Theories
Holtz has come up with several new theories and hypotheses about the dinosaurs' classification. For example, he coined the terms Maniraptoriformes and Arctometatarsus. He also proposed two classification systems for theropods. The first is the clade Arctometatarsalia, made up of tyrannosauroids, ornithomimosaurs, and troodontids, because all of these coelurosaurs had pinched middle metatarsal bones in their feet. In this proposed classification system, the tyrannosauroids were supposedly basal to a clade known as Bullatosauria, which was made up of the Troodontidae and the Ornithomimosauria. These two groups were purported to form a clade, because they both shared a common characteristic; which was a skull capsule. However, later on, both of these classification systems were found to be paraphyletic, or "artificial", clades. For example, troodontids are now known to be deinonychosaurs, or "raptors", closely related to dromaeosaurids and birds. The discovery of basal tyrannosauroids, such as Guanlong, which lacked an arctometatarsus, also helped to disprove this theory. And, eventually, the "skull capsule" in troodontids and ornithomimosaurs was found to be an example of convergent evolution, causing the clade Bullatosauria to be abandoned.
Holtz was also a key figure in the discovery that tyrannosauroids were not carnosaurs, as had been previously believed by most palaeontologists, but rather large coelurosaurs. One of the first scientists to theorize this, Holtz contributed greatly to the debunking of a monophyletic Carnosauria that contained Megalosauroidea, Allosauroidea and Tyrannosauroidea. However, fellow vertebrate paleontologist Oliver Rauhut is known to recover a monophyletic Carnosauria that contains both Megalosauroidea and Allosauroidea in many of his publications, although like most researcher he agrees that Tyrannosauroids are likely Coelurosaurians as based on the conclusions made by Holtz.
