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.
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| Holotype specimen of Sanjuansaurus gordilloi (PVSJ 605) |
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.
Gahhh, when I see cases like this, I can't help but feel very annoyed.
ReplyDeleteTheir matrix is straight from Langer and Benton (2006) - with Eoraptor and Panphagia added from Martinez and Alcober (2009). That means 98 characters, there's likely very little data supporting alternative placements- since Benton's matrices aren't usually known for that.
I agree with your remarks regarding including Sanjuansaurus into Nesbitt et al. 2009.
To expand on that would be more interesting - Guaibasaurus and the other "guaibasaurids" as well as the characters relative to that part of the basal sauropodomorph tree (Ezcurra 2010 - the description of Chromogisaurus) and any characters which are possibly missing in Nesbitt et al. 2009 which would support placing herrerasaurs outside of the Theropoda...
Ultimately, cladistics is a tool. You can use it right or you can use it wrong. Using very exclusive datasets that don't include enough characters to pit alternative ideas against each other, excluding relevant taxa, limiting your analysis to a small subset and only focusing on results that support your preferred topology is wielding it wrong. I'm not saying this to suggest the authors have ulterior motives or anything sinister like that, just you can't just drop a taxon into a matrix and plug it away and say at the end you did good by it systematically, and this seems to be a presumption that a lot of authors seem to make.
It's worth observing that whether herrerasaurs are basal saurischians or theropods, we're still looking at a very rapid early diversification of dinosaurs (consider the roughly coeval - I think - early sauropodomorphs), so the position of herrerasaurs doesn't change this narrative one bit.
That's enough ranting out of me for today, and on theropods too. ;)
"only focusing on results that support your preferred topology is wielding it wrong"
ReplyDeleteI meant to write "only focusing on data that support your preferred topology is wielding it wrong".... pulled the trigger too quickly.
Sorry, -N.
It possible that this analysis would end up supporting the "herrerasaurs are theropods" hypothesis anyway, Bonaparte et al. (2007) notes that the Guaibasaurids seem to share several characteristics with theropods that they do not share with sauropodomorphs, suggesting that there is an unlikely chance that the group could end up as basal theropods (or closer to theropods than sauropodomorphs). The shared characteristics could also give a false idea of a Guaibasauridae+Theropoda clade if there aren't non-Guaibasaurid sauropodomorphs included in the analysis.
ReplyDelete