Features/Mesozoic

From Sedes Draconis

Dinosaurs and the New World Order

Collected Ramblings on the Mesozoic

As a child, dinosaurs were never a great interest of mine. Perhaps I was jaded, after all, dinosaurs were something small boys were supposed to be interested in. But many years later, when I had come to paleontology by way of science fiction (the ancient faces of the Earth being for the most part stranger than any world I found in fiction), I rediscovered dinosaurs.

I'm still somewhat scornful of interest in the "classic dinosaurs": the great sauropods, Stegosaurus, Tyrannosaurus rex, and the like. But the world of the Mesozoic is a compelling and alien world. And other dinosaurs, such as oft forgotten ornithopods (esp. the hadrosaurs) who, to my mind, really represent the pinnacle of dinosaur evolution, are amazing animals. They are the "other us", like the Neanderthals. The line that reached similar heights as ourselves, but in different ways. The advanced alien that provides insight into ourselves and our world.

So now, I carry around a great deal of information in my head about dinosaurs and the Mesozoic. Having been immersed in story-telling my whole life, I often express these in a kind of informal narrative. These are the stories that I tell best and I enjoy casting them in narrative form. That, and I take great delight in using phrases like the Shark Renaissance; and I describe the rise of the flowering plants alternatively as "the Floral Revolution" and the "New World Order".

I was thinking about writing up one of these story-arcs, but decided that it would be repetition of things I had written before. So I collected some previous pieces instead: a few stories I have written about the Mesozoic, collected from some of the internet forums I write on. (Minorly edited for information quality.)

2002.01.20, Wikipedia (Dinosaur:Talk)[1]

Written in the context of a discussion about whether it was accurate to say that the Dinosaurs went extinct in the K-T mass extinction because they were less adaptable than mammals.

Okay, here's my knowledge/reasoning on that:

In the Permian, the dominant phylogeny of large land animals was the Synapsida, (pelycosaurs, in particular) the ancestors of mammals, or in the case of the pelycosaurs, great-uncles more like. The end of the Permian was marked by the Permian-Triassic extinction event, largest mass extinction known, in which it is estimated that more than 90% of all animal species went extinct. Certainly at least part of the cause was the formation of Pangaea at that time and the climatic change that would have been caused. Because a) the land fauna had been devestated and b) there was only one continent, the very early Triassic had pretty much only one set of land animals that was found all over the world. A little later several new groups arose to move in the gaping holes. The foremost among these (again only speaking of terrestrial animals) were the crocodiles, the dinosaurs, and the mammals (and their close relatives among the therapsids).

In the middle of the Triassic, another mass extinction, wiped out the major contenders from Crocodilia and Mammalia. This left the dinosaurs to inherit all the Earth. Since Pangaea was still all together they spread across the whole continent, leaving no place a competing group might be able to develop from. So the dinosaurs were extremely succesful and utterly dominant as land animals for the 150 odd million years, while the crocodiles and the mammals hung on on the margins by staying out the way of the dinosaurs, concentrating on semi-aquatic ambushing, and nocturnal insectivory, respectively.

The Dinosaur species were thus able to monopolize abundant resources and grow big, and specialize. The other groups, did not have access to the abundant resources and had to remain flexible enough to utilize anything that came their way, and generally stayed rather smaller and/or slower metabolismed than the dinosaurs.

Thus when the disruption that caused the Cretaceous-Tertiary extinction event, probably significantly including the Chixculub meteor, the dinosaurs fared worse.

That is, the dinosaurs, being succesful, were able to be phylogenetically adaptable, which made them individually more conservative.

The crocodiles and mammals, on the other hand, having lost out in the Triassic-Jurassic lottery, had never had the opportunity that the dinosaurs had to be phylogenetically adaptable and diverse, had retained relatively conservative body plans throughout the Jurassic and Cretaceous, but had to be individually more adaptable in more precarious niches than those enjoyed by many dinosaurs.

Which is all to say, yes there is a kernel of truth to the statement, and yes it does need considerable modification.

2002.11.22, ZBB (Solitary Intelligence)[2]

Written to clear up questions about the meaning of "reptile" and the relationships between various groups. Contains some repetition from the previous piece.

One of the big problems in understanding the relationships at this level is that word "reptile". Basically if you going to look at phylogenetic relationships, forget you ever heard it. "Amphibian" is almost as bad, but in different ways.

So, you start with amniotes, tetrapods ("four-feets", as distinct from fish) that have water-retaining skin, and water-tight eggs, which distinugishes them from their non-amniote tetrapod ("amphibian") ancestors. Often the tetrapods are called "the invaders of land", but the amniotes are the "conqerors of land", that were able to truly break their dependance on standing water. (The millipedes, by the way, were the first land animals, and made it onto land as much as 100 million years earlier than the tetrapods.)

Three major branches of the amniotes develop, which are named for the patterns of holes ("fenestra") in the skulls: the anapsids ("no windows"), the diapsids ("two windows") and the synapsids ("joined windows").

The anapsids are the most primitive in a lot of ways, and they haven't done so well in the last 300 million years. The only living anapsids are turtles. Except, I think the icthyosaurs might have been anapsids. They did very well during the mid-Jurassic.

The synapsids gained some early dominance in the Permian, when pelycosaurs (a kind of synapsid) were among the biggest, meanest animals around. Other synapsids were among the major herbivore groups. A few synapsids, notably the cynodonts, started specializing in hunting small animals at night. Mammals developed from these cynodonts. I can't remember who all is part of the sub-group "therapsids", but the cynodonts (and thus the mammals) definitely are.

Then there are the diapsids. There are two major branches of the diapsids; the archosaurs and the lepidosaurs. The lepidosaurs ("scaly reptiles"), the lizards and snakes, are relative latecomers, only really starting to become as diverse as they are today in the last, oh, 80 million years about, IIRC.

Then there are the archosaurs ("ruling reptiles"). The main early distinction in archosaurs which sets them apart from other diapsids, is their hip arrangement. They moved their legs more directly underneath them, which allows for faster movement in large animals. Crocodiles have since mostly gone back to the primitive, sprawled posture, but they can get up and gallop if they want to, believe it or not. In fact several close relatives of the crocodiles were bipeds, or scampering, lightly built quadrupeds.

So then came the Permian-end extinction, the only Class 1 mass extinction known, something 98% of all animal species went extinct, and hit even worse on land than in the sea.

This cleared the stage and began the next era of life, the Mesozoic. In the early Triassic at the beginning of the Mesozoic, there were three main contenders for the dominant large animal position. Part of the cause of the Permian exinction was the formation of Pangaea, so it was an all or nothing deal, no seperate continents to split up, like the mammals and birds did at the end of the Mesozoic.

The three contenders at the beginning of the Mesozoic were the dinosaurian archosuars, the crocodylomorph archosaurs, and the cynodont-mammal therapsids.

Then came the mid-Triassic (Carnian-Norian) extinctions, during which the dinosaurs won out, and the crocodiles and mammal survivors of the other groups were relegated to peripheral niches, semi-aquatic ambushing, and nocturnal insectiovry, respectively. About another 40-50 million years later birds developed from dinosaurs.

This is really interesting because we're often taught that the mammals replaced the dinosaurs because they were more advanced in some way. But really mammals are still around because when we went up against the dinosaurs in a fair fight we lost, which required us to stay small and generalized, which later let us survive the KT mass extinction, when all the non-avain dinosaurs, who had become fat and sassy in the intervening 160 million years, went extinct. (Where "fat and sassy" means "more invested in the status quo".)

The pterosaurs are close relatives of the dinosaurs. Best guess to date is that they are descended of the lagosuchids ("rabbit-crocodiles"), almost the nearest outgroup to the dinosaurs. But it's hard to tell, since we have basically no specimens of transition pterosaurs.

One, bizarre little bit of information, that confused me for a while when I was first learning this stuff. The dinosaurs split into two major branches, the saurishians, the "lizard-hips"), and the ornithischians, the "bird-hips". But the birds evolved not from the bird-hips, but, as Glenn notes, from the lizard-hips. Odd. Well, much later one branch of the lizard-hipped dinosaurs developed bird-like hips, too; truly bird-like hips in this case (being the same bird-like hips as their descendants the birds have), rather than the approximately bird-like hips of the ornithischians. (Bird-like hips being hips in which the pubis bone points backward).

Icthyosaurs were another branch of anapsid, if I recall. And the plesiosaurs are part of the nothosaur line of marine reptiles that split off the line leading to the lepidosaurs, but pretty soon after the split with the archosaurs. So they're "lepidosauromorphs". Icthyosaurs and plesiosaurs and pliosaurs were the major big marine carnivores of the Mesozoic, what whales, sharks, and the big seals do now. The sharks regained some of their prominance in the Cenozoic, after the great marine reptiles went extinct, but they still are nowhere near their former splendor in the mid-Paleozoic, say, 300-400 million years ago.

2002.12.15, ZBB (Solitary Intelligence)[3]

Written in response to questions about the relative smallness of modern animals. Only the parts relevant to the Mesozoic are here re-published.

But back in the Mesozoic, the age of the Archosaurs, these were bigger. I think an important part of it is momemtum and escalation.

By momentum I mean, there're always small animals around, and when something wipes out the big animals, there're already the small animals. But it takes along time for animals to get big again. And big animals are more invested in the status quo, which is why, as I said above, large scale climate fluctuations favor smaller, generalist animals.

By escalation I mean, one herbivore will get exceedingly large for reasons of its own, and that will start pushing both the other herbivores and the carnviores around it to start getting big, too. (At least some of them, as I've said there're always small animals).

Land animals, at least, were in the process of de-escalating in the Cretaceous, but de-escalation is, of course, always a slow process. But that's later in the story. First why things got so big in the Jurassic. I came across a really brilliant theory a little while ago, which I think has alot of merit, on this topic. Basically it had a lot to do with the plantscape of the Triassic and the Jurassic.

These were quite dry eras, and the dominant plants of the wetter Permian, just before, such as the cycads and the tree-ferns, faired poorly. Flowering plants wouldn't come along until later, but early flowering plants wouldn't have done well in these times, either.

What did do well were the gymnosperms, which is primarily to say the conifers. Conifers were the only thing abundant enough to feed big animals. Well, maybe ferns, too; but not to the degree that some people have suggested, is my opinion. The problem is that conifers are slow-and-steady growers, as opposed to the riotous explosions the later flowering plants can produce, or even to a lesser extent, the early cycads. This was in fact closely tied to why conifers did better in the dryer world of the Triassic and Jurassic eras.

But being slow growers meant an individual tree couldn't afford as much of its biomass geetting eaten, as a faster growing plant could. So conifers take a very hostile approach to animals; in short, they make terrible eating. The only way for a decently-large animal to make a living of conifers, is to be indecently large; because they must not only reach the branches the conifers are trying to hold up out of reach, they must also eat a huge amount of the rough, poor-nutrition leaves, and have an enormous gut to be able to digest them well enough to make any of it worthwhile.

Thence came the sauropods. Sauropods were the biggest land animals that ever lived, and structural calculations show that some of them very nearly approached the maximum possible size for a Earth-walking quadruped. At 140 tons, the differences in scaling volume to surface area no longer allow a quadruped to walk. (A biped of course would be cut off much sooner, a hexapod would presumable be able to get a little bigger, but it's never been an issue on Earth). And some of the sauropods like Ultrasauros have been estimated to have been pushing that limit, at weights of over a hundred tons.

So plants had it rough in the Jurassic, which meant they did their best to fight off animals. Which meant the dominant herbivores of the Jurassic were, perforce, enromous. There were also no very large herds (in numbers) in the Jurassic. suaropod herd sizes were probably never more than twenty or so in the Jurassic. All evidence of very large congregations of dinosaurs come from the mid- to late-Cretaceous, after what I think of as the New World Order kicked in. Which is to say, the flowering plants.

90% of known dinosaur diversity comes from the Creataceous, too. Some of that's maybe a sampling artifact: we've just found more dinosaurs from the more recent time period, but most of it has to do with an enormous shift in the way that plants and herbivores interacted.

So then you hit the Cretaceous, and it starts getting wetter again. After a while the fast-growing angiosperms (flowering plants) come along to take advantage of this. Angiosperms could be more forgiving of herbivores, because they could grow foliage back rapidly enough that it made sense for them to direct more energy into growth and less energy into fighting off any herbivore that came their way. Angiosperms also had the advantages of rapid reproduction, both sexual and vegetative (sending out root systems and putting up new shoots from them).

In fact angiosperms actively court animals. First pollinators to their eponymous flowers, but they also extended it to fruit, and many angiosperms even grow better if they are periodically browsed. (Particularly grass, but grass didn't come along until well into the Cenozoic).

So the best strategy for an herbivore was not that of a "whole-plant predator" like the sauropods who simply stripped whole branches at a time, but rather a much choosier strategy of going in and picking out the best part of the plant to eat.

Directly and indirectly this leads to smaller, nimbler, more numerous, more diverse, more intelligent herbivores. And the carnivores, of course, had to follow suit.

The scattered family groups of a score of species of slow, stupid, 20-100 ton sauropods were replaced by enormous herds of smarter, more selective, more social, 1-15 ton Ornithopods (mostly, the "duck-billed" dinosaurs), plus some Thyreophorans and Marginocephalians of similar sizes (best known for Ankylosaurus and Triceratops. respectively). By the end of the Creataceous land herbivores had reached sizes that were much more similar to those of today, or at least those of the Pleistocene. By that time even the few remaining sauropods were a fraction of their previous sizes.

There were still land predators bigger than anything we have today, but I think they would have soon followed suit, if the whole point hadn't been made moot by a 10km meteor, and some massive volcaninc activity that may or may not have been triggered by the meteor. And the dinosaurs paid dearly for their success, while the mammals (and crocodiles) they had beat out 180 million years ago were the meek that inherited the Earth.

Actually given how much the dinosaurs had changed over the Cretaceous, it astonishes me that not a single non-avian dinosaur survived. Sure, the sauropods were lumbering, obselete beasts; but not enough credit is given to the Ornithopods. Ornithopods were where it was at in the late Cretaceous, and they were modern animals in most respects. I am truly astonished that not even a few hypsilophodonts survived the K-T event.

So there you have it. I'm not much good at telling stories about people, but I like to think I tell a mean account of the Mesozoic.

That Solitary Intelligence thread, it was a blast. The main point of the thread was to explore, with my good friend Eodrakken, the designing of a sentient albatross-like animal. I'm rather fond of albatross, ever since spending a fair amount time learning about them as part of my research into the suite of traits that go along with intense pair-bonding. In the course of the discussion, I not only produced these pieces but was also called upon to hold forth on solitary intelligence, pair-bonding, and the size of flying animals, past and present.