The heavily armored ankylosaurid dinosaurs are famous for the powerful clubs they had
at the end of their tails, which made them a challenge for even the deadliest predators.
While these dinosaurs have a substantial media presence, the long-necked sauropod dinosaur
Shunosaurus possessed a similar tail-club long before the ankylosaurids developed such
weaponry.
In spite of its obscurity compared to them or fellow sauropods like Apatosaurus, numerous
Shunosaurus skeletons have been found, many of them nearly complete, making it one of
the most important sauropod dinosaurs ever discovered.
Shunosaurus’s fossils are from the Lower Shaximiao Formation in China, which dates
to the Middle and Late Jurassic.
They were first found in the Zigong Dashanpu quarry during 1977, and Shunosaurus was formally
named and described in 1983.
“Shunosaurus” means "Shu reptile”, “Shu” being an old name for Sichuan, the province
the Zigong Dashanpu quarry belongs to.
The area was originally excavated to mine natural gas, which the name of another dinosaur
found there, the theropod Gasosaurus, is in reference to.
Today, the Zigong Dinosaur Museum has been built around the quarry, and while some Shunosaurus
skeletons are displayed there in a traditional manner, many more articulated Shunosaurus
skeletons can be seen laying in the same place they were fossilized millions of years ago.
Over a hundred dinosaur skeletons were excavated at the Zigong Dashanpu quarry before construction
of the museum even began in 1985, and ninety percent of them belong to Shunosaurus.
Both their abundance and completeness are nothing short of extortionary for any dinosaur,
let alone a sauropod.
The Lower Shaximiao Formation is sometimes even called the Shunosaurus-Omeisaurus assemblage,
although half of the species of the sauropod Omeisaurus are actually found in the Upper
Shaximiao Formation, which is sometimes called the Mamenchisaurus assemblage.
Different species of the large, Allosaurus-like carnosaur Yangchuanosaurus are likewise found
in both subunits of the Shaximiao Formation.
While most Shunosaurus fossils belong to the type species, Shunosaurus lii, a second species,
Shunosaurus jiangyiensis, has been found in the Yunnan Province; which is just to the
south of Sichuan.
Shunosaurus jiangyiensis was larger and the shape of its pectoral girdle was different,
but it was otherwise similar to Shunosaurus lii.
Shunosaurus lii fossils have been found at the same site, meaning that the two species
coexisted.
Shunosaurus lii’s localized abundance at the Zigong Dashanpu quarry is strong evidence
that it lived in massive herds.
While this was a common behavior among sauropods, it was all the more important for Shunosaurus
lii due to its relatively small body size.
It was about ten meters long and weighed only about three tones.
While not quite a dwarf, and not small by any measure, its weight was well below that
of a typical African forest elephant.
The weight of other sauropods is usually quantified in terms of multiples of the larger African
bush elephant.
The massive mega-theropods like the carcharodontosaurids wouldn’t evolve for some time, but the world
was already full of predators who had evolved to hunt its titanic cousins, so it is little
wonder Shunosaurus evolved its iconic club tail to fight them off.
Shunosaurus jiangyiensis was substantially larger, but it was still far from the biggest
herbivore in Jurassic China, so would have likewise benefited greatly from its weaponized
tall.
Shunosaurus still grew quickly as the larger sauropods did.
Its bones lack signs of arrested growth, indicating they grew throughout the year rather than
slowing down during less productive seasons.
Unlike birds, but like crocodilians and many other extinct dinosaurs, Shunosaurus continued
to grow after reaching adulthood, albeit at a slower rate.
Still, even the oldest Shunosaurus had no hope of approaching the size of more typical
sauropods.
Despite the comparisons made between them, Shunosaurus’s tail club was not made the
same way as those of ankylosaurids.
Theirs’ were made by large bony plates called osteoderms which surrounded the end of the
tail.
Shunosaurus’s club was instead formed by several enlarged vertebrae, although osteoderms
were still part of the structure, forming two pairs of spikes.
It should be noted that the bone at the end is not another spike, but a final vertebra.
The spikes of this sauropod resembled the thagomizer’s of the stegosaurs, who were
close relatives of the ankylosaurs.
Unlike the tails of most other sauropods, Shunosaurus vertebrae retain a complex shape
right until they reach the club.
They served as attachment points for muscles, increasing both the power and motor control
Shunosaurus had over its club.
It was proportionally smaller than those of ankylosaurs, and its spikes were shorter than
stegosaur thagomizers, but Shunosaurus’s club had the advantage of height.
Rather than breaking shins, Shunosaurus could strike its attackers right in the face!
As dangerous as its tail was, carnivorous theropods wouldn’t have had any better luck
attacking it from the front.
Shunosaurus’s legs were proportionally long for its size, and their kicks could have easily
fended off assailants, especially since the forelimbs had the large thumb spikes typical
of other early sauropods.
Although a healthy Shunosaurus was not virtually immune to predation as the largest sauropods
were, every one of those giants were once the size of Shunosaurus lii, and they would
have been far more vulnerable at that stage.
Furthermore, with their deadly arsenal, juvenile Shunosaurus would have had a better chance
of reaching adulthood than their potentially larger peers.
Finite element analysis has proven that the larger clubs found in the ankylosaurid dinosaurs
were capable of being used as effective weapons, but the same techniques haven’t yet been
tested on Shunosaurus.
However, Shunosaurus wasn’t the only sauropod with a tail club.
Although not as well known, the sauropods Omeisaurus and Mamenchisaurus had similar
club-like structures at the ends of their tails, although they were smaller and lacked
Shunosaurus’s spikes.
The Nigerian sauropod Spinophorosaurus was once thought to have had tail spikes, but
they were not actually found in association with the tail.
They are now thought to have merely been misidentified clavicles, bones belonging to the pectoral
girdle.
A finite element analysis of Mamenchisaurus’s genuine tail club concluded that it was not
an effective weapon.
The clubs of Shunosaurus and Omeisaurus were quite different from the one belonging to
Mamenchisaurus, so the study has little applicability toward the functions of their clubs.
Even the applicability towards Mamenchisaurus itself is questionable, since only one tail
club belonging to the genus has been found, and given the differences between it and those
of the closely related Omeisaurus, it cannot be ruled out that its shape was the result
of a detrimental mutation specific to that individual.
While the role of Mamenchisaurus’s tail club is currently in doubt, the presence of
spikes on Shunosaurus’s larger club make it seem highly unlikely that its primary function
wasn’t as a weapon.
Although they were all basal sauropods with tail clubs, Shunosaurus was not closely related
to Omeisaurus or Mamenchisaurus.
Beyond that, it is unclear who Shunosaurus’s closest relatives were.
Even though its entire skeleton is known, the same cannot be said for most other early
sauropods, leaving their precise interrelationships uncertain.
Shunosaurus was about the same size as the first sauropods, so rather than shrinking
in size as dwarf sauropods like Europasaurus and Magyarosaurus did, it may have simply
stayed the same size.
While most sauropods adapted to the increasingly deadly meat-eating theropod by growing larger,
Shunosaurus’s tail-club offered it a different way to deal with them.
Additionally, the Shaximiao Formation was a forested environment, where Shunosaurus’s
relatively smaller size may have actually given its an edge over its larger, more cumbersome
cousins.
Even though they weren’t related to it, both of the other club-tailed sauropods are
also found in the Shaximiao Formation, although Mamenchisaurus is restricted to the upper
portion of the formation.
The leading hypothesis is that, like the ankylosaurs, this was the result of evolutionary convergence.
It could have been in response to local conditions specific to Jurassic China, such as the unique
strategies of a native carnivore.
As will be elaborated on later, the fauna found in the Shaximiao Formation was unique
compared to other Jurassic localities.
On the other hand, since they are all basal sauropods, it is possible that tail clubs
like these may have been more common among early sauropods than was previously thought,
and only later lost in the more derived species.
These sauropods were not the only ones to converge upon traits typically associated
with ankylosaurs.
Osteoderms, the same structures which make up the armor of the dinosaurian tanks, have
been discovered on some titanosaurs like Saltasaurus.
Despite the similarities, not enough have been found to determine if their primary purpose
was actually protection.
They are heavily vascularized, so some scientists have proposed they may have instead been used
to shed to shed excess heat.
However, like Shunosaurus, Saltasaurus was small by sauropod standards, if still large
in absolute terms, so increased protection from predators would have been a greater concern
for it than thermoregulation.
Shunosaurus’s tail club may be its most iconic trait, but its fossilized skulls are
also exceptional finds.
Unlike the rest of their otherwise robust skeletons, sauropod skulls were built delicately
to avoid placing to much strain on their famously long necks.
This meant they quickly broke apart when the animal died, and their disarticulated fragments
were unlikely to endure the elements long enough to be fossilized, and are even less
likely to be complete.
Shunosaurus is one just a handful of exceptions, and provides one of the few insights into
what the skulls of basal sauropods were like.
Like those of most other sauropods, adult Shunosaurus heads were wide and blunt.
It had more teeth than most sauropods, with twenty-five in the lower jaw, and atypically
for the clade, its lower jaw contains more teeth than the upper jaw.
The jaws curve upwards at the anterior end of the skull, creating a shape which has been
compared to garden shears, although the teeth themselves curve inwards.
Most of them were long and cylindrical with spatulate, spoon-shaped crowns.
However, Shunosaurus’s lower jaw contained a second type of tooth, which was compressed
with serrations on the sides.
These teeth resembled those of its prosauropod ancestors, and had already been lost in more
derived sauropods.
Like other sauropods and most other dinosaurs, Shunosaurus replaced its teeth throughout
its life.
However, they lasted longer than normal, and each tooth only spent about 20% of its lifecycle
in the replacement stage.
This was because Shunosaurus’s teeth had evolved to fit together so precisely that
their operation would be comprised if too many were stuck in the replacement phase.
Still, like other sauropods, it did not chew its food, but merely chomped it into pieces
small enough to shallow.
Shunosaurus coexisted with a number of other sauropods, and they had evolved to divide
the limited resources of their environment between them, which is called niche partitioning.
Mamenchisaurids like Omeisaurus had the proportionality longest necks of all sauropods, which allowed
them to reach the highest tree branches.
Shunosaurus instead had one of the proportionally shortest necks of any sauropod.
It was only outdone by the bizarre Argentinian dicraeosaurid Brachytrachelopan.
Unlike its more extreme cousin, Shunosaurus’s unusually long legs would have partially compensated
for its shortened neck, allowing it to browse from the lower tree tops.
This would have placed it in a niche between the other sauropods and the smaller stegosaurs.
Its teeth were robust, so Shunosaurus is thought to have been a generalist whose diet often
consisted of course vegetation.
Supporting this, its jaws were capable of a wide gap, allowing the club-tailed sauropod
to fill its mouth with as much food as possible.
However, juvenile skulls were notably narrower, with a more pointed shaped, suggesting they
had a more selective diet than the adults.
The time Shunosaurus lived is also noteworthy.
The Shaximiao site in the Yunnan Province is dated to one hundred and sixty-six million
years ago, during the Middle Jurassic.
Few Middle Jurassic dinosaur fossils have been excavated, and Shunosaurus’s native
environment is the best understood from that time.
The fossils from the Zigong Dashanpu quarry were once considered to have also been Middle
Jurassic in age due to similarities between them and other Middle Jurassic dinosaurs.
However, more recent research using radiometric dating has determined that the dinosaurs preserved
there died about one hundred and fifty-nine million years ago, at the start of the Late
Jurassic.
This has some pretty major evolutionary implications.
By the Late Jurassic, all sauropods found outside of Asia, from Camarasaurus to the
famous Brontosaurus, belonged to the clade Neosauropoda.
However, Shunosaurus and the majority of the other Chinese sauropods are almost universally
basal sauropods.
This trend continues into the Upper Shaximiao Formation, which extends to the end of the
Late Jurassic, showing that Shunosaurus’s home in prehistoric China was anomalous among
Late Jurassic ecosystems, being something of a sauropod equivalent of Australia.While
not a giant, with its spiked tail club, Shunosaurus is one of the most distinctive sauropods,
and a stark example of the remarkable diversity contained within the clade.
Despite Shunosaurus being one of the rare sauropods known from complete skeletons, a
lot more remains to be learned about it.
For instance, a detailed description of its post-cranial anatomy has yet to be conducted,
and a finite element analysis could be done to model the strength and mobility of its
tail club.
Like the rest of the animal, the findings of such research will certainly be extraordinary.
Thank you for watching, and a thank you to the themattalorian for narrating this video.
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