Sunday, January 9, 2011

"Microdontosaurus" - Zhao's nomina nuda part 3

"Microdontosaurus" Zhao, 1983
"Microdontosaurus dayensis" Zhao, 1985
Etymology- The genus name refers to small teeth, while the species name references the Daye township in Qamdo.
Middle Jurassic
Middle Dapuka Group, Dabuka, Qamdo County, Tibet, China
- specimen seemingly including teeth
Comments- As Gilmore (1902) already named an ichthyosaur Microdontosaurus petersonii (currently a junior synonym of Baptanodon discus), Zhao's sauropod will need a new name if it is officially described in the future.   Glut (1997) incorrectly dates the species to 1983 and places it in the Late Cretaceous. 
Relationships- Stated by Zhao (1983) to be a homalosauropodoid, in which he includes peg-toothed taxa like diplodocoids (including mamenchisaurids in his view) and titanosaurs.  Similarly, Zhao (1985) states it is a primitive peg-toothed sauropod.  Chure and McIntosh (1989) listed it as a cetiosaurid, Lambert (1990) listed it as a diplodocid, and Olshevsky (1991) as a melanorosaurid.  Glut (1997) merely refers  it to Sauropoda incertae sedis.  Most recently, Fang et al. (2006) place it in Brachiosauridae.
A placement outside of Sauropoda is unlikely for a Middle Jurassic taxon (though not unheard of- see Yunnanosaurus? youngi), suggesting Olshevsky's assignment is incorrect.  While traditionally only diplodocoids and titanosaurids were seen as having nonspatulate teeth, they are also found in more basal brachiosaur-grade titanosauriforms and even some basal eusauropods like Shunosaurus.  This suggests Fang et al.'s assignment to Brachiosauridae may be most accurate, especially as Zhao is a coauthor while Chure, McIntosh and Lambert have not seen the specimen.  Since Brachiosauridae as traditionally conceived is paraphyletic, "Microdontosaurus" is here tentatively assigned to Titanosauriformes incertae sedis.
References- Gilmore, 1902. Discovery of teeth in Baptanodon, an ichthyosaurian from the Jurassic of Wyoming. Science. 16(414), 913-914.

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