It is probably safe to say that to many, the idea that dinosaurs were slowly in decline, or that they weren't 'evolving' as fast as they should have been, is an uncomfortable thought.
I thought some context will make this idea easier to appreciate.
Palaeontologists are generally happy to say that Velociraptor is a very close relative of birds. It is morphologically very similar to birds, and there is even strong evidence that it possessed secondary feathers on its forearms (Turner et al., 2007. Science 317: 1721).
Velociraptor lived during the Campanian Stage of the Late Cretaceous, 83.6 - 72.1 million years ago (Ma).
On the other hand, the oldest bird known to date, Archaeopteryx, is known from the Tithonian Stage of the Late Jurassic, 152.1 - 147.7 Ma.
So that's on average about 72 million years (Myr) between these two dinosaurs, with a maximum of ~80 Myr and minimum of ~64 Myr.
We Homo sapiens are on average about 78 Myr apart from Velociraptor in time. That's about the same amount of time that separates Velociraptor from Archaeopteryx.
However, look how far mammals have come in that time from our rat-like mammal ancestors. We had less time to evolve than Velociraptor did from its last common ancestor with Archaeopteryx, yet mammals radiated into virtually every ecological niche conceivable, including flighted bats, giant terrestrial elephants, insectivorous anteaters, hypercarnivorous cats, deep-sea cetaceans, eusocial naked mole rats, bipedal tool-making humans, etc.
Compare that with Velociraptor, which basically looks just like any other big dromaeosaur, and very similar to Archaeopteryx - Velociraptor hadn't evolved much.
This goes for dinosaurs like Tyrannosaurus as well - despite its colossal size, the rate at which tyrannosaurs evolved in size is as one would expect given the amount of time they've been around (this is based on unpublished results from our lab; but also with respect to their biting performance).
So remember, the next time you talk about dinosaur evolution, keep in mind the passage of time - dinosaurs have been around for a very very long time, and what may seem like extreme specialisations can, more often than not, be explained by the passage of time.
I thought some context will make this idea easier to appreciate.
Palaeontologists are generally happy to say that Velociraptor is a very close relative of birds. It is morphologically very similar to birds, and there is even strong evidence that it possessed secondary feathers on its forearms (Turner et al., 2007. Science 317: 1721).
Velociraptor lived during the Campanian Stage of the Late Cretaceous, 83.6 - 72.1 million years ago (Ma).
 |
| Velociraptor from Wikimedia Commons |
On the other hand, the oldest bird known to date, Archaeopteryx, is known from the Tithonian Stage of the Late Jurassic, 152.1 - 147.7 Ma.
 |
| Archaropteryx as a Blue Jay |
 |
| Archaeopteryx with red plumage |
So that's on average about 72 million years (Myr) between these two dinosaurs, with a maximum of ~80 Myr and minimum of ~64 Myr.
We Homo sapiens are on average about 78 Myr apart from Velociraptor in time. That's about the same amount of time that separates Velociraptor from Archaeopteryx.
However, look how far mammals have come in that time from our rat-like mammal ancestors. We had less time to evolve than Velociraptor did from its last common ancestor with Archaeopteryx, yet mammals radiated into virtually every ecological niche conceivable, including flighted bats, giant terrestrial elephants, insectivorous anteaters, hypercarnivorous cats, deep-sea cetaceans, eusocial naked mole rats, bipedal tool-making humans, etc.
Compare that with Velociraptor, which basically looks just like any other big dromaeosaur, and very similar to Archaeopteryx - Velociraptor hadn't evolved much.
This goes for dinosaurs like Tyrannosaurus as well - despite its colossal size, the rate at which tyrannosaurs evolved in size is as one would expect given the amount of time they've been around (this is based on unpublished results from our lab; but also with respect to their biting performance).
So remember, the next time you talk about dinosaur evolution, keep in mind the passage of time - dinosaurs have been around for a very very long time, and what may seem like extreme specialisations can, more often than not, be explained by the passage of time.
Comments
I have always followed the general idea of punctuated equilibrium. You don't see radiations or major faunal replacements (in my admittedly biased and limited selection of topic research) until there are major extinction/ecological changes requiring a filling in or replacement of "jobs" in nature (as I teach me kids).
So while you can point to Tyrannosaur or Maniraptorid slowly changing, overall the late Cretaceous was stable, and only changed on the whole slowly thus causing slow changes in organisms (till flowers take off anyways). Meanwhile simplifying mammal evolution to rapid form changes without taking major ecology and climate changes into account also presents a false picture too. The Cenozoic has seen some crazy and extreme climate/ecology shifts in its 65 million year run. Not to mention a complete wipe out of the large animal environmental "jobs"... We'd expect major changes and unpredictablity. Much like the Permian to Triassic and from the Triassic to the Jurassic. We're only entering the Jurassicish point of the mammal reign... Dinosaur stabilized a lot after the mid Jurassic ;)
Always a pleasure to read your comments!
In this blog post I didn't go into the extrinsic effects on evolutionary rates (whether trait or speciation), but that's kind of implied in some way. And yes, the Mesozoic was a massively stable time period so dinosaurs 'didn't need to change' to keep up with changing environments.
I'm trying to highlight the immense time scales involved and how much dinosaurs hadn't changed during the latter half of their reign. So if you keep that in your mind, then the idea that speciation rates decreased to the point that it got overtaken by extinction rate = dinosaurs in gradual decline, would not be such a drastic and frightening thought. It's almost a given if you take the passage of time into consideration.
I'm in the middle of teaching my kids the rough biology of vertebrates (I'm beefing up the pathetic textbook with my own stuff), and the theme I find is when I go over my materials (which again are admittedly limited and biased) is that Reptiles and Mammals seem to be more dynamic in their evolution in the Permian and periods of the Cenozoic due to more major global shake ups, and Dinosaurs are only trubulent in a few odd patches due to a more overall stable patch of deep time (I'm finding ocean shrinking or expansion are a common theme is turbulent times).
Gould does a good job selling punctuated equilibrum at the popular level (and I'm only at the technical level with a few areas)... so I know that theory has a real advantage in my own head space. How much merit do you feel punctuated equilibrum has? Obviously probably not to the extremes Gould argues in a Wonderful Life (given all the Burgress fauna have since been shown to actually below to existing phylum), but do you feel it holds some power to explain trends of long term evolution?
A pleasure as always. Was thinking back to that sketch of Centrosaurus of yours that launch ART Evolved the other day. Good times ;) Hoping the academic life is treating you well, and I'm sure I'll pesture you as often as you post :P