Remarks on titanosaur pelvic girdle

SEBASTIÁN APESTEGUÍA; LEONARDO SALGADO

Buenos Aires

Jornada; XIX Jornadas Argentinas de Paleontologia de Vertebrados; 2003

Museo Argentino de Ciencias Naturales

The six-vertebrae titanosaur sacrum has two large and wide anterior vertebrae and a 3rd centrum that narrows posteriorly and bear ribs extending obliquely toward the ilium pubic peduncle. The 4th and 5th centra are the narrowest, except in the unnamed titanosaur MCT1536R from Peiropolis, Brazil, where the 5th, caudal in origin, expands posteriorly. The 6th sacral centrum, added in somphospondylans, is variable in shape but stays always confronting the ischiatic peduncle. In ‘Titanosaurus’ colberti and the unnamed titanosaur MCT1488R, it is probably biconvex and expands posteriorly, allowing an articulation with a larger procoelous or biconvex 6th sacral vertebra, which in Neuquensaurus is already included in the sacrum. Considering the 3rd sacral as a landmark, it is possible to understand the sacral structure of the Epachthosaurus paraplastotype, where the 4th to 6th vertebrae strongly narrow, as in Argentinosaurus. Although the sacrum of the later is poorly preserved, the lack of one more large anterior vertebra, the position of the largest sacral fenestra and the narrowing, allows to suggest that the preserved vertebrae are not 1 to 5 but 2 to 6. In the Epachthosaurus paraplastotype, an additional re-expanding vertebra, equivalent in position to the 7th Neuquensaurus sacral, allowed the contact with a larger first caudal vertebra. Although not preserved, it probably existed also in Argentinosaurus. Derived titanosaurs show as probably related features, a lengthening of the sacral count plus laterally projected ilium anterior lobes. Additionally, its transverse pubic peduncles do not allow anteroposterior movements of femora when horizontally positioned, confirming that the sacral axis was carried obliquely.