CLASSIFICATION

Comments by Mickey Mortimer

Agustinia Bonaparte 1999 = Augustia Bonaparte 1998 (preoccupied Zariquiey 1927) A. ligabuei  Bonaparte 1999 = Augustia ligabuei Bonaparte 1998

Etymology

"Honoring Agustin and Ligabue", Agustin Martinelli being discoverer of the specimen and Dr. Giancarlo Ligabue being an active philanthropist who supported the 1997 Patagonian expediation.

Holotype

(MCF-PVPH-110) (15-16 m) last three dorsal neural spines, six sacral neural spines, eleven caudal neural spines, incomplete tibia, fibula (895 mm), metatarsal I (155 mm), metatarsal II (155 mm), metatarsal III (210 mm), metatarsal IV (205 mm), proximal metatarsal V, nine dermal ossifications. note: a femur was found with the specimen represented by hundreds of small weathered fragments and was not collected.

Diagnosis

Fibula wraps anteromedially around cnemial crest of tibia; dermal ossifiations with thick proximal/ventral processes, starting as rhomboid transversely oriented midline plates, proceeding to rectangular transversely oriented midline plates with dorsolateral spikes, then to dorsolaterally projecting flattened spines.

Description

Scaling the fibula from Rebbachisaurus gives a length of 16 meters while scaling from Saltasaurus gives a length of 15 meters, so Agustinia was about 15-16 meters. The last three dorsal neural spines are preserved.  They are transversely rectangular in dorsal view and possess four laminae.  The supradiapophyseal laminae are shortest, the prespinal lamina is longer and the postspinal lamina is longest.

The six sacral neural spines are fused to one another along their pre- and postspinal laminae and similar to the dorsal spines in morphology, being transversely expanded and rectangular, with four laminae each.

The first three caudal neural spines are represented, along with what probably represents the neural spines of caudals 6-13. These start out transversely expanded in caudals 1-3, then changes to longitudinally elongate.  The lateral laminae are prominent until the eighth caudal, then become rugosities.  The prespinal laminae are much more prominent than the postspinal laminae in the proximal caudals.

Both lower limb elements are crushed lateromedially, while the metatarsals are deformed.  The tibia has a pronounced, thin crest on the proximoposterior corner and a poorly developed cnemial crest.  The fibula wraps anteromedially around the cnemial crest, which is an autapomorphy, and is placed anterior to the tibia in it's distal half.

The metatarsals are very robust and decrease in thickness towards metatarsal V.  The first two metatarsals are subequal in length and shorter than III and IV, which are also subequal.  There may be an incipient laterodistal process on metatarsal I, but the deformation makes this uncertain.  Both II and III have distal condyles for a phalanx, while I lacks them.  Metatarsal III is transversely wider (78%) compared to I and II than all sauropods except Vulcanodon and Shunosaurus (should have <65%) according to Sereno and Wilson (1998, Omeisaurus + Neosauropoda, character # 73).

The osteoderms are truly unique and come in four types. 

The first type is roughly rhomboid and probably had a median position.  It resembles a Stegosaurus plate that's been rotated so that the faces of the plate face anterodorsally and posteroventrally.  The anterodorsal face is convex, while the posteroventral face is concave.  At the base of the posteroventral face is a thick process with a sagittal ridge and concave posterolateral areas adjacent to the ridge.  It was probably positioned on the midline of the dorsal area and is 200 mm on it's sagittal axis.

The second type also had a median position, but is rather different otherwise.  It is a large rectangular plate, with the faces pointing roughly anteriorly and posteriorly.  There are two large spikes projecting dorsolaterally.  The spikes are anteroposteriorly flattened and conical.  Between the spikes and plate body is a ridge, the area between the ridges is concave.  There are also two thick processes ventrolaterally that probably contacted the dorsolateral edges of the dorsal or sacral neural spines.  This ossification is 640 mm wide, 260 mm between ridges.

The third type is a split version of type 2, so it was paired.  It has a dorsoventrally broad and bifurcated medial section, with a thick ventral process.  There are several ridges on the proximal section, between which muscles probably attached.  Again, there are two spikes (one on each type 3).  The spikes are dorsolaterally projected and most are blunt, with subparellel edges, while one is conical and pointed.  These are 460 m long.

The fourth type is very similar to the third type, but with less expanded proximal ends and smaller proximoventral processes.  They are also longer than type three, being 760 mm.

Relationships

Agustinia is a sauropod based on- more than three sacral vertebrae; metatarsals I and V with proximal ends subequal in area to metatarsals II and IV.  It is eusauropod based on- dorsal neural spines broader transversely than anteroposteriorly; metatarsal III 25% or less of tibial length; minimum shaft width of metatarsal I greater than II-IV.  Sauropod systematics are currently unresolved regarding basal forms such as cetiosaurs and euhelopodids, but Agustinia shares several characters with diplodocoids and titanosaurids, which would place it in the Neosauropoda.

Titanosaurids and diplodocoids both have prespinal and postspinal laminae in the posterior dorsal vertebrae and dorsalized proximal caudal vertebrae (with laminae), but only diplodocoids have posterior dorsal vertebrae with craniocaudally compressed neural spines and only Euhelopus and titanosaurids have six sacral vertebrae.  Diplodocoids and some titanosaurids (Saltasaurus and Opisthocoelicaudia, not Alamosaurus, Malawisaurus or Titanosaurus) have craniocaudally compressed proximal caudal neural spines.  Only titanosaurids have been discovered with dermal ossifications. 

Bonaparte states the four perpendicular laminae with postspinal laminae longest are more similar to rebbachisaurids than titanosaurids.  This makes it apparent that Agustinia is either a diplodocoid or advanced titanosaurid, with the evidence about equal for either.  Both groups were common in Early Cretaceous South America.  A titanosaurid affinity is suggested by six sacral vertebrae and dermal ossifications, while diplodocoid affinity is suggested by craniocaudally compressed posterior dorsal neural spines and the orientation of the laminae.  Thus, I recommend placing Agustinia as Neosauropoda incertae sedis, as either a diplodocoid or advanced titanosaurid.

References

Bonaparte, J., 1998. An armoured sauropod from the Aptian of northern Patagonia, Argentina, in Tomida, Y., Rich, T. H. & Vickers-Rich, P., eds., 1998. Second Symposium Gondwana Dinosaurs, 12-13 July 1998, Abstracts with Program, National Science Museum, Tokyo: 10.

Bonaparte, J. F., 1999. An armoured sauropod from the Aptian of northern Patagonia, Argentina, in Tomida, Y., Rich, T. H. & Vickers-Rich, P., eds., 1999. Proceedings of the Second Gondwanan Dinosaur Symposium, National Science Museum Monographs #15, Tokyo: 1-12.