Plant-Eating and the First Large Land Apex Predator in the Early Permian

The narration places the start of an ecological revolution around 300 million years ago, at the dawn of the Permian period, on the newly formed supercontinent of Pangaea. It describes herbivory as a lifestyle that had never existed before invertebrates emerged, and one that has been extremely common ever since.

Plant-eating had to begin at some point. The narration says the early experiments probably started just before the Permian, in the swamps of the late Carboniferous. By the early Permian, only a few million years later, these experiments had clearly succeeded and plant-eating was spreading fast.

One of the first large herbivores on land was Adafosaurus. The narration notes that it looks like a strangely built protodinosaur but is more closely related to mammals than to dinosaurs or other reptiles. It belonged to a group informally called pelycosaurs, which diversified rapidly as the late Carboniferous gave way to the early Permian. By some estimates, pelycosaurs made up around 70% of all known amniotes from this time.

The narration says the Early Permian ended with most pelycosaurs vanishing under mysterious circumstances and a new group rising to replace them. This major evolutionary shift is described as a milestone in the story of the ancestors of true mammals.

The same moment is also one of the period's biggest enigmas, because the fossil record suddenly goes dark right around this turnover.

The narration describes the basic structure of modern land ecosystems as a simple pyramid: plants at the base, herbivores above them, carnivores above those, and apex predators at the peak. It states that this classic structure did not emerge all at once and has not always been the ecological rule.

It developed in phases over hundreds of millions of years and came together fully only in the early Permian, just under 300 million years ago. Plants had to colonize land first, which they did by at least 470 million years ago in the Ordovician. Invertebrates followed, becoming established on land by the Silurian, more than 420 million years ago, but they probably ate little living plant material and were mostly carnivores, scavengers, or detritivores feeding on dead biomass.

Early four-legged amphibious vertebrates began crawling out of the water around 375 million years ago in the late Devonian, and they too had an essentially all-meat diet. Through most of the Carboniferous, the dominant amphibians of the tropical swamps still lived in a meat-eating world.

As the Carboniferous came to a close and gave way to the early Permian, herbivory suddenly exploded onto the scene, according to the narration.

The episode then cuts to a visit to a small museum in Seymour, Texas, near the Oklahoma border. The host says the site is unique because it offers hands-on encounters with the Permian. In conversation, a guest asks how a group of organisms gained the ability to digest plant material. The answer given is that in the early Permian and late Carboniferous an ecological arms race unfolded: conifers began to appear and spread, and animals developed adaptations to process these new plants.

About 300 million years ago, early land animals began eating living leaves, roots, and seeds. The narration says herbivory evolved independently in multiple lineages around the same time, with similar evolutionary effects, because a carnivore cannot simply start eating plants full time. Meat-eaters shed teeth while feeding, which would not help a plant-eater chewing conifers and roots.

As a result, herbivores developed deep-rooted teeth with thicker, stronger enamel that did not fall out consistently. Several of the earliest herbivore lineages, such as pelycosaurs, also developed much bigger, wider, barrel-shaped bodies. Their expanded guts house trillions of symbiotic microbes that break down indigestible parts of plants, such as cellulose in plant cell walls.

The narration says that developing a huge gut to use as a bacterial fermentation chamber is an adaptation that many large land herbivores have repeatedly evolved since then, to squeeze as many nutrients as possible out of a plant-based diet. Examples given are triceratops from the Mesozoic, and bison and rhinos today.

The narration adds that Adafosaurus was not alone in growing large at this time as its lineage made the switch from insects to plants. Other vertebrate lineages also became large on land as they independently moved toward herbivory. These included Kotylorhinus, another pelycosaur from the synapsid side of the family tree, and caseids such as Cotylorhynchus, which made the switch independently of their edaphosaurid cousins. The two pelycosaur lineages shared traits such as big sprawling barrel-shaped bodies with relatively tiny heads. Diadectes, another early herbivore, filled a similar niche in this period.

Although the earliest herbivores converged on similar traits such as large, bulky bodies, they evolved different ones on their independent paths to plant-eating. The narration compares their teeth as an example. Adafosaurus had dense clusters of peg-like teeth in the lower jaw and on the roof of the mouth, called dental batteries. The amount of wear shows it used them to grind and crush plant material, and the teeth were continually replaced over its life.

Kotylorhinus had a completely different setup. Instead of massive dental batteries, it had fewer leaf-shaped teeth that show no wear from contact with each other, and they do not seem to have been regularly replaced. Some scientists think it cropped a mouthful of plant material, then used a large, tough tongue to press the food against several large, slender teeth on the roof of its mouth to break it up for digestion.

The narration concludes that there is no single way of being an herbivore, and that different lineages found different solutions to similar problems during the transition.

Pelycosaurs also gave rise to the first large land apex predators, which arrived alongside the new herbivores. The narration says their ancestors remained meat-eaters, growing in size in parallel with their newly herbivorous cousins. The most iconic of these was Dimetrodon.

The narration states that Dimetrodon is the world's first known fully terrestrial carnivore to fill the apex predator role, rising to the top of the new complex food chain that had emerged on land. A guest notes that if you see a Dimetrodon skeleton in a museum anywhere in the world, there is a good chance it came from this area.

On the dig, a geologist explains that the site is in the Lower Clear Fork, the lower Permian, roughly 287 million years ago. Dimetrodon had been around for about 20 million years, and its size had been increasing over the last 15 to 20 million years. At this site the team is at the last breath of Dimetrodon, seeing the last big push of its size. The largest form, Dimetrodon grandis, represents its biggest size, and about 10 million years later they were all gone.

During the dig, the team finds a small bone that is identified as a baby Dimetrodon toe.

The host says that, unlike her co-hosts and colleagues, she is not a scientist. Her background is in science writing and science journalism, and she describes herself as not a skilled fossil hunter. She says her colleagues find fossils with ease, and at first she could not tell a bone from a stick.

Her first Permian find was a Dimetrodon bone, identified as a rib. By the end of the day, she could generally tell which part of the Dimetrodon a fossil came from. A claw was also found and described as a killing claw, and the narration notes that these animals had evolved claws able to do what they needed in seconds after only about 20 million years on the planet.

The narration calls Dimetrodon the T-Rex or tiger of its time. Like the earliest herbivores, it moved into a groundbreaking ecological niche with radical physical adaptations. It developed a big, powerful skull with deep jaws lined with several types of teeth suited to different parts of its predatory lifestyle.

These included large, sharp canines for striking and stabbing prey, and shearing teeth for slicing meat into digestible chunks. Differentiated teeth are found in many modern and ancient animals, but this was another innovation that first appeared in Dimetrodon and its relatives. The narration notes that these differences are less obvious than in carnivorous mammals today, but they had to start somewhere.

For the past couple of weeks the team had been slowly digging a shelf, where a colleague named Holly had found incredible skeletons and parts of bones. The team said that Permian clays are easy to dig, which makes it quick to expose bones.

While digging, a participant finds a large bone that turns out to be a joint, then a skull. The host remarks that the digger has been digging the Permian for four minutes and has found a Dimetrodon skull. The site is described as very fossiliferous, with a rib in front of a spine and a skull nearby. The mudstone crumbles easily, which makes the bones simple to reach and dig through. Mudstone forms in a gentle environment, such as a lake bed where there is little energy, so no big waves disturbed the remains left behind.

Asked about Dimetrodon's role in the ecosystem, a team member describes it as the king of the food pyramid. It eats everything, recycles organisms in the ecosystem by putting them back into it, and its role here is to keep the population down, which is population control.

The early Permian saw a range of pelycosaurs with different lifestyles dominate the landscape during the ecological revolution. But around 273 million years ago, their reign ended under suspicious circumstances. Dimetrodon goes extinct right at the end of the early Permian, along with the adaposaurs and diadectids.

The fossil record of land animals then appears almost blank for about 5 million years. When it resumes in the middle Permian, most of the once-abundant pelycosaurs have disappeared, and a new thriving group of synapsids called therapsids is present. The narration says therapsids evolved from a pelycosaur lineage that seems to have emerged, diversified, and taken over ecosystems during those 5 million years, while almost all older pelycosaur lineages vanished. This turnover was a crucial plot twist in the later rise of mammals, because therapsids had a host of new traits compared with their pelycosaur ancestors and relatives. They were often faster and more agile, with legs positioned more directly under their bodies rather than sprawled to the sides. Their skulls became stronger and more robust, with teeth increasingly specialized for different roles, and they may also have had better hearing, a more sensitive sense of smell, and a faster growth rate.

In a discussion of jaw joints, a speaker explains that the joints of these animals were starting to shrink and would move up into the back of the skull to become the eardrum. Shrinking the jaw joints made room for a larger brain, so the animals were adapting quickly to their surroundings and becoming predators in a new way, not only apex predators. In short, they were becoming increasingly mammal-like.

But how and why the transition from pelycosaurs to therapsids actually occurred has been a long-standing mystery in paleontology. The void in the fossil record is known as Olson's gap, named after Everett C. Olson, who first proposed this ecological changeover. The first hypothesis holds that the gap results from geological bad luck: good fossil sites covering this period are simply missing. The narration notes that fossil sites on either side of the gap also come from different places. Earlier Permian sites with pelycosaurs come mostly from North America and Western Europe, which were near the equator at the time, while middle Permian fossils with the first therapsids mostly come from higher-latitude areas in what is now Russia and South Africa. So the apparent disappearance and appearance could reflect a shift in the environments being preserved, giving an illusion of a stark change when there may be a longer, more gradual transition. The narration adds that some scientists say there is no gap and others say there is, and that they are looking at different fossil records around the world.

Under the second hypothesis, Olson's gap should really be thought of as Olson's extinction. According to this idea, the global shift in life on land observed between the early and middle Permian is not just a result of sampling bias. It represents a genuine mass extinction that caused a global turnover in terrestrial animal life.

Paleontologists are still debating whether the gap or the extinction hypothesis has more support. The narration says the mystery may be solvable through more digging and more discoveries in places that have historically been under-sampled, which may yet turn up fossils that fill the gap.

During a dig, a speaker explains a practical rule: when a bone is seen in the side of a hill, digging should stop, and diggers should work from above the bone rather than underneath it, so it does not collapse and fall out of the wall.

The narration says the Early Permian was a time of firsts for planet Earth. Life on Pangaea gained its first big herbivores, its first big apex predators, and a new ecological framework that has stuck around ever since. Permian life may also have faced its first major challenge. Out of that period emerged the therapsids, whose origins may be blurry and mysterious, but whose later story is one of enormous ecological success, diversification, and evolutionary innovation. The narration says this story is still unfolding through the last surviving therapsid lineage, mammals.

The narration says that many twists and turns were still in store for the early therapsids in the middle Permian, as they were about to enter a golden age. But at the end of the Permian, they, along with every other living thing on Earth, would have to face and survive the most severe mass extinction of all time.

The episode closes with a view of mountains built from the ruins of a vast tropical barrier reef. The narration says complex traits and behaviors now seen as typical of modern mammals were emerging for the first time, and higher up the food chain were the biggest and most dangerous terrestrial apex predators of the entire period. The reef, along with much middle Permian life, was doomed.