WEBVTT - The Artifact: The Theropod Gap 0:00:03.279 --> 0:00:05.640 Welcome to Stuff to Blow Your Mind, a production of 0:00:05.640 --> 0:00:10.600 I Heart Radio. Hi, my name is Joe McCormick, and 0:00:10.680 --> 0:00:13.960 this is the Artifact, a short form series from Stuff 0:00:14.040 --> 0:00:17.560 to Blow Your Mind, focusing on particular objects, ideas, and 0:00:17.680 --> 0:00:23.800 moments in time. Despite what you might see in some 0:00:23.880 --> 0:00:28.000 classic picture books, not all species of dinosaurs lived together 0:00:28.160 --> 0:00:31.120 at the same time. You might have seen an illustration 0:00:31.160 --> 0:00:34.520 where a tyrannosaurus is opening its jaws to clamp down 0:00:34.520 --> 0:00:37.839 on the neck of a lone stegosaurus while the stegosaurus 0:00:37.960 --> 0:00:41.159 raises the end of its spiked tail and defense. But 0:00:41.280 --> 0:00:44.400 this never happened. We know it never happened because the 0:00:44.400 --> 0:00:48.400 Tyrannosaurus lived about sixty six million years ago and the 0:00:48.400 --> 0:00:51.920 Stegosaurus genus lived a hundred and fifty million years ago. 0:00:52.640 --> 0:00:56.080 The time between them was more than eighty million years, 0:00:56.240 --> 0:00:59.240 which is actually greater than the time between the extinction 0:00:59.280 --> 0:01:03.080 of the non avian dinosaurs and us. If you show 0:01:03.080 --> 0:01:05.920 a t rex fighting a stegosaurus, you might as well 0:01:05.920 --> 0:01:10.000 show a group of humans hunting at triceratops. Just like us, 0:01:10.120 --> 0:01:15.120 dinosaurs lived within particular ecological communities, groups of organisms that 0:01:15.200 --> 0:01:18.760 all occupied the same time and place, and when you 0:01:18.800 --> 0:01:22.959 look at fossils in terms of individual communities, interesting questions 0:01:23.000 --> 0:01:26.520 can arise. This week, I came across a study published 0:01:26.520 --> 0:01:30.320 in the journal Science in February by a group of 0:01:30.319 --> 0:01:35.840 paleoecologists named Katlin Schroeder, S, Kathleen Lyons, and FELICEA. A. Smith, 0:01:36.200 --> 0:01:40.280 which discussed a potentially mysterious gap that emerges when you 0:01:40.319 --> 0:01:43.400 try to put together a picture of these dinosaur communities 0:01:43.440 --> 0:01:47.360 from the fossil record. Most of the time these communities 0:01:47.400 --> 0:01:52.960 are missing something medium sized carnivores. This gap in medium 0:01:53.000 --> 0:01:58.120 sized animals seems specific to dinosaurs and carnivorous dinosaurs in particular, 0:01:58.240 --> 0:02:01.200 rather than something that is general really observed in nature. 0:02:01.800 --> 0:02:03.880 For example, if you look at the carnivores that live 0:02:03.920 --> 0:02:07.440 on the African savannah today, you'll find small carnivores like 0:02:07.480 --> 0:02:11.240 the mongoose, large carnivores like the lion, and in between 0:02:11.400 --> 0:02:15.040 plenty of medium sized carnivores like the wild dog. The 0:02:15.160 --> 0:02:18.399 environment allows niches for meat eaters of all these sizes 0:02:18.440 --> 0:02:21.640 because each can evolve to specialize in prey that's best 0:02:21.720 --> 0:02:24.800 suited for its body mass. But when it comes to 0:02:24.840 --> 0:02:28.560 communities of dinosaurs, the distribution often appears to have this 0:02:28.639 --> 0:02:32.000 whole In the middle, there were giant predators, the ones 0:02:32.040 --> 0:02:35.680 we know very well, the mega therapods like Terrannosaurus rex. 0:02:36.240 --> 0:02:39.800 The class known as megatherapods includes any therapod predator that 0:02:39.840 --> 0:02:42.960 grew to over a thousand kilograms, and there were some 0:02:43.000 --> 0:02:46.919 small predators as well with body masses under one kilograms, 0:02:47.400 --> 0:02:51.040 but there was often very little in between. These communities 0:02:51.080 --> 0:02:54.840 mostly didn't have predators that were over a hundred kilograms 0:02:54.960 --> 0:02:58.520 but under a thousand. To use a comparison cited in 0:02:58.600 --> 0:03:02.680 the study itself, quote, if the modern mammal carnivore assemblage 0:03:02.680 --> 0:03:06.360 of the Krueger National Park were similarly structured, there would 0:03:06.400 --> 0:03:09.880 be no carnivores between the size of an African lion 0:03:10.200 --> 0:03:13.480 at a hundred nine rams and a bat eared fox 0:03:13.639 --> 0:03:17.600 at four kilograms. So what accounts for the dinosaur gap? 0:03:20.600 --> 0:03:23.120 When there's an apparent gap in the fossil record, there's 0:03:23.160 --> 0:03:26.880 always a possibility that what paleontologists are seeing is merely 0:03:26.919 --> 0:03:31.320 a bias in the fossilization process. Most of the animals 0:03:31.320 --> 0:03:35.440 that live and die never become fossils. Fossilization is a 0:03:35.480 --> 0:03:39.880 special process that only takes place under unique conditions, for example, 0:03:40.080 --> 0:03:43.440 when the bones are rapidly buried after the animal's death. 0:03:44.040 --> 0:03:46.160 This is much more likely to happen in a wet 0:03:46.280 --> 0:03:48.840 environment like a sea floor, which is why we have 0:03:48.960 --> 0:03:52.520 far more fossils of prehistoric marine organisms than we do 0:03:52.560 --> 0:03:55.600 of land animals. And when we do have the fossils 0:03:55.600 --> 0:03:58.960 of a land animal, it's often because their bones somehow 0:03:59.080 --> 0:04:02.000 fell into a water source or we're covered by a flood. 0:04:02.400 --> 0:04:04.480 But shrowd to read all in their paper argue for 0:04:04.520 --> 0:04:08.480 a different explanation. After they examined forty three communities of 0:04:08.520 --> 0:04:12.119 dinosaurs across a hundred and thirty six million years, they 0:04:12.120 --> 0:04:15.680 concluded that the meso carnivore gap is real, and the 0:04:15.760 --> 0:04:20.239 explanation has to do with how dinosaurs reproduced and grew. 0:04:21.360 --> 0:04:25.400 The largest therapod predators were gigantic once they reached full size. 0:04:25.839 --> 0:04:28.880 An adult terinosaurs Rex probably grew to more than twelve 0:04:28.960 --> 0:04:32.200 meters in length and weighed somewhere between four thousand and 0:04:32.279 --> 0:04:35.599 seven thousand kilograms. They're often compared to the size of 0:04:35.680 --> 0:04:39.039 large vehicles, but there's a fact that limits how big 0:04:39.080 --> 0:04:42.040 the offspring of even the biggest dinosaurs could be when 0:04:42.080 --> 0:04:49.359 they entered the environment. Dinosaurs are oviparous, they hatch from eggs. Mammals, 0:04:49.440 --> 0:04:52.400 on the other hand, are viviporous, giving birth to live young, 0:04:52.720 --> 0:04:56.440 and the newborns could be pretty stout. African elephant calves 0:04:56.440 --> 0:04:59.520 are sometimes already over a hundred kilograms at the time 0:04:59.520 --> 0:05:03.040 they're born. This gives those calves a leg up on survival, 0:05:03.120 --> 0:05:06.880 but it also comes with biological costs. Mammals usually have 0:05:07.040 --> 0:05:10.560 fewer offspring than oviporous species, and they have to invest 0:05:10.680 --> 0:05:14.520 more resources into each one. An African elephant mother is 0:05:14.560 --> 0:05:17.839 pregnant with each calf for almost two years, and even 0:05:17.880 --> 0:05:21.600 after the calf is born, it's been several more years nursing. 0:05:22.160 --> 0:05:25.040 But while egg laying species don't have these problems, there 0:05:25.040 --> 0:05:29.200 are also drawbacks to egg based reproduction. For example, there 0:05:29.240 --> 0:05:33.680 are harsh physical constraints on the size of eggs. The 0:05:33.760 --> 0:05:35.680 shell of an egg has to be thin enough to 0:05:35.720 --> 0:05:39.600 allow the permeation of gas so oxygen can get inside 0:05:39.600 --> 0:05:42.720 and reach the developing embryo, and the shell also has 0:05:42.760 --> 0:05:44.880 to be thin enough to allow the hatchling to break 0:05:44.920 --> 0:05:49.160 out once it reaches maturity. But as eggs increase in size, 0:05:49.560 --> 0:05:53.440 thin shells become less and less tenable. A huge egg 0:05:53.480 --> 0:05:56.680 with a wafer like shell would break too easily. As 0:05:56.720 --> 0:05:59.960 a result, even gigantic dinosaurs would have to lay pretty 0:06:00.040 --> 0:06:03.919 small eggs shredder at all. Right. That oviparity meant even 0:06:03.960 --> 0:06:08.360 the largest dinosaur species were limited to about fifteen kilograms, 0:06:08.480 --> 0:06:11.240 which is roughly about thirty three pounds at the time 0:06:11.320 --> 0:06:14.120 that they hatched. No matter how big the adult, the 0:06:14.120 --> 0:06:16.920 new hatchlings were never going to be any bigger than 0:06:16.960 --> 0:06:21.159 your average Welsh corky. A freshly hatched Tyrannosaurus rex was 0:06:21.200 --> 0:06:24.240 probably about the size of a house cat. So these 0:06:24.279 --> 0:06:28.680 mega theropods started very small, grew enormous in adulthood, and 0:06:28.760 --> 0:06:31.279 had a lot of eating and growing to do along 0:06:31.279 --> 0:06:35.200 the way, and there were lots of them. Ultimately, this 0:06:35.279 --> 0:06:38.760 is the explanation offered by the authors of paper for 0:06:38.760 --> 0:06:42.560 the meso carnivorre gap. The gap was filled by growing 0:06:42.720 --> 0:06:46.560 mega therapods. In other words, these communities did have their 0:06:46.600 --> 0:06:51.040 own medium sized carnivores, but they weren't separate species. They 0:06:51.040 --> 0:06:56.080 were the rampaging juveniles of giant megatherapod predators. The young 0:06:56.240 --> 0:06:59.800 functioned in the community almost as a species of their own, 0:07:00.520 --> 0:07:03.840 and by functioning like different species in their juvenile phases. 0:07:03.920 --> 0:07:08.560 Mega theropods limited species diversity. There's just less room in 0:07:08.600 --> 0:07:12.240 the environment for a predator that reaches three ds as 0:07:12.280 --> 0:07:15.840 an adult if there are lots of three t rex 0:07:15.920 --> 0:07:22.560 teenagers running around competing for the same prey. Tune into 0:07:22.560 --> 0:07:25.720 new editions of the Artifact every Wednesday, hosted by either 0:07:25.840 --> 0:07:28.960 Robert or myself. As always, you can email us at 0:07:29.160 --> 0:07:40.360 contact at stuff to Blow your Mind dot com. Stuff 0:07:40.400 --> 0:07:42.600 to Blow Your Mind is production of I Heart Radio. 0:07:42.960 --> 0:07:45.080 For more podcasts for my heart Radio, visit the i 0:07:45.120 --> 0:07:47.960 heart Radio app, Apple Podcasts, or wherever you listen to 0:07:47.960 --> 0:07:48.800 your favorite shows.