Therizinosauridae
Extinct family of large, herbivorous theropod dinosaurs with scythe-like claws.
Therizinosauridae, whose name means "scythe lizards," is an extinct family of therizinosauroid dinosaurs. Their fossils come mainly from the Late Cretaceous, and complete specimens have only turned up in Asia and North America. As of 2025, the family includes nine confirmed genera.
The group was named in 1954 by Soviet paleontologist Evgeny Maleev, based on the huge, scythe-like claws of the type species *Therizinosaurus cheloniformis*. These animals were generally large and heavily built, with a body plan that converged strongly on that of ground sloths. The biggest genera, *Therizinosaurus* and *Segnosaurus*, reached about 10 meters (33 feet) and 7 meters (23 feet) long, respectively. Their physique featured a broad, rounded belly supported by a wide, sturdy pelvis; thick hind limbs with stout, four-toed feet; strong arms with flexible hands and elongated claws; and a highly specialized, almost bird-like inner ear. Other well-known traits include a long neck, a prominent keratinous beak, and teeth similar to those of prosauropods—unlike any other theropod. Earlier, more primitive therizinosaurs like *Beipiaosaurus* or *Jianchangosaurus* likely had less advanced feathers than later members of the family.
Because of their odd features, researchers have focused heavily on therizinosaurid paleobiology and ecology, especially their diet. They are considered the best theropod candidates for herbivory. While other theropods were fully carnivorous, therizinosaurids shifted to a plant-based—and possibly omnivorous—lifestyle, a conclusion supported by their unusual anatomy. Foot structure and footprints from Asia indicate they may have walked plantigrade (flat-footed), though more study is needed. They were oviparous, nesting in colonies and laying clutches of eggs. Their eggs, often assigned to the oofamily Dendroolithidae, are common in Late Cretaceous formations, particularly in Asia; some of the first dendroolithid eggs were found in the Bayan Shireh and Nanchao formations.
Early research on therizinosaurid relationships was muddled by their bizarre traits. Alternative classifications were proposed—such as Segnosauridae in 1979—until more complete specimens in the 1990s confirmed them as theropods.
Quick Facts
- Taxon
- Therizinosauridae
- Type Species
- Therizinosaurus cheloniformis
- Type Species Authority
- Maleev, 1954
Facts from the source article.
Lore & Background
The first definitive therizinosaurid discovered was Therizinosaurus, whose fossil remains were unearthed in 1918 during a Mongolian field expedition in the Gobi Desert. Several claws were described by Soviet paleontologist Evgeny Maleev in 1954; he considered them to belong to a giant marine turtle and named the new genus and species Therizinosaurus cheloniformis, also erecting the Therizinosauridae. In the 1970s more findings were made, including the naming of Segnosaurus by Altangerel Perle in 1979, who also coined the newer Segnosauridae (now a junior synonym of Therizinosauridae). In the same year, Dong Zhiming described Nanshiungosaurus but wrongly interpreted it as a dwarf sauropod.
Reader's Guide
Therizinosauridae holds significant importance in paleontology as the best-regarded theropod candidates for herbivory, demonstrating a major dietary shift from the fully carnivorous ancestors of most theropods. Their unique and bizarre features—including a broad rounded belly, robust pelvis, four-toed feet, elongated neck, keratinous beak, prosauropod-like dentition, and highly derived near-avian inner-ear—have encouraged extensive research into their paleobiology and paleoecology. The group's relationships were initially confusing, with alternative classifications such as Segnosauridae proposed until more complete specimens in the 1990s confirmed them as theropods. Current phylogenetic consensus places therizinosaurids within Maniraptora, evolving from small, bird-like maniraptorans, and they are considered the first of five major groups to diverge within that clade. Their eggs, attributed to the oofamily Dendroolithidae, are particularly common in Late Cretaceous formations, mainly in Asia, and they nested in colonies. The exponential sizes achieved across the family appear linked to their bulk and specialized diet.
Did You Know?
- Therizinosauridae was named in 1954 by paleontologist Evgeny Maleev after the large claws of Therizinosaurus cheloniformis, which he initially thought belonged to a giant marine turtle.
- The largest therizinosaurid, Therizinosaurus, reached up to 10 m (33 ft) long and weighed over 5 t (11,000 lb), making it among the largest-known theropods.
- Therizinosaurids were oviparous animals that nested in colonies and laid egg clutches; their eggs are often attributed to the oofamily Dendroolithidae.
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