Saturday, August 19, 2017

[Paleontology • 2017] A Dinosaur Missing-Link? Chilesaurus and the Early Evolution of Ornithischian Dinosaurs

Chilesaurus diegosuarezi 
Novas, Salgado, Suárez, Agnolín, Ezcurra, Chimento, de la Cruz, Isasi, Vargas & Rubilar-Rogers, 2015

DOI: 10.1098/rsbl.2017.0220  & DOI: 10.1038/nature14307
Illustration: Gabriel Lío


The enigmatic dinosaur taxon Chilesaurus diegosuarezi was originally described as a tetanuran theropod, but this species possesses a highly unusual combination of features that could provide evidence of alternative phylogenetic positions within the clade. In order to test the relationships of Chilesaurus, we added it to a new dataset of early dinosaurs and other dinosauromorphs. Our analyses recover Chilesaurus in a novel position, as the earliest diverging member of Ornithischia, rather than a tetanuran theropod. The basal position of Chilesaurus within the clade and its suite of anatomical characters suggest that it might represent a ‘transitional’ taxon, bridging the morphological gap between Theropoda and Ornithischia, thereby offering potential insights into the earliest stages of ornithischian evolution, which were previously obscure. For example, our results suggest that pubic retroversion occurred prior to some of the craniodental and postcranial modifications that previously diagnosed the clade (e.g. the presence of a predentary bone and ossified tendons).

KEYWORDSArchosauria, Dinosauria, Ornithischia, systematics: phylogeny, Jurassic

Figure 2. Ornithischian features of Chilesaurus. (a) Simplified tree with key acquisitions marked on;
(b) right dentary of Chilesaurus (SNGM-1935) in lateral view; (c) right dentary of Heterodontosaurus (SAM-PK-K1332) in lateral view; (d) pelvic girdle of Chilesaurus SNGM-1936 in lateral view; (e) pelvic girdle of Agilisaurus (ZDM T6011) in lateral view; (f) right femur of Chilesaurus (SNGM-1935) in anterior view; (g) right tibia and fibula of Chilesaurus (SNGM-1935) in posterior view.
Numbers indicate the acquisition of key ornithischian synapomorphies within the clade: 1, complete loss of recurvature in maxillary and dentary teeth; 2, edentulous anterior end of the dentary; 3, predentary bone at the anterior end of the lower jaw; 4, retroversion of the pubis; 5, rod-like pubic shaft; 6, pubic symphysis restricted to the distal end; 7, anteriorly elongate preacetabular process; 8, broadened, wing-like anterior trochanter; 9, fibula less than half the width of the tibia at midshaft. Dark grey circles denote unknown in Pisanosaurus.

This study identifies Chilesaurus as a transitional ornithischian taxon and suggests that the unique suite of anatomical features it possesses could be informative not only in unravelling dinosaur interrelationships, but also in shedding light on the evolution of the anatomical peculiarities that characterize ornithischians. Paradoxically, this early diverging lineage is of Late Jurassic age, implying an extensive ghost lineage between it and other ornithischians and basal theropods. If this hypothesis is correct, this ghost lineage suggests that other similar animals await discovery in Late Triassic–Middle Jurassic deposits. This study highlights the importance of broad taxon sampling when attempting to assess the phylogenetic affinities of enigmatic taxa such as Chilesaurus and also demonstrates the utility of this new early dinosaur dataset for testing the relationships proposed for other problematic dinosauromorph taxa.

Matthew G. Baron and Paul M. Barrett. 2017. A Dinosaur Missing-Link? Chilesaurus and the Early Evolution of Ornithischian Dinosaurs. Biology Letters. 13(8); 20170220. DOI: 10.1098/rsbl.2017.0220.

Abstract: Many dinosaur skeletons show evidence of behavior, including feeding, predation, nesting, and parental care. The resting posture of the forelimbs has been studied in some theropod species, in relation to the acquisition of flight in advanced maniraptoran theropods. Chilesaurus diegosuarezi is a bizarre tetanuran recently described from the Toqui Formation (latest Tithonian) of southern Chile that is represented by multiple well-preserved and articulated specimens. The aim of the present work is to analyze the forelimb posture of four articulated specimens of Chilesaurus: SNGM-1935 (holotype), SNGM-1936, SNGM-1937, SNGM-1938; focusing on its anatomical description, and phylogenetic and behavioral implications. All the preserved specimens of Chilesaurus show strongly ventrally flexed arms with the hands oriented backwards, an arrangement that closely resembles those in dinosaur specimens previously described as preserving resting posture, such as Mei long, Sinornithoides youngi, and Albinykus baatar. As a result, it seems that individuals of Chilesaurus have been in passive activity (e.g. feeding, resting) when they were buried quickly, allowing their fossilization in life position and preserving the forelimb resting posture. The arrangement of the forelimb bones in Chilesaurus could show the first evidences of the structures linked to the muscles that flex the forearms, features related with the acquisition of flying control in advanced maniraptorans.

Nicolás R. Chimento, Federico L. Agnolin, Fernando E. Novas, Martín D. Ezcurra, Leonardo Salgado, Marcelo P. Isasi, Manuel Suárez, Rita De La Cruz, David Rubilar-Rogers and Alexander O. Vargas. 2017. Forelimb posture in Chilesaurus diegosuarezi (Dinosauria, Theropoda) and its behavioral and phylogenetic implications. Ameghiniana. in press. DOI: 10.5710/AMGH.11.06.2017.3088.

Study identifies dinosaur ‘missing link’

Fernando E. Novas, Leonardo Salgado, Manuel Suárez, Federico L. Agnolín, Martín D. Ezcurra, Nicolás R. Chimento, Rita de la Cruz, Marcelo P. Isasi, Alexander O. Vargas and David Rubilar-Rogers. 2015. An Enigmatic Plant-eating Theropod from the Late Jurassic period of Chile. Nature. (2015) DOI: 10.1038/nature14307

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