Those Freaky Therizinosaurs
Frustratingly enigmatic theropod dinosaurs
These things are weird. Period. They have variously been classified as sauropod dinosaurs, a separate group of saurischian dinosaurs, and most recently theropod dinosaurs closely related to birds! You won't realize how bizarre they are without placing them into the context of phylogenetic systematics. No matter where therizinosaurs were placed, they used to seem too out of place to fit in with their purported closest relatives. However, new studies of new specimens confirm that therizinosaurs are maniraptorans. These "transitional" therizinosauroid fossils include Alxasaurus and the recently named Beipiaosaurus inexpectus, which has "protofeather"-like filaments on its arms! As such, basal therizinosaurs are a testament to our ever-improving understanding of the fossil record and macroevolutionary patterns.
Therizinosaurs (formerly known as segnosaurs) include a few poorly known taxa such as Therizinosaurus cheloniformis, Alxasaurus elesitaiensis, Erlikosaurus andrewsi and Segnosaurus galbinensis, all of which are sufficiently similar to be placed in this enigmatic clade. Therizinosaur fossils have been found in Early through Late Cretaceous deposits in Mongolia and the People's Republic of China.
The physical characteristics of therizinosaurs are one source of the mystery surrounding their phylogenetic relationships (until recently). They provide a good example of homoplasy (such as convergent evolution). Therizinosaurs had an opisthopubic pelvis (that is, the pubis pointed backwards) like birds, and had pneumatized hollowed bones like all theropods. Except for the basal form Beipiaosaurus inexpectus, they were quite large and bulky. Like sauropods, they had four main toes on their feet, and a small head with large leaflike teeth. Unlike most other dinosaurs, they had enormous claws on their manus (hands) up to 3 feet long! Also, their tooth and jaw morphology suggests that they might have been herbivorous (unlike most other theropods).
Considering the newest discoveries of fossil therizinosaurs, various features of the forelimbs, skull, and pelvis unite them quite comfortably within the Maniraptora, as close relatives to birds. This makes them seem even stranger ,Unfortunately, well preserved and articulated, complete skeletons are not yet known, which makes interpreting their anatomy and resolving their relationships difficult. Yet the fact that several independent scientific analyses of their relationships all have arrived at similar conclusions offers some comfort and hope of eventual resolution.