The long-necked sauropod dinosaurs were the largest creatures to ever walk on Earth.
They were the descendants of bipedal animals from the Triassic Period who belonged to the
larger clade Sauropodomorpha. Nicknamed the “prosauropods”, they were already some
of the largest animals the world had yet seen, despite the fact that during the Triassic Period
most dinosaurs were still second fiddle to other reptiles. One of the best examples of how the
transition from prosauropods like Plateosaurus to the titans of the Jurassic and Cretaceous periods
took place is Lessemsaurus. Lessemsaurus was one of the closest relatives of the true sauropods.
In many ways, what has been found of it resembles sauropods more than other prosauropods,
but Lessemsaurus still lacked some traits that were critical to the sauropods obtaining
their incredibly massive size. Lessemsaurus was first discovered in the Los Colorados Formation
in Argentina, which was formed during the Late Triassic Period. Lessemsaurus
is named after Don Lessem, who is the author of numerous books, primarily about dinosaurs.
Based on the neck vertebrae that have been found, Lessemsaurus’s neck was shorter
than most sauropods. While this would have limited the height it could browse from,
it was still higher than most other creatures. Lessemsaurus had also high neural spines
which may have formed some sort of ridge along its back. Although prosauropods were already
some of the largest animals during the Triassic Period, Lessemsaurus was one of the largest.
Estimates suggest it could reach seven tons. This is twice as heavy as almost all other
Triassic prosauropods and is also heavier than the average weight of a bull African Elephant. Still,
even Lessemsaurus was dwarfed by most of the later sauropods. To reach their massive sizes,
even the earliest sauropods grew rapidly, and didn’t slow down even during unfavorable seasons.
In contrast, Lessemsaurus’s growth, while fast, did slow down annually.
Even though it was one of the largest animals the world has yet seen, this shows how Lessemsaurus
was not yet as utterly devoted towards attaining gigantic size as the true sauropods were.
Another example of how much Lessemsaurus was like the sauropods but still different are its limbs.
While often portrayed as quadrupedal, most prosauropods were actually obligate bipeds,
although some bipedal species such as Mussaurus hatched as quadrupeds. On the other hand,
Lessemsaurus was quadrupedal as an adult, which allowed it to support its greater weight.
However, it still offers insight into how the arms of the prosauropods evolved into the front legs
of the sauropods. Sauropods limbs were built like columns, straight and directly below their bodies.
Such legs are very effective at supporting massive weight and similar legs are also present
in today’s largest animals. Lessemsaurus’s forelimbs were instead flexed. These bent
forelimbs may have allowed Lessemsaurus to run faster or they have simply been the result of
them still not being optimized for weight bearing since they had only recently evolved into legs.
Lessemsaurus’s vertebrae also reveal much about the evolution of the sauropod respiratory system.
The airflow in the respiratory systems of the first archosaurs was unidirectional,
which is more efficient than the respiratory system of mammals. However, like birds,
the sauropods had an expansive system of air sacs which invaded the bones themselves.
These air sacs reduced the weight of their bones and helped the sauropods to cool off,
a crucial trait given their size. Lessemsaurus’s neck vertebrae already had the cavities made by
these air sacs, proving that the close relatives of the sauropods had already evolved this more
advanced respiratory system. It certainly helps to explain how Lessemsaurus itself
was able to grow larger than almost any proceeding species. No skulls belonging to Lessemsaurus or
any close relatives have been found. This is important because the skulls of early sauropods
and prosauropods were actually quite different from each other. Sauropod skulls were shorter,
had rounder snout, and lacked the overbite present in their ancestors. Assuming Lessemsaurus’s skull
didn’t evolve in a completely different direction, it was probably more like either the sauropods
or the prosauropods than transitional between them. This is because, with the exception of the
overbite, the skulls prosauropod hatchlings were actually very similar to the future sauropods.
This suggests the sauropod skull was the result of perhaps a single mutation
preventing the skull from acquiring its adult characteristics rather than several mutations.
Just as the sauropods later propelled some predatory theropod dinosaurs to also evolve
massive size, Lessemsaurus had the same effect on its predators. The Los Colorados Formation also
contains fossils of the carnivorous pseudosuchian Fasolasuchus, a distant relative of crocodilians.
Fasolasuchus is the largest terrestrial predator known from the entire Triassic Period.
It is estimated to have been between eight to ten meters long and to have weighed at least two tons.
Even the gigantic Lessemsaurus would not have been safe from it. However,
a mere subadult Lessemsaurus was still immune to attacks from other contemporary predators, such
as Riojasuchus and Zupaysaurus, one of the largest Triassic theropods. Lessemsaurus is the namesake
of the clade Lessemsauridae, which is usually considered to be the sister group to Sauropoda.
Like Lessemsaurus, the other lessemsaurids outclassed all other sauropodomorphs in size
besides the sauropods themselves. The only other animal from the Triassic Period who
even approached their size was the dicynodont Lisowicia, which grew between five to six tons.
The only Triassic dinosaur that was likely larger than Lessemsaurus itself was the lessemsaurid
Ingentia, who weighed between seven to ten tons. The largest lessemsaurid was Ledumahadi, who able
to reach a staggering twelve tons. However, it lived during the Early Jurassic period. This
doesn’t mean that Ledumahadi or another Jurassic lessemsaurid was the ancestor of the sauropods.
Lessemsaurids seem to have been more closely related to each other than to the sauropods.
Additionally, while no sauropod bones have been found dating to the Triassic Period, fossil tracks
laid by sauropodomorphs with columnar limbs have been found in Late Triassic Greenland.
Even if these Triassic footprints were not left by actual sauropods, their makers were at least
more closely related to them than even the lessemsaurids where. Although Lessemsaurus and its
relatives shared many features with the sauropods, they still lacked some of the traits that allowed
the true sauropods to grow so large. Over time, the lessemsaurids and the other prosauropods were
steady outcompeted by the sauropods until they had been completely replaced by the Late Jurassic.
Despite this, Lessemsaurus was quit the exceptional dinosaur. It was not only
one of the largest animals of its own time, but was one of the largest animals the Earth had
yet seen, while foreshadowing the even larger giants to come. Thank you for watching. I hope
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