, and position to those of most other dromaeosaurids ( Norell & Makovicky 2004): the external naris is located relatively posteriorly, with most of its area lying posterior to the anterior extremity of the maxilla; the maximum anteroposterior diameter of the antorbital fossa is less than half the snout length; the obliquely oriented oval orbit extends further dorsally than the sub-triangular antorbital fenestra; the infratemporal fenestra is kidney-shaped; and the quadrate foramen is large.

Skull length (from snout tip to ventral end of quadrate) 225 Cervical series 320 Trunk length (first dorsal to acetabulum) 365* Anteriormost caudal vertebra 20

A middle caudal vertebra (the longest preserved one) 30

Right scapula 160* Right humerus 155* Right radius 110 Metacarpal II 25

Metacarpal III 63

Metacarpal IV 52

Manual phalanx II-1 49

Manual phalanx III-1 37 * Manual phalanx III-2 52

Manual phalanx III-3 41

Manual phalanx IV-1 25

Manual phalanx IV-2 12

Manual phalanx IV-3 36 * Sternal plate length 80

Sternal plate width (each plate) 55

Right pubis 240 Right femur 230 Right tibiotarsus 255 Right metatarsal II 105 Right metatarsal III 125 Right metatarsal IV 110 Pedal phalanx II-2 25

Pedal phalanx II-3 75

Pedal phalanx III-1 50

Pedal phalanx III-2 25 * Pedal phalanx III-3 20 * Pedal phalanx III-4 45 * Pedal phalanx IV-4 22

Pedal phalanx IV-5 18

The premaxilla bears a long subnarial process that excludes the maxilla from the external naris, as in most dromaeosaurids ( Barsbold & Osmólska 1999; Norell & Makovicky 2004), but in relative length this process is intermediate between the extremely long one in Velociraptor ( Barsbold & Osmólska 1999) and the relatively short ones in most other taxa such as Deinonychus ( Ostrom 1969) and Tsaagan ( Norell et al. 2006) . An important similarity to Tsaagan ( Norell et al. 2006) , and a probable synapomorphy for Linheraptor and Tsaagan amongst dromaeosaurids, is that the large maxillary fenestra is anteriorly located and extends to the anterior border of the antorbital fossa. In other dromaeosaurids, including the two Velociraptor species ( Godefroit et al. 2008), the maxillary fenestra is located considerably posterior to the anterior border of the antorbital fossa. However, the maxillary fenestra of Linheraptor is narrower and more slit-like than that of Tsaagan , which is relatively round. Unlike in Tsaagan ( Norell et al. 2006) , a small promaxillary fenestra is visible in lateral view and is located ventral to the maxillary fenestra. Comparatively, the promaxillary fenestra is larger and more dorsally located in the two Ve lo c ir a pt o r species ( Godefroit et al. 2008). Unlike in Velociraptor ( Barsbold & Osmólska 1999) , Tsaagan , and several other dromaeosaurids ( Norell et al. 2006), the ventral margin of the antorbital fossa is sharply rimmed laterally. As in most other non-avian theropods, Linheraptor has a fully open antorbital fenestra. In Velociraptor and Tsaagan ( Norell et al. 2006) , the anterior portion of the antorbital fenestra has a small osseous medial wall; in some Liaoning deinonychosaurian specimens such as the Mei long holotype ( Xu & Norell 2004), the antorbital fenestra has an even larger osseous floor. Unlike in Tsaagan , in which the nasal is only shallowly inflected, the nasal of Linheraptor is dorsally concave in lateral view. It bears a row of distinctive foramina close to the lateral edge as in Deinonychus ( Ostrom 1969) , Ve l o c i r a p t o r ( Barsbold & Osmólska 1999), and Tsaagan ( Norell et al. 2006) . The T-shaped lacrimal is similar to that of Tsaagan ( Norell et al. 2006) in lacking a lateral flange over the descending process, and in bearing a relatively broad medial lamina that floors the posterodorsal corner of the antorbital fossa. The robust jugal bears a few relatively large foramina on its lateral surface. The postorbital process of the jugal contacts the squamosal to exclude the postorbital from the infratemporal fenestra, a feature otherwise known only in Tsaagan ( Norell et al. 2006) . In Linheraptor , the frontal and jugal processes of the postorbital are angled at slightly more than 90 degrees to each other, while in Tsaagan the angle between these processes is approximately 135 degrees ( Norell et al. 2006). As in other dromaeosaurids ( Norell et al. 2006), the quadratojugal is shaped like an inverted T, with the posterior process being much larger than the anterior process. As in Tsaagan , the angle between the anterior and ascending processes is smaller than 90 degrees. The quadratojugal forms the anterior and lateral borders of the quadrate foramen, which is proportionally even larger than in other dromaeosaurids except Ve lo c ir a pt o r ( Barsbold & Osmólska 1999). The ratio of the maximum diameter of the quadrate foramen to the height of the quadrate is 0.56 in Linheraptor , but significantly smaller than 0.5 in other dromaeosaurids except Velociraptor . The presence of a greatly enlarged and more laterally-facing foramen in Linheraptor and Velociraptor is associated with a number of features that distinguish these taxa from most other dromaeosaurids. These include proportionally longer ascending and posterior processes of the quadratojugal and a dorsoventrally shorter lateral flange of the quadrate. The posterior edge of the quadrate is less concave than in other dromaeosaurids ( Norell et al. 2006). As in other dromaeosaurids, the squamosal has an anteroventrally oriented quadratojugal process that is inset from the lateral margin of the bone. This process is considerably shorter than that of Tsaagan ( Norell et al. 2006) . The postorbital process of the squamosal curves distinctly ventrally as in Velociraptor and Deinonychus ( Ostrom 1969) , unlike the straight postorbital process of Tsaagan ( Norell et al. 2006) . As in Velociraptor and other dromaeosaurids ( Barsbold & Osmólska 1999), the paroccipital processes are posterolaterally oriented. In Tsaagan , the processes are pendant and are proximodistally longer ( Norell et al. 2006), whereas in Mahakala they are intermediate in condition ( Turner et al. 2007b). The braincase is currently under preparation, so it is not possible to determine if the basipterygoid processes are elongate as in Tsaagan .

The mandible has a slightly downturned anterior end, a feature described in some basal dromaeosaurids such as Microraptor ( Xu 2002) . It is nevertheless similar to those of other dromaeosaurids in possessing the following features: symmetric concave dorsal margin and convex ventral margin, two rows of mental foramina on the dentary, and prominent surangular crest overhanging an enlarged surangular foramen. A second surangular foramen appears to be present, a feature also seemingly present in Tsaagan ( Norell et al. 2006) . Relative to other dromaeosaurids, other distinctive features of the mandible of Linheraptor include a greater posterior extension of the angular that approaches the posterior level of the mandibular glenoid fossa, and a considerably taller posterior end of the mandible such that the glenoid fossa is approximately level with the tooth row (in Tsaagan and Dromaeosaurus the posterior end of the mandible is shallow, and the glenoid fossa lies considerably ventral to the tooth row: Currie 1995; Norell et al. 2006).

Poor preservation of the dentition precludes exact counts of the premaxillary, maxillary, and dentary teeth. However, the dentary is inferred to bear about 15 teeth, a number similar to the counts recorded in Tsaagan , Velociraptor , and some other dromaeosaurids ( Barsbold & Osmólska 1999; Norell et al. 2006). The maxillary and dentary teeth are relatively sparsely distributed. As in Tsaagan ( Norell et al. 2006) and also most basal dromaeosaurids ( Xu 2002), all teeth lack serrations along the anterior carina.

There are 10 cervical vertebrae including the atlas and axis. Corresponding to the S-shaped neck, the neural spines are oriented as follows along the cervical series: posteriorly in the second (axis), third and fourth cervicals; subvertically in the fifth cervical; anteriorly in the sixth through ninth cervicals; and nearly vertically again in the tenth. The neural spines of nearly all the presacral vertebrae other than the axis show little transverse expansion distally. The axis bears a large pneumatic foramen in the center of the lateral surface of the centrum, as in Mahakala ( Turner et al. 2007b), Velociraptor and Deinonychus , but not Tsaagan ( Norell et al. 2006) . Distinct pneumatic foramina are not visible on the lateral central surfaces of the middle and posterior cervical vertebrae, although deep lateral fossae are present. Epipophyses are well developed on the anterior cervical vertebrae, and the fifth vertebra bears an epipophysis that extends posteriorly to the level of the posterior edge of the postzygapophysis. Most cervical rib shafts, except the two most posterior ones, span nearly two cervicals. The sixth and seventh cervical ribs are the most robust, while the ninth and tenth (and possibly the eighth) are relatively short anteroposteriorly. In the middle of the cervical series, the cervical rib heads are partially fused to the corresponding vertebrae. The posterior surfaces of the dorsal ribs bear long, slender uncinate processes, which arise from the mid-shaft region in each case. The uncinate processes angle dorsally, and each process extends posteriorly beyond the posterior margin of the next rib in the series, as in Velociraptor ( Norell & Makovicky 1999) . As in most dromaeosaurids except the unenlagiines and Mahakala ( Makovicky et al. 2005; Turner et al. 2007b), the caudal vertebrae possess extremely elongated prezygapophyses and chevrons. The middle and posterior caudal vertebrae are not significantly elongated. The longest middle caudal vertebrae are proportionally much shorter than those of basal paravians including basal dromaeosaurids ( Xu 2002). They are approximately 170% as long as the most anterior caudal vertebrae, a condition similar to derived dromaeosaurids such as Velociraptor ( Norell & Makovicky 1999) . The lateral surface of each middle caudal vertebra is flat and lacks a longitudinal groove or ridge near the neurocentral suture as in some basal dromaeosaurids such as Mahakala , Buitreraptor and Rahonavis ( Makovicky et al. 2005; Turner et al. 2007b). Distinct, strap-like transverse processes are present on at least the first 12 caudal vertebrae. In Ve l o c i r a p t o r, transverse processes are present on only 11 caudal vertebrae ( Norell & Makovicky 1999). Interestingly, the transition is sudden, with no vertebrae intermediate between those with distinct transverse processes and those that lack transverse processes entirely. Long, low, distinct neural spines are present back to at least the 15th caudal vertebra.

The two sternal plates are fused anteriorly but separate posteriorly. The sternal plates are transversely curved, and the midline of the entire bipartite structure forms a low carina.

The humerus is less than 70% the length of the femur. A prominent longitudinal groove is present along the posterior surface immediately distal to the deltopectoral crest, a feature shared with Ve l o c i r a p t o r and Deinonychus ( Norell & Makovicky 1999) . This groove is much wider proximally than distally. The radius bears a semilunate lateral flange distally. On the basis of positional homologies, we identify the manual digits of Linheraptor as corresponding to the middle three digits of ancestral archosaurs, as is currently hypothesized for all maniraptorans (Xu et al. 2009). While the manus is in general similar to that of derived dromaeosaurids, manual phalanx IV-1 is more than twice as long as IV-2, a condition shared with basal dromaeosaurids such as Microraptor ( Xu 2002) . Although IV-1 is considerably longer than IV- 2 in all dromaeosaurids ( Xu 2002), IV-1 is less than twice the length of IV- 2 in Archaeopteryx ( Wellnhofer 2008) and in derived dromaeosaurids like Deinonychus ( Ostrom 1969) and Velociraptor ( Norell & Makovicky 1999) .

The posteriorly projecting pubis is longer than the femur as in some other dromaeosaurids, such as Velociraptor and Deinonychus ( Ostrom 1976) . This feature may characterize derived dromaeosaurids, given that basal dromaeosaurids resemble most other non-avian theropods in having a pubis shorter than the femur ( Xu 2002). A longer pubis relative to the femur appears to be correlated with a full posterior rotation of the pubis in derived dromaeosaurids. The pubis is straight in lateral view, and although it lacks a distinct pubic boot, the distal quarter is anteroposteriorly enlarged relative to the shaft. Proximal to the anteroposteriorly expanded distal region, the pubic shaft forms a considerable lateral expansion as in Veloc irap tor. This condition is somewhat similar to that of basal dromaeosaurids from Liaoning ( Xu 2002), in which a lateral projection occurs near the midpoint of the pubic shaft. The hindlimb has proportions intermediate between those of basal and derived dromaeosaurids ( Table 1): the tibiotarsus and metatarsus are respectively 111% and 54% the length of the femur. In derived dromaeosaurids such as Velociraptor ( Norell & Makovicky 1997, 1999), the distal segments of the hindlimb are proportionally shorter. The astragalus and calcaneum are fused to each other and appear to be co-ossified with the tibia, forming a true tibiotarsus. The tibia bears a distinct tuberosity medial to the distal end of the fibular crest, along the anteromedial margin of the shaft. The two hemicondyles of the astragalus-calcaneum complex are transversely narrow.