Sunday, June 3, 2018

Bagualosaurus agudoensis: another sauropodomorph ahead of its time

Our subject today is the newly described Bagualosaurus agudoensis from lower Upper Triassic rocks of southern Brazil. If I'd known back in March 2016 that I'd have the opportunity in a couple of years to write about another "prosauropod" that was "ahead of its time", and that it would include a partial skull justifying a terrible "head" pun, maybe I'd have come up with another title then. Oh, well; spilled milk and all that.

Genus and species: Bagualosaurus agudoensis; "Bagual" is a regional Brazilian word for "an animal or person of strong build or valour" (Pretto et al. 2018), and "agudoensis" refers to the town of Agudo, the source of the type and only known specimen, giving us the "valorous lizard of Agudo" or something similar. (I went with "valorous" instead of "strong build" because how often do you get to use "valorous" these days, and also because it's on the gracile side compared to other later "prosauropods".)
Citation: Pretto, F. A., M. C. Langer, and C. L. Schultz. 2018. A new dinosaur (Saurischia: Sauropodomorpha) from the Late Triassic of Brazil provides insights on the evolution of the sauropodomorph body plan. Zoological Journal of the Linnean Society in press.
Stratigraphy and geography: Santa Maria Formation, Upper Triassic (correlated to the Candel├íria Sequence, Hyperodapedon Assemblage Zone), Janner outcrop, about 2 km from Agudo, central Rio Grande do Sul, southern Brazil; Pretto et al. (2018) quoted a late Carnian age, but a date of 233.23 ± 0.73 million years ago published this year for a bed near the top of the Santa Maria (Langer et al. 2018) sounds rather more middle Carnian according to current Triassic divisions (as of this writing the Carnian is placed at approximately 237 to 227 million years ago).
Holotype: UFRGS-PV-1099-T (vertebrate paleontology collection at Universidade Federal do Rio Grande do Sul), a partial specimen including a partial skull (the lower, bitey part) and jaws, dorsal, sacral, and caudal vertebrae, some ribs, gastralia, and chevrons to go with them, partial hips, and most of the hind legs except the right foot and part of the left.

Bagualosaurus is known from one individual at this point, although pelvic material from the same site shares a feature with the type individual and may represent another, larger example of the same species. The type specimen was somewhat eroded when found, and unusually the carcass was buried on its back with the legs laying forward (picture how the legs would be positioned if you dragged the body by the tail, and you'll have the general idea). Inconveniently the skull was right along the line of modern erosion, causing the loss of the skull roof and most of the sides. Short strings of vertebrae were found between the skull and legs. The overall body size was modest for a dinosaur, with the femora estimated at about 215 mm (about 8.5 inches; for comparison, the femur of the holotype of our mascot Thescelosaurus is 355 mm), and the authors' reconstruction of the whole animal coming in a little shy of 2 m long. For a very early dinosaur, though, 2 m is pretty darn big.

The known bones offer an interesting mix of features. The pelvis and legs are fairly typical for a basal-dinosaur-slash-basal-saurischian. The long bones are more gracile than those of later "prosauropods", and the ilium has a low profile and is not all that large. There is evidence of a "prosauropodan" "pubic apron", where the two pubic bones are wider side-to-side than front-to-back and contact each other for much of their length. Meanwhile, some parts of the skull appear notably more derived than those of contemporaneous sauropodomorphs (Buriolestes, Pampadromeus, Panphagia, Saturnalia, etc.). The lower jaw is somewhat deeper than in those other forms, and the teeth seem to be more firmly in the herbivore camp than, say, Buriolestes or Panphagia, giving Bagualosaurus a face more noticeably like the "prosauropods" that showed up a few million years later. The eye socket is large, leading me to wonder if perhaps the type specimen had some growing left to do. The authors ran their new species through the data matrices of three different authors and in each case Bagualosaurus gravitated toward being the most derived of the early (Carnian) sauropodomorphs.

The source of the Bagualosaurus holotype, the Janner outcrop, has a lot more going on than just dinosaur, as is usual for Upper Triassic dinosaur-bearing. The most abundant animal is Exaeretodon riograndensis, a herbivorous traversodont (a kind of non-mammalian cynodont, first cousin a few generations removed from mammals). The site has also yielded specimens of the small beaked rhynchosaurian reptile Hyperodapedon, as-yet undetermined dinosauromorphs, and the type specimens of the sauropodomorph Pampadromaeus barberenai (which differs from Bagualosaurus in several ways, such as having a proportionally large skull and a tibia significantly longer than the femur) and the carnivorous cynodont Trucidocynodon riogranensis (Pretto et al. 2018).

References

Langer, M. C., J. Ramezani, and A. A. S. Da Rosa. 2018. U-Pb age constraints on dinosaur rise from south Brazil. Gondwana Research 57:133–140.

Pretto, F. A., M. C. Langer, and C. L. Schultz. 2018. A new dinosaur (Saurischia: Sauropodomorpha) from the Late Triassic of Brazil provides insights on the evolution of the sauropodomorph body plan. Zoological Journal of the Linnean Society in press.

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