Therizinosauria

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Therizinosaurs
Temporal range:
Ma[1][2][3]
Possible Early Jurassic record[4]
Collection of five therizinosaurs, clockwise from top left: Suzhousaurus, Erliansaurus, Nothronychus, Falcarius and Jianchangosaurus.
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Clade: Saurischia
Clade: Theropoda
Clade: Maniraptora
Clade: †Therizinosauria
Russell, 1997
Subgroups
Synonyms
  • Segnosauria Barsbold, 1980
  • Segnosaurischia Dong, 1987

Therizinosaurs (

dinosaurs whose fossils have been found across the Middle Jurassic to Late Cretaceous deposits in Europe,[1] Asia and North America. Various features of the forelimbs, skull and pelvis unite these finds as both theropods and maniraptorans, making them relatives of birds. The name of the representative genus, Therizinosaurus, is derived from the Greek θερίζω (therízō, 'to reap' or 'scythe')[6] and σαῦρος (saûros, 'lizard'). The older representative, Segnosaurus, is derived from the Latin
sēgnis ('slow') and the Greek σαῦρος.

History of research

Main article: Timeline of therizinosaur research
Forelimbs of Therizinosaurus, specimen IGM 100/15 displayed at Nagoya City Science Museum

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,

infraorder within Saurischia (one of the two main divisions of dinosaurs, the other being Ornithischia).[8]

In 1980, Barsbold and Perle named the new theropod infraorder Segnosauria, containing only Segnosauridae. In the same article, they named the new genus Erlikosaurus (known from a well-preserved skull and partial skeleton) which they tentatively considered a segnosaurid, and reported a partial pelvis of an undetermined segnosaurian, both from the same formation as Segnosaurus. Combined, the specimens provided relatively complete data on this group; they were united by their opisthopubic pelvis, slender mandible, and the toothless front of their jaws. Barsbold and Perle stated that though some of their features resembled those of ornithischians and sauropods, these similarities were superficial, and were distinct when examined in detail. While they were essentially different from other theropods (perhaps due to diverging from them relatively early), and therefore warranted a new infraorder, they did show similarities with them. Since the Erlikosaurus specimen lacked a pelvis, the authors were unsure if that of the undetermined segnosaurian could belong to it, in which case they would consider it part of a separate family.[9] Though Erlikosaurus was difficult to compare directly to Segnosaurus due to the incompleteness of their remains, Perle stated in 1981 that there was no justification for separating it into another family.[10]

infraorder Segnosauria; this group is now a synonym
of Therizinosauria

In 1982, Perle reported hindlimb fragments similar to those of Segnosaurus, and assigned them to Therizinosaurus, whose forelimbs had been found in almost the same location. He concluded that Therizinosauridae, Deinocheiridae, and Segnosauridae, which all had enlarged forelimbs, represented the same taxonomic group. Segnosaurus and Therizinosaurus were particularly similar, leading Perle to suggest they belonged in a family to the exclusion of Deinocheiridae (today, Deinocheirus is recognized as an

sauropods.[14]

theropods

Paleontologist

saurischian dinosaurs, paleontologist Jacques Gauthier concluded that segnosaurs were prosauropods. While he conceded they had similarities with ornithischians and theropods, he proposed these featured had evolved independently.[18] In a 1989 conference abstract about sauropodomorph interrelationships, paleontologist Paul Sereno also considered segnosaurs as prosauropods, based on skull features.[19]

Reconstructed skeleton of Alxasaurus from China, the completeness of which confirmed therizinosaurs as theropods in 1993

In a 1990

Therizinosauroidea to contain Alxasaurus and Therizinosauridae (since the new genus was somewhat different from its relatives), which they placed in the group Tetanurae within Theropoda. They considered therizinosaurs most closely related to ornithomimids, troodontids, and oviraptorids, which they placed together in the group Oviraptorosauria (since they found Maniraptora, the conventional grouping of these, invalid, and the higher level taxonomy of theropods was in flux at the time).[21][22]

Partial forelimb of the basal therizinosaur Beipiaosaurus with impressions of feather structures, Paleozoological Museum of China

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 (since they did find Maniraptora to be valid through their analysis). They also discussed the previous ornithischian and sauropod hypotheses for therizinosaur affinities in detail and demonstrated various faults with them.

Ornithomimosaurian theropods during the year 1999.[27]

By the early 21st century, many more therizinosaur taxa had been discovered, including outside Asia (the first being Nothronychus from North America), as well as various basal taxa that helped understanding of the early evolution of the group (such as Falcarius, also from North America). Therizinosaurs were not considered as rare or aberrant anymore, but more diverse than previously thought (including in size), and their classification as maniraptoran theropods was generally accepted.[28][29][30] The placement of Therizinosauria within Maniraptora continued to be unclear; in 2007, paleontologist Alan H. Turner and colleagues found them to group with oviraptorosaurs, while Zanno and colleagues found them to be the most basal clade within Maniraptora in 2009, bracketed by Ornithomimosauria and Alvarezsauridae.[31][32] Despite the additional fossil material, the interrelations within the group were also still uncertain by 2010, when Zanno conducted the most detailed phylogenetic analysis of the Therizinosauria until that point. She cited the inaccessibility, damage, potential loss of holotype specimens, scarcity of cranial remains, and fragmentary specimens with few overlapping elements as the most significant obstacles to resolving the evolutionary relationships within the group.[33]

Wills, Underwood & Barrett (2023) assigned specimen GLCRM G167-32, a tooth from the Bathonian-aged Chipping Norton Limestone in England, to the Therizinosauroidea, making this the oldest record of Therizinosauroidea and also the first record of Therizinosauroidea in Europe.[1]

Uncertain species

In 1979 Dong Zhiming named a new species of the

megalosaurid Chilantaisaurus, C. zheziangensis, based on specimen ZhM V.001. This specimen was recovered in 1972 from the Tangshang Formation and consists of a partial tibia and partial right pes (foot) largely lacking metatarsals. Dong referred it to the genus mainly based on similarities between the unguals of this specimen and those of C. tashuikouensis.[34] Barsbold and Maryanska in 1990 considered C. zheziangensis as a tentative segnosaur (later known as therizinosaurs) based on its relatively short and robust pedal phalanges and enlarged, strongly curved unguals, mostly similar to Segnosaurus. As this taxon may lie outside the genus Chilantaisaurus, they listed this species as "Chilantaisaurus" zheziangensis.[35] Although Glut (1997) stated this specimen may have been based on part of the holotype of Nanshiungosaurus brevispinus (based on a pers. comm from Dong to Molnar in 1984),[36] Dong in 1979 described both taxa from largely different formations and localities.[34] Zanno in 2010 argued that the notorious side to side compression of the unguals reflect therizinosaur affinities, although examinations to the preserved tibia are required for further conclusions.[33] In 2012 Mai-Ping Qian and colleagues placed "C". zheziangensis in the family Therizinosauridae based on its pes morphology, which is consistent to other therizinosaurids. They also illustrated most of the preserved pes.[37] Hartman with team in 2019 added "C". zheziangensis to a phylogenetic analysis and recovered it within Therizinosauroidea in a polytomy with Alxasaurus, Enigmosaurus and therizinosaurids.[38]

In 1997 Dong Zhiming and You Hailu named and described a supposed second species of

ribs. In order to contain both N. brevispinus and N. bohlini they coined the Nanshiungosauridae family. Dong and Yu presented no clear evidence regarding the assignment of this new species to Nanshiungosaurus.[39] Li and colleagues in their 2007 description of Suzhousaurus pointed out that N. bohlini might be synonymous with the former, as both are found in the same geological group and also incompletely known. As per terms of taxonomic priority, the species name would be Suzhousaurus bohlini. However, they noted that a direct comparison between specimens is difficult to near impossible because there is no overlapping material (besides dorsal vertebrae) and the holotype of N. bohlini is apparently lost. Li and team disagree in that this species belong to Nanshiungosaurus and listed it as "Nanshiungosaurus" bohlini.[40] Zanno in 2010 indicated that the anatomical traits that were originally used to characterize "N." bohlini are now known to be present in other therizinosaur taxa.[33] Hartman with colleagues in 2019 recovered "N." bohlini as a therizinosaurid in a clade joined by Segnosaurus and Nothronychus.[38]

Around 2005 partial therizinosaur material was collected from the

Laijia Formation and later used to represent "Tiantaiosaurus sifengensis" (alternatively "Tiantaisaurus"), which is a currently unpuslihed and informal therizinosaur taxon. Qian and team in 2012 noted that a whole manuscript describing the taxon was written in 2007 but never officially published.[37]

Holotype lower jaw of Eshanosaurus

In 1998

Paul M. Barrett in 2009 examined the specimen in detail, noting six features shared with therizinosaurs but not exhibited by prosauropods, agreeing in that Eshanosaurus is a therizinosaur.[43]

In 2012 the Mongolian Academy of Sciences recovered a partial theropod specimen from the Bayan Shireh Formation at the Urlibe Khudak (also Ulribe Khuduk) locality. The specimen was in a 2015 abstract by Yoshitsugu Kobayashi and team briefly described and identified as a new therizinosaur taxon distinct from the concurring

tyrannosaurids. Such trait makes it the only known two-fingered therizinosaur.[44]

Description

Life restoration of Falcarius (primitive member)
Life restoration of Therizinosaurus (advanced member)
Size comparison of several therizinosaurids

Therizinosaurs spanned a large range of sizes, from the smaller Beipiaosaurus (2.2 m (7.2 ft) long)[26] and Jianchangosaurus (2 m (6.6 ft) long)[45] to the large-sized 6–7 m (20–23 ft) long Segnosaurus and Suzhousaurus.[46][47] Therizinosaurus itself, obtained the top dimensions of the group, growing up to 10 m (33 ft) long and weighing over 5 t (11,000 lb), dimensions that make the genus among the largest-known theropods.[47]

Therizinosaurs had a very distinctive, often confusing set of characteristics. Their long necks, wide torsos, and hind feet with four toes used in walking resembled those of basal

feathers similar to those seen in the compsognathid Sinosauropteryx, as well as longer, simpler, quill-like feathers that may have been used in display.[26][49]

Classification and systematics

Taxonomy

Hips from different genera

Barsbold and Perle named the group Segnosauria as an infraorder of Theropoda in 1980.[9] Dong Zhiming went further, placing the segnosaurs in their own order, Segnosaurischia.[50] This name has been abandoned since the discovery that segnosaurs are a specialized group within the suborder Theropoda. Clark et al. 2004 considered Segnosaurischia a synonym of Therizinosauroidea.[23]

The clade Therizinosauria was first coined by Dale Russell in 1997—effectively replacing the older name Segnosauria, which has not yet been defined as a clade—to contain all therizinosaurian dinosaurs.[51] The superfamily Therizinosauroidea was first coined in 1993 as a superfamily with no phylogenetic definition.[21] The family Therizinosauridae had been established by Maleev in 1954 to include only the bizarre, giant-clawed theropod Therizinosaurus.[6] Subsequent analyses have proven this family to be more diverse and synonymous with Segnosauridae.[33][38]

The following taxonomy follows Zanno 2010, unless otherwise noted.[33]

Phylogeny

Skeletons of various genera (not to scale)

Therizinosauria is defined as Alxasaurus, Enigmosaurus, Erlikosaurus, Nanshiungosaurus, Segnosaurus, Therizinosaurus, and all taxa closer to them than to oviraptorosaurs, ornithomimids, and troodontids.[51] Paul Sereno, in 2005, modified this definition to the most inclusive clade containing Therizinosaurus but not Ornithomimus, Oviraptor, Shuvuuia, Tyrannosaurus, or Troodon.[53]

When it was later realized that Therizinosaurus was an advanced therizinosaur more related to Alxasaurus than other dinosaur lineages, Therizinosauroidea was coined to include Alxasaurus and Therizinosauridae, 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.[21] Therizinosauroidea was first defined by Zhang et al. in 2001, as the clade containing all theropods more closely related to Therizinosaurus than to birds. 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.[54] This definition was followed by Sereno (2005), Zanno et al. (2009) and Zanno (2010),[33][53][55] though other subsequent studies, such as Senter (2007, 2012) have continued to use Therizinosauroidea for the therizinosaur "total group".[56]

The cladogram below follows the extensive phylogenetic analysis of the Therizinosauria by

Lindsay E. Zanno in 2010.[33]

Therizinosauria

Below is the recently performed phylogenetic analysis performed by Hartman et al. 2019 using the data provided by Zanno in 2010:[38]

Therizinosauria

Paleobiology

Senses

CT scans published by Stephan Lautenschlager et al. 2012 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 performing complex social behavior. These senses were also well-developed in earlier coelurosaurs and other theropods, indicating that therizinosaurs may have inherited many of these features from their carnivorous ancestors and used them for their specialized dietary purposes.[57]

Reproduction

Life restoration of an embryonic therizinosaur based on fossils from the Nanchao Formation

precocial, capable of locomotion from birth, and able to leave their nests to feed alone, independently of their parents. The development of the teeth of the hatchlings was consistent with an omnivorous diet. Subterraneously constructed nests are also indicative of a lack of parental care during the incubation period.[58][59]

In a 2013 conference abstract, paleontologist Yoshitsugu Kobayashi and colleagues reported a nesting ground of theropod dinosaurs at the

site fidelity. The discovery was the first record of colonially nesting non-avian theropods from Asia, as well as the largest known non-avian theropod colony.[60]

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External links

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