Analyses and random notes on Chilesaurus from the media

Still big news, Chilesaurus was originally considered (Novas et al. 2015) a ‘bizarre’ theropod, but nests at the base of all ornithischians in the large reptile tree.

Figure 1. Chilesaurus and kin, including Damonosaurus and basal phytodinosauria.

Figure 1. Chilesaurus and kin, including Damonosaurus and basal phytodinosauria.

Prior hylogenetic analyses
Novas et al. tested Chilesaurus into four different data matrixes with samplings focused on basal dinosauriforms (Nesbitt et al.7), basal sauropodomorphs (Otero & Pol) and basal theropods (Carrano et al.; a modified version of Smith et al.). Some were modified by the addition and removal of characters and taxa. Ultimately, none agreed with one another in the nesting of ChilesaurusThat’s a red flag. 

The results of the four analyses
did agreed in the position of Chilesaurus as a tetanuran theropod (but that’s a very large clade, and done so in the absence of its true sister taxa, see below). According to Novas et al, “Features supporting the theropodan position of Chilesaurus include: pleurocoelous cervical and cranial dorsal vertebrae; hypapophyses on ‘pectoral’ vertebrae; preacetabular wing of ilium dorsoventrally expanded; femoral fourth trochanter semicircular; and tibia distally expanded and with lateral malleolus extending strongly laterally. Tetanuran characteristics present in Chilesaurus are: scapular blade elongate and strap-like; distal carpal semilunate; and manual digit III reduced.”

Novas et al. found this combination of derived Coelurosaurian and Prosauropod-like traits specifically recalls the plant-eating Laurasian therizinosaurs, nevertheless, this set of general characteristics contrasts with the 18 derived features absent in Chilesaurus that are usually recognized as uniting therizinosaurs with derived coelurosaurs.

Furthermore, under constrained suboptimal topologies, 11 additional steps are necessary to force the position of Chilesaurus as a therizinosaur. This set of anatomical distinctions implies a phylogenetic position for Chilesaurus outside Therizinosauria, Maniraptora and Coelurosauria.

Ornithischians were considered:
Novas et al. report, “Apart from typically saurischian and theropodan characters, Chilesaurus also shows several potential apomorphies of ornithischians or subclades thereof. Unfortunately, very few basal ornithischians are currently known from good materials. The data matrix of Nesbitt et al. 2009 includes Scutellosaurus, Lesothosaurus, Eocursor, Heterodontosaurus and Pisanosaurus, and thus provides as good a taxon. This matrix discards that Chilesaurus is an ornithischian.”

As noted above,
none of the Novas analyses included the basal ornithischians, Daemonosaurus and Jeholosaurus. By excluding these taxa Novas et al. were comparing apples to oranges, something that happens far too often in paleontology as noted tar too many times in this blog.

Figure 3. Cladogram of basal dinosaurs. Note that Chilesaurus nests near the base of the Phytodinosauria and at the base of the Ornithischia, both far from the Theropoda.

Figure 1. Cladogram of basal dinosaurs. Note that Chilesaurus nests near the base of the Phytodinosauria and at the base of the Ornithischia, both far from the Theropoda.

Quotes from the PR machine:
I thought it interesting to run through some quotes from the various online news stories carried today on this new find. When you read this, keep the new phylogenetic nesting in mind and it will clarify the mysteries raised.

NBC News: “Scientists have unearthed fossils of a strange dinosaur in southern Chile that boasts such an unusual combination of traits that they are comparing it to a platypus, that oddball egg-laying, duckbilled mammal from Australia…  it ate only plants with a beak and leaf-shaped teeth, scientists said on Monday. …Its skull and neck resembled those of primitive long-necked dinosaurs, and its vertebrae those of primitive meat-eating theropods. It had robust arms, but just two blunt fingers on each hand. It was bipedal, but its wide, four-toed feet were unlike the slender, three-toed feet of most theropods. And it had a bird-like pelvis.”

“Chilesaurus constitutes one of the most bizarre dinosaurs ever found,” said paleontologist Fernando Novas. “No other dinosaurs exhibit such a combination or mixture of features.”

America Aljazeera: “Four nearly complete skeletons and dozens of bones from other individuals were found, making Chilesaurus one of the best-understood Jurassic Southern Hemisphere dinosaurs. It belongs to a previously unknown dinosaur lineage,” University of Birmingham paleontologist Martín Ezcurra said.

Brian Switek writing for Smithsonian: “In the place where Diego discovered it, there are more bones of Chilesaurus than any other creature. This is odd. In most environments of about the same age, the most common dinosaurs are beaky little herbivores that belonged to a very different lineage of dinosaurs called ornithischians. Here, for some reason, a theropod came to dominate instead.

“This is a really unusual beastie, a bit of a dinosaur Frankenstein,” says paleontologist Lindsay Zanno of the North Carolina Museum of Natural Sciences.

Reuters: “It belongs to a previously unknown dinosaur lineage,” University of Birmingham paleontologist Martín Ezcurra said. “Convergent evolution’ is a process in which two unrelated species or groups acquire similar characteristics from living in similar environments or having a similar behavior,” like the wings of a bat and a bird, Ezcurra added. “In the case of ‘mosaic convergent evolution,’ different parts of the body resemble those of other unrelated species, such as in the case of the platypus and Chilesaurus.”

Sydney Morning Herald: “Chilesaurus can be considered a ‘platypus’ dinosaur because different parts of its body resemble those of other dinosaur groups due to mosaic convergent evolution,” study author Martín Ezcurra of the University of Birmingham said in a statement. “In this process, a region or regions of an organism resemble others of unrelated species because of a similar mode of life and evolutionary pressures. Chilesaurus provides a good example of how evolution works in deep time and it is one of the most interesting cases of convergent evolution documented in the history of life”

EurkaAlert (the Global Source for Scientific News): “Chilesaurus represents one of the most extreme cases of mosaic convergent evolution recorded in the history of life. For example, the teeth of Chilesaurus are very similar to those of primitive long-neck dinosaurs because they were selected over millions of years as a result of a similar diet between these two lineages of dinosaurs.” 

Novas et al. writing in Nature: “Early theropod evolution is currently interpreted as the diversification of various carnivorous and cursorial taxa, whereas the acquisition of herbivorism, together with the secondary loss of cursorial adaptations, occurred much later among advanced coelurosaurian theropods12. A new, bizarre herbivorous basal tetanuran from the Upper Jurassic of Chile challenges this conception.”

As noted
earlier, Chilesaurus is not bizarre, convergent or enigmatic in the large reptile tree. Rather it nests at the base of the Ornithischia, close to the base of the Sauropodomorpha, together nesting near the base of the Phytodinosauria and derived from Eoraptor, Pampadromaeus and kin.

Together with Jeholosaurus, Chilesaurus provides a rare glimpse into the genesis of the ornithischian beak and pelvis, something paleontologists have been looking for for decades. This long-sought relationship was completely overlooked by the original authors. AND this is one of the holy grails of paleontology… sadly passed by due to taxon exclusion.

Gotta work on that…

Theropod database
See M.Mortimer’s take on Chilesaurus here. Mortimer found a raft of miscodings in the original paper by Novas et al.

References
Novas FE, Salgado, Suárez LM, Agnolín FL, Ezcurra MND, Chimento NSR.,de la Cruz R, Isasi MP, Vargas AO, Rubilar-Rogers D. 2015. An enigmatic plant-eating theropod from the Late Jurassic period of Chile. Nature. doi:10.1038/nature14307

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