The Artifact: The Theropod Gap

raises the end of its spiked tail and defense. But this never happened. We know it never happened because the 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

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 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 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 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 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 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 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 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 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 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 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

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. 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 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 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 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 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 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 contact at stuff to Blow your Mind dot com. Stuff to Blow Your Mind is production of I Heart Radio. For more podcasts for my heart Radio, visit the i heart Radio app, Apple Podcasts, or wherever you listen to your favorite shows.