Mamenchisaurus sp.
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publication ID |
https://doi.org/ 10.4202/app.2009.0155 |
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persistent identifier |
https://treatment.plazi.org/id/03C98268-CB72-FF86-8202-F86DFE7762FD |
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treatment provided by |
Felipe |
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scientific name |
Mamenchisaurus sp. |
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intrapostzygapophyseal
Material.—SM KS26−4, nearly complete posterior cervical vertebra; SM KS26−2, SM KS26−3, fragmentary ribs from Phu Dan Ma, Phu Kradung Formation (Upper Jurassic–?Lower Cretaceous), northeastern Thailand.
Description.—The vertebra SM KS26−4 is nearly complete and well preserved, except the right parapophysis, diapophysis, and postzygapophysis, and the posterior portion of the centrum which are damaged ( Figs. 3 View Fig , 4 View Fig ). The vertebra is
http://dx.doi.org/10.4202/app.2009.0155
Height of articulation 125
Width of articulation 165
Height of centrum at posterior 125*
Width of centrum at posterior 180*
Greatest length of centrum 170
Centrum length without articular condyle 145
Total height 340
Vertebra Centrum Centrum Elonga− Taxon
number length height tion Phu Dan Ma (SM KS26−4)? 170 125 1.4 Euhelopus zdanskyi C17 180 142 1.3 (PMU.R−233) C18 142 142 1.0
C19 128 132 0.9 Mamenchisaurus C17 550 375 1.5 hochuanensis C18 400 380 1.1 (CCG V 20401)
C19 325 350 0.9 Mamenchisaurus youngi C17 316 175 1.8 (ZDM0083) C18 260 165 1.6 Omeisaurus tianfuensis C16 500 310 1.6 (T5704) C17 335 290 1.2
distorted and slightly compressed anteroposteriorly and laterally and the postzygapophysis is bent forward. The ribs were found articulated to the vertebra. The capitulum and tuberculum processes of the right rib (SM KS26−2) are missing. The parapophyses are located on the centrum ventrolaterally and the rib shaft is bent outward oblique to the long axis of the centrum. This feature indicates an intermediate position between cervical and dorsal vertebrae. In addition, the position of the parapophyses is as low as the ventral surface of the centrum and the shape of the rib is reminiscent of cervical ribs. Thus, SM KS26−4 is identified as one of the most posterior cervical vertebrae.
The vertebra is relatively high and short anteroposteriorly ( Table 1). The centrum is strongly opisthocoelous with an Elongation Index (EI) about of 1.4 (EI = anteroposterior length/height of posterior face; sensu Upchurch 1998; Wilson 2002). The ventral surface of the centrum is concave and has a stout median keel. The centrum and neural arch are pneumatised ( Wedel 2009). The breaks of the anterior articular surface show pneumatic camellate structures, circular cells separated by thin bone laminae ( Wedel et al. 2000). Laterally, the pneumatic fossa occupies most of the lateral surface of the centrum. A horizontal lamina (or “supracentral lamina”; Osborn and Mook 1921) divides the pneumatic fossa into upper and lower parts.
The parapophysis is robust and projects ventrolaterally. Its dorsal surface is excavated by a pneumatic fossa. The parapophyseal facet is sub−circular.
The neural canal is triangular in anterior view and spindle−shaped in posterior view. The neural spine is relatively low. The U−shaped cleft between the bifid spines is shallow and has no median spine. Ventral to the cleft, a prominent scar is marked at the base of the neural spine in anterior view. It probably represents the mineralised attachment area for the elastic ligament ( Schwarz et al. 2007).
The prezygapophyseal facets are large, subrectangular and convex transversely. Triangular pneumatic fossae excavate below the prezygapophysis and postzygapophysis and are limited medially by the intraprezygapophyseal and intrapostzygapophyseal laminae, respectively.
The diapophysis projects outward from the neural arch and curves ventrally toward its distal end. The diapophysis is flattened anteroposteriorly, with internal pneumatic cavities extending both anteriorly and posteriorly. A large pneumatic fossa excavates the diapophysis ventrally between the anterior and posterior centrodiapophyseal laminae.
Both ribs (SM KS26−2 and SM KS26−3) are preserved but not complete ( Fig. 5). They show a sharp angle (45 °) between the capitulum and tuberculum with a median ridge. The tuberculum is transversely wide and has a rectangular facet, while the capitulum is a peduncle−like structure with a circular facet.
Discussion.—The EI of the centrum of SM KS26−4 is reminiscent of the long−necked Chinese sauropods: it is close to that of the 17 th cervical vertebra of Omeisaurus tienfuensis and that of the 18 th cervical vertebra of Mamenchisaurus hochuanensis and is between those of the 17 th and 18 th cervical vertebrae of Euhelopus zdanskyi ( Table 2). The large pleurocoel with successive generations of smaller chambers of SM KS26−4 is similar to that of Camarasaurus ( Osborn and Mook 1921: pl. 69) and differs from Euhelopus , in which the pleurocoel is a simple and shallow concavity. Ouyang and Ye (2002) noted that there is no pleurocoel in the cervical vertebrae of M. youngi , but in fact the lateral surface of the centrum of Mamenchisaurus is excavated by a large concavity ( Young and Zhao 1972: pl. 14; Ouyang and Ye 2002: pl. 8). Moreover, the centrum and neural arch of SM KS26−4 are entirely filled with numerous small pneumatic camellae like those of Mamenchisaurus (Russell and Zhang 1993; Ouyang and Ye 2002), Omeisaurus ( He et al. 1988) , and a vertebra from the Jurassic of Southern Thailand ( Buffetaut et al. 2005). This cancellous inner structure differs from the spongy bones of somphospondylans such as Euhelopus and the saltasaurid Ampelosaurus Le Loeuff, 1995 , the latter having more irregularly shaped and relatively smaller camellate cells. The camellate structure of SM KS26−4 is different from the solid bone of Phuwiangosaurus , from the overlying Sao Khua Formation (Martin 1994). Wedel et al. (2000) proposed that the pneumatic camellate structure in vertebrae (in both the centrum and neural spine) evolved independently at least twice in the Sauropoda: in Mamenchisaurus and in the Somphospondyli, a group defined as titanosauriforms more closely related to Saltasaurus than to Brachiosaurus ( Wilson and Sereno 1998) .
capitulum anterior process tuberculum 10 cm posterior process
The neural spine of SM KS26−4 is reminiscent of the bifid spine of Mamenchisaurus hochuanensis and M. youngi ( Young and Zhao 1972; Ouyang and Ye 2002). The cleft is broader with a U−shape in Opisthocoelicaudia Borsuk−Białynicka, 1977 and a V−shape in Apatosaurus ajax ( Upchurch et al. 2004b) , whereas it is deep and steep in Camarasaurus ( Osborn and Mook 1921: pl. 69: 13) and Phuwiangosaurus Martin, Buffetaut, and Suteethorn, 1994 . The bifid spine of Euhelopus has a median tubercle, which is an autapomorphy of the genus ( Wilson 2002). Omeisaurus differs from the others as its spine is undivided ( He et al. 1988). The short neural spine in the posterior cervical vertebra with a shallow U−shaped cleft is considered as an autapomorphy of Mamenchisaurus .
Mamenchisauridae indet. A
Fig. 6A–C View Fig .
Material.—SM MD3−53, SM MD3−54, SM MD3−62, isolated teeth from Dan Luang, Phu Kradung Formation, northeastern Thailand.
Remarks.— Buffetaut and Suteethorn (1998a, 2004) described spatulate teeth from the Dan Luang locality in the Phu Kradung Formation ( Fig. 6 View Fig ), and recognized the resemblances between the Thai specimens and Omeisaurus based on the presence of denticles on unworn teeth ( Buffetaut and Suteethorn 2004: figs. 1, 3). However, denticles are also present in Mamenchisaurus ( Russell and Zheng 1993) . Teeth from Thailand are similar to those of Mamenchisaurus fuxiensis (= Zigongosaurus fuxiensis Hou, Chao, and Chen, 1976 ) from the Wujiaba locality in the lower part of the Upper Shaximiao Formation of Zigong, China (Haiyan Tong and Hui Ouyang, personal communication 2009). Although Ouyang and Ye (2002) regarded the latter as indeterminate, because of the presence of a mosaic of characters found in Omeisaurus and Mamenchisaurus , these teeth belong to the family Mamenchisauridae as shown below.
The presence of a lingual boss has been recognized in various sauropods such as Mamenchisaurus ( Russell and Zheng 1993: pl. 2; Ouyang and Ye 2002: figs. 9–12), Omeisaurus ( He et al. 1988: fig. 16), Euhelopus ( Wiman 1929: pl. 2), Camarasaurus ( Osborn and Mook 1921: pl. 60) and isolated teeth from Thailand ( Buffetaut and Suteethorn 2004: fig. 1) and Spain ( Canudo et al. 2002: fig. 2). It seems that among these sauropods, those from younger formations, namely Euhelopus and the Spanish specimen ( Wiman 1929; Canudo et al. 2002; Barrett and Wang 2007), show a complex pattern of cingular structure. Some of their teeth ( Wiman 1929: pl. 2: 12, 15, 17, 18, 23) have a lingual boss associated with a cingulum, a horizontal ridge and some ( Wiman 1929: pl. 2: 14, 16,19, 21; Canudo et al. 2002: figs. 2, 3) have two lingual
http://dx.doi.org/10.4202/app.2009.0155
bosses, mesially and distally, associated with a medial ridge or (third) boss forming a pseudo−cingulum. These differences possibly result from their positions in the tooth row such as anterior−posterior or upper−lower teeth. Teeth of sauropods from older formations, namely Mamenchisaurus , Omeisaurus , and Thai specimens ( Russell and Zheng 1993: fig. 4; Zhang et al. 1998: pl. 1; Ouyang and Ye 2002: figs. 11, 12; He et al. 1988: pl. 2; Buffetaut and Suteethorn 2004: fig. 1), show a single distal boss with a smoothly curved lingual concavity. We agree with Canudo et al. (2002) that the complexity of the cingular structures (a cingulum and boss) possibly represents a derived state ( Fig. 6 View Fig ). Regardless, it should be stated that this feature is not present in all preserved specimens and is sometimes present in more primitive sauropods such as Camarasaurus ( Osborn and Mook 1921: pl. 60: 6a). The spatulate teeth from Dan Luang are thus similar to those of Mamenchisaurus and Omeisaurus on the basis of primitive characters: presence of tooth denticles, single boss on lingual surface.
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