Therizinosauridae
Therizinosaurids | |
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Mounted skeletons of Nothronychus mckinleyi and N. graffami | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Clade: | Dinosauria |
Clade: | Saurischia |
Clade: | Theropoda |
Clade: | Maniraptora |
Clade: | †Therizinosauria |
Superfamily: | † Therizinosauroidea
|
Family: | †Therizinosauridae Maleev, 1954 |
Type species | |
† Therizinosaurus cheloniformis Maleev, 1954
| |
Genera | |
Synonyms | |
Therizinosauridae (meaning 'scythe lizards')
Therizinosauridae was named in 1954 by paleontologist
The unique and bizarre features of the group has encouraged research into the
Their relationships were confusing and obscure on the early years of research mainly because of the unusual traits among members. Several alternative classifications were proposed (such as the naming of Segnosauridae in 1979) until more complete specimens and other taxa were described during the 1990s, which confirmed them as
History of discovery
The first definitive therizinosaurid discovered was
In the 1970s more findings were made including the naming of more taxa. Additional specimens of Therizinosaurus were referred by the Mongolian paleontologist Rinchen Barsbold in 1976. He described a set of partial arms from the Nemegt Formation and given the similarities within the claws, he referred this specimen to the genus.[2] The Mongolian paleontologist Altangerel Perle described and named the new genus Segnosaurus in 1979, based on lower jaws and much of the hindlimbs. He also coined the newer Segnosauridae (now synonym of Therizinosauridae) to contain this species.[3] In the same year, paleontologist Dong Zhiming described the genus Nanshiungosaurus, but wrongly interpreted the remains to have pertained to some kind of dwarf sauropod.[4] In the following year, Barsbold and Perle coined the family Segnosauria (now Therizinosauria) to contain the Segnosauridae and kin. Also, they named and briefly described Erlikosaurus, a new genus smaller than Segnosaurus.[5] Confusingly, Perle redescribed Erlikosaurus treating the taxon as a new genus and species in 1981.[6] Also, Perle described another specimen of Therizinosaurus in 1982, this time a partial hind limb. He referred the hindlimb to the genus based on the similarities with Segnosaurus.[7]
In 2001, the new therizinosaurids Neimongosaurus and Nothronychus (N. mckinleyi) were described and named,[8][9] and in the following year Erliansaurus was also described and named.[10] Although a new species of Nothronychus (N. graffami) was named in 2009.[11] With this, both species make Nothronychus the most complete therizinosaurid known.[12]
Description
Therizinosaurids were quite advanced and robustly-built animals that reached multi-ton dimensions which make them among the largest maniraptorans. The smallest known therizinosaurids are Erliansaurus, Erlikosaurus and Neimongosaurus which were around 3–4 m (9.8–13.1 ft) long, furthermore the holotype skull of Erlikosaurus is about 25 cm (250 mm). Segnosaurus and the possible therizinosaurid Suzhousaurus were about the same size, 6–7 m (20–23 ft) in length and weighing 1.3 t (2,900 lb). Slightly smaller members are represented by Nanshiungosaurus and Nothronychus, estimated at 5–5.3 m (16–17 ft) long and 600–1,000 kg (0.60–1.00 t) in weight, respectively.[13][14] The related "Nanshiungosaurus" bohlini was similar in size (about 6 m (20 ft) long), however, its assignment to Nanshiungosaurus is unlikely and it may or not represent another species or specimen of the better known Suzhousaurus.[14][15] Therizinosaurus were the largest representatives, reaching the top dimensions of the group, they grew up to 10 m (33 ft) long weighing over 5 t (11,000 lb). These dimensions that make the genus among the largest-known theropods.[14] The exponential sizes obtained across the Therizinosauridae appear to have been triggered by the bulk and specialized diet within the group and early members of the Therizinosauria.[16]
Skin impressions from
Skull
Therizinosaurids had more specialized
The
Postcranial skeleton
In therizinosaurids, the
The manual
The
Therizinosaurids had wide
Classification
The family Therizinosauridae was coined by
In 1982, Perle described a third specimen of the therizinosaurids Therizinosaurus. He referred this material to the genus based on the striking similarities with the specimens of Segnosaurus. Additionally, Perle compared the forelimbs in these two taxa and concluded that they were characterized by elongated arms, possibly belonging to a single taxonomic group.
With the description of the therizinosauroids
In 2010, Lindsay Zanno conducted the most detailed phylogenetic analysis of the Therizinosauria to 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. The position of Segnosaurus and those of some other Asian therizinosaurids was affected by these factors; Zanno stated more well-preserved specimens and the rediscovery of missing elements would be necessary. Zanno also revised Therizinosauroidea to exclude Falcarius and retained it in the wider clade Therizinosauria, which became the senior synonym of Segnosauria. In addition, she defined Therizinosauridae as the least inclusive clade containing Erlikosaurus, Nothronychus, Segnosaurus and Therizinosaurus.[36] Falcarius and Jianchangosaurus are now regarded as the most primitive therizinosaurs while therizinosauroids are considered to be more derived that these two taxa, but less derived than therizinosaurids.[19][37]
Most phylogenetic analyses recover the family Therizinosauridae containing Erlikosaurus, Erliansaurus, Nanshiungosaurus, Neimongosaurus, Nothronychus, Segnosaurus and Therizinosaurus. However, Suzhousaurus is commonly recovered within the group.
The cladogram below follows the 2019 phylogenetic analysis by Scott Hartman and colleagues, which is largely based on the work from Zanno in 2010. While most therizinosaurids are recovered in relatively traditional, well-established positions, the therizinosauroid "N." bohlini was unusually recovered as a therizinosaurid taxon:[37]
Therizinosauridae |
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Paleobiology
Feeding
In 2009, Zanno and colleagues stated therizinosaurs were the most-widely regarded candidates for herbivory among theropods based on the small, densely packed, coarse serrations; lance-shaped teeth with a relatively low replacement rate; a well-developed keratinous beak; long neck for browsing; relatively small skulls; a very large gut capacity as indicated by the rib circumference at the trunk and the outwards flaring processes of the ilia; and the notable lack of
As indicated by their respective dental morphologies, the
However, advanced
Foraging
Among therizinosaurids, the forelimbs were increased in robustness and the flexibility of the
Brain and senses
The group is also notable for adaptations to the structures of their
Locomotion
In 1964, Zakharov described and named the particular
However, several other footprints may indicate a more digitigrade stance. Such is the case the footprints from the Cantwell Formation labelled under the numbers DMNH 2010-07-01, 2013-08-04, 2013-08-06 and 2014-11-05. These impressions are composed by four toes with the first digit slightly smaller than the others which is attributed to therizinosaurids. At least one footprint, DMNH 2010-07-01, was directly compared to the relatively complete right pes of Erlikosaurus revealing a consistent morphology. However, most of the footprint was made (apparently) by the fingers and a foot pad, indicating a more upright position of the metatarsals.[54][55] Nevertheless, Botelho and colleagues have also considered a plantigrade stance for therizinosaurids in 2016. Accordingly, members of the Therizinosauridae (such as Neimongosaurus or Nothronychus) re-evolved a fully functional first digit that articulates to the ankle.[56]
Reproduction
Nesting
Some of the first
In a 2013 conference abstract, paleontologist
The prominent nesting site of the Javkhlant Formation indicates that colonial nesting first evolved in non-avian dinosaur species to increase hatching success in ecosystems subject to high nest predation pressure (such as this formation). Lastly, though dendroolithid eggs are also attributed to megalosauroids, and therizinosaurids are not known from the Javkhlant Formation, it overlies the Bayan Shireh Formation, where other dendroolithid eggs and the therizinosaurs Enigmosaurus, Erlikosaurus and Segnosaurus were found. Moreover, the absence of megalosauroids on these formations is notable.[64]
Ontogeny
At least four developmental embryonic stages were reported from the Nanchao embryos; Kundrát and colleagues classified them from stages A to D depending on the development. Stage A is the earliest of all and is characterized by the poor ossification of bones and the porous structure of the centra in the vertebral column. During stages B and C the bones become slightly more articulated and ossified. These seem to correspond with developmental levels of 45–50, and 64-day-old embryos of the american alligator. The last and more advanced stage is D where the embryos had completely ossified vertebral centra and a partially reduced neurocentral suture in their cervical vertebrae. Additionally, most therizinosaurian characters are more notorious in this phase, such as the edentulous premaxilla. The most mature embryo is represented by CAGS-01-IG-5, which had a clearly more developed ossification than alligator hatchlings. This indicates that embryonic therizinosaurids reached a more mature skeleton than other archosaur hatchlings in ovo and stayed within the egg for a longer period to enlarge their proportions despite the advanced ossification. Based on the developmental states preserved within embryos, Kundrát and colleagues suggested a rough incubation period between 1.5 and 3 months.[62]
Given that some embryos had their bones ossified and the fact that no adults were found in association with the nests, therizinosaurid
Paleoecology
Therizinosaurids were very specialized herbivores that evolved a convergent life-style to the more recent
As reflected by at least 31 therizinosaurid footprints at the
Distribution
Although the vast majority of all therizinosaurids have come out of
The idea of a land bridge is even more supported by the multiple co-occurrence of
See also
References
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- ^ a b c Barsbold, R. (1976). "New data on Therizinosaurus (Therizinosauridae, Theropoda)". Joint Soviet-Mongolian Paleontological Expedition (in Russian). 3: 76–92.
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