Recovering Missing Morphological Data in Vertebrate Animals

An interesting new paper that proposes a method using regression equations to determine the position of preserved vertebrae in the caudal region (tail) of the herrerasaurid Staurikosaurus pricei and also provides estimates for the number and dimensions of missing vertebral elements.

Grillo, O. N., and S. A.K. Azevedo. in press.Recovering missing data: estimating position and size of caudal vertebrae in Staurikosaurus pricei Colbert, 1970. Anais da Academia Brasileira de Ciências ahead of print Epub Feb 04, 2011. doi: 10.1590/S0001-37652011005000003  



Abstract - Missing data is a common problem in paleontology. It makes it difficult to reconstruct extinct taxa accurately and restrains the inclusion of some taxa on comparative and biomechanical studies. Particularly, estimating the position of vertebrae on incomplete series is often non-empirical and does not allow precise estimation of missing parts. In this work we present a method for calculating the position of preserved middle sequences of caudal vertebrae in the saurischian dinosaur Staurikosaurus pricei, based on the length and height of preserved anterior and posterior caudal vertebral centra. Regression equations were used to estimate these dimensions for middle vertebrae and, consequently, to assess the position of the preserved middle sequences. It also allowed estimating these dimensions for non-preserved vertebrae. Results indicate that the preserved caudal vertebrae of Staurikosaurus may correspond to positions 1-3, 5, 7, 14-19/15-20, 24-25/25-26, and 29-47, and that at least 25 vertebrae had transverse processes. Total length of the tail was estimated in 134 cm and total body length was 220-225 cm.

Sanjuansaurus gordilloi a new Herrerasaurid from Argentina

Here is a new herrerasaurid from the the Ischigualasto Formation of Argentina. Looks much more similar to Herrerasaurus than to Staurikosaurus or Chindesaurus, although the latter taxon was not included in the phylogenetic analysis.  This analysis finds Herrerasauridae to be the sister taxon to Eoraptor + (Guaibasaurus + Neotheropoda), so we are back to the position of herrerasaurids still being dinosaurs but as basal saurischians rather than theropods.  Comparison is provided in the text to Tawa hallae, but it is not included in the phylogenetic analysis.  Too bad.  However, I'm pretty certain that based on the current material coding Sanjuansaurus into the analysis of Nesbitt et al. (2009) would not change the position of herrerasaurids as basal theropods as recovered in that analysis. So whether or not herrerasaurids are theropods or not appears to be entirely dependent on the base matrix one uses. Thus, unfortunately, the analysis in this paper does not appear to offer anything particularly game-changing or significant in that regards. Also unfortunate is that there is no discussion of these differing hypotheses in the paper.

The further recognition of the increased diversity of basal saurischian dinosaurs in the latest Carnian is interesting and provides more circumstantial support for the earlier diversification of Archosauria in the Early Triassic as hypothesized by Nesbitt (2009) based on body fossils and more recently by Brusatte et al. (2010) based on footprint evidence. 

Alcober O.A., and R. N. Martinez. 2010. A new herrerasaurid (Dinosauria, Saurischia) from the Upper Triassic Ischigualasto Formation of northwestern Argentina. ZooKeys 63 : 55 – 81 . doi: 10.3897/zookeys.63.550

Abstract - Herrerasauridae comprises a basal clade of dinosaurs best known from the Upper Triassic of Argentina and Brazil, which have yielded remains of Herrerasaurus ischigualastensis and Staurikosaurus pricei, respectively. Systematic opinion regarding the position of Herrerasauridae at the base of Dinosauria has varied. Here we describe a new herrerasaurid, Sanjuansaurus gordilloi gen. n., sp. n., based on a partial skeleton from Carnian-age strata of the the Upper Triassic Ischigualasto Formation of northwestern Argentina. The new taxon is diagnosed by numerous features, including long, band-shaped and posterolaterally oriented transverse process on the posterior cervical vertebrae; neural spines of the sixth to eighth dorsal vertebrae, at least, bearing acute anterior and posterior processes; scapula and coracoid with everted lateral margins of the glenoid; and short pubis (63% of the femoral length). Phylogenetic analysis placed Sanjuansaurus within a monophyletic Herrerasauridae, at the base of Theropoda and including Herrerasaurus and Staurikosaurus. The presence of Sanjuansaurus at the base of the Ischigualasto Formation, along with other dinosaurs such as Herrerasaurus, Eoraptor, Panphagia, and Chromogisaurus suggests that saurischian dinosaurs in southwestern Pangea were already widely diversified by the late Carnian rather than increasing in diversity across the Carnian-Norian boundary.


Holotype specimen of Sanjuansaurus gordilloi (PVSJ 605)

REFERENCES
Brusatte, S. L., Niedźwiedzki, G., and R. J. Butler. 2010. Footprints pull origin and diversification of dinosaur stem lineage deep into Early Triassic. Proceedings of the Royal Society B. doi: 10.1098/rspb.2010.1746


Nesbitt, S. J. 2009. The antiquity of Archosauria and the origin of Late Triassic archosaur assemblages. Journal of Vertebrate Paleontology 29:155A.

Nesbitt, S. J., Smith, N. D., Irmis, R. B.,Turner, A. H., Downs, A., and M. A. Norell. 2009. A Complete skeleton of a Late Triassic saurischian and the early evolution of dinosaurs. Science 326: 1530–1533.