Sauropod dinosaurs are famous for two things,
their long necks and their massive size.
While there were numerous species of sauropods throughout the Mesozoic, almost all sauropods retained these two traits.
This makes the sauropod Brachytrachelopan truly unique.
At only 10 meters long, it was rather small, at least sauropod standards
But much more striking, Brachytrachelopan had a relatively short neck, almost comedically small compared to the rest of its body.
Brachytrachelopan was first named in 2005. As of yet it is only know from one specimen.
Unfortunately, the skeleton is not complete. with the head, tail, and most of the limb bones missing.
This skeleton is from the Argentinian Cañadón Calcáreo Formation,
which was formed at some point in the Late Jurassic period, roughly 163 to 145 million years ago.
Therefore, it is roughly contemporaneous with the more famous Morrison Formation.
As Pangea had just split up, there was less difference in the faunal composition between continents at the time
Brachytrachelopan lived alongside relatives of the more famous Stegosaurus, Brachiosaurus, and Camarasaurus.
Likewise, one of Brachytrachelopan’s own relatives, Suuwassea, lived in North American alongside these more famous species.
Brachytrachelopan and Suuwassea were part of the sauropod clade Dicraeosauridae.
Many dicraeosaurids had large neural spines and Brachytrachelopan was no exception.
This trait was taken to an extreme in the dicraeosaurids Amargasaurus and Bajadasaurus.
Like Brachytrachelopan, other dicraeosaurids were also smaller than to most other sauropods, though few were as small as Brachytrachelopan
However, while dicraeosaurids had short necks compared to other sauropods, none compared to Brachytrachelopan’s.
Despite a total length of roughly 10 meters, its neck was only two meters long.
This length in comparison to the rest of its body is roughly 40% shorter than other dicraeosaurids.
The anatomy of its cervical vertebrae also would have also made it difficult to lift its already short neck very high.
With these considerations, Brachytrachelopan is estimated to have been only able access foliage of a height of one to two meters.
This suggests a very different diet from most other sauropods, who were generally high browsers,
eating foliage from the tops of tall trees that was unavailable to most other herbivores.
It is speculated that Brachytrachelopan filled the same niche as the iguanodontian dinosaurs that were evolving in the other continents.
Unfortunately, this cannot yet be confirmed since Brachytrachelopan’s skull is still currently unknown.
Brachytrachelopan’s diet is also thought to be why it was so small.
Part of the reason sauropods evoled their massive size was to better access higher foliage.
With a change in diet, Brachytrachelopan would have no longer needed to access such great heights.
Still, its small size may have come with a cost.
Another purpose of large size in sauropods is that it actually increased how much energy could be extracted from their food.
Sauropods are thought to have been hindgut fermenters, storing large amounts of plant matter in their guts which would then be slowly digested by microorganisms.
The longer the plant matter remained there, the more energy could be extracted from it, and the larger the hindgut, the longer it could remain
As growing larger allowed them to gain more energy from their food,
hindgut fermentation is thought to be part of what lead to the evolution of large size in sauropods in the first place,
Brachytrachelopan’s smaller size could suggest it was eating plants that were easier to digest
or had greater nutritional value than the conifers eaten by most other sauropods.
Alternatively, it may simply be that small size was simply worth sacrificing the efficiency of its digestion.
While its would extract less energy from its food, less energy would be required to support its smaller body.
An additional benefit from large size that Brachytrachelopan would have lost was safety from predators.
With a few exceptions, sauropods were reliant on their size for defense.
Most small sauropods were early species or island dwarfs that lived with proportional smaller predators.
But while rare, Brachytrachelopan wasn’t the only sauropod this small that lived in a mainland environment.
Both species of the closely related Dicraeosaurus were of similar sized.
More distantly related examples are Nigersaurus
and Shunosaurus.
While Shunosaurus had a spikes at the end of its tail for defense
and Nigersaurus may have been able to seek refuge in the water of its wetland habitat
how Brachytrachelopan defended itself remains a mystery. Perhaps the answer may lay in the missing parts of its skeleton.
Perhaps the answer may lay in the missing parts of its skeleton.
While the iguanodontians would become incredibly diverse elsewhere in the world,
Brachytrachelopan was long extinct when the first large iguanodontians like Secernosaurus
finally arrived in South America during the Late Cretaceous
While some dicraeosaurids survived into the Early Cretaceous, Brachytrachelopan seems to have remained little more than an interesting evolutionary dead end.
Brachytrachelopan seems to have remained little more than an interesting evolutionary dead end.
Still, very few sauropods showed as much adaptability as Brachytrachelopan.
While it eventually went extinct, it does show that even the most specialized clades have a chance to adapt and diversify in surprising ways.
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