A bizarre new species of ankylosaur dinosaur, named Stegouros elengassen, has recently been
found in Chile. It lived roughly between 72 to 75 million years ago during the Late Cretaceous
period, at a time when Chile bordered Antarctica. Not only is Stegouros the first named South
American ankylosaur, who were previously only known from fragmentary fossils, but its skeleton,
which is 80% complete, reveals a novel new weapon among dinosaurs. While ankylosaurs
are famous for the club tails present in some species, Stegouros’s tail ended in a very
odd weapon. It has been compared to a battle axe, fern fronds, and a most commonly a macuahuitl,
a weapon once used by many Mesoamerican cultures. Stegouros’s club consisted of seven pairs
of large, flattened, frond-like bony osteoderms. As its tail was fortunately articulated, it
is clear these were held in a lateral position to the tail. Compared to more typical ankylosaur
clubs, less mass was concentrated at any one point. However, Stegouros’s macuahuitl club
would have been both harder to avoid and able to inflict damage across a wider area. Depending
on the shape of its keratin sheaths, it may have even been able to pierce flesh like stegosaur
thagomizers. However, the range of the macuahuitl club was shorter than the clubs of other ankylosaurs
on account of Stegouros’s remarkably sort tail. Only eighteen tail vertebrae have been
found in Stegouros, with an estimated maximum of twenty-six tail vertebrae. Even the maximum
estimate is shorter than the tail of any other thyreophoran, the group of armored dinosaurs
that includes ankylosaurs and stegosaurs. The previous record holder, the basal thyreophoran
Scelidosaurus, has 35 tail vertebrae. The reason Stegouros’s tail was so short compared
to other ankylosaurs was likely to support its heavier club. Notably, only thirteen tail
vertebrae were even flexible while the rest were encased by the macuahuitl-like weapon.
While its tail is unlike anything seen before, the rest of Stegouros’s skeleton is strange
as well. Stegouros was small for an ankylosaur, only about 2 meters long. It legs were also
atypically slender, ending in hoof-like claws. Like other ankylosaurs, its body was covered
in smaller osteoderms that formed armor. However, Stegouros had fewer osteoderms than most of
its cousins. Its skull, which has a curved beak, is also proportionately large by ankylosaur
standards. Overwise, it contains most of the traits unique to ankylosaurs. On the other
hand, the same is not true of the rest of the skeleton, which is more like that of basal
thyreophorans and even contains some stegosaurian traits, such as a wide pelvis. While Stegouros
was an ankylosaur, it was a very basal species. Since clubbed tails were otherwise only present
in the derived ankylosaur clade Ankylosaurinae, this means Stegouros’s clubbed tail was
the result of convergent evolution. That is not to say its clubbed tail was a complete
coincidence, because osteoderms were present on the tails of early thyreophorans, allowing
for the evolution of more specialized weapons. This is also how stegosaurs evolved their
spiked tails. Stegouros is part of a newly recognized clade basal of ankylosaurs named
Parankylosauria. This clade split off from the other ankylosaurs very earlier in their
evolution. With its largely complete skeleton, Stegouros has served as an “ankylosaur Rosetta
Stone”, allowing the close relationship of the parankylosaurs to be recognized. The
other known members of Parankylosauria are Kunbarrasaurus from Australia and Antarctopelta
from Antarctica. Osteoderms similar to those in Stegouros’s odd club have also been found
in Antarctopelta. Combined with similarities in their tail vertebrae, it seems Antarctopelta
also had a macuahuitl club. Unfortunately, not enough of Kunbarrasaurus’s tail has
been found to determine if it had one as well. Like Stegouros, all other known parankylosaurians
lived during the Cretaceous period. However, Parankylosauria must have evolved during the
Jurassic period as the more derived ankylosaurs had already evolved by at least 167 million
years ago. Parankylosaurians seem to have been restricted to the southern continents,
which were formerly part of the mega continent Gondwana. Only one Gondwanan ankylosaur, the
Australian Minmi, hasn’t been placed in Parankylosauria, at least yet. Either way,
it was clearly at most only a distant relation to their northern species. Therefore, it seems
the northern and southern ankylosaurs had split into separate, distinct lineages. In
general, after Pangea spilt into Gondwana and Laurasia, the now isolated dinosaurs became
more divergent from each other. Other examples include how the southern continents possessed
their own distinct clade of dromaeosaurids named Unenlagiinae, the top predators were
abelisaurids and megaraptorans instead of tyrannosaurids, and how sauropods were still
the top herbivores while hadrosaurids were rare and ceratopsians were completely absent.
Whether or not the parankylosaurs coexisted with more derived ankylosaurs, Stegouros has
revealed yet another example of a dinosaur lineage unique to the southern hemisphere.
While Stegouros’s macuahuitl club is extortionary, it has also helped science to better understand
greater evolutionary and biogeographical trends during the Mesozoic era. This makes Stegouros
a very important discovery. Thank you for watching. I hope you learned something interesting.
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