[D] Opisthocoelicaudia skarzynskii [sG] [T]
Cretaceous Late ?Campanian Maastrichtian
Saurischia Sauropodomorpha Sauropoda Titanosauria
Nemegt Formation, Altan Uul IV, Omnogov, Mongolia
Genus - Typespecies
Skeleton lacking skull and cervical series
Opisthocoelicaudia is diagnosed by dorsal and sacral centra having a concave ventral surface that bears a ventral midline keel; opistocoelous proximal caudal centra (i.e., their cranial articular surfaces bear hemispherical articulations and their caudal articular surfaces are cupped); and pubes and ischia co-ossified throughout their lengths and closing the opening normally present in the ventral floor of the pelvis.
Opisthocoelicaudia is known from an articulated partial skeleton that is nearly complete apart from the loss of the skull and neck and from a scapulocoracoid belonging to a smaller individual. Opisthocoelicaudia was originally considered to be camarasaurid by Borsuk-Bialynicka (1977) and McIntosh (1990).
This taxon, however possesses many of the synapomorphies of Titanosauria and [Lithostrothia], including six co-ossified sacral vertebrae, a manus that lacks phalanges and has an elongate metacarpal I, and a shortened ischium with a transversely expanded distal blade (Gimenez, 1992; Salgado, 1993; Upchurch, 1995, 1998; Wilson and Sereno, 1998), and is therefore now widely considered to be an advanced lithostrotian closely related to either Alamosaurus or Saltasaurus. (Upchurch, Barrett & Dodson, 2004)
ZPAL MgD-I/48 housed in the Institue of Geology, Mongolian Academy of Sciences, Ulaanbataar. The incomplete skeleton of Opisthocoelicaudia was discovered in 1965 during a Polish-Mongolian Expedition to the Gobi Desert. Neither the head nor the neck was recovered from the site.
The skeleton of Opisthocoelicaudia is rather like that of most non-brachiosaurid sauropods in its general shape and proportions. The tail as preserved consists of 34 or 35 vertebrae. These bear chevrons which are similar to those of Camarasaurus rather than the skidd-like chevrons characteristic of diplodocids. The structure of the back vertebrae, particularly those to the neck led Borsuk-Bialynicka to suggest that the neck was not only flexible but that the vertebrae were relatively short, as they are in the neck of Camarasaurus. A scapulocoracoid of a young individual (ZPAL MgD I25/c) is also refered to Opisthocoelicaudia skarzynskii.
Leonardo Salgado, and Rodolfo A. Coria (1993) Considerations on the phylogenetic relationships of Opisthocoelcaudia skarzynskii (Sauropoda) from the Upper Cretaceous of Mongolia.
Translated by: Jeffrey A. Wilson University of Chicago Department of Organismal Biology & Anatomy June 1997
Opisthocoelicaudia skarzynskii was originally included in the family Camarasauridae principally on the presence of low, bifurcate dorsal neural spines. The following derived characters, nevertheless, relate Opisthocoelicaudia to Titanosauridae:
[Titanosauria*] (1) femur with pronounced lateral bulge below the major trochanter; (2) elongate, strongly opisthocoelous dorsal centra; (3) dorsal vertebrae with the infradiapophyseal lamina thickening toward its base; (4) presence of a centro-diapophyseal lamina uniting the base of the infradiapophyseal lamina and the parapophysis in dorsal vertebrae; (5) absence of hyposphene-hypantrum in dorsal vertebrae; (6) dorsal vertebrae with prespinal lamina developed until the base of the neural spine; (7) six strongly fused sacral vertebrae; ( caudal neural arches displaced anteriorly; (9) summit of iliac portion of the acetabulum located at base of the pubic peduncle; (10) ilia expanded outwardly; (11) pubis longer than ischium; (12) semilunar sternal plates; (13) quadrangular coracoids; (14) absence of manual phalanges. Within Titanosauridae, the following characters relate Opisthocoelicaudia to DGM “series B” (Titanosaurinae indet. from Peirópolis, Brazil): (15) presence of accessory spinodiapophyseal laminae in dorsal vertebrae.
The presence of bifurcate presacral neural spines, fusion of the haemal arches to the caudal centra, and the presence of strongly opisthocoelous caudal vertebrae are considered autapomorphies of Opisthocoelicaudia skarzynskii.
Source: Polyglot Paleontologist