Therizinosaur

Taxonomy

Barsbold and Perle named the group Segnosauria as an infraorder of Theropoda in 1980.^[11] Dong Zhiming (1992) went further, placing the segnosaurs in their own order, Segnosaurischia. This name has been abandoned since the discovery that segnosaurs are a specialized group within the suborder Theropoda. Clark et al. in 2004 considered Segnosaurischia a synonym of Therizinosauroidea.

The superfamily Therizinosauroidea had been established by Maleev in 1954, to include only the bizarre, giant-clawed theropod Therizinosaurus. When it was later realized that Therizinosaurus was an advanced segnosaur, Therizinosauroidea was given a phylogenetic definition to include both groups, and has largely replaced the use of the older name Segnosauria in phylogenetic studies, mainly because of the association of the name Segnosauria with the discredited idea that these animals were relatives of prosauropods.

The following taxonomy follows Zanno, 2010 unless otherwise noted.^[12]

• Branch Therizinosauria
  • Genus Eshanosaurus? (could be relative of Chilesaurus^[13])
  • Genus Falcarius
  • Genus Martharaptor
  • Genus Jianchangosaurus (Pu et al., 2013)
  • Superfamily Therizinosauroidea
    • Genus Beipiaosaurus
    • Genus Enigmosaurus
    • Genus Erliansaurus
    • Genus Neimongosaurus
    • Genus Suzhousaurus
    • Family Alxasauridae
    • Family Therizinosauridae

Phylogeny

Hips from different genera.

The clade Therizinosauria was first defined by Dale Russell in 1997 as Alxasaurus, Enigmosaurus, Erlikosaurus, Nanshiungosaurus, Segnosaurus, Therizinosaurus, and all taxa closer to them than to oviraptorosaurs, ornithomimids, and troodontids. Paul Sereno, in 2005, modified this definition to the most inclusive clade containing Therizinosaurus but not Ornithomimus, Oviraptor, Shuvuuia, Tyrannosaurus, or Troodon.^[14]

Therizinosauroidea, previously named as a superfamily with no phylogenetic definition, was first defined by Zhang et al. in 2001, as the clade containing all theropods more closely related to Therizinosaurus than to birds (effectively replacing the older name Segnosauria, which has not yet been defined as a clade). This definition, however, defines the same group as the pre-existing Therizinosauria. An alternate definition was given by Clark in 2004 (as the last common ancestor of Therizinosaurus and Beipiaosaurus and all its descendants), comprising a narrower group that excludes more primitive therizinosaurs, such as Falcarius, and allows the name Therizinosauria to remain in use for the larger group comprising all therizinosaurs. This definition was followed by Teresa Maryańska and Barsbold (2004), Sereno (2005), Zanno et al. (2009) and Zanno (2010),^{[12][14][15][16]} though other subsequent studies, such as Senter (2007, 2012) have continued to use Therizinosauroidea for the therizinosaur "total group".^[17]

The following cladogram follows an analysis by Phil Senter, 2007.^[17]

The cladogram below follows the extensive phylogenetic analysis of Therizinosauria, by Lindsay E. Zanno, 2010.^[12]

The following cladogram is based on the phylogenetic analysis by Phil Senter et al., 2012.^[18]

The cladogram below is the most recent cladogram based on the phylogenetic analysis of Therizinosauria conducted by Hanyong Pu et al., 2013.^[19]

Paleobiology

Therizinosauroid behavior is quite poorly understood, but 20th-century studies and subsequent finds have revealed some aspects of their behavior. Nests with sub-spherical eggs have been found, and evidence points to the eggs being buried and abandoned by the parents. And studies of neonates indicate they were well developed and likely precocial—able to leave the nest shortly after birth and suggesting little to no parental care.^[20]

CT scans published in 2012 by Stephan Lautenschlager et al. focused on the skull and brain cavity of Erlikosaurus, revealing it to have a large forebrain, and suggesting it had well developed senses of balance, hearing and smell, all of which would have been useful in evading predators, finding food, or in performng complex social behavior.^[21]

In 2011, a nesting ground containing 17 clutches of eggs was found in Mongolia's Gobi Desert, with a total of 75 eggs uncovered during excavation. The eggs were 5 inches in diameter and contained no embryos; and there was evidence—in the form of egg shells that had been broken out of—that the young had hatched and left, presumably with their parents. The presence of so many fossilized eggs in one venue implies that therizinosaurs probably were social animals that came together for nesting; and that some genera may have performed parental care. This find, described by Yoshitsugu Kobayashi et al., and mass-death quarries such as those containing Falcarius, augments evidence that therizinosauroids were social, herding animals. Adult therizinosaurs are estimated to have weighed around 500 kg (1,100 lb).^[22][23]