TROODONTIDAE, GILMORE, 1924

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 : 54-64

TROODONTIDAE

Troodontids were first recognized in the late 19th century in North America and it is on that continent and in Asia where most fossils have been found (fig. 1; table 4). Troodontids are otherwise scarce and have been traditionally thought of as a Laurasian group, but a single tooth now suggests that troodontids were possibly present in Gondwana (fig. 1; table 4).

NORTH AMERICA: The first troodontid genus Troodon was given to a tooth discovered in the Campanian Judith River Formation of Montana in the mid-19th century (Leidy, 1856). Originally thought to belong to a fossil lizard and then a pachycephalosaur, this is one of three historic North American troodontid genera, alongside Polyodontosaurus (Gilmore, 1932) and Stenonychosaurus (Sternberg, 1932) . North American Campanian- and Maastrichtian-aged troodontids have experienced a prolonged period of taxonomic instability, including the role of Troodon as a wastebasket taxon (see Zanno et al., 2011, for further details) once it was recognized as a theropod (Sternberg, 1945). Campanian material referred to this genus comes from the Judith River Formation (Lawver and Jackson, 2017) as well as the Dinosaur Park (Brown et al., 2013) and Oldman formations of Alberta, Canada (Ryan and Russell, 2001), the Two Medicine Formation of Alberta, Canada, and Montana (Foreman et al., 2008), the Kaiparowits and Wahweap formations of Utah (Eaton et al., 1999), the El Gallo Formation of Baja California, Mexico (Weishampel et al., 2004) and the Campanian-Maastrichtian Wapiti, Horseshoe Canyon and St. Mary River formations of Alberta, Canada (Ryan et al., 1998; Ryan and Russell, 2001). The Two Medicine material includes eggs, some with embryos, and nests (Varricchio and Jackson, 2016) as well as skeletons. Troodon has been reported from Maastrichtian strata including the Ferris Formation of Wyoming (Lillegraven and Eberle, 1999), the Hell Creek Formation of Montana, Wyoming, North Dakota, and South Dakota (Fastovsky and Bercovici, 2016), the Prince Creek Formation of Alaska (Fiorillo et al., 2016), the Lance Formation of Wyoming (Carpenter, 1982), and the Scollard Formation of Alberta, Canada (Weishampel et al., 2004). Troodon has even been assigned to material from the Lower Cretaceous Dakota Formation of Utah (Eaton et al., 1999), although this rock unit, now known as the Naturita Formation, has been reassigned to the early Late Cretaceous (Tucker et al., 2020). Material from the Dinosaur Park Formation has been assigned a different species name, T. inequalis , from the original T. formosus (Currie, 2005) . The discovery of Talos , a partial postcranial skeleton from the Campanian Kaiparowits Formation of Utah, provided a chance to reappraise North American troodontid material, which led to the suggestion that Troodon is a nomen dubium and support for the genus Pectinodon (Longrich, 2008; Zanno et al., 2011). The latter, known from teeth and juvenile skeletal material from the Maastrichtian Lance Formation of Wyoming, was originally described as an additional species of Troodon , T. bakkeri (Carpenter, 1982) . Continued efforts to address the taxonomic confusion arising from North America’s problematic, highly fragmentary historic holotypes led to the resurrection of the genus Stenonychosaurus for some troodontid skeletal material from the Dinosaur Park Formation (Evans et al., 2017). This analysis was supported by subsequent work that assigned some of this Stenonychosaurus material to the new genus Latenivenatrix (van der Reest and Currie, 2017) . Albertavenator was named from a distinctive partial left frontal recovered from the Maastrichtian Horseshoe Canyon Formation of Alberta, Canada (Evans et al., 2017). “ Saurornitholestes” robustus from the Campanian Kirtland Formation of San Juan Basin, New Mexico, is an indeterminate troodontid frontal (Evans et al., 2014), originally referred to a new species of the dromaeosaurid Saurornitholestes (Sullivan, 2006) . Geminiraptor , an incomplete maxilla from the Cedar Mountain Formation of Utah is arguably one of the most important North American troodontid specimens because, as the only Early Cretaceous record, it provides a crucial point of comparison with better-known Chinese contemporaries (Senter et al., 2010). A tooth that is the holotype of Koparion (Chure, 1994) , and the partial articulated skeleton that forms the type of Hesperornithoides miessleri (Hartman et al., 2019) are possible Jurassic troodontid records, both from the Morrison Formation of the western United States.

ASIA: The Gobi Desert of Mongolia provided the first Asian record of troodontids: Saurornithoides from the Campanian Djadokhta Formation of southern Mongolia (Osborn, 1924). Its reasonably complete skull and partial postcranium was particularly important in the early days of troodontid research. This animal was known from one species, S. mongoliensis , that was later joined by a second species, S. junior , from the younger Maastrichtian Nemegt Formation (Barsbold, 1974), although S. junior is now ascribed to Zanabazar (Norell et al., 2009) . Other Djadokhta taxa include Byronosaurus , which is known from a large amount of cranial material and some postcranial material (Norell et al., 2000; Makovicky et al., 2003) including, perhaps, two perinates (Bever and Norell, 2009; but see Pei et al., 2017a). Gobivenator and Almas are well-preserved, recently described specimens from this formation, with Gobivenator one of the best three-dimensionally preserved troodontids in existence (Tsuihiji et al., 2014; Pei et al., 2017a). Linhevenator tani , known from a partial, eroded skeleton, was discovered from the similar Campanian Wulansuhai (Bayan Mandahu) Formation across the border in Nei Mongol, northern China (Xu et al., 2011b, 2012b). A single leg from the same formation was originally identified as a juvenile Saurornithoides specimen (Currie and Peng, 1993) and was later assigned to the new taxon Philovenator (Xu et al., 2012b) . Mongolia and Russia provide the latest Cretaceous records. Borogovia and Tochisaurus are known from fragmentary hind-limb elements (Osmólska, 1987; Kurzanov and Osmólska, 1991), and like Zanabazar , were recovered from the Maastrichtian Nemegt Formation of southern Mongolia. “ Troodon ” records from the Maastrichtian Kakanaut and Udurchukan formations of Russia are expected to belong to one or more new genera given the recent revisions to Troodon taxonomy in North America (Averianov and Sues, 2007; Zanno et al., 2011; Evans et al., 2017; van der Reest and Currie, 2017). A single tooth from the Maastrichtian Kallamedu Formation of India potentially represents the only troodontid record from Gondwana (Goswami et al., 2013), despite the group being known for over 150 years. Occurrences from China and Uzbekistan extend the Asian troodontid record back into the earliest Late Cretaceous as well as the Early Cretaceous, providing the only described taxa from these time intervals worldwide. Xixiasaurus is from the Coniacian-Santonian Majiacun Formation (Lü et al., 2010) of Henan, China, and Urbacodon is from the Cenomanian Dzharakuduk Formation of Navoi Viloyat, Uzbekistan (Averianov and Sues, 2007). The Early Cretaceous troodontid record of Asia is well represented in China by at least eight named genera. The oldest record is Jinfengopteryx from the Hauterivian-Barremian Huajiying (Qiaotou) Formation of Hebei, China, that was originally described as an avialan and whose stomach may contain preserved seeds (Ji et al., 2005; Pan et al., 2013). Sinovenator , Mei , Sinusonasus , Daliansaurus , Liaoningvenator , and Jianianhualong were all discovered from the Barremian-Aptian Yixian Formation of northern China ( Xu et al., 2002 b, 2017; Xu and Norell, 2004; Xu and Wang, 2004b; Pan et al., 2013; Chang et al., 2017; Shen et al., 2017a, 2017b). This formation and the Djadokhta Formation represent the most important sources of troodontid material globally. Sinovenator was the first troodontid reported from the Yixian Formation ( Xu et al., 2002 b). Initially represented by a partial skull and a few incomplete postcranial skeletons ( Xu et al., 2002 b), later material included a partial skull with a well-preserved braincase (Yin et al., 2018). Mei was first described on the basis of an exquisitelypreserved skeleton with a bird-like sleeping posture, which is arguably the most complete Early Cretaceous troodontid specimen known (Xu and Norell, 2004; Pan et al., 2013). Sinusonasus , Daliansaurus , and Liaoningvenator all have a similar size as Sinovenator , and each of them were reported from a single, near com-

TABLE 5 Mesozoic avialan fossil record Geological Unit Country Period Age Age Reference Taxa Reference Gao and Shubin, 2012; Liu et al., Xu and Zhang, 2005; Xu et al., Anchiornis , Aurornis (possibly a Haifanggou and 2012; Wang et 2009, 2011a; Hu et al., 2009, 2018; Middle-Late Bathonian- synonym of Anchiornis ), Caihong, Tiaojishan China al., 2013c; Godefroit et al., 2013a, 2013b; Jurassic Oxfordian Eosinopteryx , Pedopenna , Serikornis , formations Sullivan et al., Lefèvre et al., 2017; Pei et al., Xiaotingia (possibly all troodontids) 2014; Tian et al., 2017b 2015 Hauterivian- Zhang et al., Zhang et al., 2008b; Wang et al., Dabeigou Formation China Early Cretaceous Jinguofortis , Eoconfuciusornis Barremian 2008b 2018 Hou, 1997a; Zhang and Zhou, 2000; Zhang et al., 2004; Jin et al., Archaeornithura , Cruralispennia , 2008; Wang et al., 2014a,,, 2015a, Huajiying Hauterivian- Eoconfuciusornis , Eopengornis , China Early Cretaceous Pan et al., 2013 2017; Pan et al., 2016; Zheng et Formation Barremian Hebeiornis ( Vescornis ), Jibeinia , al., 2017; Navalón et al., 2018, Orienantius , Protopteryx Chiappe et al., 2019a ; Liu et al., 2019 Wang et al., Qiaotou Formation China Early Cretaceous Barremian Shenqiornis Wang et al., 2010 2010 Hou et al., 1995, 1996, 1997b, 1999a, 1999b, 2002, 2004; Hou, Archaeorhynchus , Changchengornis , 1996, 1997b; Chiappe et al., 1999, Confuciusornis , Dalingheornis , Dingavis , 2007, 2014, 2019b; Ji et al., 1999, Eoenantiornis , Eogranivora , 2002a, 2002b; Xu et al., 1999; Grabauornis , Gretcheniao, Zhang et al., 2006 , 2009; Zhou Hongshanornis , Iteravis , Jeholornis , and Zhang, 2005, 2006a, 2006b; Jehol Group (Yixian Chang et al., Jixiangornis (likely a synonym of Gao et al., 2008, 2012; O’Connor Formation; Jiufotang China Early Cretaceous Barremian-Aptian 2009, 2017; Pan Jeholornis ), Junornis , Liaoningornis , et al., 2009, 2010, 2011a, 2013, Formation) et al., 2013 Longicrusavis , Longirostravis , Mirusavis , 2016c; Li et al., 2010; Wang et al., Monoenantiornis , Paraprotopteryx , 2013d, 2013e, 2019c;; Zheng et Sapeornis , Shanweiniao , Sulcavis , al., 2007, 2013, 2014, 2018; Tianyuornis , Xinghaiornis , Yangavis, Dalsätt et al., 2014 ; Lefèvre et al., Yanornis , Zhongornis (possibly a 2014; Zhou et al., 2014a; Hu and scansoriopterygian) O’Connor, 2017; Liu et al., 2017; Wang and Zhou, 2018 5 continued Continent Geological Unit Country Period Age Age Reference Taxa Reference Asia Sereno and Rao, 1992; Zhou et Alethoalaornis , Archaeorhynchus , al., 1992, 2004, 2008, 2009, 2010, Bellulornis, Bohaiornis , Boluochia , 2012, 2013, 2014; Hou and Cathayornis ( Largirostrornis, Zhang, 1993 ; Zhou, 1995, 2002; Longchengornis ), Chaoyangia, Hou, 1997a ; Hou et al., 2002; Chiappeavis , Chongmingia, Czerkas and Ji, 2002 ; Ji et al., Confuciusornis , Dalianraptor (might be 2002a; Zhou and Hou, 2002; a chimera), Dapingfangornis, Zhou and Zhang, 2001 , 2002a, Eocathayornis, Fortunguavis , Gracilornis , 2002b, 2003a, 2003b,, 2006a; Houornis , Huoshanornis , Jeholornis Gong et al., 2004 ; Dalsätt et al., ( Shenzhouraptor ), Jianchangornis , 2006;; Li et al., 2006, 2007, 2008, Linyiornis , Longipteryx , Longusunguis , 2010a, 2010b, 2011, 2012, 2014b; Mengciusornis , Parabohaiornis, Morschhauser et al., 2009 ; Yuan, Parahongshanornis , Parapengornis , 2010; Hu et al., 2010, 2011, 2015a, Pengornis , Piscivoravis , 2015b; Li and Hou, Piscivorenantiornis , Pterygornis , 2011O’Connor et al., 2011a, 2012, Rapaxavis , Sapeornis , Schizooura , 2016a; Pu et al., 2013; Wang et al., Shangyang , Shengjingornis , Sinornis , 2010b, 2014b, 2014d, 2014e, Songlingornis , Xiangornis , Yanornis , 2016b, 2016c, 2016d, 2019d; Yixianornis , Yuanjiawaornis, Zhang et al., 2001 , 2013; Wang Zhongjianornis , Zhouornis and Zhou, 2018, 2019 Otogornis (originally assigned to the Jingchuan Lockley et al., Hou, 1994; Li et al., 2008b; Zhang China Early Cretaceous Barremian-Aptian? Yijinhuoluo Formation), Formation 2012 et al., 2010; Wang and Liu, 2015 enantiornithines Hou and Liu, 1984; You et al., Avimaia , Changmaornis , Dunhuangia , 2006, 2010; Ji et al., 2011; Wang O’Connor et al., Xiagou Formation China Early Cretaceous Aptian Feitianius , Gansus , Jiuquanornis , et al., 2013b, 2013f, 2015b; 2016b Qiliania , Yumenornis O’Connor et al., 2016 b ; Bailleul et al., 2019 Wang et al., Jiangdihe Formation China Late Cretaceous Turonian-Santonian Parvavis Wang et al., 2014 c 2014 c Campanian- Qiupa Formation China Late Cretaceous Jiang et al., 2011 enantiornithines Xu et al., 2011c Maastrichtian Kuwajima Sano and Yabe, Japan Early Cretaceous Barremian enantiornithines Matsuoka et al., 2002 Formation 2017

5 continued

5 continued

Calizas de la

Huérguina Spain Early Cretaceous Formation

Buscalioni and

Barremian Fregenal- Concornis , Eoalulavis , Iberomesornis Martínez, 2010

Melovatskaya Kurochkin et al.,

Russia Late Cretaceous Cenomanian Cerebravis

Formation 2007

Csehbánya Dyke and Ösi,

Hungary Late Cretaceous Santonian Bauxitornis

Formation 2010

Grès à Reptiles Campanian- Walker et al.,

France Late Cretaceous Gargantuavis , Martinavis Formation Maastrichtian 2007

“Fox-Amphoux Buffetaut et al.,

France Late Cretaceous Maastrichtian? unnamed taxon

basin” 1995

Sanz and Bonaparte, 1992; Sanz and Buscalioni, 1992; Sanz et al., 1996, 2002; Sereno, 2000; Navalón et al., 2015

Kurochkin et al., 2007

Dyke and Ösi, 2010

Buffetaut, 1998; Walker et al., 2007

Buffetaut et al., 1995

5 continued

5 continued

Continent Geological Unit Country Period Age Age Reference Taxa Reference

North Belle Fourche

Canada Late Cretaceous Cenomanian Clarke, 2004 Ichthyornis -like material Clarke, 2004

America Formation

Woodbine Tykoski and

U.S. Late Cretaceous Cenomanian Flexomornis Tykoski and Fiorillo, 2010

Formation Fiorillo, 2010

Kaskapau Formation Canada Late Cretaceous Turonian Clarke, 2004 Ichthyornis -like material Clarke, 2004 “Canadian Arctic

Canada Late Cretaceous Turonian Bono et al., 2016 Tingmiatornis Bono et al., 2016

circle”

Mancos Shale

U.S. Late Cretaceous Formation

Turonian Clarke, 2004 Ichthyornis

Lucas and Sullivan, 1982; Clarke, 2004

Austin Chalk

U.S. Late Cretaceous Formation

Coniacian-

Clarke, 2004 Ichthyornis Santonian

Apatornis , Baptornis , Hesperornis , Coniacian- Da Gama et al.,

Niobrara Formation U.S. Late Cretaceous Iaceornis , Ichthyornis , Parahesperornis, Campanian 2014

hesperornithiforms

Belly River Group Canada Late Cretaceous Campanian Longrich, 2009 ornithurines

Clarke, 2004

Marsh, 1872, 1877, 1880; Martin and Tate, 1976; Martin, 1984; Clarke, 2004; Bell and Chiappe, 2015; Field et al., 2018 b

Longrich, 2009

Dinosaur Park Canada Late Cretaceous Formation

Northumberland

Canada Late Cretaceous Formation

La Bocana Roja

Mexico Late Cretaceous Formation

Kaiparowits U.S. Late Cretaceous Formation

Mooreville Chalk

U.S. Late Cretaceous Formation

Two Medicine U.S. Late Cretaceous Formation

Brown et al.,

Campanian enantiornithines 2013

McLachlan et

Campanian Maaqwi

al., 2017

Peecook and

Campanian Alexornis

Sidor, 2015

Zanno et al.,

Campanian Mirarce

2011

Campanian Clarke, 2004 Halimornis , Ichthyornis

Foreman et al.,

Campanian Gettyia

2008

Buffetaut, 2010

Morrison et al., 2005; McLachlan et al., 2017

Brodkorb, 1976

Atterholt et al., 2018

Chiappe et al., 2002; Clarke, 2004; Field et al., 2018 b

Atterholt et al., 2018

5 continued plete skeleton (Xu and Wang, 2004b; Shen et al., 2017a, 2017b). Jianianhualong is only known from a single flattened specimen from the Yixian Formation, which preserves the first record of asymmetrical feathers in troodontids (Xu et al., 2017). Sinornithoides is known from the Aptian-Albian Ejinhoro Formation of Nei Mongol, China, based on a near complete skeleton (Russell and Dong, 1993). Across the border in Mongolia, unnamed Early Cretaceous troodontids have been reported, including the well-known “Early Cretaceous troodontid” MPC-D 100/44 and MPC-D 100/140 (Barsbold et al., 1987; Tsuihiji et al., 2016).

In northeastern China, the Middle Jurassic Bathonian Haifanggou Formation yields Pedopenna (Xu and Zhang, 2005) , while the Late Jurassic Oxfordian Tiaojishan Formation yields Anchiornis, Auronis , Caihong , Eosinopteryx , Serikornis , and Xiaotingia (Xu et al., 2008, 2011a; Godefroit et al., 2013a, 2013b; Lefèvre et al., 2017; Hu et al., 2018). These taxa have been proposed as members of the Anchiornithinae, a controversial clade of long-tailed, early-diverging paravians. Anchiornithines were first described as early avialans, but their phylogenetic placement lacks consensus (Hu et al., 2009; Lee and Worthy, 2011; Xu et al., 2011a; Agnolín and Novas, 2013; Godefroit et al., 2013a, 2013b; Foth and Rauhut, 2017; Lefèvre et al., 2017; Pei et al., 2017b, in press). If they are troodontids as recovered in some recent works, they would be the oldest fossils of these animals (Hu et al., 2009). Hesperornithoides , from the Morrison Formation of Wyoming, is also a potential Jurassic troodontid (Hartman et al., 2019). Two Early Cretaceous taxa from the Yixian Formation, Liaoningvenator and Yixianosaurus , have been recovered as anchiornithines and are the only anchiornithine taxa besides Ostromia (previously an archaeopterygid) that have been found outside the Tiaojishan Formation (Cau et al., 2017; Foth and Rauhut, 2017; Shen et al., 2017b). However, the phylogenetic placement of Yixianosaurus remains controversial (Dececchi et al., 2012; Xu et al., 2013a; Cau et al., 2017; Foth and Rauhut, 2017; Lefèvre et al., 2017).