Dryptosaurus, SYSTEMATICS

Brusatte, Stephen L., Benson, Roger B. J. & Norell, Mark A., 2011, The Anatomy of Dryptosaurus aquilunguis (Dinosauria: Theropoda) and a Review of Its Tyrannosauroid Affinities, American Museum Novitates 2011 (3717), pp. 1-56 : 41-42

DRYPTOSAURUS SYSTEMATICS : HISTORICAL OVERVIEW

When Cope (1866, 1867, 1868a, 1868b, 1869) first described the holotype of Dryptosaurus , it was among the most complete skeletons of a large theropod dinosaur known to science. With little comparative material at hand, Cope and other contemporary paleontologists noted similarities between Dryptosaurus and other fragmentary remains of large theropods, including Megalosaurus from the United Kingdom (e.g., Cope, 1866; Leidy, 1868; Lydekker, 1888). Dryptosaurus was often allied with Megalosaurus and Deinodon , both wastebasket genera that subsumed material from what are now known as numerous theropod taxa widely spread in time and space, in the family Megalosauridae or Deinodontidae (e.g., Osborn, 1902; Hay, 1902; Gilmore, 1920; Huene, 1926; Romer, 1956, 1966; Kuhn, 1965). Other authors, however, argued that Dryptosaurus was unusual enough to warrant its own family, Dryptosauridae , which in time became a wastebasket taxon for fragmentary theropod remains from across North America (e.g., Marsh, 1890, 1896). This concept has remained in favor until relatively recently, as Carpenter et al. (1997) retained a monotypic Dryptosauridae after they were unable to confidently place Dryptosaurus within any other theropod clade.

Beginning in the middle of the 20th century, and especially gaining steam in the 1970s, paleontologists began to recognize similarities between Dryptosaurus and the characteristic Late Cretaceous tyrannosaurids Albertosaurus and Tyrannosaurus from western North America (e.g., Gilmore, 1946; Steel, 1970; White, 1973). Baird and Horner (1979) formally referred Dryptosaurus to Tyrannosauridae , but this was based on the tyrannosaurid affinities of a femur from North Carolina that was tentatively referred to Dryptosaurus , not the holotype. Molnar (1980) regarded Dryptosaurus as an “albertosaur,” an informal term used to group this genus with the tyrannosaurids Albertosaurus and Daspletosaurus . However, despite these referrals, the tyrannosaurid affinities of Dryptosaurus remained controversial. Russell (1970) disputed any connection between Dryptosaurus and Late Cretaceous tyrannosaurids, arguing that the larger hand and more gracile femur of Dryptosaurus precluded assignment to Tyrannosauridae . Carpenter et al. (1997: 571) followed suit, asserting that the teeth and astragalus of Dryptosaurus differed in morphology from those of tyrannosaurids, thus “cast(ing) doubt on the affinity of Dryptosaurus with the tyrannosaurids.” In his review of problematic large theropods, Molnar (1990) noted a few features shared between Dryptosaurus and tyrannosaurids, but concluded that Dryptosaurus could not be placed into any currently known theropod clade.

One of the primary problems confronting theropod paleontologists, from the time of Cope (1866) until the more recent redescription of Carpenter et al. (1997), was poor sampling, specifically the lack of basal tyrannosauroid taxa—species intermediate in phylogenetic position, body size, and morphological features between generalized coelurosaurs and the derived, colossal Late Cretaceous tyrannosaurids. Authors such as Russell (1970), Molnar (1990), and Carpenter et al. (1997) were correct in identifying both similarities between Dryptosaurus and tyrannosaurids, as well as differences. This confounded many researchers, including Carpenter et al. (1997), who used a typological argument to exclude Dryptosaurus from Tyrannosauridae based only on the absence of characters, ignoring shared derived features that unite these taxa. The discovery of numerous basal tyrannosauroids during the first decade of the 21st century, which have been studied in a rigorous phylogenetic context, illuminate an important pattern: the mixture of similarities and differences between Dryptosaurus and tyrannosaurids are expected in an “intermediate” tyrannosauroid taxon on the stem toward the large-bodied Tyrannosauridae . In other words, Dryptosaurus shares many derived characters with tyrannosaurids not seen in non-tyrannosauroid coelurosaurs and some basal tyrannosauroids such as Guanlong , Dilong , and Eotyrannus ( Hutt et al., 2001; Xu et al., 2004, 2006). It lacks other characters that unite taxa such as Albertosaurus , Daspletosaurus , and Tyrannosaurus into Tyrannosauridae proper. Therefore, it possesses a mixture of primitive and derived features, which in the context of a phylogenetic analysis support the placement of Dryptosaurus on the tyrannosaurid “stem” (e.g., Holtz, 2004; Brusatte et al., 2009, 2010; Carr and Williamson, 2010).

The first authors to recognize this emerging pattern were Holtz (2004) and Carr et al. (2005). The discovery of the Campanian tyrannosauroid Appalachiosaurus , also a midsized taxon from eastern North America showing both similarities and differences with derived tyrannosaurids, was a key to realizing the tyrannosauroid affinities of Dryptosaurus . Carr et al., (2005: 139) argued that Dryptosaurus was a tyrannosauroid based on the possession of two unique, derived characters seen only in this clade: an enlarged posterior surangular foramen and a bifurcated iliofibularis scar on the fibula. In addition, the generalized morphology of metatarsal IV was described as “comparable” to that of tyrannosaurids. Carr et al. (2005) also included Dryptosaurus in a phylogenetic analysis, which placed it as the most basal tyrannosauroid (however, it is worth noting that other basal tyrannosauroids such as Eotyrannus , Dilong , and Guanlong were not included, and several recently described basal tyrannosauroids were unknown at the time). Holtz (2004), who based his analysis on the characters discussed in a prepublication version of Carr et al. (2005), also included Dryptosaurus in a phylogenetic analysis, finding it in a comparable basal tyrannosauroid position. More recently, Brusatte et al. (2009, 2010) and Carr and Williamson (2010) have recovered similar results. These latter studies include a full array of basal tyrannosauroids, and place Dryptosaurus as intermediate between basal taxa (i.e., Guanlong , Proceratosaurus , Dilong , Eotyrannus ) and Tyrannosauridae proper (i.e., Albertosaurus , Alioramus , Daspletosaurus , Gorgosaurus , Tarbosaurus , Tyrannosaurus ). Contemporary theropod paleontologists, therefore, have reached a consensus: Dryptosaurus is an intermediate-grade tyrannosauroid ( fig. 23).