The name Tyrannosauridae was first coined by Osborn (1906:283). 1905 is often incorrectly cited as the date of authorship of the taxon, but that only refers to Osborn’s initial short note on Tyrannosaurus rex. The family has historically been used to incorporate all large theropods closely related to Tyrannosaurus rex. Various definitions have been proposed as reviewed below.
Sereno (1998) was first to define Tyrannosauridae as a stem-based taxon excluding three relatively poorly known genera (Alectrosaurus, Aublysodon, Nanotyrannus), several of which may owe their distinction to immaturity (Carr 1999, Currie et al. 2003). Two node-based definitions were subsequently coined without reference to Sereno’s prior definition. Both specify more inclusive clades, Holtz (2001) including Aublysodon and Brochu (2003) listing Alectrosaurus as internal specifiers. These poorly understood basal taxa do not function well as specifiers when other basal tyrannosauroids from the Early Cretaceous are being described, such as Eotyrannus lengi (Hutt et al. 2001) and Dilong paradoxus (Xu et al. 2004).
Brochu (2003:2) criticized Sereno’s (1998) stem-based definition for excluding some poorly known and potentially basal forms from Tyrannosauridae but then erected a definition for Tyrannosauridae that does the same (The last common ancestor of Tyrannosaurus rex, Tarbosaurus bataar, Albertosaurus sarcophagus, Gorgosaurus libratus, Alectrosaurus olseni, Alioramus remotus, and Daspletosaurus torosus and all of its descendants). Several authors have subsequently used Tyrannosauridae to circumscribe a group within Tyrannosauroidea (Brochu 2003, Xu et al. 2004).
In The Dinosauria, Holtz (2004) presented two very different definitions for Tyrannosauridae. He first listed a node-based definition (Tyrannosaurus rex, Tarbosaurus bataar, Daspletosaurus torosus, Albertosaurus sarcophagus, Gorgosaurus libratus, their most recent common ancestor and all of its descendants), and two pages later presented a stem-based definition (those theropods more closely related to Tyrannosaurus rex than to Eotyrannus lengi). Although referring to similar clades, Holtz’ former definition excludes the basal tyrannosauroids Alectrosaurus and Alioramus from the Tyrannosauridae, while his latter definition includes them within Tyrannosauridae.
There are two basic taxnomic needs that these definitons are attempting to adddress: the need for a catch-all stem-based taxon and another node within the group for the better known, large-bodied advanced species. Tyrannosauroidea as defined by Sereno (1998) and in this compilation is a stem-based definition that performs the catch-all function. Tyrannosauridae ought to circumscribe a more advanced clade within Tyrannosauroidea. To achieve this end, the active defintion of Tyrannosauridae is node-based and constitutes a first-order revision of one of the first of two definitions provided by Hotlz (2004:127). It constitutes a second-order revision of the initial stem-based definition in Sereno (1998).
As here defined, Tyrannosauridae may exclude potentially basal forms such as Alectrosaurus and Eotyrannus but includes all of the better known Asian and North American genera, such as Gorgosaurus libratus and Albertosaurus sarcophagus. These are routinely been identified as sister taxon to genera more closely related to Tyrannosaurus (Daspletosaurus, Tarbosaurus). This is active definition cites the minimum number to ensure inclusion of all of these advanced tyrannosauroids.