Adolescence is a time of great
change for most of us. But it was particularly volatile for young T rexes.
Before they became fearsome, bone-crushing adults, they had to pass through a
number of stages — 60m hatchling, gangly preteen, bulky young adult. At each
phase, they hunted different prey and filled different niches.
اضافة اعلان
As a new study in Science reveals,
juvenile T rexes and the youth of other large carnivores called megatheropods
transformed their communities as they fumbled through their own physical
changes. Their rapid shifts in size and roles shaped their ecosystems, the
study suggests, and could help to explain some of the perplexing mysteries of
dinosaurdom, from the relative lack of species diversity to the strange
preponderance of huge body sizes.
Considering dinosaurs ruled the
planet for 179 million years, there were fewer distinct species than you might
expect. While today’s world is fuzzy with mammals — at the moment, nearly 7,000
different types — we only know of about 1,500 non-avian dinosaur species, said
Kat Schroeder, a PhD student at the University of New Mexico and a co-author
of the new paper.
There are also “some very weird
things about their mass distribution,” Schroeder said. Within contemporary
animal classes, small-bodied species tend to vastly outnumber big ones. (For
instance, there are currently 20 species of elephant shrew, and just three
species of elephant.) But for dinosaurs, it’s the opposite: “Most of them are
large,” she said.
Some paleontologists looking into
these dynamics over the years have tentatively blamed the youth. Juvenile T rexes
were light and agile before they leveled up into the adults we’re more familiar
with. (The physical discrepancies between younger and older T rexes can be so
vast that experts have argued over whether certain specimens are different
species altogether, rather than different ages.) Other megatheropods, such as
abelisaurs, also grew from turkeylike hatchlings to bus-sized behemoths over
the course of their lives.
For this reason, the presence of
just one of these species in an ecosystem meant that “a large number of
different-sized predators existed” there, hunting progressively larger prey as
they themselves grew up, said Dr. Marcus Clauss, head of research at the Clinic
for Zoo Animals in Zurich, who has published theoretical work on this concept
but was not involved in the new study. Perhaps the ecological real estate that
might have been filled by midsized dinosaur species was instead taken up by
these in-betweeners.
To test this hypothesis, Schroeder
and her co-authors examined 43 dinosaur communities. By cross-referencing
scientific papers with a public paleobiology database, they deduced which
species were likely to have coexisted. They then sorted the species in each
community by size.
In communities with megatheropods,
Schroeder and colleagues found what they call a “carnivore gap”: a large swath
of empty medium-size niches. For instance, the Hell Creek Formation, a fossil
area that stretches from modern day Utah to Alberta, was once home to 286kg
grown-up dromaeosaurs, 7 ton adult T rexes — and no mature meat-eaters in
between.
Though the gap in Hell Creek is
extreme, most of the communities with megatheropods the researchers studied had
no carnivores between 100kg and 300kg. (For a modern comparison, this is as if
the adult carnivores in South Africa’s Kruger National Park were all either
larger than a lion or smaller than a bat-eared fox, they write.)
The carnivore gaps were more
pronounced in these individual communities than in those that lacked
megatheropods, supporting the idea that the young megatheropods were filling
them, Schroeder said. The gaps also didn’t apply to herbivores. This suggests
that the juvenile carnivores’ inability to hunt the same food as adults forced
them to carve out their own niche, which had a strong influence on the
ecosystem, she said. (A baby sauropod, in contrast, could eat on the bottom
branches of a tree while an adult consumed the top.)
The new study “represents an
enormous feat in testing this concept,” Clauss said. Theoretically, he said,
these same dynamics might have made it harder for dinosaurs to repopulate large
niches after a mass extinction event: When the big dinosaurs died, the relative
lack of small and medium-sized species meant that mammals were better
positioned to take over.
Broad analyses like this one are
“truly transforming the field” of paleontology, said Lawrence M. Witmer, a
professor of anatomy and paleontology at Ohio University who was not involved
in the study.
“The notion that youngsters were
different kinds of predators than their monster parents was out there,” Witmer
said. But while many paleontologists had been focusing on one species at a time
in addressing this question, this study instead connects “thousands of dots,”
he said, to show “how whole communities of dinosaurs evolved.”
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