Troodontidae Gilmore 1924

Troodontidae Gilmore, 1924 Z ? Included taxa. Borogovia gracilicrus Osmólska, 1987; Byronosaurus jaffei Norell, Makovicky and Clark, 2000; Saurornithoides junior Barsbold, 1974; Saurornithoides mongoliensis Osborn, 1924; Sinornithoides youngi Russell and Dong, 1993 b; Tochisaurus nemegtensis Kurzanov and Osmólska, 1991; Troödon formosus Leidy, 1856. Temporal range. ?Aptian-Maastrichtian. Occurrence. Nemegt Formation, Omnogov, Mongolia; Nemegt Svita, Bayankhongor, Mongolia; Djadokhta Formation, Omnogov, Mongolia; Bayan Mandahu Red beds, Nei Mongol Zizhiqu, China; Ejinhoroqi Formation, Nei Mongol Zizhiqu, China; Xinminbao Group, Gansu, China; Judith River Formation and Horseshoe Canyon Formation, Alberta, Canada; Judith River Formation, wo Medicine Formation, and Hell Creek Formation, Montana; Lance Formation, Wyoming, USA. Diagnosis. Large maxillary fenestra that is more than half the size of the internal antorbital fenestra; bulbous parasphenoid capsule connected with internal chambers in the basi sphenoid and basipterygoid processes; nutrient foramina in lower jaw placed in a longitudinal groove; interdental plates absent and teeth held in place by interdental bone; teeth with enlarged, apically hooked denticles with distinct ‘blood ­ pits ’ at their base; Mt IV very robust, and more than twice as wide (in anterior view) as the Mt II at midshaft. Remarks. Although Troödon formosus was one of the first dinosaurs to be described from North America (Leidy 1856), the affinities and anatomy of this animal remained enigmatic until very recently, mainly due to the fact that the type material of T. formosus consists of a single tooth. The family name was proposed by Gilmore (1924 b), who at that time believed Troödon to be a pachycephalosaur. In the same year, Osborn (1924) described a new small theropod dinosaur from the Upper Cretaceous of Mongolia, Saurornithoides mongoliensis. Sternberg (1932) described some new remains of a small theropod dinosaur from the Upper Cretaceous of Canada as Stenonychosaurus inequalis, and later (1945) noticed the theropod affinities of Troödon. The latter was subsequently confirmed by a find of a dentary referable to this taxon (Russell 1948). Strangely enough, these interesting taxa received little attention until Russell (1969) noticed the similarities between Saurornithoides and Stenonychosaurus, placing them in the family Troodontidae sensu Russell, 1948 (Russell 1969, p. 595), and considered Troödon to be probably a closely related, or even synonymous taxon. In contrast to this view, Barsbold (1974) considered Troödon to be distinct from the closely related Saurornithoides and Stenonychosaurus, and proposed a new family, Saurorn ithoididae, for the latter two taxa. This assignment was accepted by many authors, and the name Sauro rn ithoididae became widely used (e.g. Barsbold 1983; Carpenter 1982; Currie 1985; Wilson and Currie 1985) until Currie (1987) showed that Troödon Leidy, 1856, is a senior synonym foStenonychosaurus Sternberg, 1932, Polyodontosaurus Gilmore, 1932, and Pectinodon Carpenter, 1982, and proposed to use the family name TroÖdontidae again, to include Troödon and Saurornithoides. Since then, the name TroÖdontidae has become widely accepted, and several new taxa have been referred to this family (Barsbold et al. 1987; Osmolska 1987; Osmolska and Barsbold 1990; Kurzanov and Osmolska 1991; Russell and Dong 1993 b; Norell et al. 2000). Unfortunately, despite the discovery of an almost complete skeleton of a troÖdontid in the Lower Cretaceous of China (Russell and Dong 1993 b), many aspects of the anatomy of these enigmatic theropods are still poorly known. An additional comment on the anatomy of troÖdontids might be added: Russell and Dong (1993 b, p. 2169) claimed that a pubis boot is absent in the small Early Cretaceous troÖdontid Sinornithoides. However, based on my own observations of the type specimen, I believe the distal ends of the pubes to be missing; thus the absence of a pubic boot cannot be proven. A pubis referred to Troödon formosus (MOR 553S 8.3.9.387) shows a well-developed pubic boot, but this element was not found in articulation with other Troödon material, and several characters that are more typical for oviraptorosaurs (more anteriorly than posteriorly expanded pubic boot, anteriorly concave shaft) cast doubt on its referral to this taxon.

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