DROMAEOSAURIDAE

Pittman, Michael & Xu, Xing, 2020, Pennaraptoran Theropod Dinosaurs Past Progress And New Frontiers, Bulletin of the American Museum of Natural History 2020 (440), pp. 1-353 : 48-54

DROMAEOSAURIDAE

Dromaeosaurid fossils have been found on almost all modern continental landmasses including members that appear to have had volant capabilities (Turner et al., 2012; Pei et al., in press; fig. 1; table 3).

NORTH AMERICA: In 1922, Dromaeosaurus , from the Campanian Dinosaur Park Formation of Alberta, Canada, was the first dromaeosaurid to be described. It lends its name to the clade and is known from partial cranial and very fragmentary postcranial material (Matthew and Brown, 1922; Currie, 1995). The Dinosaur Park Formation has also yielded Saurornitholestes , a relatively completely known taxon thought to be from only one species, Saurornitholestes langstoni (Sues, 1978; Turner et al., 2012). This taxon lacked a proper diagnosis until recently and is likely represented by multiple partial skeletons. Recently, an exquisite skull and skeleton from Dinosaur Provincial Park were recovered allowing for a revised diagnosis (Currie and Evans, 2020). Evidence of tooth-marked bones and a broken tip of a tooth still embedded in a bone suggest that this taxon ate azhdarchid pterosaurs on occasion (Currie and Jacobsen, 1995). Hesperonychus elizabethae is known from a single incomplete pelvis and referred pedal bones recovered from the Dinosaur Park Formation (Longrich and Currie, 2009. This taxon is North America’s only named microraptorine and the youngest one worldwide by almost 45 million years (Longrich and Currie, 2009). Atrociraptor marshalli is a fragmentary taxon recovered from the similarly aged Horseshoe Canyon Formation from the same part of Canada (Currie and Varricchio, 2004). It consists of a partial rostrum, including both premaxillae, a right maxilla, and both dentaries. The snout of this dromaeosaurid appears to be quite short and deep, given the abbreviated nature of the facial process of the maxilla. Across the border in neighboring Montana, the Campanian Two Medicine Formation is home to the relatively well-preserved dromaeosaurid Bambiraptor (Burnham et al., 2000) . The holotype of Bambiraptor feinbergi is quite small and typically considered a juvenile to subadult (Currie and Varricchio, 2004; Norell and Makovicky, 2004). However, attempts at histologically sampling the single known skeleton of this taxon have been unsuccessful. It is possible that Bambiraptor is a juvenile specimen of Saurornitholestes (Burnham et al., 2000; Norell and Makovicky, 2004): both taxa lack detailed and adequate diagnoses and differ only in the length of the suborbital process of the frontal, a feature that is undoubtedly influenced by ontogeny. Furthermore, the Bambiraptor feinbergi type specimen is known to be a chimera, as there are elements of three different similarly sized lower legs included in the holotype. The youngest North American dromaeosaurids are from the Maastrichtian Hell Creek and Ojo Alamo formations of the United States. From the Hell Creek Formation: the velociraptorine Acheroraptor temertyorum , which is known from a complete right maxilla and a referred right dentary (Evans et al., 2013) as well as the significantly larger Dakotaraptor steini , which was originally described as a dromaeosaurine (DePalma et al., 2015), but recently recovered as a velociraptorine (Pei et al., in press). From the Ojo Alamo Formation: the velociraptorine Dineobellator notohesperus , which is known from fragmentary cranial and postcranial material (Jasinski et al., 2020). Deinonychus , Utahraptor , and Yurgovuchia are the oldest widely accepted dromaeosaurids from North America with an Aptian/Albian age for the former (Ostrom, 1969) and a Barremian age for the latter two taxa (Kirkland et al., 1993; Senter et al., 2012). Deinonychus and Utahraptor are known from a large amount of material, much of it undescribed (personal commun., J. Kirkland), and Utahraptor ostrommaysorum remains the largest dromaeosaurid known. Yurgovuchia doellingi is represented by associated postcranial remains. The oldest record of Dromaeosauridae in North America relates to controversial fragmentary material from the Late Jurassic Morrison Formation (Heckert and Foster, 2011). Deinonychus antirrhopus remains the bestrepresented dromaeosaurid from North America. It is known from at least eight partially articulated and disarticulated skeletons from the Cloverly and Antlers formations. A partial egg associated with an adult has also been recovered (Grellet-Tinner and Makovicky, 2006). The osteology of this taxon was described in detail in Ostrom’s monograph (Ostrom, 1969) and has been revisited by subsequent studies (Norell and Makovicky, 1997, 1999; Norell et al., 2006).

ASIA: Velociraptor , arguably the most famous dromaeosaurid, was the second dromaeosaurid to be described, in 1924 (Osborn, 1924). It is one of the best-known genera, with several complete or near-complete skeletons, and lends its name to the subfamily Velociraptorinae . Velociraptor was recovered from the Campanian Djadokhta Formation of southern Mongolia, which is among the most productive strata for dromaeosaurids anywhere on Earth. Several specimens of Velociraptor tell us much about its palaeobiology. The famous “fighting dinosaurs” appears to preserve Velociraptor attacking a large Protoceratops (Kielan-Jaworowska and Barsbold, 1972) . Another specimen shows the presence of quill knobs on the ulna (Turner et al., 2007a), and yet another preserves stomach contents that include the remains of a pterosaur (Currie and Jacobsen, 1995). A second species, V. osmolskae , is known from paired maxillae and a left lacrimal described from similar rocks across the border in Nei Mongol, China (Wulansuhai (Bayan Mandahu) Formation) (Godefroit et al., 2008). This appears to be a valid taxon despite the paucity of its preserved fossil material (Turner et al., 2012). Djadokhta Formation outcrops at Ukhaa Tolgod have yielded Tsaagan mangas , a velociraptorine larger than Velociraptor (Norell et al., 2006) that is closely related to Linheraptor exquisitus , with a nearly complete holotype skeleton from the Wulansuhai Formation (Xu et al., 2010b, 2015b). The Djadokhta Formation has also yielded the earliest diverging noneudromaeosaurian dromaeosaurid Mahakala omnogovae , which is known from a partial skeleton including the back of the skull (Turner et al., 2007b; Turner et al., 2011). It was recovered from the Tögrögiin Shiree locality in Mongolia. Recent work described an additional dromaeosaurid from the Djadokhta Formation, Halszkaraptor escuilliei , and recovered it as the sister taxon to Mahakala , although parts of the sole specimen have been forged (Cau et al., 2017). Hulsanpes is another enigmatic specimen purported to be a dromaeosaurid (Osmólska, 1982). It is from the Campanian -Maastrichtian Barun Goyot Formation at the Khulsan locality in Mongolia. It consists only of a partial right metatarsus and pes (and possibly an associated braincase). Although considered a dromaeosaurid by recent analyses (Cau et al., 2017; Cau and Madzia, 2018), because of the extremely fragmentary nature of the material this identification has been repeatedly challenged (Turner et al., 2012).

The Gobi Desert has yielded a number of taxa occupying other parts of the Late Cretaceous: Achillobator from the Cenomanian-Santonian Bayan Shireh Formation (Perle et al., 1999) and Adasaurus from the Maastrichtian Nemegt Formation (Bayankhongor) of southwestern Mongolia (Barsbold, 1983). Adasaurus was only recently well figured and described (Turner et al., 2012). IGM 100/20 is the only specimen considered to be Adasaurus and is known from a partial skull and postcranial skeleton. Additional cranial and postcranial remains (IGM 100/22 and 100/23) likely pertain to a different taxon from the older Baynshiree Formation. Shanag is the only Early Cretaceous Mongolian dromaeosaurid, belonging to the Berriasian-Barremian Öösh Formation (Turner et al., 2007c).

In contrast, China has a large number of Early Cretaceous forms, but fewer Late Cretaceous ones. The Barremian-Aptian Yixian Formation and Aptian Jiufotang Formation of northeastern China, which yield part of the Jehol Biota, are home to many microraptorines, a non-eudromaeosaurian subclade that is known only from one fragmentary specimen outside Asia (Longrich and Currie, 2009). Despite their name, microraptorines were not all small and appear to be reasonably large ancestrally (Pei et al., in press). Their well-known arm and leg feathers are exemplified in the group’s namesake Microraptor , where they are extremely long and are thought to have enabled volant capabilities, although this remains an area of intense study (Dyke et al., 2013; Dececchi et al., 2016; Pei et al., in press). Microraptor is from the Aptian Jiufotang Formation and is known from three species M. zhaoianus , M. gui , and M. hanqingi (Xu et al., 2000, 2003; Gong et al., 2012); however, the status of M. gui (Senter et al. 2004) and M. hanqingi have been questioned (Turner et al., 2012; Pei et al., 2014). The other known Jiufotang microraptorine is Wulong (Poust et al., 2020) . The Yixian Formation has the microraptorines Changyuraptor (Han et al., 2014) , Graciliraptor (Xu and Wang, 2004a) , Sinornithosaurus (Xu et al., 1999) , Zhongjianosaurus (Xu and Qin, 2017) and the larger seemingly early-diverging forms Tianyuraptor and Zhenyuanlong (Zheng et al., 2009; Lü and Brusatte, 2015; Pei et al., in press). Microraptorines are otherwise rare in Asia: IVPP V22530 is from the younger Aptian-Albian Bayan Gobi Formation of Nei Mongol, northern China (Pittman et al., 2015) and suspected microraptorine tracks have been discovered in the Aptian Jinju Formation of Gyeongsangnamdo, South Korea (Kim et al., 2018). Shanag is possibly a microraptorine as well, as found in some phylogenetic analyses (Gianechini et al., 2018). Luanchuanraptor , known from a partial skeleton, was discovered from the Campanian-Maastrichtian Qiupa Formation of Henan, central China (Lü et al., 2007), and a recent analysis found it closely related to its Late Cretaceous Mongolian relative Velociraptor (Pei et al., in press). Tracks of two differently sized coeval deinonychosaurs have been found in the Barremian-Aptian Tianjialoue Formation of Shandong, eastern China, but the identity of their makers remains elusive (Li et al., 2008a).

A small partial braincase forms the type of Itemirus medullaris from the Turonian Bissekty Formation of Uzbekistan, which was originally described as an earlier-diverging theropod (Kurzanov, 1976). More recently, two phylogenetic analyses have recovered it as a velociraptorine (Longrich and Currie, 2009) and dromaeosaurine (Sues and Averianov, 2014).

EUROPE: Variraptor was named as a dromaeosaurid from the Late Campanian–Early Maastrichtian Grès à Reptiles Formation of France (LeLoeuff and Buffetaut, 1998). However, it was shown to lack dromaeosaurid synapomorphies and was superseded by Pyroraptor (Late Campanian–Early Maastrichtian of La Boucharde, France) as the only known Late Cretaceous European dromaeosaurid taxon (Allain and Taquet, 2000; Turner et al., 2012). Prior to the discovery of Pyroraptor , only indeterminate Late Cretaceous dromaeosaurid material had been known in Europe (Allain and Taquet, 2000) from elsewhere in France (Buffetaut et al., 1986; LeLoeuff et al., 1992; LeLoeuff and Buffetaut, 1998) and from Portugal (Antunes and Sigogneau, 1992) and Romania (Weishampel and Jianu, 1996). Despite being represented by only extremely fragmentary remains, the unique biogeography of Pyroraptor and its near contemporaneousness with Late Cretaceous taxa from neighboring continents (Campanian and Maastrichtian of Provence, France) made it an important taxon (Allain and Taquet, 2000). Understanding of Late Cretaceous European dromaeosaurids dramatically increased with the discovery of Balaur , a more complete partial skeleton of an island-dwelling velociraptorine from the Maastrichtian Sebeş Formation of Alba county, Romania (Csiki et al., 2010; Brusatte et al., 2013). The animal is perhaps most distinctive for its double sickle claw on the foot, due to the unusual hypertrophy of the first pedal ungual in addition to the typically enlarged and trenchant second pedal ungual of dromaeosaurids and other deinonychosaurs. Although recently argued to be an avialan (Cau et al., 2015), its status as a velociraptorine was recently reaffirmed (Pei et al., in press).

Knowledge of Early Cretaceous European dromaeosaurids is sparse and superficial. Reexamination of historic reptilian tooth and fragmentary jaw material from the Berriasian Lulworth Formation of the U.K. led to Nuthetes being reassigned as a dromaeosaurid taxon (Milner, 2002), and then being narrowed to the subfamily Velociraptorinae (Sweetman, 2004) . However, this assignment was later contested by one of the original authors as possible tyrannosaurid material instead (Rauhut et al., 2010). Six fused sacral vertebrae from the Berriasian-Barremian Wessex Formation of the U.K.

TABLE 4

Troodontid fossil record

Geological Unit Country Period Age Age Reference Taxa Reference Ejinhoro Early

China Aptian-Albian Sereno, 2010 Sinornithoides Russell and Dong, 1993 Formation Cretaceous

Huajiying Early Hauterivian-

China Pan et al., 2013 Jinfengopteryx Ji et al., 2005 Formation Cretaceous Barremian

Majiacun Late Coniacian-

China Tan et al., 2015 Xixiasaurus Lü et al., 2010 Formation Cretaceous Santonian

Wulansuhai/

Late

Bayan Mandahu China Campanian Godefroit et al., 2008 Linhevenator , Philovenator Xu et al., 2011b , 2012b

Cretaceous

Formation

Xu et al., 2002 b, 2017; Xu

Daliansaurus , Jianianhualong , and Wang, 2003, 2004b; Jehol Group

Early Barremian- Chang et al., 2009, Liaoningvenator , Mei , Sinovenator, Xu and Norell, 2004 ; (Yixian China

Cretaceous Aptian 2017; Pan et al., 2013 Sinusonasus , Yixianosaurus (possibly Chang et al., 2017; Shen Formation)

an avialan) et al., 2017a, 2017b; Yin et al., 2018 Kallamedu Late

India Maastrichtian Goswami et al., 2013 troodontid tooth Goswami et al., 2013 Formation Cretaceous

Barsbold et al., 1987; Khamareen Us Early Makovicky and

Mongolia Cenomanian MPC-D 100/44 Makovicky and Norell, locality Cretaceous Norell, 2004

2004

Khamaryn Ar Early Barremian- Tsuihiji et al., 2016;

Mongolia MPC-D 100/140 locality Cretaceous Albian Lucas, 2006

Tsuihiji, et al., 2016

Osborn, 1924; Norell et van Itterbeeck et al., al., 2000; Makovicky et Djadokhta Late 2005; Dingus et al., Almas , Byronosaurus , Gobivenator , Mongolia Campanian al., 2003; Bever and Formation Cretaceous 2008; Hasegawa et Saurornithoides Norell, 2009 ; Tsuihiji et al., 2009 al., 2014; Pei et al., 2017a Jerzykiewicz and Barsbold, 1974; Nemegt Late Russell, 1991; Osmólska, 1987; Mongolia Maastrichtian Borogovia , Tochisaurus , Zanabazar Formation Cretaceous Shuvalov, 2000 ; van Kurzanov and Osmólska, Itterbeeck et al., 2005 1991; Norell et al., 2009

4 continued 4 continued form the type of Ornithodesmus cluniculus (Seeley, 1887) , which probably belongs to a dromaeosaurid (Norell and Makovicky, 1997; Naish and Martill, 2007). However, this specimen has a complex taxonomic history including past identifications as a bird, pterosaur, troodontid, and earlier-diverging theropod (Anonymous, 1887; Seeley, 1887; Howse and Milner, 1993; Naish et al., 2001). Dromaeosauroides bornholmensis is a taxon known from a tooth from the Early Cretaceous of Denmark (Bonde and Christiansen, 2003).

AFRICA: Rahonavis ostromi of the Maastrichtian Maevarano Formation (Rogers et al., 2013) of Madagascar’s Mahajanga Basin was first described as an avialan (Forster et al., 1998) as supported by others (Agnolín and Novas, 2013; Cau, 2018; Novas et al., 2018). However, it has also been recognized as one of the first discovered Gondwanan dromaeosaurids (Makovicky et al., 2005; Turner et al., 2012; Pei et al., in press), which we follow in this volume. A dromaeosaurid from the Albian-Cenomanian Wadi Milk Formation of Sudan ( Dromaeosauridae incertae sedis (Turner et al., 2012)) is the first and only African record reaching into the Early Cretaceous (Rauhut and Werner, 1995).

SOUTH AMERICA: The discovery of Unenlagia from the Turonian-Coniacian Portezuelo Formation of Patagonia, Argentina (Calvo et al., 2007) provides strong support that dromaeosaurids were not exclusively Laurasian, but occupied Gondwana as well (Novas and Puerta, 1997). This landmark discovery was followed by recognition of a second species, U. paynemili , in addition to the original U. comahuensis from the same formation (Calvo et al., 2004) as well as the new genus Neuquenraptor (Novas and Pol, 2005) . However, the latter might be a junior synonym of Unenlagia (Makovicky et al., 2005) , but this remains unclear (Brissón Egli et al., 2017). Buitreraptor from the Cenomanian-Turonian Candeleros Formation of Patagonia extended the South American record of dromaeosaurids into the earliest Late Cretaceous (Makovicky et al., 2005) and also provided evidence for a monophyletic Unenlagiinae in Gondwana, while Austroraptor demonstrated that their record extended to the end of the Cretaceous (Campanian-Maastrichtian Allen Formation) (Novas et al., 2009) and solidified Patagonia, Argentina, as a hotspot for dromaeosaurid fossils. Pamparaptor is based on a deinonychosaurian foot from the Portezuelo Formation that is distinct from specimens of Unenlagia (Porfiri et al., 2011) . This material has possible unenlagiine affinities, but does not nest exclusively with that clade in phylogenetic analyses (Gianechini et al., 2018). Overoraptor of the Cenomanian-Turonian Hiuncul Formation of Patagonia is known from fragmentary postcranial material (Motta et al., 2020). Described as a paravian, it was recovered as a stem avialan in a phylogenetic analysis (Motta et al., 2020). However, the closeness of its phylogenetic position to contemporaneous Patagonian unenlagiine dromaeosaurids as well as its highly modified deinonychosaurian digit II-2, suggests that Overoraptor might instead be an unenlagiine. Unquillosaurus is based on a left pubis from the Maastrichtian Los Blanquitos Formation of Patagonia (Powell, 1979). It may be a dromaeosaurid (Martínez and Novas, 2006) and was previously proposed as an indeterminate maniraptoran theropod (Novas and Agnolín, 2004) and as an earlier-diverging theropod (Powell, 1979). South American records outside Argentina are rare, but possible unenlagiine elements have been reported from the Late Cretaceous Bauru group of Brazil (Candeiro et al., 2012; Delcourt and Grillo, 2017).

ANTARCTICA: Two isolated teeth associated with a partial left foot and fragments from the right foot from the Maastrichtian Snow Hill Island Formation of James Ross Island, Antarctica were referred to Dromaeosauridae (Case et al., 2007) . These were subsequently reinterpreted as indeterminate deinonychosaurian material (Turner et al., 2012). Ely and Case (2019) have recently described this specimen as Imperobator antarcticus , and recovered it as a nondromaeosaurid paravian.