The Artifact: The Theropod Gap

In this episode of STBYM’s The Artifact, Joe looks at an intriguing question about missing carnivorous dinosaurs.

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Episode Transcript

Speaker 1 (00:03):
Welcome to Stuff to Blow Your Mind, a production of
I Heart Radio. Hi, my name is Joe McCormick, and
this is the Artifact, a short form series from Stuff
to Blow Your Mind, focusing on particular objects, ideas, and
moments in time. Despite what you might see in some

(00:23):
classic picture books, not all species of dinosaurs lived together
at the same time. You might have seen an illustration
where a tyrannosaurus is opening its jaws to clamp down
on the neck of a lone stegosaurus while the stegosaurus
raises the end of its spiked tail and defense. But
this never happened. We know it never happened because the

(00:44):
Tyrannosaurus lived about sixty six million years ago and the
Stegosaurus genus lived a hundred and fifty million years ago.
The time between them was more than eighty million years,
which is actually greater than the time between the extinction
of the non avian dinosaurs and us. If you show
a t rex fighting a stegosaurus, you might as well

(01:05):
show a group of humans hunting at triceratops. Just like us,
dinosaurs lived within particular ecological communities, groups of organisms that
all occupied the same time and place, and when you
look at fossils in terms of individual communities, interesting questions
can arise. This week, I came across a study published

(01:26):
in the journal Science in February by a group of
paleoecologists named Katlin Schroeder, S, Kathleen Lyons, and FELICEA. A. Smith,
which discussed a potentially mysterious gap that emerges when you
try to put together a picture of these dinosaur communities
from the fossil record. Most of the time these communities

(01:47):
are missing something medium sized carnivores. This gap in medium
sized animals seems specific to dinosaurs and carnivorous dinosaurs in particular,
rather than something that is general really observed in nature.
For example, if you look at the carnivores that live
on the African savannah today, you'll find small carnivores like

(02:07):
the mongoose, large carnivores like the lion, and in between
plenty of medium sized carnivores like the wild dog. The
environment allows niches for meat eaters of all these sizes
because each can evolve to specialize in prey that's best
suited for its body mass. But when it comes to
communities of dinosaurs, the distribution often appears to have this

(02:28):
whole In the middle, there were giant predators, the ones
we know very well, the mega therapods like Terrannosaurus rex.
The class known as megatherapods includes any therapod predator that
grew to over a thousand kilograms, and there were some
small predators as well with body masses under one kilograms,
but there was often very little in between. These communities

(02:51):
mostly didn't have predators that were over a hundred kilograms
but under a thousand. To use a comparison cited in
the study itself, quote, if the modern mammal carnivore assemblage
of the Krueger National Park were similarly structured, there would
be no carnivores between the size of an African lion
at a hundred nine rams and a bat eared fox

(03:13):
at four kilograms. So what accounts for the dinosaur gap?
When there's an apparent gap in the fossil record, there's
always a possibility that what paleontologists are seeing is merely
a bias in the fossilization process. Most of the animals
that live and die never become fossils. Fossilization is a

(03:35):
special process that only takes place under unique conditions, for example,
when the bones are rapidly buried after the animal's death.
This is much more likely to happen in a wet
environment like a sea floor, which is why we have
far more fossils of prehistoric marine organisms than we do
of land animals. And when we do have the fossils

(03:55):
of a land animal, it's often because their bones somehow
fell into a water source or we're covered by a flood.
But shrowd to read all in their paper argue for
a different explanation. After they examined forty three communities of
dinosaurs across a hundred and thirty six million years, they
concluded that the meso carnivore gap is real, and the

(04:15):
explanation has to do with how dinosaurs reproduced and grew.
The largest therapod predators were gigantic once they reached full size.
An adult terinosaurs Rex probably grew to more than twelve
meters in length and weighed somewhere between four thousand and
seven thousand kilograms. They're often compared to the size of
large vehicles, but there's a fact that limits how big

(04:39):
the offspring of even the biggest dinosaurs could be when
they entered the environment. Dinosaurs are oviparous, they hatch from eggs. Mammals,
on the other hand, are viviporous, giving birth to live young,
and the newborns could be pretty stout. African elephant calves
are sometimes already over a hundred kilograms at the time

(04:59):
they're born. This gives those calves a leg up on survival,
but it also comes with biological costs. Mammals usually have
fewer offspring than oviporous species, and they have to invest
more resources into each one. An African elephant mother is
pregnant with each calf for almost two years, and even
after the calf is born, it's been several more years nursing.

(05:22):
But while egg laying species don't have these problems, there
are also drawbacks to egg based reproduction. For example, there
are harsh physical constraints on the size of eggs. The
shell of an egg has to be thin enough to
allow the permeation of gas so oxygen can get inside
and reach the developing embryo, and the shell also has

(05:42):
to be thin enough to allow the hatchling to break
out once it reaches maturity. But as eggs increase in size,
thin shells become less and less tenable. A huge egg
with a wafer like shell would break too easily. As
a result, even gigantic dinosaurs would have to lay pretty
small eggs shredder at all. Right. That oviparity meant even

(06:03):
the largest dinosaur species were limited to about fifteen kilograms,
which is roughly about thirty three pounds at the time
that they hatched. No matter how big the adult, the
new hatchlings were never going to be any bigger than
your average Welsh corky. A freshly hatched Tyrannosaurus rex was
probably about the size of a house cat. So these

(06:24):
mega theropods started very small, grew enormous in adulthood, and
had a lot of eating and growing to do along
the way, and there were lots of them. Ultimately, this
is the explanation offered by the authors of paper for
the meso carnivorre gap. The gap was filled by growing
mega therapods. In other words, these communities did have their

(06:46):
own medium sized carnivores, but they weren't separate species. They
were the rampaging juveniles of giant megatherapod predators. The young
functioned in the community almost as a species of their own,
and by functioning like different species in their juvenile phases.
Mega theropods limited species diversity. There's just less room in

(07:08):
the environment for a predator that reaches three ds as
an adult if there are lots of three t rex
teenagers running around competing for the same prey. Tune into
new editions of the Artifact every Wednesday, hosted by either
Robert or myself. As always, you can email us at

(07:29):
contact at stuff to Blow your Mind dot com. Stuff
to Blow Your Mind is production of I Heart Radio.
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