Wednesday, May 20, 2015

Is Chilesaurus a basal tetanurine?

I've been away for a few weeks, but came back to find the weird new dinosaur Chilesaurus exists.  Just looking at it, any dinosaur worker would notice an odd mix of characters, making me curious exactly what it's related to.  Novas et al. (2015) used four analyses to determine it belonged in non-orionidan Tetanurae.  To use a Peters-ism, that's one strange bed fellow.  So let's go in for a closer look.

Their analyses

The first analysis was Nesbitt et al.'s (2009) Tawa analysis (315 characters), which focused on basal theropods and saurischians.  This found Chilesaurus to be a coelurosaur, though so was Ceratosaurus.  The authors rightly note the matrix was not designed to test neotheropod phylogeny, and the low Bremer supports show this, though I note the pairing of Chilesaurus with the only included actual coelurosaur Velociraptor is better supported than the other neotheropodan nodes. 15 more steps were needed to constrain it as sister to Avepoda, 16 more to constrain it as a sauropodomorph (ended up sister to Plateosaurus instead of Saturnalia or Efraasia), and 14 more as an ornithischian (as the basalmost one or a heterodontosaurid).  These sound impressive until you realize that the matrix wasn't designed with ornithischian or sauropodomorph apomorphies in mind.  Also, I wonder how many more steps it took to place as a basal tetanurine where Novas et al. ultimately conclude it goes?  And why not use Nesbitt's (2011) larger anlysis which expanded on this and has had a major update by Langer and Ferigolo (2013), or at least Sues et al.'s (2011) version of Nesbitt et al.'s that added Daemonosaurus?  The latter genus also has three premaxillary teeth, a short snout, broad posterolateral premaxillary process and elongate cervicals with pleurocoels, so might help move Chilesaurus out of Avepoda.

The second analysis is a logical follow up, coding Chilesaurus in the sauropodomorph-focused matrix of Otero and Pol (2013) (with Tawa added; 353 characters), that is itself basically Yates' 2007 'prosauropod' matrix.  This finds Chilesaurus in a polytomy with Avepoda, Tawa and Chindesaurus, but placing it in Sauropodomorpha (as the most basal member) is now only five steps longer.  I'd say that's not strong support for Chilesaurus being a theropod, as numerous relationships rejected by five steps in one matrix end up being supported in other or later matrices.  It now takes 11 more steps to place it sister to Ornithischia, but that OTU isn't divided up, making the situation more problematic than above.

The third analysis uses a revised version of Smith et al.'s (2007) theropod matrix that is terribly mis/un-coded.  Not encouraging, though Novas et al. "deeply rescored" "several taxa" "based on new available data" including at least Eoraptor and Megaraptor.  They also added Tawa, Aerosteon, Falcarius, Jianchangosaurus and Therizinosauridae and 56 new characters (total now 412 characters).  Here, Chilesaurus emerges as sister to Piatnitzkysauridae+Orionides.  However, only TWO steps are needed to constrain it as the sister of Avepoda*, and only three are needed to place it as the basalmost coelurosaur.  So even if the authors fixed the matrix, it only very weakly supports a basal tetanurine position.  Suspiciously, Plateosaurus was deleted from the matrix "because of the morphological gap present between the very early sauropodomorphs present in the data set (e.g. Saturnalia) and Plateosaurus."  This is rich when they left Velociraptor in Nesbitt et al.'s matrix, which is at least as different from Allosaurus.  As the only sauropodomorphs left were the incomplete Saturnalia and controversial Eoraptor, and considering Chilesaurus emerged sister to Plateosaurus when constrained as a sauropodomorph in their first analysis, I can't help but wonder if Chilesaurus was sister to Plateosaurus here too until the latter was deleted.  Novas et al. never say how many more steps it takes to place Chilesaurus in Sauropodomorpha in this matrix, and ornithischians weren't included.

* Just looking through their list of characters supporting the position, astragalar ascending process height is miscoded in Smith et al.'s (it's clearly not "higher than the astragalar body, typically covering only lateral half of anterior surface of distal tibia"), as is "ridge on lateral side of tibia for connection with fibula present and clearly separated from proximal articular surface."  So there's the two steps we need to move it outside Avepoda.

Finally, the fourth analysis used Carrano et al.'s (2012) basal tetanurine-focused analysis (351 characters).  Now Chilesaurus emerges in a polytomy with Monolophosaurus, Chuandongocoelurus, Megalosauroidea (including piatnitzkysaurids) and Avetheropoda.  Again, only two more steps are necessary to place it in Megalosauroidea, so its precise position here is very poorly supported.  Seven more steps are needed to place it in Coelurosauria (emerges sister to Compsognathus), but only three coelurosaurs are included in the matrix and they're heavily miscoded.  I added a lot of basal coelurosaurs to it when testing Bahariasaurus and found e.g. Compsognathus has 112 miscodings (of 351 characters).  That these include at least 7 that are actually more similar to Chilesaurus seems likely, though of course they probably also include at least 7 less similar to it, with the conclusion that the original matrix can't tell us how Chilesaurus compares to coelurosaurs.  As no maniraptoriforms were included, it can't tell us if Chilesaurus might belong there either.  No sauropodomorphs except the controversial Eoraptor were included, and no ornithischians.  Though like above matrix, this one wasn't made to test such basal nodes.

Given these results, Novas et al.'s conclusion seems to be stated far more strongly than their evidence indicates.  They say "the four independent phylogenetic data matrix [sic] favours a position as a neotheropod [avepod in my terminology], and particularly as a basal tetanuran", "The results of the four analyses are detailed below, but all of them agree in the position of Chilesaurus as a tetanuran theropod", and "Remarkably, all these analyses placed Chilesaurus as a member of Theropoda, near the origin of tetanurans."  Only the first two matrices have the needed basal taxa to test whether Chilesaurus is in Avepoda, and the second doesn't place it any closer to avepods than Tawa or Chindesaurus and supports Sauropodomorpha as a highly plausible alternative.  Further, the first places it as a ceratosaur not a tetanurine, the second doesn't even split Avepoda into multiple OTUs so can't weigh in, and the third is basically ambiguous whether Chilesaurus is actually sister to Avepoda or a coelurosaur.  Only the fourth analysis may strongly support placing Chilesaurus as a monolophosaur-piatnitzkysaur grade tetanurine, though we don't know how easy it is to place outside Avepoda in that one.

I'd say that even if we trusted these matrices accuracy 100%, they only tell us it's quite possible Chilesaurus is the basalmost sauropodomorph or coelurosaur and is about equally likely to be sister to Avepoda, a non-orionidan tetanurine or a basal megalosauroid.  Not actually much agreement there.

My analyses

Will come next...

References- Smith, Makovicky, Hammer and Currie, 2007. Osteology of Cryolophosaurus ellioti (Dinosauria: Theropoda) from the Early Jurassic of Antarctica and implications for early theropod evolution. Zoological Journal of the Linnean Society. 151, 377-421.

Yates, 2007. The first complete skull of the Triassic dinosaur Melanorosaurus Haughton (Sauropodomorpha, Anchisauria). Special Papers in Palaeontology. 77, 9-55.

Nesbitt, Smith, Irmis, Turner, Downs and Norell, 2009. A complete skeleton of a Late Triassic saurischian and the early evolution of dinosaurs. Science. 326, 1530-1533.

Nesbitt, 2011. The early evolution of archosaurs: Relationships and the origin of major clades. Bulletin of the American Museum of Natural History. 352, 292 pp.

Sues, Nesbitt, Berman and Henrici, 2011. A late-surviving basal theropod dinosaur from the latest Triassic of North America. Proceedings of the Royal Society B. 278(1723), 3459-3464.

Carrano, Benson and Sampson, 2012. The phylogeny of Tetanurae (Dinosauria: Theropoda). Journal of Systematic Palaeontology. 10(2), 211-300.

Langer and Ferigolo, 2013. The Late Triassic dinosauromorph Sacisaurus agudoensis (Caturrita Formation; Rio Grande do Sul, Brazil): Anatomy and affinities. Geological Society, London, Special Publications. 379(1), 353-392.

Otero and Pol, 2013. Postcranial anatomy and phylogenetic relationships of Mussaurus patagonicus (Dinosauria, Sauropodomorpha). Journal of Vertebrate Paleontology. 33(5), 1138-1168.

Novas, Salgado, Suarez, Agnolın, Ezcurra, Chimento, Cruz, Isasi, Vargas and Rubilar-Rogers, 2015. An enigmatic plant-eating theropod from the Late Jurassic period of Chile. Nature. doi:10.1038/nature14307