Chilesaurus was a very weird dinosaur. At a distance, it may have looked almost normal: a
small biped with a break at the end of its mouth, something that evolved independently in multiple
dinosaur lineages. However, a closer look at Chilesaurus’s anatomy reveals a mixture of traits
from all three major branches of Dinosauria: Theropoda, Sauropodomorpha, and Ornithischia.
Not only it is unclear which of the three major clades this dinosaur platypus belongs to, but
it has called into question how these clades are related to each other. Chilesaurus is held by some
paleontologists to be a “missing link” between the theropods and ornithischians, two dinosaur clades
previously thought to be only distantly related. Even the very definition of Dinosauria had to be
revised due to the taxonomic chaos unleashed by the discovery of this unusual dinosaur.
As its name implies, Chilesaurus was found in the country of Chile. Its fossils were first found in
2004 by Diego Suárez, who was only seven years old at the time. The name of the only known species of
Chilesaurus, Chilesaurus diegosuarezi, is in his honor. Due to just how strange it was, Chilesaurus
wasn’t described until 2015. The site turned out to contain multiple Chilesaurus skeletons,
which seem to have been part of a single herd or family group. Given how Chilesaurus possesses a
mosaic of traits from various different types of dinosaurs, one could be forgiven for thinking it
was a chimera, and its remains were initially mistaken for those of multiple new species.
However, the mostly complete and articulated Chilesaurus holotype proved they belong to a
single, if confusing, species. This skeleton was one and a half meters long, but the largest adult
Chilesaurus had a length of over three meters. The arms of all specimens are in a resting position,
suggesting they were killed and buried suddenly. The fossils are from the Toqui Formation,
which was formed one hundred and forty-seven million years ago during the Late Jurassic.
The fragmentary fossils of at least three species of sauropods have been found there, but so far
Chilesaurus is the only named dinosaur from the Toqui Formation. However, it did coexist with the
crocodylomorph Burkesuchus. Like Chilesaurus, this small reptile is also thought to be a “missing
link”, in its case between the terrestrial and semi-aquatic ancestors of today’s crocodilians.
Chilesaurus had a small, short head and a long neck, an arrangement similar to that of early
sauropodomorphs, but also some theropods. Its snout ended in a small beak, much like those
found in ornithischians and many herbivorous theropods. It notably lacked a predentary bone,
which is found at the end of the lower jaw of all known members of Ornithischia and is seen as a
defining feature of the clade. Chilesaurus’s teeth were tall, leaf-shaped, pointed slightly forwards,
and had denticules on the tops. For all the mystery surrounding this dinosaur,
something all scientist agree on is that with teeth like these, it was clearly a plant-eater.
Chilesaurus’s forelimbs were quite odd, resembling the two fingered arms of the later tyrannosaurids,
such as Tyrannosaurus rex. Most dinosaurs had five digits on their forelimbs, while
the theropods of the Jurassic had three or four. Chilesaurus technically had three fingers, but the
third digit had atrophied to the point that it only consisted of a single, tiny nub of a bone.
It is likely the vestigial remnants of the third finger were entirely obscured in life.
Some reconstructions of Chileaurus portray this finger as much larger than it really was.
The real bone is so small that some paleoartists have apparently either
not noticed it or assumed it was a claw, mistaking the metacarpal, a hand bone, for the third finer.
Why Chilesaurus evolved tyrannosaurid-like hands remains unknown, but a few other dinosaurs,
such as the oviraptorosaur Oksoko, also only had two functional fingers.
Despite the visual similarity with tyrannosaurids, Chilesaurus’s arms don’t seem to have been used
for the same purposes. They were proportional to its size, and the claw on the first digit
was greatly enlarged, much like those of the early sauropodomorph dinosaurs.
These odd forelimbs could have been used to bring tree branches closer to Chilesaurus’s mouth or for
warding off predators and rivals. When resting, Chilesaurus folded its arms in a bird-like
position, with the hand facing backwards. Although such a resting posture is typical
of the bird-like coelurosaurs, it has also been found in the Triassic sauropodomorph Guaibasaurus
and even Scleromochlus, who was not a dinosaur but a relative of pterosaurs. Although not as visually
striking, Chilesaurus’s legs were also strange. They were similar to those of the basal members of
the theropod clade Tetanurae, but they were stout like those of sauropodomorphs. As a consequence,
Chilesaurus would have been slower than most of the contemporary theropods and ornithischians.
A major difference between Chilesaurus and most theropods can be found in its toes.
Theropods typically had three main toes, while Digit 1 was smaller and held off of the ground.
This arrangement can be seen today in many modern dinosaurs.
In contrast, Digit 1 in Chilesaurus was close to the size of Digit 2, and it seems to have been
supporting the body’s weight like the other toes. This trait also evolved in the sauropodomorphs,
but the only theropods known to have such feet are the derived therizinosaurs,
who were contenders for the title of strangest dinosaurs before the discovery of Chilesaurus.
Chilesaurus’s pelvis was largely similar to those of the bird-like coelurosaurs, a derived clade
of theropods it has never been placed within. However, while the pubises of most coelurosaurs,
and other theropods, point forwards, Chilesaurus’s pubis points backwards, towards the tail. This
condition is typical of the ornithischian dinosaurs, and has often been used to
support its placement within the clade. However, some derived coelurosaurs (the therizinosaurs,
dromaeosaurids, and birds) also had, or in the case of birds, have, backwards pointing pubises.
Birds and dromaeosaurids are thought to have evolved this condition to make room for larger
lungs, which were necessary to support the greater activity levels that eventually lead to flight.
Therizinosaurs and ornithischians are believed to have instead reversed their pubises to allow
for the evolution of longer digestive tracts as they shifted towards herbivorous diets.
Given its similar diet and slower speed compared to other small dinosaurs,
this is considered to have been the case for Chilesaurus as well.
Chilesaurus possessed numerous other traits that link it to theropods, sauropodomorphs,
and ornithischians. Some must have been the result of evolutionary convergence, but determining where
the dinosaur platypus belongs has been difficult, and it is in many ways more perplexing than
the actual platypus. The paper that originally described Chilesaurus concluded it was a theropod,
with its sauropodomorph and ornithischian-like features being attributed to evolutionary
convergence resulting from their similar diets, much what happened with the therizinosaurs.
There were other theropods who became specialized herbivores, but Chilesaurus was not closely
related to any of them. It was placed as a basal member of the clade Tetanurae, which includes
both the diverse coelurosaurs and the iconic carnosaurs, like Allosaurus and Torvosaurus.
The first tetanurans were carnivores, and even the authors were a bit surprised that
Chilesaurus was placed as a basal member of the clade instead of within Coelurosauria,
which contains the majority of the plant-eating theropods.
The only herbivorous theropods known outside of Coelurosauria are Limusaurus and Berthasaura,
but these dinosaurs were ceratosaurs, not tetanurans. Rather than being an unusual theropod,
Chilesaurus may hold a much more important place in the history of Dinosauria. A different
hypothesis is that Chilesaurus was the most basal ornithischian, and the “missing link” between
Theropoda and Ornithischia, who are envisioned as forming the larger clade Ornithoscelida.
It is generally accepted that the earliest members of all three branches of Dinosauria started out as
superficially theropod-like carnivores, but the previous consensus of dinosaur relationships held
that the true theropods were most closely related to Sauropodomorpha, forming the clade Saurischia.
Ever since the Ornithoscelida hypothesis was first published in 2017, there really hasn’t
been a firm consensus of how the three major branches are related to each other,
with roughly equally statistical support for Ornithoscelida, the traditional Saurischian
model, and even Phytodinosauria, a proposed grouping of Ornithischia and Sauropodomorpha.
Although it may rewrite what we know about the dinosaurs, Chilesaurus’s identify may not be
able to be solved without first resolving the greater interrelationships within Dinosauria.
A major issue when discussing the classification of Chilesaurus is
that all of the datasets used to test its relationships were inadequate.
The original description didn’t include many ornithischians in its phylogenetic analysis,
while the analyses that concluded Chilesaurus was an ornithischian didn’t include many
tetanuran theropods. Essentially, neither was capable of refuting the findings of the other.
For all the uncertainty surrounding this dinosaurian platypus, an undeniable and
lasting impact it has had on dinosaur science is that it caused the actual definition of “dinosaur”
to be revised. Dinosauria was previously defined as last common ancestor of the ornithischians,
represented by Triceratops, and the theropods, represented by the house sparrow,
and all the descendants of that common ancestor. Sauropodomorpha had always been intended to be
part of Dinosauria, but it hadn’t been added to the formal definition since it was assumed that
the theropods had shared a more recent common ancestor with them than with the ornithischians.
The discovery of the small, unassuming Chilesaurus and the resulting Ornithoscelida
hypothesis jeopardized the status of an entire branch of Dinosauria.
Even Brontosaurus, who had ironically been reinstated as a valid genus the same year that
Chilesaurus was described, wouldn’t be considered a dinosaur if the Ornithoscelida model proved
correct. To avoid creating unnecessary confusion, it was decided that it was easier to just add
the sauropod Diplodocus as one of the defining members of Dinosauria. Regardless of the status of
Ornithoscelida or Saurischia, Chilesaurus may not be the “missing link” it was once thought to be.
More recent research has proposed that the most basal ornithischians were instead the silesaurs,
small reptiles from the Triassic Period who were previously not
even been considered true dinosaurs, but instead closely related dinosauromorphs.
One of the papers which came to this conclusion was notably coauthored by Dr. Matthew Baron,
one of the most notable architects and major proponents of the Ornithoscelida hypothesis.
Although both Chilesaurus and silesaurs lacked a predentary bone, the lower jaws of the silesaurs
had a similar shape to it and seem to be its evolutionary predecessor. The silesaurs have
more primitive anatomy compared to Chilesaurus, including a forward-facing pubis and a primarily
quadrupedal stance, but also lack Chilesaurus’s purported transitional, theropod-like traits.
The paper recently co-authored by Dr. Baron instead placed Chilesaurus in
the clade Heterodontosauridae, a slightly more derived position within Ornithischia.
Heterodontosaurids were small bipedal omnivores and herbivores. They did not share
Chilesaurus’s eccentricities, but had large, fang-like tusks at the ends of their mouths.
If this placement is accurate, then Chilesaurus not only lost ornithischian predentary bone,
but these trademark tusks as well. Rather than representing the early transition
from theropod-like forms to the more traditional ornithischians, the heterodontosaurid hypothesis
considers Chilesaurus to simply be an aberrant member of its clade,
much as the theropod hypothesis considers it to be a highly divergent basal tetanuran.
Although Chilesaurus has some traits similar to the sauropodomorphs, the possibility that
it belongs to the clade has received the least support. The only phylogenetic analysis which
initially favored this conclusion turned out to have been based on erroneous data. The authors
of the study tried to use the same data set as the paper which initially proposed the Ornithoscelida
hypothesis, to see if they could replicate the results. Unfortunately, the authors of the first
paper had accidently uploaded an older version of their dataset, one which contained errors.
Once the corrected data was used, the authors of the second study ended up with to the same
conclusion of Chilesaurus being the most basal ornithischian, even if they found the
results debatable due to the lack of tetanuran theropods in the dataset. If Chilesaurus was a
sauropodomorph, it would be quite an exceptional example for its time. It most closely resembles
early members of Sauropodomorpha, who they were just starting to become specialized herbivores.
Only larger, more derived species are known to have survived into the Jurassic Period,
and sauropods are thought to have been the only remaining sauropodomorphs by the Late Jurassic.
Even if Chilesaurus is the most basal known ornithischian, it would not be a very good
example of what the first ornithischians looked like. Although there are a few other dinosaurs
whose placement within Dinosauria is also unclear, they are all early dinosaurs, from the time when
the various dinosaur lineages hadn’t yet changed that much from each other. The same cannot be
said for Chilesaurus. This “Frankensaur” lived during the Late Jurassic, and despite the wildly
different classifications it has received, all paleontologists agree that its ancestors diverged
from all other known dinosaurs no later than the Early Jurassic, nearly fifty million earlier.
In all likelihood, the Chilesaurus ghost lineage dates all the way back to the Triassic Period.
It would have changed greatly during that time, and it clearly acquired a number of
traits found in a number of different derived dinosaur clades. It took less time for small,
bipedal ornithischians to become armored behemoths like Stegosaurus. In any event, although it lacked
showy horns or crests, this small herbivore is the strangest dinosaur known to science.
This is true regardless of whether it was a late survivor representing the transition to the more
familiar ornithischians or sauropodomorphs, a highly divergent theropod or heterodontosaurid,
or something else entirely. No matter where it fits within Dinosauria, Chilesaurus
had an unprecedented collection of traits convergent with many of its distant relatives.
The debate about its relationships, and those of Dinosauria as a whole,
remain unresolved, so much more research awaits regarding the dinosaur platypus.
Its long ghost linage also suggests that many new types of closely related dinosaurs await
discovery, but for now, Chilesaurus remains unique from all other known dinosaurs.
Thank you for watching, and a thank you to the themattalorian for narrating this video.
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