Monday, June 16, 2008

Triassic Park

The more I read about it, the more I find the Triassic fascinating. When it started, 251 million years ago, the world was still reeling from the single most devastating mass extinction ever: "the day the earth almost died." For millions of years afterward—the entire Early Triassic epoch and possibly much of the Middle Triassic—the entire biosphere can be fairly called "impoverished." Some therapsids bounced back—Lystrosaurus was an early Triassic animal that's well known for it's astonishing recovery, and other dicynodonts seem to have followed in short order. Carnivorous therocephalians and cynodonts spread early as well. One very interesting development was the rise of the thecodonts. (I realize that under cladistic nomenclature, Thecodontia no longer exists. However, its often necessary to refer to paraphyletic groups and even paleontologists do it all the time. I also don't like having to use cumbersome technical jargon when an existing term works quite well. So rather than say "non-dinosaurian and non-crocodilian archosauriomorphs" I still use the word thecodont. I use therapsid and other terms the same way. Trust me: its better than the alternatives.)

Very early on, big-headed "red crocodiles" like the erythrosuchians became the apex predators of the early Triassic. Because environmental conditions were similar, this may have been like in the Australian Pliocene when pristichampsian crocodiles and giant komodo dragons became apex predators, lording it over the mammals. Very likely, these large thecodont predators were cold-blooded as well, although many of the therapsids may no longer have been.

Anatomical clues suggest that the thecodonts may have been developing warm-bloodedness throughout this time as well. By the time they became true archosaurs, they may well have already been well on that path, since crurotarsians and ornithodirans both show anatomical evidence associated with warm-bloodedness. Certainly later ornithodirans had it; birds, dinosaurs and pterosaurs.

Modern crocodilians are a strange case, and throw a potential monkey wrench in the idea of warm-blooded archosaurs since they are a fairly late group and are clearly cold-blooded. However, they also have several anatomical adaptations that serve no purpose for a cold-blooded creature, and Triassic crocodilians have even more such adaptations (fully erect gait, active terrestrial lifestyle, etc.) Most likely, as crocodilians inherited the semi-aquatic ambush predator role from their cousins, the spectactularly misnamed phytosaurs, they lost their warm-bloodedness as a result.

In fact, that constant state of transition is characteristic of the Triassic and what makes it so fascinating. During the Jurassic and Cretaceous, individual species and genera may have had a relatively rapid turnover, but entire dynasties did not. We see, for example, a steady, uninterrupted progression from primitive ornithopods through iguanodonts and on into duck-billed dinosaurs; an arc that lasted well over 100 million years and that was full of highly successful animals throughout. While this is probably the most extreme example amongst dinosaurs, the basic pattern is visible throughout—dinosaur dynasties were long-lived. Crocodilians even have them beat; they took over the semi-aquatic ambush predator role at the very beginning of the Jurassic 200 million years ago and still have the niche locked up.

This was not true of the Triassic. Looking at the big picture, of course, the Triassic is the story of the gradual replacement of therapsid-held guilds with thecodont-held ones. As the Triassic wound down, both megadynasties produced their best designs: specifically mammals and dinosaurs respectively. The dinosaurs inherited (or wrested control of) an empire that their thecodont ancestors had built over the course of less than 40 or so million years and held it for over 160 million years while the mammals—who's ancestors had been so dominant since the Permian—lost theirs and had to bide their time until the post-Cretaceous extinctions opened the door for them to spread again.

But on the smaller scale, dynasties turned over with astonishing frequency during the Triassic. The semi-aquatic ambush predator guild was held by presterosuchids and champsosuchids in the early Triassic, who gave way to the only distantly related phytosaurs by the mid-Triassic, just a few million years later, who themselves ceded it to the crocodilians by the end of the period. Apex predators at the beginning were erythrosuchids who ceded to rauisuchians (themselves possibly a polyphyletic group) and ornithosuchians who moved over for therapod dinosaurs. Large herbivores were initially dicynodonts who faded quickly to be challenged by aetosaurs and others, who in turn were challenged by the rhynchosaurs, who in turn were replaced by revueltosaurs and silesaurs, which were replaced by true dinosaurian prosauropods and ornithischians.

Everywhere you look during the Triassic, we see churn and change. Entire dynasties rise up only to last a few million years before going extinct and being replaced by unrelated (or at beast very distantly related) new ones. There's also still a lot we don't know about the Triassic. The relationship between various thecodont groups is poorly understood and entire clans like the revueltosaurs and silesaurs were completely unknown and unguessed at as recently as five years ago—their importance to the larger faunal picture even less time than that—at most a year or two.

Although compared to the gigantic and dramatic fauna of later dinosaur eras the Triassic seems understated, in my opinion its got a unique and very compelling charm all its own.

1 comment:

Will Baird said...

Hi Joshua!

It's always kewl to run across some of the more obscure sections of Deep Time in the blogosphere. I've written a tad on the paleo arena.

You might want to try submitting your post to the . The point man for the carnival is Brian at .

Just a thought.