Therizinosauria
Herbivorous theropod dinosaurs related to birds.
Therizinosaurs, originally known as segnosaurs, were large, plant-eating theropod dinosaurs. Their fossils come mainly from Cretaceous rocks in Asia and North America, with possible fragmentary remains also found in Jurassic deposits in Asia and Europe. Shared features of their forelimbs, skulls, and pelvises place them among theropods and maniraptorans, making them relatives of birds. The name *Therizinosaurus* comes from Greek words meaning "reaping lizard" or "scythe lizard," while the older name *Segnosaurus* combines Latin for "slow" and Greek for "lizard."
For decades after their discovery, therizinosaurs puzzled paleontologists. Their mix of traits from different dinosaur groups, along with scarce fossils, sparked long debate over their relationships. The first known genus, *Therizinosaurus*, was initially thought to be a turtle when described from arm bones in 1954. In 1979, Perle suggested that *Segnosaurus* fossils might represent a new dinosaur family, which he tentatively classified as theropods—traditionally considered meat-eaters. He named this family Segnosauridae, with *Segnosaurus* as its only member. He distinguished it from the families Deinocheiridae and Therizinosauridae (then known only from large forelimbs found in Mongolia) by features of the upper arm bones and hand claws. Later that year, Barsbold and Perle argued that the pelvises of segnosaurids and dromaeosaurids were so unlike those of "true" theropods that they should be placed in separate infraorders within Saurischia.
In 1980, Barsbold and Perle created the new theropod infraorder Segnosauria, containing only Segnosauridae. That same article named a new genus, *Erlikosaurus* (known from a well-preserved skull and partial skeleton), which they tentatively considered a segnosaurid, and reported a partial pelvis from an unidentified segnosaurian, both from the same rock formation as *Segnosaurus*. Together, these specimens gave a fairly complete picture of the group: they shared an opisthopubic pelvis, a slender lower jaw, and toothless front jaw tips. Barsbold and Perle noted that while some features resembled those of ornithischians and sauropods, these similarities were superficial and differed in detail.
Quick Facts
- Taxon
- Therizinosauria
Facts from the source article.
Lore & Background
Therizinosaurs were long considered an enigmatic group, whose mosaic of features resembling those of various different dinosaur groups, and scarcity of their fossils, led to controversy over their evolutionary relationships for decades after their initial discovery. The first genus, Therizinosaurus, was originally identified as a turtle when described from forelimb elements in 1954. Perle noted in 1979 that the Segnosaurus fossils were possibly representative of a new family of dinosaurs, which he tentatively classified as theropods. He named the family Segnosauridae, with Segnosaurus as type genus and sole member. Later in 1979, Barsbold and Perle found the pelvic features of segnosaurids and dromaeosaurids so different from those of 'true' theropods that they proposed placing segnosaurs and dromaeosaurs in separate infraorders within Saurischia.
Reader's Guide
Therizinosaurs represent a significant chapter in the history of dinosaur paleontology due to the prolonged debate over their classification. Initially misidentified as turtles, then as prosauropods or ornithischians by various researchers, they were eventually recognized as specialized herbivorous theropods closely related to birds. The synonymy of Segnosauridae with Therizinosauridae was accepted by Perle himself and co-authors of a redescription of the holotype skull of Erlikosaurus in 1994, and they considered therizinosaurs maniraptoran theropods, the group that also includes modern birds. Their mosaic anatomy—combining a toothless beak, opisthopubic pelvis, and large claws—challenged traditional views of theropod evolution and highlighted the diversity of feeding strategies among dinosaurs. The group's eventual placement within Maniraptora solidified the understanding that many theropods were not exclusively carnivorous.
Did You Know?
- Paleontologist Gregory S. Paul concluded in 1984 that segnosaurs did not possess any theropodan features, but were instead derived, late-surviving Cretaceous prosauropods.
- Potential fragmentary remains of therizinosaurs have been found in Jurassic deposits of Asia and Europe.
Frequently Asked Questions
What are therizinosaurs?
Therizinosaurs were large, plant-eating theropod dinosaurs best known for their enormous scythe-like forelimb claws. They represent one of the most unusual body plans within the theropod lineage.
What did therizinosaurs eat?
Despite being theropods, therizinosaurs were strict herbivores that fed on vegetation. Their massive curved claws are generally thought to have helped them pull down or strip plant material for consumption.
Where and when did therizinosaurs live?
The bulk of therizinosaur fossils come from Cretaceous-age deposits in Asia and North America. A few fragmentary remains have been tentatively linked to Jurassic strata in Asia and Europe, suggesting they may have appeared earlier than once thought.
What does the name Therizinosaurus mean?
The name blends Greek words meaning 'reaping lizard' or 'scythe lizard,' a direct nod to their oversized curved claws. The earlier genus name Segnosaurus pairs the Latin word for 'slow' with the Greek word for 'lizard.'
How are therizinosaurs related to birds?
Anatomical similarities in their forelimbs, skulls, and pelvises confirm that therizinosaurs belong to the theropod and maniraptoran lineages. This makes them distant relatives of modern birds, despite their bizarre body plan.
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