Segnosaurus

Phylogeny: Dinosauria > Saurischia > Theropoda > Neotheropoda > Tetanurae > Avetheropoda > Coelurosauria > Maniraptora > Therizinosauria > Therizinosauroidea > Therizinosauridae

Overview: Back in the 1970’s, when the first fossils of this dinosaur were discovered, nobody was entirely certain what it was. Collected fossils included a lower jaw, neck vertebrae, parts of the shoulder and arms, a claw, some leg bones, an unusual pelvis, and most of the tail. These were all collected by a joint team of Soviet and Mongolian paleontologists from Mongolia’s Bayan Shireh Formation, in the Gobi Desert. In 1979, these fossils were used as the basis for a brand new genus, Segnosaurus, the “slow lizard”. The name refers to its bulky body and supposed sluggish nature. It had an unusual mix of traits from many distinct dinosaurian lineages. When it was described, it was tentatively classified as a theropod, but others suggested this clearly herbivorous animal was closer to sauropods and possibly represented a surviving lineage of the “prosauropods”, such as Plateosaurus. Others, noting its supposed ornithischian traits, thought that Segnosaurus could’ve been proof that sauropodomorphs and ornithischians shared a closer common ancestor than previously thought.

Paleontologists would refer Segnosaurus to its own group, the Segnosauria, as well as to the family Segnosauridae. Other similarly bizarre and poorly known dinosaurs would be referred to this group, often being restored in art as sauropodomorph-like animals. By the 1990’s and 2000’s, it was clear that the “segnosaurs” were actually members of the Therizinosauria, based on new and thankfully more complete discoveries. This made Segnosaurus a maniraptoran theropod, closer to birds than it was to Plateosaurus or the ornithischians. Therizinosaurus itself was a close relative, as a fellow member of the family Therizinosauridae. Segnosaurus would’ve had a long neck, a narrow-snouted skull, huge arms, sickle-like claws, and a semi-upright posture. Its claws were used defensively, but mainly for manipulating branches or digging up roots. Being known from the Bayan Shireh, it likely lived around ninety million years ago, alongside dinosaurs like Achillobator and Graciliceratops, in a semi-arid and seasonal river plain. Sources of food included ferns, cycads, and conifers.