Biology 103
2003 First Paper
On Serendip

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I hope you guys have as much fun reading this as I did writing it. ^_^

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The Amazing Cheesy Adventures of Professor Sanderson's Paleobiology Class!

Investigation 1: Where did mammals come from? Or: Therapsids!

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Professor Sanderson's class was popular. Partly this was because he was a well-meaning psyinstructor; the images he crafted were neat, cohesive, and usually entertaining. Mainly, though, it was because he was a young male teacher at an all-girls' college, who had the fortune to resemble Jai from "Queer Eye For the Straight Guy." These two factors led to an unnaturally high enrollment in Paleobiology 101. No less than fifty-two girls sat chittering in the classroom before he appeared each day, punctually, at 10 a.m. Most were more intrigued by *his* anatomy than that of the long-dead tetrapods to which he devoted his lectures.

Today's attendance was especially high. It was a Field Trip day. The term wasn't literal. There was no trip involved---simply the students closing their eyes and falling into the trance-like state from which the professor led their excursions. There, in the collective canvass of their psyches, he would build that day's lesson, sculpting visceral images from his expansive knowledge of biology and his even more expansive creativity.

Today's lesson was mammalian origins.

"Where do we come from?" he had asked, by way of preamble. "We all know the basic answer, or think we do. Apes, right? And apes from primates, and primates from mammals, all well and good. But where do mammals come from? I mean, what did mammals evolve from, and what were the major evolutionary steps they took to get there? Doesn't that sound fascinating? "

The class eyed him warily. A few actually listened. The rest swooned.

"Today we're going to try and explore that question. If you'll all take out your textbooks, flip to page 137, lean back, orient your touchpads, and close your eyes..." he waited while the actions were performed. "We'll be going to the Permian. That's the time right before the Triassic period, which started the age of the dinosaurs. It's approximately 300 million years in the past." (1)

He briefly surveyed the class, then looked thoughtful for a moment. "Wait. I guess I should give you a little background first," he relented. "The main thing we're going to see today is a group of animals called the therapsids. They were precursors to the mammals. Both they and reptiles were tetrapods, a category created for the earliest four-legged land animals. The therapsids lived in the Permian era, mainly, and were a hugely diverse group of animals. We'll see just how diverse in a moment." (2) If the class had been watching, they would have seen him shut his eyes and tap his temple, once, gently. "Hm, guess that's about it for now. Here we go."

**Click.**

The jungle was humid but sparse. No flowers, little underbrush; only thick conifers and ferns, and rock-hugging carpets of moss. Rivulets of water veined the damp soil. Professor Sanderson was standing casually next to a sprawling, eleven-foot-long lizard-like beast whose ridged sail-back opened like a fan into the heavy sunlight.

"This," he said, gesturing vaguely, "Is modern-day Texas. Currently it's squashed in the middle of the supercontinent Pangea, which only recently formed. We're actually around the equator. This---" he pointed out the creature at his knees--- "is your great-great-great-great... well, it keeps going. He's a pelycosaur, more specifically a Dimetrodon, and he's our collective ancestor." (2)

The class, arrayed in a circular fashion before him, looked skeptical.

"Looks like a lizard to me," one girl muttered.

The professor caught it. "I know he looks like one," he replied. He indicated the animal's parabolic dorsal ridge. "See this? It's like a solar conductor. He uses this thing to soak up sun radiation, and also to provide him with surface area for temperature control. But he's not a lizard."

The thing grunted and scratched itself. The girl raised an eyebrow. "Why?"

Sanderson snapped his fingers and the class started with disgust. The skin, muscle, and guts of the pelycosaur had disappeared, leaving only a brown-white skeleton which continued placidly scratching itself. Unruffled, he pointed to the skeleton's jaw. "Under the jaw here, he's only got one hole for his jaw muscles to go through. Reptiles have two---he's really primitive. (3) Also, he's more mobile than reptiles. If he hoists himself up a little from a squat, his back legs can run straight-legged. Makes sense, considering he's endothermic like his mammal descendents. They were all less climate-dependent, and so they could afford to move more."

A different student raised a hand. "Uh, but if he's endothermic, why does he have that sail?"

Sanderson looked uncomfortable. "Don't rightly know," he admitted. "Some pelycosaurs didn't even have one."

"Was that group more advanced?"

He shrugged. "No, not from all we know so far. The group of pelycosaurs that turned into mammals, the sphenacodonts, included Dimetrodon. (3) But some sphenacodonts didn't have sails. We figure maybe they lived in cooler environments, like thicker forests, where they'd need less temperature regulation. Some even had smaller sails."

The student sniffed skeptically. "So they were like what, half warm blooded? A quarter less warm blooded? Why are we descended from the most cold-blooded one?"

Sanderson held up his hands. "Again, sorry, I don't know. Maybe we're not---the fossil record is sketchy here. We may well be descended from one of the more forest-dwelling types. Or maybe the sails weren't used for temperature regulation at all, but for something else, like display." Wryly, he snapped his fingers again and the pelycosaur regained flesh, this time of a strikingly iridescent color. "But don't say that around any paleontologists."

The pelycosaur yawned. Sanderson glanced at it, then back at his class. "In any case, this is still as far from a mammal as any Synapsid---that's the largest grouping we'll see here, it's a vertebrae-bearing collective of which mammals are the only surviving members---has a right to be. (3) We won't learn much about mammals from studying him. Let's get a little closer."

He raised a hand.

**Click.**

The landscape was much the same: level, grass-less, braided with rivers, shaded by conifers. However, the air had a different quality. It was cooler and drier, as if the altitude has increased. The students discovered that they were now wearing windbreakers.

Again, Sanderson stood about ten feet in front of them.

"This," he explained, reviewing the landscape with a smile, "is the central plains of Russia during the middle-to-late Permian, around 255 million years ago. This is where it really gets good."

The students blinked, glanced around. Apart from a few disturbingly-large insects, there was nothing in sight.

"Um, sir?" one student raised her hand tentatively. "What are we here to see?"

"Something very cool!" he chirped, looking for all the world like a kid in a candy shop. "Our first therapsids. They're the descendants of the pelycosaur you just saw, and the biggest significant jump towards mammals we'll see today." (2) He turned and began striding through the brush, beckoning one hand loosely behind him. "Come on, follow me."

They wove after him between the conifers, which were thicker and more complex than the ones in "Texas." After about a minute the ground began to slope upwards, and soon they emerged on a ridge whose treeless banks tumbled down into a muddy river valley.

"I thought it would be better if we watched them from a distance," the professor said.

Below, in the bowl-like crevasse the river had chewed into the floodplain, a bizarre and eclectic group of animals was busily going about its business.

"Those are therapsids," began Sanderson. "More specifically, a group of gigantic, somewhat primitive therapsids called dinocephalians. Name means 'ugly head.' (2) Accurate, eh?"

It was. The closest animal was a lumbering, hippo-like herbivore with a fat, stiff tail. Its head resembled that of a noseless pit-bull with an overbite. Despite its bowling-ball rotundity, it walked upright on a columnar pair of hind legs---clearly more mobile than the pelycosaur. Sanderson, for some inscrutable reason, had decided to color the entire species pink.

"Those pink ones are Ulemosaurs," he said, pointing. "Notice how much more they look like mammals! There's a definitive movement towards true endothermic metabolism here. See the short neck and tail, and how fat they all are? That's to reduce surface area, to conserve heat. They're beginning to regulate their own internal temperatures, since now the climate's a lot cooler." (2)

He paused for a second, watching the Ulemosaur wade heavily towards a group of water-ferns. "Unfortunately, all of that gut makes them sort of slow..."

The girl who had questioned endothermism raised an eyebrow. "Sounds like a premonition to me."

"Um," said Sanderson, one hand behind his back.

Not a moment later, something large and purple and striped wriggled from behind a clump of ferns and galloped at the Ulemosaurus. It attacked in comic slow-motion. The beast's gallop was actually more of a trot: although it bounced confidently along on four upright legs, its body was too thick-limbed for litheness.

Nevertheless, the Ulemosaurus was far outmatched. It tried pivoting its elephantine body, but only succeeded in turning broadside in the mud, widening the attacker's target. The striped animal drove into the Ulemosaurus's side, snapping, and bore it down beneath the brown water.

"Um," Sanderson repeated, glancing sheepishly at his students' shocked expressions, "Maybe planning that wasn't really a good idea. Uh...." The Ulemosaurus was making obese thrashings beneath the water. Sanderson's discomfort increased, and, decisively, he clicked his fingers. The Ulemosaurus disappeared, leaving its attacker nuzzling confusedly around the riverbed. "See, that carnivore there is called Inostrancevia," (4) he explained quickly. "It's the most advanced type of therapsid we've yet seen, from a group called the theriodonts that appeared right at the end of the dinocephalian reign. Theriodont means 'beast-toothed....'" (4) The class was still gaping at the water. He continued loudly, "Can, er, anyone tell me what they noticed about Inostrancevia's looks that was different from dinocephalian features?"

After a few moments, one of the more strong-stomached girls raised a tentative hand. "The way it ran," she said. "It was upright. It could trot. And the shape of its head was like, I dunno, a really ugly hairless dog."

Sanderson nodded palely. "Both right. By this point in their evolution, the therapsids were moving mainly like modern mammals---with legs beneath the body. This made them faster. They were the first animals that could trot. (4) And the head's very important as well. Theriodonts had shorter heads and better jaw muscles, which meant they could chew more." He patted the underside of his own jaw. "They had a bigger gap for muscles to go in, under here.

"Theriodonts also had more differentiated teeth---specialized for different jobs. (5), (6) Dinocephalians, on the other hand, couldn't chew much." He was calming down some, regaining his joviality. "It's why they had such big guts, to digest all that rough plant material." Sanderson mimed a sagging belly. "Theriodonts, I'm extrapolating here, could afford to be sleeker because they chewed more. And maybe it's that sleekness that allowed them the more erect form, which in turn gave them the speed. All because of chewing."

"But-" the girl furrowed her brows---"it couldn't have chewed that much. You agreed that it looked like a dog, and dogs don't really chew."

"Point taken," admitted Sanderson. "I should have said comparatively. This is just my own theory, understand. Theriodonts chewed more than dinocephalians, not more in any modern sense. Modern mammals, especially those with molars, are still tops in the chewing department." He rubbed the back of his neck, looking a little overwhelmed. "Speaking of modern mammals, you saw how comparatively fast the thing was?" The class nodded vaguely. "That's from internal temperature regulation again. The theriodonts were another step in the warm-blooded direction." (7)

In the basin, Inostrancevia finally realized that its prey had eluded it. Honking a snort of defeat, it begrudgingly extracted itself from the mud and trotted off. The class let it disappear into the foliage before raising more questions.

A student who hadn't spoken before raised a hand. "Is that, I mean, the temperature regulation, why they eventually got fur?"

The professor nodded. "I would guess so. Although we can't know for sure when or where or even exactly how. For example, some paleontologists depict Inostrancevia with fur. I don't think it appeared that early on, so I didn't."

"Yeah. You made them purple," she noted dryly.

"I like purple," he defended.

The class had no further questions. Most were too preoccupied with examining the underbrush for concealed predators.

"Right then," Sanderson declared. "Since I guess we're done here, it's time for us to move to our final stop on this field trip... the closest therapsid-mammal ancestor we'll be seeing today. This guy is pretty famous, so try to remember him."

He looked markedly relieved as he raised his hand.

**Click.**

The forest was dense, coniferous, and dark. Clumps of waxy ferns splayed upwards between mounds of rotting logs. The air, though cool, had a spicy humidity. The class was standing tightly-packed in the pit of a deep gully. Sanderson perched nearby on the arm of a fallen tree. He was gazing intently at a spot about ten feet away.

Beneath the shadow of a gutted tree-trunk lay a squiggling mass of stiff fur. The animal, which lay serenely on its side, was house-cat sized, with a mole's pinpoint head and a dog's protruding claws. Four or five smaller tufts of fur were tugging at its middle: pups, nursing.

"This little family," introduced the professor quietly, "belongs to the species Dvinia prima. They're one of the most advanced type of therapsid, from a group called the cynodonts. They're classified as theriodonts as well, although they're obviously a lot more advanced than Inostrancevia. Cynodont means 'dog-tooth,' by the way," he added, catching the pensive look on one of the Latin major's faces. "They're our most direct Permian ancestors." (6)

"So this is still the Permian?" a student whispered.

"Probably," Sanderson answered. "Dvinia lived in both the late Permian and early Triassic. This could be either, although I'm inclined towards Permian, since there's still a lot that looks alive around here."

The student crinkled her nose in preemptive distaste. "What do you mean, *alive*?"

"The Permian extinction. Wiped out 70% of land species around 251 million years ago. (8) I probably would've simulated a lot less diversity if this were the early Triassic, even plant-wise." He tilted his head to gaze upwards into the forest canopy. "But I sort of like trees, so I stuck with Permian. Dvinia lived in this sort of cool forest, anyway."

Her nose remained crinkled. "They look like rats," she said.

Sanderson nodded acquiescingly. "They do. But they're not mammals yet. Very, very close, but no cigar. Later cynodonts like Dvinia here have most of the main traits we call mammalian. Fur, most obviously. Diversified teeth, for chewing. Almost exclusive endothermism. And a bigger brain, although they're still pretty dull compared to modern mammals." He paused to smile fondly at the little family. "They also nurse their young." (7)

The "rat"-comment girl was deadpan. "You think they're cute, don't you?"

"I do."

"You *are* Jai."

He blinked, puzzled, but opted to ignore the comment. "Most of the differences that separate Dvinia from 'true' mammals are trivial," he continued. "Things like a fused brain-case, the arrangement of the middle ear, sweat glands, etcetera." He ticked the traits off on his fingers. "It gets pretty hard to tell the difference from here on out. Externally, it's near impossible." (7)

One of the nursing furballs raised its snout in a miniscule yawn. "Aww," murmured the class, save the girl who had termed them rats, who scowled.

The student who had questioned endothermism waved her hand for the third time. "So if the differences between these guys and mammals are so trivial, how are we to say what's a mammal and what's not?" she asked. "I mean, I read in my college seminar that some scientist in the 1700's coined the term 'mammal.' How do we know it's not just an arbitrary grouping?" She waved a hand at the Dvinia family. "How are we to say that those aren't mammals?"

Sanderson shrugged, smiling. "I don't know. They certainly elicit the same response a group of baby mammals would. I mean, they're so cute! Our emotions identify them as mammals. Science says they're not. Who's to say who's right?" He turned to the bundle of tiny furballs, still grinning. The mother Dvinia, blissfully unaware of his presence, flicked one fleshy ear and wiggled to better accommodate her children.

The student mimicked the professor, shrugging. "Does it matter?" she asked.

The class seemed to mull this for some time. "I don't think so," another student finally answered, quietly. It was the girl who had called the theriodont an "ugly hairless dog." "I mean, we're going to say 'aww' at both these guys and puppies, and 'ugh' at those Inostrancevias, no matter what science tells us. I think scientists only make those distinctions because they're interested in how we got where we are. They just need some method of labeling the steps."

Sanderson shifted on his log, then nodded. "Works for me," he asserted. "I mean, that's basically what I meant to show you all today: label, and see, some of the steps on the way towards what we call mammals." He craned his had back towards the class. "Did I do ok? How did you all like it?"

Most of the class gave perfunctory nods. A few were still swooning. The remainder were still cooing over the Dvinia family.

The professor's thin face relaxed into a smile. "That's great!" he enthused. "Because tomorrow, how cool is this, we're doing Lepidosaurs!" (5)

The response this time was slightly less enthusiastic. Sanderson didn't seem to notice.

"Well, that's it for today then," he concluded sunnily. "I'll see you all tomorrow at 10 a.m. Class dismissed!"

He raised his hand.

**Click.**

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References

Works Cited

1) Palaeos, a great all-around paleontology site with a bunch of great graphics

2) The Therapsids!, an illustrated tour-de-force of the therapsid group

3) Pelycosaurs!, all about Pelycosaurs

4) The big bad Inostravencia, what it sounds like...

5) The Permian Slide Show!, a cute, concise, illustrated slide show of predominant Permian/Triassic life

6) BBC, the BBC's take on the Permian world

7) Theriodonts in Detail, a huge bunch of technical jargon concerning theriodonts with a few laymen's terms thrown in for good measure

8) The Permian Extinction, an overview of the Permian extinction


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