Obligatory disclaimer: real life sucks, so we will talk about a magazine – i.e., ‘Scientific American’ – instead.
There is an article in the ‘Scientific American’ Sep
2023 issue, discussing sauropod dinosaurs and as to why some of them got
super-big. The author proclaims that it is a mystery; I am no paleontologist,
but I feel that I have a theory, here: it was a number of factors.
Why do animals do anything? Moreover, why do they
evolve in the wild? To secure an advantage over competitors and predators/prey;
a bigger body size is par de course here.
What are the advantages of being big, (super-big)? You
get access to food sources that are unavailable to other herbivores, and
predators are not as big of a threat to you, (pun intended). Conversely,
though, you also need more food than the other herbivores do, and carnivores
will eventually evolve their own adaptations, physical or behavioristic, that
will even the score between them and a super-big herbivore. Let us widen the
query.
What are the factors that allow the initial growth
of super-size? An abundance of food, plant matter in the sauropods’ case. Wait.
There is an arms’ race between plants and herbivores, just as there is one between
them and the carnivores. In addition, just as some herbivores, (i.e. African [and
Asian] elephants), get big to escape predators, (i.e. African lions and
tigers), so do some plants get big in an attempt to ‘escape’ the elephants,
(such as the baobab trees). The twist? This strategy does not work on all of
the levels: under right conditions, elephants can bring down a baobab tree,
(literally, topple it over), while a pride of lions can bring down an African
elephant, (again under right conditions and circumstances). What is absent
here?
Space. The more space there is, the more individual
specimens of any given species occupy it. True, it does not necessarily mean getting
big: a pine tree in a forest grows tall and thin, with its’ top high above the
ground – a pine tree in a clearing grows shorter and squatter, with its’ top
spread out wider than its’ crowded counterpart’s… but we digress. The point of
this discussion is that just as modern elephants, (and baobab trees), got big
because of several factors, so did the prehistoric sauropods…and trees, (or
tree-like plants), that they ate. What else?
Right, not all of the sauropods evolved into
super-sized plant eaters, some remained smaller. What about it?
First: sauropods evolved as bulk-feeders: their
teeth and jaws were not designed for chewing, but for stripping foliage from
branches, and for uprooting other plants whole. Another part of the reason as
to why sauropods became the largest of the dinosaurs, extinct and modern, was
because they had to become big to accommodate large and massive digestive systems
that were almost constantly busy, because foliage, (as well as grass), isn’t
very nutritious at all, and it has to be consumed in large amounts to satisfy
not just hunger, but nutritious requirements of an organism.
Well, yes, but again, not all of the herbivorous
dinosaurs got so big: even Jurassic ornithischians, such as Stegosaurus, never
got as big as the sauropods did. That is correct, and it is competition again:
by becoming big, the sauropods overshadowed their competitors: they could feed
in places unavailable to the bird-hipped herbivores, and they were relatively
immune to attacks to such theropods as Allosaurus, Torvosaurus, and
Saurophaganax. That said, there was differentiation between the Jurassic
sauropods themselves: some, like Diplodocus, were longer than they were tall,
and their hips were taller than their shoulders – these dinosaurs could sit
down on their hind legs alone, (forming a tripod with their tails), and be,
well, construction cranes.
Other sauropods, like Brachiosaurus, were taller than
they were long, and their shoulders were taller than their hips. They probably
could also form a fulcrum tripod, but less well than Diplodocus and its’
relatives could, not that they needed too – they already were tall, taller than
Diplodocus normally was.
Finally, there were less specialized sauropods, such
as Camarasaurus, which were neither too tall nor too long, but just big, and
fed on whatever Diplodocus would feed, but less well, and on whatever
Brachiosaurus would feed, but less well. What is the moral?
We move on from the Jurassic to the Cretaceous and
the world of the dinosaurs’ changes. The single super-continent of the Triassic
and part of the Jurassic is gone, there are now two landmasses, Laurasia in the
north and Gondwana in the south, and they are splitting as well. In the north,
sauropods are practically gone, with some singular generalized species, such as
Alamosaurus, remaining. In the south, they are flourishing, however…
However, Gondwana would eventually fall apart, as
did Laurasia. The latter formed Eurasia and North America; the former – Africa,
Australia, Antarctic, and South America, and it is in the last continent that
the sauropods would reach their last peak of variety. Why? Because there were fewer
bird-hipped dinosaurs in South America than elsewhere. In Africa and Australia,
for example, there were such dinosaurs as Ouranosaurus and Muttaburrasaurus,
smaller plant-eaters that were intermediate between the earlier iguanodonts and
the true hadrosaurs, (like Edmontosaurus). If the sauropods depended on their
guts to grind down and digest their food, the bird-hipped dinosaurs, (and
especially the hadrosaurs and their kin), had more effective chewing systems
than the sauropods did; they were better adapted to digest the new plants, (the
first flowering plants appeared in the Early Cretaceous, just as the sauropods
began to disappear), and they were able to outcompete the sauropods – just
enough for them to die-out first, before the theropods and the bird-hipped
herbivores, and before the K/T extinction.
In South America, it was slightly different. There,
the sauropods remained dominant herbivores, and they began to compete with each
other. As a result, some became extra-large, just as the African bush elephants
are today. Others remained small; some, like Amargasaurus developed spiny
crests, while others, like Saltasaurus, evolved bony armor, reminiscent of Ankylosaurus
and co. These adaptations were defense mechanisms against their predators –
carnosaurs, (Giganotosaurus and co.), and abelisaurs, (Abelisaurus and co.). Did
they work…technically they did, though most paleontological texts give-off a
feeling that the South American sauropods died-out before the K/T Extinction,
again. Also, the biggest sauropods of them all, such as Argentinosaurus, never
developed any bony armor or spikes or anything – it was too big to need this
sort of armor, and not even the biggest South American theropods, (Mapusaurus,
Giganotosaurus, etc.) were able to take it down…unless the circumstances were
just in their favor.
Pause. We have come full circle. A learned person, a
publicator in ‘Scientific American’ proclaimed that there is no idea as to why
some sauropods became super-sized. For the same reason that some of the mammals
did during the Cenozoic – to get advantage over their competitors, over their
carnivores, and over their food source… and at the same time, to keep
themselves alive and breeding, because bulk-food feeding comes with its own
catches. Not such a mystery after all.
End
