Speaker 1: Welcome to Stuff to Blow Your Mind, a production of
Speaker 1: I Heart Radio. Hi, my name is Joe McCormick, and
Speaker 1: this is the Artifact, a short form series from Stuff
Speaker 1: to Blow Your Mind, focusing on particular objects, ideas, and
Speaker 1: moments in time. Despite what you might see in some
Speaker 1: classic picture books, not all species of dinosaurs lived together
Speaker 1: at the same time. You might have seen an illustration
Speaker 1: where a tyrannosaurus is opening its jaws to clamp down
Speaker 1: on the neck of a lone stegosaurus while the stegosaurus
Speaker 1: raises the end of its spiked tail and defense. But
Speaker 1: this never happened. We know it never happened because the
Speaker 1: Tyrannosaurus lived about sixty six million years ago and the
Speaker 1: Stegosaurus genus lived a hundred and fifty million years ago.
Speaker 1: The time between them was more than eighty million years,
Speaker 1: which is actually greater than the time between the extinction
Speaker 1: of the non avian dinosaurs and us. If you show
Speaker 1: a t rex fighting a stegosaurus, you might as well
Speaker 1: show a group of humans hunting at triceratops. Just like us,
Speaker 1: dinosaurs lived within particular ecological communities, groups of organisms that
Speaker 1: all occupied the same time and place, and when you
Speaker 1: look at fossils in terms of individual communities, interesting questions
Speaker 1: can arise. This week, I came across a study published
Speaker 1: in the journal Science in February by a group of
Speaker 1: paleoecologists named Katlin Schroeder, S, Kathleen Lyons, and FELICEA. A. Smith,
Speaker 1: which discussed a potentially mysterious gap that emerges when you
Speaker 1: try to put together a picture of these dinosaur communities
Speaker 1: from the fossil record. Most of the time these communities
Speaker 1: are missing something medium sized carnivores. This gap in medium
Speaker 1: sized animals seems specific to dinosaurs and carnivorous dinosaurs in particular,
Speaker 1: rather than something that is general really observed in nature.
Speaker 1: For example, if you look at the carnivores that live
Speaker 1: on the African savannah today, you'll find small carnivores like
Speaker 1: the mongoose, large carnivores like the lion, and in between
Speaker 1: plenty of medium sized carnivores like the wild dog. The
Speaker 1: environment allows niches for meat eaters of all these sizes
Speaker 1: because each can evolve to specialize in prey that's best
Speaker 1: suited for its body mass. But when it comes to
Speaker 1: communities of dinosaurs, the distribution often appears to have this
Speaker 1: whole In the middle, there were giant predators, the ones
Speaker 1: we know very well, the mega therapods like Terrannosaurus rex.
Speaker 1: The class known as megatherapods includes any therapod predator that
Speaker 1: grew to over a thousand kilograms, and there were some
Speaker 1: small predators as well with body masses under one kilograms,
Speaker 1: but there was often very little in between. These communities
Speaker 1: mostly didn't have predators that were over a hundred kilograms
Speaker 1: but under a thousand. To use a comparison cited in
Speaker 1: the study itself, quote, if the modern mammal carnivore assemblage
Speaker 1: of the Krueger National Park were similarly structured, there would
Speaker 1: be no carnivores between the size of an African lion
Speaker 1: at a hundred nine rams and a bat eared fox
Speaker 1: at four kilograms. So what accounts for the dinosaur gap?
Speaker 1: When there's an apparent gap in the fossil record, there's
Speaker 1: always a possibility that what paleontologists are seeing is merely
Speaker 1: a bias in the fossilization process. Most of the animals
Speaker 1: that live and die never become fossils. Fossilization is a
Speaker 1: special process that only takes place under unique conditions, for example,
Speaker 1: when the bones are rapidly buried after the animal's death.
Speaker 1: This is much more likely to happen in a wet
Speaker 1: environment like a sea floor, which is why we have
Speaker 1: far more fossils of prehistoric marine organisms than we do
Speaker 1: of land animals. And when we do have the fossils
Speaker 1: of a land animal, it's often because their bones somehow
Speaker 1: fell into a water source or we're covered by a flood.
Speaker 1: But shrowd to read all in their paper argue for
Speaker 1: a different explanation. After they examined forty three communities of
Speaker 1: dinosaurs across a hundred and thirty six million years, they
Speaker 1: concluded that the meso carnivore gap is real, and the
Speaker 1: explanation has to do with how dinosaurs reproduced and grew.
Speaker 1: The largest therapod predators were gigantic once they reached full size.
Speaker 1: An adult terinosaurs Rex probably grew to more than twelve
Speaker 1: meters in length and weighed somewhere between four thousand and
Speaker 1: seven thousand kilograms. They're often compared to the size of
Speaker 1: large vehicles, but there's a fact that limits how big
Speaker 1: the offspring of even the biggest dinosaurs could be when
Speaker 1: they entered the environment. Dinosaurs are oviparous, they hatch from eggs. Mammals,
Speaker 1: on the other hand, are viviporous, giving birth to live young,
Speaker 1: and the newborns could be pretty stout. African elephant calves
Speaker 1: are sometimes already over a hundred kilograms at the time
Speaker 1: they're born. This gives those calves a leg up on survival,
Speaker 1: but it also comes with biological costs. Mammals usually have
Speaker 1: fewer offspring than oviporous species, and they have to invest
Speaker 1: more resources into each one. An African elephant mother is
Speaker 1: pregnant with each calf for almost two years, and even
Speaker 1: after the calf is born, it's been several more years nursing.
Speaker 1: But while egg laying species don't have these problems, there
Speaker 1: are also drawbacks to egg based reproduction. For example, there
Speaker 1: are harsh physical constraints on the size of eggs. The
Speaker 1: shell of an egg has to be thin enough to
Speaker 1: allow the permeation of gas so oxygen can get inside
Speaker 1: and reach the developing embryo, and the shell also has
Speaker 1: to be thin enough to allow the hatchling to break
Speaker 1: out once it reaches maturity. But as eggs increase in size,
Speaker 1: thin shells become less and less tenable. A huge egg
Speaker 1: with a wafer like shell would break too easily. As
Speaker 1: a result, even gigantic dinosaurs would have to lay pretty
Speaker 1: small eggs shredder at all. Right. That oviparity meant even
Speaker 1: the largest dinosaur species were limited to about fifteen kilograms,
Speaker 1: which is roughly about thirty three pounds at the time
Speaker 1: that they hatched. No matter how big the adult, the
Speaker 1: new hatchlings were never going to be any bigger than
Speaker 1: your average Welsh corky. A freshly hatched Tyrannosaurus rex was
Speaker 1: probably about the size of a house cat. So these
Speaker 1: mega theropods started very small, grew enormous in adulthood, and
Speaker 1: had a lot of eating and growing to do along
Speaker 1: the way, and there were lots of them. Ultimately, this
Speaker 1: is the explanation offered by the authors of paper for
Speaker 1: the meso carnivorre gap. The gap was filled by growing
Speaker 1: mega therapods. In other words, these communities did have their
Speaker 1: own medium sized carnivores, but they weren't separate species. They
Speaker 1: were the rampaging juveniles of giant megatherapod predators. The young
Speaker 1: functioned in the community almost as a species of their own,
Speaker 1: and by functioning like different species in their juvenile phases.
Speaker 1: Mega theropods limited species diversity. There's just less room in
Speaker 1: the environment for a predator that reaches three ds as
Speaker 1: an adult if there are lots of three t rex
Speaker 1: teenagers running around competing for the same prey. Tune into
Speaker 1: new editions of the Artifact every Wednesday, hosted by either
Speaker 1: Robert or myself. As always, you can email us at
Speaker 1: contact at stuff to Blow your Mind dot com. Stuff
Speaker 1: to Blow Your Mind is production of I Heart Radio.
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