Monday, April 13, 2009

Expansion of dinosaur ages spreadsheet

A while ago, I found myself increasingly frustrated by the inability to find a good chart that showed all of the divisions of the Dinosaur Age (The Mesozoic). Even when you did find such a chart, it was invariably "upside down"; that is, it started from the most recent age and went backwards rather than starting from the beginning and going forward. That makes some sense, I'll grant you, since these are all brought to us by rock stratigraphy, but from a chronological perspective, I didn't like it. Plus, just knowing what those ages were and what their cut-off dates were, wasn't all that helpful without some context. So, my chart had all of the subdivisions, starting from the earliest Triassic going down to the end of the Cretaceous, with notes on which formations feature in each age. After all, the whole point of this was to figure out which dinosaurs (and other animals) were likely to have crossed each other's paths on the morning commute, right? I've found this increasingly helpful to me over the years with understanding the global context on dinosaurs, where they came from, how they evolved and spread, and whatnot.

However, it also occurred to me that to greater enhance its use as a tool for context, to say nothing of the fact that lately I've found pre-dinosaurian tetrapods more interesting than actual dinosaurs (probably due to my own personal familiarity with dinosaurs---thecodonts and therapsids are like the exciting mistress that I'm still discovering, I suppose) I recently went back and added the Permian, Carboniferous and even the Devonian ages to the chart as well.

I could have gone back and added the Silurian, Ordovician and even the Cambrian too, but the Devonian, the so-called "Age of Fishes", is a good starting point for a number of reasons. The earliest Devonian is when vertebrates actually became an important and significant faunal component overall. Vascular plants had started some tentative colonization of terrestrial environments in the earlier Silurian period, but it was really the Devonian where land plants came into their own, and the first forests started spreading across at least the wetter terrestrial regions. And, at the end of the Devonian, the first tetrapods evolved from some lobe-finned fish; guys like Acanthostega and Ichthyostega took their first shuddering steps out of the water and onto the mud.

That said, it's really after the first diapsids and synapsids appeared in the Carboniferous that things started getting interesting. The Permian seems like a transitional period; one that perhaps doesn't actually belong thematically in the Paleozoic after all. From the Permian on down, the history of vertebrate life on land is the history of the dynastic struggles of the synapsids vs. the diapsids. The synapsids took an early lead, but lost ground steadily after the Permian-Triassic extinction event (the worst in the history of the world) and were displaced completely by the diapsids before the Jurassic even arrived. The diapsids' major success was in the run-up to, and subsequent domination by the dinosaurs. The degree and longevity of their reign is somewhat startling, really.

Of course, as we all know, it didn't last either. The diapsid dominance came to a screeching halt at the Cretaceous-Tertiary boundary and the synapsids put their newer models, the true mammals, up in a bid for a hostile takeover that seems to be going strong still today. It's interesting to note that if you count the Cenozoic Era plus the Permian period as times when synapsids dominated the terrestrial megafauna together (113 million years), they still don't equal the amount of time that the diapsids dominated the terrestrial megafauna during the Jurassic and Cretaceous periods (134 million years.) We've got a ways to go if we want to prove that we're even the equal of dinosaurs in that regard. Note: I didn't count the Triassic, because it was a contested period of just over 50 million years, but if I did, I'd probably have to give most if not all of it to the diapsids anyway. The apex predators of the early Triassic were archosauromorphs like the eurythrosuchids, and by the middle of the period, diapsids were knocking on the doors of, if not already dominating, almost every large-bodied animal ecological niche, while the increasingly mammal-like therapsids were getting smaller, and smaller, and more and more migrating into small-bodied, nocturnal animals rather than contenders for the sexy positions of apex predators, large-bodied herbivores, or even medium sized animals, for that matter.

What is the secret of the diapsid success during the Triassic, anyway? Nobody's really sure. Part of that hinges on the still mysterious, yet tantalizing question of the physiology of the earlier non-dinosaurian therapsids and thecodonts. Were they warm-blooded? If so, to what degree? In any case, this is exactly why I've found the Triassic to be one of my most fascinating periods of life on earth. It starts off with a world in recovery, after the greatest natural disaster (still!) in the history of our planet, and throughout the Triassic, environmental conditions were difficult. Even so, the rapid turnover of dynasties as one group of animals after another put in their bids for supremacy in guild after guild of large-bodied terrestrial and semi-aquatic ecological niches, only to be (relatively) quickly turned over and replaced by a new king on the hill is fascinating to reconstruct. Looking at this broader context, the story of the Triassic, and the story of the diapsids and synapsids as a "big picture" dynastic conflict, is one of the most interesting I can think of in the natural history of our planet.

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