How to Build a Giant Dinosaur
Sauropods were humongous creatures, but how they got 
so large is a mystery that paleontologists are still 
trying to unravel
By Brian Switek
Smithsonian.com, 
April 29, 2011
http://www.smithsonianmag.com/science-nature/How-to-Build-a-Giant-Dinosaur.html 

They were the most gigantic animals ever to walk the
earth. Sauropod dinosaurs-"thin at one end; much, much
thicker in the middle; and then thin again at the far
end," as comedian John Cleese described them-were titans
that thrived for more than 130 million years. The
largest known species, such as Argentinosaurus and
Futalognkosaurus from prehistoric South America,
stretched more than 100 feet long and weighed in excess
of 70 tons. Bones found in the 1870s (and since somehow
lost) hint that an enigmatic species dubbed Amphicoelias
may have been even bigger still.

No land mammal has ever come close to the size of these
gargantuan dinosaurs. The prehistoric hornless rhino
Paraceratherium-the largest land mammal ever-was a mere
40 feet long and weighed a paltry 17 tons, and today's
African bush elephants, at 5 tons, would look dainty
next to the largest sauropod dinosaurs. (Blue whales, at
100 feet and 200 tons, are a bit more massive than
sauropods, but it's easier, physiologically, to be large
in an aquatic environment.)

What was it about these dinosaurs that allowed them to
become the biggest terrestrial animals of all time?
Paleontologists have been puzzling over the question for
more than a century. Even relatively modest-sized giants
such as Apatosaurus and Diplodocus, early naturalists
believed, were so huge that they must have been confined
to rivers and lakes deep enough to support the
dinosaurs' bulk. On land, the argument went, these
dinosaurs would collapse under their own weight. By the
1970s, skeletal evidence and preserved footprints in
trackways confirmed that sauropods were land-dwellers.
But it has only been recently that paleontologists have
been able to start unlocking the secrets of how these
seemingly improbable animals developed over their
lifetimes and how they evolved in the first place.

Understanding the natural history of sauropods has been
crucial to figuring out how they got so big. Though some
of the earliest members of the sauropod lineage-such as
the 230 million-year-old Panphagia from Argentina-were
less than five feet long, even they possessed a unique
combination of traits that eventually allowed the group
to attain huge sizes.

The way sauropods reproduced may have been a key to
their ability to grow to such prodigious sizes. Mother
sauropods laid about 10 eggs at a time in small nests;
scores of fossilized egg clutches have been found, as
have thousands of eggs from sites all over the world.
(Some even preserved embryos inside, allowing
paleontologists to definitively identify sauropod eggs
by their shape.) That means these dinosaurs grew outside
of their mother's bodies. According to Christine Janis
of Brown University and Matthew Carrano of Smithsonian's
National Museum of Natural History, laying eggs opened
up evolutionary possibilities for these dinosaurs.

For large mammals, carrying a fetus is a major
investment. Developing African bush elephants gestate
inside their mothers for a staggering 22 months, for
example, and the larger mammal species get, the longer
their offspring have to develop before birth. A lot can
go wrong during a long gestation, including miscarriage,
and nourishing such a large embryo for so long is a huge
energy drain on an expectant mother (to say nothing of
nursing the baby and providing care after birth). As
mammals get larger, the risks and costs of carrying
offspring increase, and so there might be some kind of
size threshold that land mammals can't cross.

Mother sauropods, on the other hand, did not have to
carry their developing babies for nearly two years, and
they could lay numerous eggs at relatively short
intervals. Some species may have provided parental care
after hatching: rare trackways show that some herds
likely included sauropods of different ages. But fans of
the animated movie The Land Before Time may be
disappointed to know that others probably didn't care
for their young. Paleontologists have also found bone
beds that contain only young sauropods of species such
as Alamosaurus, indicating that these dinosaurs were on
their own after leaving the nest.

Regardless of whether juvenile sauropods hung out in big
herds or in smaller groups of dinosaurs their own age,
though, the young dinosaurs were probably picky eaters.
They had to be if they were to grow to adult size.
Diplodocus is one of the most iconic sauropod dinosaurs,
and adults of this Jurassic herbivore had broad,
squared-off muzzles indicative of an indiscriminate
diet. In addition to energy-rich ginkgo trees and
conifers called monkey puzzles, they could have also
survived on lower-quality food like cycads and the tough
parts of conifers. The skull of a juvenile, described by
John Whitlock, Jeffrey Wilson and Matthew Lamanna last
year, hints that young Diplodocus had different tastes.

Paleontologists have recognized that the differences in
menu choice between grazing and browsing herbivores can
generally be seen in skull shape. While grazers have
broad muzzles to scarf up a wide variety of food,
selective browsers have narrower and rounded snouts that
make it possible for them to pick specific plants or
plant parts. (Some fanciful reconstructions gave
Diplodocus and other sauropods elephant-like trunks with
which to pluck food, but this idea has been thoroughly
debunked.) Since the juvenile Diplodocus skull had a
more rounded shape, Whitlock and colleagues proposed
that it selected the juiciest browse - juvenile
Diplodocus may have focused on foods like horsetails,
ferns and high-energy evergreens, instead of sucking
down whatever was available, as adults did.

From an energy perspective, it made sense for young
sauropods to be choosy. Small dinosaurs required the
most bang for their buck in terms of food; they were
specialized to pick high-energy plants to fuel their
rapid growth. Adults, which were already large and
merely had to maintain-rather than grow-large bodies,
could afford to hork down large amounts of lower-quality
fuel. While they consumed more food in absolute terms,
adult sauropods could eat lower-quality foods, whereas
smaller sauropods required high-quality food. (This is a
common pattern seen among animals even today: a tiny
shrew has to eat nutritious insects almost constantly,
but African elephants can subsist on a diet of lower-
quality grass and other plant food.) The dietary
difference may have allowed young and mature Diplodocus
to live in the same area through a phenomenon ecologists
call "niche partitioning." The specialization of the
juveniles and the more generalist diet of the adults
kept them out of constant competition for food, meaning
that the young and old Diplodocus fed almost as if they
were two different species.

In order to consume all that food, though, sauropods had
to reach it. Long necks were a critical, early
adaptation that allowed sauropods to attain large body
sizes, according to a recent review by Martin Sander and
15 other scientists. Think of an Apatosaurus standing at
the edge of a prehistoric forest. The dinosaur's long
neck would allow it to reach a wide swath of vegetation-
high and low, left and right-without moving its body at
all. From early on in sauropod evolution, long necks
made these dinosaurs efficient feeders able to reach
resources that were inaccessible to other herbivores,
and even with tiny heads, big sauropods would have
easily been able to vacuum up huge quantities of food.

Just how these dinosaurs converted all this green food
into energy and tissue is a trickier matter. Sauropods
did not have robust batteries of molars to chew their
food. Many had only a few pencil- or spoon-shaped teeth
to pluck food before swallowing it whole. Given
sauropods' poor table manners, scientists used to think
that the dinosaurs might have swallowed stones to grind
up food still in the stomach the way some birds do.
Paleontologists Oliver Wings and Martin Sander have
argued that this probably wasn't the case-so-called
"stomach stones" found with some sauropod fossils do not
show a pattern of wear consistent with what would be
expected if they were being used this way. Instead, the
dinosaurs extracted as much nutrition as possible from
their food by retaining it for long periods in their
digestive systems.

A few details of sauropod digestion were experimentally
modeled by Jürgen Hummel and colleagues in 2008. The
scientists placed modern-day samples of the most
abundant sauropod chow from the Mesozoic-ferns,
horsetails, ginkgoes and conifers-in simple artificial
stomachs. They inoculated the fake guts with microbes
taken from the part of sheeps' digestive systems where
plant food is initially broken down. As the plants
fermented, the scientists tracked how much nutrition
they released.

Contrary to what had been assumed, many of these plants
degraded relatively easily in the crude stomach
environments. Horsetails and monkey puzzles were
especially nutritious. Actual dinosaur stomachs might
have been even better equipped at breaking down these
plants, and there was certainly enough available energy
in the plants of the time for sauropods to grow large.
Sauropods probably did not require extraordinary gut
architecture to survive.

Another major feature allowed these titans to balloon in
size. It is a trait they share with birds. Birds are the
direct descendants of small theropod dinosaurs related
to species like Velociraptor and Anchiornis, but they
are not very closely related to sauropod dinosaurs; they
last shared a common ancestor more than 230 million
years ago. Even so, both the theropod and sauropod
lineages shared a peculiar trait that was extremely
important in their evolution-a network of internal air
sacs connected to the lungs.

The soft air sacs haven't been seen directly in the
fossil record, but the structures left telltale pockets
where they invaded bones. Naturalists recognized the
indentations more than a century ago, but modern
paleontologists are only just beginning to understand
their significance. As in birds, the lungs of sauropods
were probably connected to a series of air sacs, and
attached to these organs was a network of smaller
pockets-called diverticula-that infiltrated the bones in
the neck, chest and abdomen of the dinosaurs. From a
structural point of view, this network of air-filled
structures lowered the density of the sauropod skeleton,
and allowed these dinosaurs to have a relatively
lightweight construction for their size. Rather than
having extra-strength bones, as had once been suggested,
sauropod skeletons were made lighter by a trait they
share with birds, and the network of air sacs probably
had other benefits, too.

In birds, air sacs are part of a flow-through breathing
arrangement that is far more efficient at extracting
oxygen than is the respiratory system of mammals. We
don't yet know if sauropods breathed the same way birds
did-the degree to which their skeletons were modified by
air sacs varied across species-but it is likely that the
air sacs of the giant dinosaurs were better equipped at
delivering oxygen to their bodies than the alternative
seen in giant mammals. Birds have a high metabolic rate
that requires a great deal of oxygen for sustained
flying; similarly, the size and active lives of
sauropods would have required a great deal of oxygen,
and the air sac system would have provided them with
essential breathing benefits.

Not all sauropod dinosaurs were giants. Some species-
such as Magyarosaurus from the strata of Romania-were
small descendants of much larger species. They shrunk in
size because of their isolation on islands, though the
exact reason why such island dwarfs evolve is debated by
scientists. Still, sauropods weighing more than 40 tons
evolved independently in at least four lineages during
the long tenure of this dinosaur group, all thanks to a
suite of characteristics that made large body size
possible.

Paleontologists are still investigating the evolutionary
pressures that made such large forms advantageous. Their
size gave them some protection from predators,
presumably, and their long necks let them reach food
that smaller creatures looked at hungrily but couldn't
reach. What other advantages giant size might have
provided remain unclear. Nevertheless, sauropods were
astounding creatures that could only have existed thanks
to a peculiar confluence of events. They were fantastic
forms unlike anything that came before or has evolved
since.
 
 
 

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