Mamenchisaurus

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Mamenchisaurus
Temporal range: Late Jurassic-Early Cretaceous,[1] 161–114.4 Ma
Mamenchisaurus in Japan.jpg
Mounted skeleton of M. sinocanadorum, Japan
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Clade: Saurischia
Suborder: Sauropodomorpha
Clade: Sauropoda
Clade: Eusauropoda
Family: Mamenchisauridae
Genus: Mamenchisaurus
Young, 1954
Type species
Mamenchisaurus constructus
Young, 1954
Other species
  • M. hochuanensis Young & Zhao, 1972
  • M. sinocanadorum Russell & Zheng, 1994
  • M. youngi Pi, Ouyang & Ye, 1996
  • M. anyuensis He et al., 1996
  • M. jingyanensis Zhang, Li & Zeng, 1998
  • M. yunnanensis Fang et al., 2004[2]

Mamenchisaurus (/mɑːˈmɛniˈsɔːrəs/ mah-MUN-chi-SAWR-əs,[1] or spelling pronunciation /məˌmɛniˈsɔːrəs/) is a genus of sauropod dinosaur known for their remarkably long necks[3] which made up nearly half the total body length.[4] Numerous species have been assigned to the genus; however, many of these might be questionable. Fossils have been found in the Sichuan Basin and Yannan Province in China. Several species are from the Upper Shaximiao Formation whose geologic age is uncertain. However, evidence suggests that this be no earlier than the Oxfordian stage of the Late Jurassic. M. sinocanadorum dates to the Oxfordian stage (158.7 - 161.2 mya) and M.anyuensis to the Aptian stage of the Early Cretaceous ~114.4 mya.[1] Most species were medium to large size sauropods 15 to 26 meters (49 to 85 ft) in length.[5][6] Two as yet undescribed cervical vertebrae, which might belong to M. sinocanadorum, suggest one of the largest dinosaurs known; estimated at 35 metres (115 ft) in length and possibly weighed 60 and 80 tonnes (66 and 88 short tons).[6][7]

Description[edit]

Mounted M. hochuanensis skeleton, Field Museum

It's not certain that all the species that have been assigned to Mamenchisaurus should belong to the genus.[8][9][6][10] Some Mamenchisaurus species are almost completely known, and others are fragmentary. The species vary in size and specific skull and skeleton features but share typical sauropod traits; like large bodies with long tails, and are quadrupedal. However, mamenchisaurids most distinctive feature is their exceptionally long necks, approaching half their body length.[5] Complete necks are known in M. youngi and M. hochuanensis, consisting of 18 and 19 vertebrae respectively.[11][12] Mamenchisaurus cervical vertebrae are elongated, lightly constructed and highly pneumatic.[10] The neural spines on their posterior cervical and anterior dorsal vertebrae are bifurcated.[11][13][14] Their shoulders were somewhat higher than the hips. The different species vary in length, from around 15 metres (49 ft) to at least 26 metres (85 ft), and maybe up to 35 metres (115 ft) based on two undescribed vertebrae.[6][7]

The neck posture of sauropod dinosaurs is a controversial topic. Andreas Christian and colleagues analysed the neck of Mamenchisaurus youngi. They found the neck was nearly straight with a slight upward bend at the base and a slight downward bend towards the head when articulated in a neutral posture. The base of the neck has comparatively high upward flexibility but limited downward flexibility. The region near the head had better downward flexibility and low upward flexibility. In the mid-region, downward flexibility was high, which led the authors to conclude that M. youngi frequently fed at low levels. Long overlapping cervical ribs may have limited flexibility. The authors also estimated the stress on the intervertebral joint cartilage. Their results suggest the neck was kept mostly straight, with the possible exception of the neck base and near the head.[15]

Mamenchisaurus have forked chevrons (or sled chevrons) starting around the middle of the tail, similar to those seen in diplodocids; these chevrons curve strongly backwards and add a forward projection.[16][17] Gregory S. Paul has argued that these types of chevrons are adaptations linked to rearing behaviour. In a tripodal stance, the tail acts as a prop, and the forked chevrons would help distribute the weight evenly. Paul also notes that the pelvises of Mamenchisaurs are retroverted (tilted), which may have allowed slow-walking whilst bipedal.[18][17]

A large ulna (GPIT SGP 2006/10) measuring >96 centimetres (38 in) in length, referred to Mamenchisaurus sp. from the Shishugou Formation, was used in a bone histology analysis. By sectioning the bone and counting the growth rings, the age at death was estimated at 43 years.[19]

Discovery and species[edit]

Size comparison of M. constructus, M. youngi, M. hochuanensis and M. sinocanadorum

Mamenchisaurus was first discovered in 1952 on the construction site of the Yitang Highway in Sichuan Province of China. The fossil site belonged to the Upper Shaximiao Formation, dating to at least the Late Jurassic.[1] The partial skeleton fossil was later studied and named Mamenchisaurus constructus in 1954 by the renowned Chinese paleontologist Professor C. C. Young.

The type specimen (IVPP V. 790) was fragmentary, disordered, and not excavated in a technical way. Material included five dorsal vertebrae, 30 caudal vertebrae, rib fragments, dorsal neural spines, and chevrons. Fourteen neck vertebrae were preserved, but none were complete. Young noted that some neck vertebrae might have been missing, and estimated the whole neck at 4.67 metres (15.3 ft). Limb material included; two pieces of a femur, a complete tibia, fibula, astragalus, metatarsals, phalanges, and claws. The skull, forelimbs and pelvic girdle were missing.[20] M. constructus has been estimated around 13 to 15 meters (43 to 49 ft) in length with a mass of 5 tonnes (5.5 short tons).[20][6]

Mamenchisaurus means 'Mamenchi lizard', from the Chinese Pinyin (马 'horse') and mén (门 'gate'), while chi is a transliteration of (溪 'stream' or 'brook'), combined with the suffix -saurus (from Greek sauros meaning 'lizard'). The intention was to name the genus after the place where its fossil was first found. However, due to an accentual mix-up by Young, the location name míng (马鸣溪 'horse-neighing brook') was mistaken as mén (马门溪 'horse-gate brook').[21] The fact that the first Mamenchisaurus fossil was found due to construction work led to Young's naming the type species as Mamenchisaurus constructus.[20]

In 1958, Young described additional sauropod remains collected from Gansu Province. The remains consisted of various partial specimens (IVPP V. 945, V. 946, V. 947, V. 948) most of which were assigned to M. constructus.[22] In 1972, one of these specimens was reassigned to M. hochuanensis.[11]

The holotype skulls of M. youngi, M. jingyanensis, and M. sinocanadorum, and a skull referred to M. hochuanensis

Mamenchisaurus hochuanensis[edit]

In 1972, a second species Young and Xijin Zhao described Mamenchisaurus hochuanensis. The fossils were found near a village in Hechuan, north of Chongqing (originally part of the Sichuan Province), China; 200 metres (660 ft) above the Fu River on the slope of a mountain.[11] The M. hochuanensis fossil site also belonged to the Upper Shaximiao Formation, very close to the M. constructus type specimen's location, dating to at least the Late Jurassic.[1] Locals first discovered the remains sometime before the Chinese Revolution. However, the remains were ultimately abandoned and left to weather in situ. Excavation didn't begin on the site until 1957.

The holotype specimen (CCG V 20401) consisted of an almost complete and articulated vertebral column; including 19 elongated cervical vertebrae which were almost entirely preserved, 12 dorsal vertebrae, four sacral vertebrae, and 35 caudal vertebrae with only the last several missing. Also missing from the skeleton was the majority of the forelimbs and skull. The total length of M. hochuanensis is about 21 to 22 meters (69 to 72 ft) with a 9.3 metres (31 ft) long neck.[6][11][23] When M. hochuanensis was first described, it was the largest sauropod known from China.[24] Young and Zhao estimated the mass of M. hochuanensis at 45 tonnes (50 short tons).[11] However, later volumetric mass estimates are lower at 14–18.2 tonnes (15.4–20.1 short tons).[6][25][26]

In 1958, Young described a mamenchisaur specimen (IVPP V. 946) from the Haishiwan region of Yongdeng, Gansu Province. This specimen was initially assigned to M. constructus. However, in 1972 it was reassigned to M. hochuanensis as a paratype. This specimen was slightly smaller than the holotype and consisted of less material overall. However, it contained some anatomical details missing in the type specimen.[11]

In 2001, another specimen (ZDM0126) was described and referred to M. hochuanensis. It was nearly complete and mostly articulated. Found in 1995 at a construction site in Huidong New District, Zigong City, Sichuan Province. This specimen also preserved a skull, pectoral girdle and forelimb material, all of which were missing from the holotype. Four fused tail vertebra from the tip of the tail have expanded neural arches and taller neural spines. It's thought that these could be a weapon, such as a tail club, or a sensory organ. Other Chinese sauropods, Shunosaurus and Omeisaurus, are also known to have had ’tail clubs’, but they differ in shape to that of M. hochuanensis.[27][28] Phylogenetic analyses have cast doubt on the referral of this specimen to M. hochuanensis.[10]

Mamenchisaurus sinocanadorum[edit]

In 1993, M. sinocanadorum was described by Dale Russell and Zhong Zheng. The remains were found in 1987 when a cervical rib was seen projecting out of a cliff. The fossil site was located in the Junggar Basin, Xinjiang; from the upper part of the Shishugou Formation (between 161.2 Ma and 158.7 Ma in age).[5][1] The specimen (IVPP V10603) consisted of an articulated series of the first four cervical vertebrae, a well-preserved mandible, and other skull material. The mandible was 60.3 centimeters (23.7 in) in length and had 19 teeth. In contrast to the more squared-off jaws of diplodocids, the front of the mandibles met at an oblique angle. The neural arches of the vertebrae were well fused to the centra suggesting that the animal was mature.[5] This species possessed the longest cervical rib of any described sauropod dinosaur, measuring 4.1 metres (13 ft). For comparison, a Sauroposeidon cervical rib measures 3.42 metres (11.2 ft).[5][29] The cervical vertebrae are on average 1.19 times longer than those of M. hochuanensis; based on this, Russel and Zheng estimated the type specimen at 26 metres (85 ft).[5] The neck has been estimated at 12 metres (39 ft) in length.[30] The species name refers to the China-Canada Dinosaur Project.[5]

The authors noted that the teeth are fully erupted but unworn, suggesting the animal starved. Due to there being little bone at the cliff base, the authors proposed that the cervicals broke away before full decomposition. The head and part of the neck then drifted downstream until they came to rest in shallow water on a point bar and eventually berried.[5]

Two large cervical vertebrae have since been found from the same formation as M. sinocanadorum and might belong to the species. However, these vertebrae have yet to be formally described. Gregory S. Paul has suggested these may represent one of the largest dinosaurs known, with an estimated length of 35 metres (115 ft) and 60 and 80 tonnes (66 and 88 short tons)[6][31]

M. jingyanensis skeleton, Beijing Museum of Natural History

Mamenchisaurus jingyanensis[edit]

M. jingyanensis was described in 1998 by Yihong Zhang, Kui Li, and Qinghua Zeng. The type specimen (CV00734) was located 200 meters away from the administrative headquarters of the town of Meiwang, Jingyan county. Another specimen (JV002) was found in the village of Sanjiang approximately 10 km from Meiwang and became the paratype. A third specimen (CV00219) found near the village of Dujia, about 10 km from Sanjiang.[13] The fossils were located in the Sichuan basin, from the Upper Shaximiao Formation.[1][13] The species name refers to Jingyan County from which the majority of specimens were excavated.[13]

The type specimen includes a partial scapula and complete coracoid, forelimb material, a complete ischium, a relatively complete skull, and a hyoid bone. The paratype specimen included three anterior cervical vertebrae, several weathered dorsal vertebrae, various caudal vertebrae including a nearly articulated column, relatively complete hind and forelimbs, and isolated teeth. The third specimen preserved a column of cervical vertebrae with articulated ribs, four fused sacrals, several caudals, a complete scapulocoracoid, various limb bones, and isolated teeth. The skull was restored at 55 centimeters (22 in) in length. There were four teeth in the premaxilla, 14-16 in the maxilla, and 17-19 in the dentary. Zhang and colleagues estimated this species between 20 to 26 meters (66 to 85 ft) in length.[13] Paul estimated it at 20 metres (66 ft) with a mass of 12 tonnes (13 short tons).[6]

Mamenchisaurus anyuensis[edit]

M. anyuensis was described in 1996 by Xinlu He and colleagues. The remains were discovered in 1987 from two locations near the town of Longchiaoxiang in the Sichuan Basin. At one quarry, at least 5-6 individuals were found. At a second quarry, four individuals were found. One of these became the holotype, AL001, representing two-thirds of an articulated skeleton. Other specimens were also reported, AL002, AL003, and AL101-106, which provide more skeletal information. The species name references Anyue County where it was discovered.[14] M. anyuensis is known from both the top of the Suining Formation and the bottom of the Penglaizhen Formation. Uranium-lead dating places M. anyuensis in the Suining Formation at 114.4 Ma in age; as this would make it roughly 30 million years younger than the other Mamenchisaurus species, it is unlikely that M. anyuensis is actually a member of the genus.[1]

The holotype specimen preserved eight posterior cervical vertebrae, 12 dorsal vertebrae, five sacral vertebrae, several caudals, a complete pelvis. Another specimen (AL102) preserved five articulated cervicals from the middle of the neck. Except for the digit bones, the forelimb is completely known and represented by multiple individuals. This species shares a lot of morphological similarity to M. hochuanensis. He and colleagues estimated the length of this species at 21 meters (69 ft).[14] Paul estimated it longer at 25 meters (82 ft) with a mass of 25 tonnes (28 short tons).[6]

Artist's impression of M. youngi

Mamenchisaurus youngi[edit]

Also described in 1996 was Mamenchisaurus youngi from the Upper Shaximiao Formation.[12][1] A local quarrying stone near a village in Zigong, Sichuan Province, found the remains in 1987. The species was named in honour of C. C. Young.[12]

The holotype specimen (ZDM 0083) was very complete and mostly articulated, preserving all the vertebra from the head down to the 8th tail vertebra; this species had 18 neck vertebra. Also preserved was, the pectoral girdle, pelvic girdle, and material from all four limbs. This specimen also preserved a nearly complete skull. There were four teeth in the premaxilla, 18 in the maxilla, and 22-24 in the dentary.[12] An unusual part of the skeleton is the wedge-shaped sacrum; this causes the sacrum and the base of the tail to pitch up relative to the rest of the vertebral column.[18] At 16 meters (52 ft) long with a 6.5 meters (21 ft) neck, M. youngi is one of the smaller species of Mamenchisaurus.[12][32] The mass of M. youngi was estimated at 7.87 tonnes (8.68 short tons) using volumetric techniques.[26]

Other Mamenchisaurus species and material[edit]

Other species of Mamenchisaurus have been named over the years. In some cases, species from other genera have been transferred to Mamenchisaurus, but there is disagreement with the referral's validity. Some of these species are based on fragmentary remains and have been considered undiagnostic. Others are considered as invalid or as nomina nuda.[1][28]

In 1976, Hou, Chao and Chu named a new genus, Zigongosaurus fuxiensis.[33][24] Known from at least four specimens from the Upper Shaximiao Formation. The type specimen (CV 00261??) included skull material; maxilla, dentary, basioccipital. Additional material includes dorsal vertebrae, pubis, and ischium. Since then other researchers have disagreed as to whether the genus is valid. Dong, Zhou and Zhang assigned the remains to the similarly named Omeisaurus fuxiensis in 1983. Zhang and Chen assigned the remains to Mamenchisaurus as M. fuxiensis in 1996.[34] Li and Cai considered it a nomen nudum in 1997.[24] In 1999, Valérie Martin-Rolland considered Zigongosaurus a valid genus. Wang and colleagues considered it undiagnostic in 2019.[9][28][1]

M. guangyuanensis was briefly mentioned by Zhang, Li, and Zeng in 1998 with the description of M. jingyanensis, but no details are given.[13] Li and Cai considered it a nomen nudum.[28] Wang and colleagues considered it undiagnostic in 2019.[1] M. yannanensis was described by Fang and colleagues in 2004, from the Anning formation Yunnan in the Sichuan Basin. The type specimen consisted of disarticulated forelimb, hindlimb, and pelvic material.[35] Wang and colleagues questioned the assignment to Mamenchisaurus in 2019.[1] M. yaochinensis is a nomen nudum.[36]

Omeisaurus changshouensis described by Young in 1958 and Omeisaurus gongjianensis were considered to be species of Mamenchisaurus by Zhang and Chen in 1996.[37] Upchurch considered O. changshouensis undiagnosable.[9]

NSM PV17656

In 1978, an incomplete sauropod humerus (NSM PV17656) found in a layer of the Early Cretaceous-aged Miyako Group of Japan was considered to probably belong to Mamenchisaurus. In 1991, it was referred to ?Mamenchisaurus sp. by Hasegawa and colleagues.[38][39] However, the humerus was reassessed by Azuma & Tomida in 1998, and Barrett and colleagues in 2002. They couldn't find any distinguishing features that could place the humerus into a specific sauropod group. They regarded it as an indeterminate sauropod.[40][41] The remains were given an informal name ''Moshi-ryu''; ''Moshi'' being the local name for the location it was discovered, and ''ryu'' being Japanise for dragon[38] — also referred to as ''Moshisaurus".[39]

Classification[edit]

Mamenchisaurus is sometimes referred to as a 'wastebasket taxon', with researchers questioning the number of species and fragmentary remains assigned to the genus.[8][9][6][10][42] The genus is poorly defined with an increasingly confused taxonomy which makes understanding phylogenic relationships difficult. Several analyses have failed to show Mamenchisaurus as monophyletic, suggesting the need to revise the genus.[10] Additional research on the type species, M. constructus, is required to better understand the genus.[9]

When M. constructus was first described, Young noted that the chevron bones indicated an affinity with Diplodocidae, but was uncertain to its exact position.[20] In 1958, Young assigned Mamenchisaurus to the Titanosauridae.[22] With the description of M. hochuanensis, Young and Zhao created the family Mamenchisauridae in 1972.[11] In 1978, when no Mamenchisaurus skulls were known, Berman and McIntosh assigned the genus to diplodocidae based on diplodocid-like vertebral features such as the forked chevrons. In 1990, McIntosh assigned Mamenchisaurus to a subfamily Mamenchisaurinae, which was placed inside Diplodocidae.[39]

An analysis by Upchurch found Mamenchisaurus in the family Euhelopodidae. Euhelopodidae, being named first, would take priority over Mamenchisauridae.[43] Several later analyses found Euhelopus to be a more distantly related macronarian, with Mamenchisaurus in Mamenchisauridae just outside of Neosauropoda.[10][8]

Sekiya in 2011 and Moore and colleagues in 2020 treated M. constructus, M. hochuanensis, ZDM 0126 (M. hochuanensis referred), M. sinocandadorum, and M. youngi separately in their analyses.[10][44] Moore and colleagues analyses found the position of M. constructus to be unstable, probably due to the limited character information in its original description. Depending on the dataset used, Euhelopus would either be within Macronaria as other studies have found or outside Neosauropoda in a more traditional position, grouped with other Mamenchisaurus-like taxa. The latter scenario would make mamenchisaurids members of Euhelopodidae.[10] The analyses of Sekiya (2011) and Moore and colleagues (2020) didn't recover ZDM 0126 as a sister taxon to the holotype of M. hochuanensis, questioning its referral to the species.[44][10]

The cladogram below shows a possible phylogenetic position of the genus within Sauropoda, from Wilson 2002:[45]

Sauropoda

Melanorosaurus

Camelotia

Blikanasaurus

 

Lessemsaurus

Antetonitrus

Gongxianosaurus

Gravisauria

Vulcanodon

Tazoudasaurus

Isanosaurus

Eusauropoda

Shunosaurus

Patagosaurus

Omeisaurus

Mamenchisaurus

Barapasaurus

unnamed

Cetiosaurus

Neosauropoda


Below, two phylogenetic trees show the internal relationships of Euhelopodidae/Mamenchisauridae in the two analyses Moore and colleagues deemed most favorable, the implied-weights and Bayesian analyses of the Gonzàlez Riga dataset.[10]

Topology A: Implied-weights analysis, Gonzàlez Riga dataset[10]

Mamenchisauridae

Tienshanosaurus chitaiensis

Omeisaurus junghsiensis

Core Mamenchisaurus‑like taxa

Wamweracaudia keranjei

Qijianglong guokr

Mamenchisaurus sinocanadorum

Chuanjiesaurus anaensis

Analong chuanjieensis

Mamenchisaurus hochuanensis (holotype)

Mamenchisaurus hochuanensis (referred)

Mamenchisaurus youngi

Shishugou cervicodorsals

Phu Kradung taxon

Klamelisaurus gobiensis

Topology B: Time-calibrated Bayesian analysis, Gonzàlez Riga dataset[10]

Euhelopodidae

Omeisaurus junghsiensis

Cetiosauriscus stewarti

Omeisaurus maoianus

Omeisaurus tianfuensis

Tienshanosaurus chitaiensis

Core Mamenchisaurus‑like taxa

Chuanjiesaurus anaensis

Analong chuanjieensis

Qijianglong guokr

Shishugou cervicodorsals

Mamenchisaurus sinocanadorum

Mamenchisaurus hochuanensis (holotype)

Mamenchisaurus constructus

Phu Kradung taxon

Klamelisaurus gobiensis

Mamenchisaurus hochuanensis (referred)

Mamenchisaurus youngi

Wamweracaudia keranjei

Xianshanosaurus shijiagouensis

Daxiatitan binglingi

Euhelopus zdanskyi

Dongbeititan dongi

Paleochronology[edit]

Mamenchisaurus was originally thought to have ranged from the Middle to Late Jurassic.[46][1] However, there isn't reliable dating for the Upper Shaximiao Formation, where many of the Mamenchisaurus species are found.[1]

A study published in 2018 used Uranium-lead dating on the underlying Omeisaurus bearing beds of the Lower Shaximiao Formation, previously thought to belong to the Middle Jurassic. However, the radiometric dating found the Lower Shaximiao Formation dated to the Late Jurassic, Oxfordian Stage, ~159 million years ago (mya). This finding suggests a younger age for the overlying Mamenchisaurus bearing rocks of the Upper Shaximiao; implying them to be no older than the Oxfordian.[1][47]

M. sinocanadorum was found from the Middle to Upper Shishugou Formation. Radiometric dating suggests this formation dated to the Oxfordian, ranging from 158.7 - 161.2 mya.[1][47] M. anyuensis, was found in the Suining Formation, thought initially to be Middle to Late Jurassic. A 2019 study found these rocks belonged to the Early Cretaceous, Aptian Stage, with an average age of ~114.4 mya. This indicates that mamenchisaurids might have existed around 30 million years longer than previously thought.[1]

References[edit]

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