The timeline of therizinosaur research is a chronological listing of events in the history of paleontology focused on therizinosaurs. They were unusually long-necked, pot-bellied, and large-clawed herbivorous theropods most closely related to birds. The early history of therizinosaur research occurred in three phases. The first phase was the discovery of scanty and puzzling fossils in Asia by the Central Asiatic Expeditions of the 1920s and Soviet-backed research in the 1950s. This phase resulted in the discovery of the Therizinosaurus cheloniformis type specimen. Soviet paleontologist Evgeny Maleev interpreted these unusual remains as belonging to some kind of gigantic turtle.
The second major phase of therizinosaur research followed the discovery of better preserved remains in the 1970s by collaborative research between the Soviets and Mongolians. These finds revealed the true nature of therizinosaurs as bizarre dinosaurs. However, the exact nature and classification of therizinosaurs within Dinosauria was controversial as was their paleobiology. When Rozhdestventsky first reinterpreted therizinosaurs as dinosaurs he argued that they were unusual theropods that may have used their clawed arms to break open termite mounds or collect fruit. Osmolska and Roniewicz also considered therizinosaurs to be theropods.
In 1979, Altangerel Perle named the new species Segnosaurus galbinensis, which although he recognized was an unusual theropod, he did not recognize as a therizinosaur. Consequently, he named the new family Segnosauridae and, in 1980, Segnosauria. Two years later, Perle recognized commonalities between Therizinosaurus and segnosaurs, reclassifying the former as a member of the latter. From hereout therizinosaur research was considered "segnosaur" research. Perle himself thought that his "segnosaurs" were semi-aquatic fish-eaters. However, in the early 1990s, researchers like Rinchen Barsbold and Teresa Maryańska cast doubt on the connection between therizinosaurs and segnosaurs altogether.
Nevertheless, the description Alxasaurus elsitaiensis provided more evidence for a close relationship between the therizinosaurs and "segnosaurs" and led to a revision of their classification. The discovery of this and other primitive therizinosaurs in China formed the beginnings of the third major wave of therizinosaur research. That same year Russell and Russell reinterpreted therizinosaurs as herbivorous foragers like mammalian chalicotherium. Other significant finds of the 1990s include therizinosaur eggs with embryos preserved inside and the first known therizinosaur with feathers, Beipiaosaurus, which was described from China in 1999.
20th century 1950s
1954 Evgeny Maleev described the new genus and species Therizinosaurus cheloniformis. from the Nemegt Formation He interpreted it as a gigantic turtle. Maleev also named the family Therizinosauridae to contain this species. Translated paper
1960s
1964 Zakharov described and named the genus and type ichnospecies Macropodosaurus gravis, which is represented by a series of four-toed footprints.
1970s
1970 Rozhdestventsky first proposed the idea that therizinosaurids were actually theropod dinosaurs. He thought they used their large claws to tear open termite mounds or collect fruit from trees. Osmolska and Roniewicz also interpreted Therizinosaurus as a carnosaur theropod.
1976 Barsbold proposed the Deinocheirosauria and included Therizinosaurus as a member. In another paper during the same year, Barsbold referred a shoulder and forearm found in the same strata as the Therizinosaurus type specimen to that genus because of the resemblance between the specimens claws. He observed that the anatomy of the arm and shoulder remains suggested that it belonged to a theropod dinosaur. Barsbold also remarked on similarities it shared with Deinocheirus, another mysterious dinosaur from the same rock unit.
1979 Dong described the new genus and species Nanshiungosaurus brevispinus based on a vertebral column and pelvis. He interpreted Nanshiungosaurus as a new genus of dwarf sauropods. He also described the new species Chilantaisaurus zheziangensis but interpreted it as a carnosaur. Translated paper Perle described the new genus and species Segnosaurus galbinensis based on mostly complete limbs and girdles. He erected a new family, the Segnosauridae, for this unusual dinosaur. He tentatively regarded it as a theropod. Translated paper
1980s
1980 Barsbold and Perle named the Segnosauria and described the new genus and species Erlikosaurus andrewsi. They also described in brief detail an unknown segnosaur. Barsbold and Perle thought segnosaurs were slow, semi-aquatic animals.
1981 Perle redescribed the holotype of Erlikosaurus but this time in more detail and misspelled Erlicosaurus. Translated paper
1982 Perle reported an unusual partial four-toed hind limb from Hermiin Tsav, Nemegt Formation. Because this partial leg was found not far from where Barsbold reported the shoulder and humerus he referred to Therizinosaurus, Perle thought that his specimen also probably belonged to that taxon. Since this leg was similar to those of segnosaurs, he classified Therizinosaurus as a segnosaur. Translated paper
1983 Barsbold described the new genus and species Enigmosaurus mongoliensis based on a partial pelvis from the previously unknown segnosaur. Barsbold regarded Enigmosaurus as so unusual that he gave it its own family, the Enigmosauridae. Translated paper
1984 Paul suggested that segnosaurs shared an evolutionary relationship with prosauropods and ornithischians. He thought this implied that they were probably herbivores.
1986 Gauthier considered segnosaurs to be relatives of sauropodomorphs.
1989 Paul Sereno also followed this new interpretation of segnosaurs. Bryn J. Mader and colleagues re-examined the specimens assigned to Alectrosaurus and concluded that the large humerus and unguals used by Gilmore in 1933 did not come from a tyrannosauroid, but a therizinosaurid.
1990s
1990 Barsbold and Maryanska reinterpreted the theropod Nanshiungosaurus and the carnosaur Chilantaisaurus as segnosaurs. They agreed with Perle that the partial hind limb from Hermiin Tsav he described in 1982 was segnosaurian, but cast doubt with his referral of it to Therizinosaurus, and therefore with his subsequent conclusion that Therizinosaurus was a segnosaur. Barsbold and Maryanska also disagreed with previous researchers who classified Deinocheirus as a segnosaur. David B. Norman considered Therizinosaurus to be a theropod of uncertain classification.
1992 A collaborative expedition between Chinese and Japanese scientists discovered the type specimen of the new segnosaur species. In addition, Dong named the Segnosaurischia to place segnosaurs on an equal rank with Saurischia and Ornithischia.
1993 Russel and Dong described the new genus and species Alxasaurus elsitaiensis. They considered it distinct enough to warrant its own family, the Alxasauridae. They pointed out similar anatomical traits in the anatomy of the hand of the partial forelimb Barsbold referred to Therizinosaurus in 1976 and those of segnosaurs. They concluded that therizinosaurids and segnosaurids were theropods and almost identical in traits, and synonymized the Therizinosauridae and Segnosauridae, with the former having nomenclatural priority and coined the superfamily Therizinosauroidea to contain Alxasaurus and its family. The relative completeness of the specimens concluded the debate on whether or not segnosaurs were theropods in the affirmative. Russell and Russell noticed that segnosaurs had similar body plans to chalicotheres and ground sloths and concluded that they may have used their large forelimbs to forage in a similar manner.
1994 Clark and others redescribed the skull of Erlikosaurus. They found more evidence that segnosaurs were theropods and classified them as maniraptorans.
1995 Nessov speculated that segnosaurs may have hung from trees using their large claws like sloths do and fed on wasp nests. He reported the discovery of segnosaur remains in Kazakhstan and Uzbekistan. Translated paper
1996 Currie attributed some Late Cretaceous eggs and their embryos from the Nanchao Formation of Henan, China to segnosaur dinosaurs. These spherical eggs still preserve the fossilized remains of their developing embryos. In a second paper, Currie attributed another kind of fossil egg from an entirely different oofamily to segnosaurs. Unlike the spherical eggs of his first paper, these huge "elongated" eggs are classified as members of the Elongatoolithidae and could reach lengths of up to 50 cm.
1997 Manning and others did not agree with Currie's referral of elongatoolithid eggs to segnosaurs. Dong and Yu described the new species "Nanshiungosaurus bohlini" discovered during the 1992 Sino-Japanese expedition, and coined the Nanshiungosauridae to contain it and Nanshiungosaurus. Russell coined the Therizinosauria in order to contain all segnosaurs. This new infraorder was composed of Therizinosauroidea and the more advanced Therizinosauridae. With this, the terms segnosaur and Segnosauria became synonyms to therizinosaur and Therizinosauria, respectively.
1998 Zhao and Xu reported the existence of a possible therizinosaur dentary dating back to the Early Jurassic.
1999 Xu, Tang, and Wang described the new genus and species Beipiaosaurus inexpectus. The type specimen actually preserves impressions of the animal's feathered integument. It was also the oldest known therizinosaur. Carpenter reported that the embryos Currie considered therizinosaurian had teeth in their premaxillae, unlike any known member of the group. This could be evidence that the egglayer was actually primitive for a therizinosaur. He also did not confirm Currie's referral of elongatoolithid eggs to therizinosaurs.
21st century
2000s
2000 Manning and others observed that the embryos identified by Currie in 1996 as therizinosaurian had "an unusual pattern of tooth replacement in which a slender, elongate tooth is replaced by a symmetrical, denticulate tooth". Contrary to Carpenter's claim in 1999, Manning and his colleagues reported the embryos premaxillae as toothless.
2001 Xu, Zhao, and Clark described the new genus and species Eshanosaurus deguchiianus based on the Early Jurassic dentary. This species was the probably oldest known therizinosaur. Kirkland and Wolfe described the new genus and species Nothronychus mckinleyi. This was the first definitive therizinosaurid discovered outside of Asia. They also found that Eshanosaurus had a similar dentition to prosauropods. Zhang and others described the new genus and species Neimongosaurus yangi.
2002 Xu and others described the new genus and species Erliansaurus bellamanus.
2003 Xu and team described new pelvic and tail material from the holotype of Beipiaosaurus. These were discovered during a re-excavation of the fossil quarry were the first elements of the holotype were found.
2004 Kirkland and colleagues briefly reported and described primitive remains of therizinosaurs found in Utah. The same team made comparisons with braincases found among the remains and that of Nothronychus mckinleyi. Lindsay Zanno continue with these short descriptions but this time focused on forelimb elements. Zanno discussed in more detail these elements in her Ph.D.
2005 Kirkland and others described the primitive new genus and species Falcarius utahensis based on the multiple remains previously described.
2006 Zanno and Erickson briefly discussed the growth of Falcarius which was represented by adult and juvenile specimens. Zanno formally published and described the pectoral anatomy of Falcarius. Bursh reported the inferred range of motion in the therizinosaurids Neimongosaurus, which was very pronounced and circular.