Chilesaurus diegosuarezi Transitional Fossil – Root and Branch Reform of the Dinosaur Family Tree

A bizarre Late Jurassic dinosaur called Chilesaurus diegosuarezi had been described as a member of the Theropoda group, but this strange little dinosaur that seemed to possess anatomical characteristics reminiscent of Sauropods, Ornithischian dinosaurs as well as meat-eating Theropods, has been re-described, this time as a “missing link” between the Theropods and the bird-hipped, Ornithischians.  The re-think has to do with the use of datasets to assess the taxonomic relationships between different types of dinosaur.  Chilesaurus may be the first dinosaur to be reassessed in the light of a new way of looking at the dinosaur family tree, chances are, it won’t be the last.

An Illustration of the Bizarre Late Jurassic Dinosaur from Southern Chile C. diegosuarezi

Chilesaurus illustration.

An illustration of Chilesaurus, once classified as a Theropod now regarded as a transitional fossil towards the Ornithischia.

Picture Credit: Nobumichi Tamura

Chilesaurus diegosuarezi Discovery and Description

The first fossils of this three-metre long dinosaur were found by a pair of geologists who were hiking in the remote Aysén region of southern Chile.  Scientists have mapped and explored these deposits (the Toqui Formation – Upper Jurassic) and a description of this dinosaur was published in the journal “Nature” in 2015.

To read Everything Dinosaur’s article written in 2015 reporting the discovery of Chilesaurus: Chilesaurus – a Dinosaur Designed by a Committee

The dinosaur had some strange features that set it apart from other dinosaurs.  Some of these features, such as the over-sized claw which could be extended outwards, on the first digit on the hands of Chilesaurus were reminiscent of the large claws found on the first digits of the front limbs of primitive Sauropods.  However, it had a skull similar to that seen in Theropods, but the jaws were lined with spoon-shaped teeth that pointed outwards at a slight angle.  These teeth were unique in the Theropoda and suggested a plant-eating diet.  In addition, the pubic bone in the pelvis was pointing backwards not forwards as in the Theropods, this pelvic arrangement was typical of a bird-hipped dinosaur, an Ornithischian.  These and other anatomical features made Chilesaurus into a bit of a conundrum for the palaeontologists studying it.  This dinosaur was difficult to place on the Dinosauria family tree, because of its combination of characteristics.  In the original 2015 paper, Chilesaurus was described as a Tetanuran Theropod, a member of the “stiff-tailed” group of bipedal, mainly carnivorous dinosaurs, meaning that it was distantly related to Megalosaurus, the ornithomimids and the Tyrannosaurs.

The Teeth in the Lower Jaw of Chilesaurus were Unlike Any Other Teeth of a Theropod

The fossilised jaw of Chilesaurus.

Teeth adapted for cropping plants.

Picture Credit: Dr Fernando Novas (Museo Argentino de Ciencias Naturales (Buenos Aires, Argentina)

New Dataset – New Classification

Researchers Matthew Baron (Cambridge University) and Paul Barrett (Natural History Museum), writing in the Royal Society journal “Biology Letters” describe how they applied a different dataset to assess the phylogenetic relationship between Chilesaurus and other dinosaurs.  When C. diegosuarezi was first described, back in 2015 the researchers used several datasets to test the interrelationships within the Dinosauria, but crucially, the focus of this analysis was on looking at the relationships within the Saurischia.  In March of this year, Matthew and his co-researchers proposed re-drawing the dinosaur family tree, in essence, resurrecting work undertaken by Thomas Huxley in the late 19th Century, that unites the Saurischia and the Ornithischia together into a new clade called the Ornithoscelida.

Theropods Grouped with Ornithopods

Under this revision, the Theropoda, which are closely related to extant birds and classified as Saurischian dinosaurs (lizard-hipped), were united with the bird-hipped dinosaurs, the Ornithopods, Thyreophora, Ceratopsian, Hadrosaurs etc.

When Chilesaurus was examined again, using the taxonomic relationships proposed by the newly drawn dinosaur cladogram, Chilesaurus was placed in a new position.  The idea that it was a Tetanuran Theropod based on this dataset could be discounted.  Instead, Chilesaurus is placed at a point in the dinosaur family tree where the Ornithischia diverged from their close relatives.  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 the Theropoda and the Ornithischia.

Chilesaurus “Missing Link” on the Road to the Ornithischia

Chilesaurus consensus tree.

In this new Chilesaurus study, the consensus tree formed via the phylogenetic analysis indicates this dinosaur was a basal Ornithischian.

Picture Credit: Royal Society Biology Letters

In the diagram above, the Chilesaurus (red star) is seen as a potential link between the Saurischian Theropoda and the Ornithischian dinosaurs.  Chilesaurus may therefore provide an insight into the evolutionary origins of the bird-hipped dinosaurs.  It may also have an extensive ghost lineage (thin black line), going back to the Middle Triassic.

To read the Everything Dinosaur article, published in March 2017, about the redefinition of the Dinosauria: Root and Branch Reform in the Dinosaur Family Tree

Co-author of the research, Professor Paul Barrett (Natural History Museum) explained the significance of this new paper:

“Chilesaurus is one of the most puzzling and intriguing dinosaurs ever discovered.  Its weird mix of features places it in a key position in dinosaur evolution and helps to show how some of the really big splits between the major groups might have come about.”

Finding a Better Fit within the Dinosauria – Chilesaurus diegosuarezi 

The bizarre Chilesaurus.

An illustration of Chilesaurus that shows a suite of dinosaur traits.

Picture Credit: Gabriel Lio

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