Here we have an article about the disappearance of the dinosaurs. Seems we – you know, that we – now know that dinosaurs were petering out for 40 million years before the asteroid/comet wiped them out.
There are a couple of problems with this article and the study it is based on. The first is just silly:
Around 66 million years ago, the sky fell on the dinosaurs’ heads. An asteroid smashed into the Yucatan Peninsula, causing cataclysmic climate changes that marked the end of the Cretaceous period, and killed off some three-quarters of animal species.
Climate change? The way the sentence is structured, one can read it as saying that climate change killed off the dinosaurs. Really? This reminds me of a quip a friend once made, when a report came out saying the World Trade Center collapse on 9/11 was due to substandard steel used in the buildings. He opined that a couple fully-fueled airliners crashing into them also had something to do with it.
Let’s recap that fateful day 66 million years ago: A significant percentage of dinosaurs were pulverized on impact (1); many were crushed or otherwise killed by the debris and shock waves; another chunk were asphyxiated as dust filled the air; another bunch buried in ash; more crushed and drowned by tsunamis; the earthquakes that result from having a large mass smash into the earth’s crust took out some more. THEN many starved as the dust high in the atmosphere killed off what was left of their food supplies. Some tiny percentage that made it through all this then had to deal with a world in which the sun was blotted out for nobody knows how long, trying to find food in a dark and growing cold.
Some creatures, like Coelacanths, who happen to live in caves deep in the ocean, survived. Others were able to travel around easily – birds, say – and could expand the range in which they could look for food. But larger creatures would need to be very, very lucky to avoid all the things that would kill them and still find enough food to not starve. Goodbye all the classic dinosaurs we loved as kids (those that had not died off before the asteroid, that is).
After all this, the post asteroid impact climate was different than the pre-impact climate, because few things can change your climate like a giant chunk of rock hitting your planet at high speeds. Saying the climate killed them is a little like saying blood loss killed somebody who was shot by a murderer – kind of missing the point, even if not entirely untrue.
The second problem is more subtle and ubiquitous if not any less damaging to modern science properly understood. Here’s the abstract from the paper that occasioned the article in the Atlantic linked above:
Whether dinosaurs were in a long-term decline or whether they were reigning strong right up to their final disappearance at the Cretaceous–Paleogene (K-Pg) mass extinction event 66 Mya has been debated for decades with no clear resolution. The dispute has continued unresolved because of a lack of statistical rigor and appropriate evolutionary framework. Here, for the first time to our knowledge, we apply a Bayesian phylogenetic approach to model the evolutionary dynamics of speciation and extinction through time in Mesozoic dinosaurs, properly taking account of previously ignored statistical violations. We find overwhelming support for a long-term decline across all dinosaurs and within all three dinosaurian subclades (Ornithischia, Sauropodomorpha, and Theropoda), where speciation rate slowed down through time and was ultimately exceeded by extinction rate tens of millions of years before the K-Pg boundary. The only exceptions to this general pattern are the morphologically specialized herbivores, the Hadrosauriformes and Ceratopsidae, which show rapid species proliferations throughout the Late Cretaceous instead. Our results highlight that, despite some heterogeneity in speciation dynamics, dinosaurs showed a marked reduction in their ability to replace extinct species with new ones, making them vulnerable to extinction and unable to respond quickly to and recover from the final catastrophic event.
(Full study behind a paywall, darn it!)
The problem in its most general form: you don’t get more certain results by applying statistical analysis to uncertain, speculative data (2). The most well know and egregious example would be the Drake Equation, which purports to tell us something about how common intelligent life is in the Universe, when we have no data to support any of the values used in the equation itself. In other words, *if* we know how often life arises under the right conditions (we don’t) and what those conditions are (we don’t) and how common those conditions are (we don’t) and on and on, *then* we can apply a little math and – voila! – we commit a Sagan and start talking about a universe just crawling with inevitable, mathematically demonstrated intelligent aliens.
So, before we head down this bunny trail, let’s recap what we know about dinosaurs. Not what we speculate, however reasonably, but what we know in any demonstrable sense.
The direct evidence for dinosaurs is almost entirely stones that look like bones – fossils. We think (I certainly think) that these fossils are the evidence of creatures, some of which were enormous, that lived millions of years ago. We can even, usually, make an educated guess as to how old fossils are, within a few million years either way.
The stone fossils are very real, very solid. Sometimes, experts are even able to arrange sets of fossils so that they look like a more or less complete skeleton (3) which are then clothed in imaginative flesh and skin. When we see those impressive reproductions – artist’s impressions, really – of dinosaurs, we would be prudent to keep in mind that it’s all pretty speculative. Sure, it’s all reasonable-sounding: big sharp teeth indicate a carnivore, for example, and it would need to be able to move in such a way as to use them, and probably be camouflaged like modern predators and – and pretty soon, we’ve got semi-upright, beady-eyed T-Rex with tiger stripes running down prey. Which might be right – and might not. Nature is full of stuff that doesn’t show up in a skeleton and doesn’t make any sense until you see it in action – could anyone tell how a skunk or a porcupine or a squid looks and operates from fossilized remains?
This is all peripheral to the issue at hand, except to point out that, while it’s wonderful and good fun to find dinosaur fossils and set up museum exhibits and shoot big-budget fantasy movies about them, in the end, we just don’t know very much about dinosaurs in general. For example, there is no reason to suppose that the fossils we have are anything like a representative sample of dinosaurs and contemporary creatures in general, either over time or at some specific period.In fact, there are plenty of good reasons to suppose that any sample we get is wildly unrepresentative.
It is near-miraculous that any fossils survive. Something like 99.9999% (guessing here – numbers look impressive, don’t they?) of animals that die leave no trace within a very short time. they rot, are eaten and torn apart by scavengers, their bones, if any, strewn and crushed to dust. This is a good thing, too: life gets recycled, or the entire mass of the earth’s crust would need to have been converted to living matter millions of years ago(4). Sounds kind of squishy and smelly…
So, no, discovered fossils are the very, very rare exception: a moment among millions of similar moments that, by some freak confluence of forces, got preserved in stone – and found by a fossil hunter. Perhaps gigantic creatures with huge bones are more likely to get fossilized than smaller more fragile creatures? Or creatures who live on silting mud flats, where they could sink and get buried before they were destroyed? Perhaps there were entire families of species who, for some unknown reasons, left few or no fossil remains?
The proper answer here, as in so much of science, is: we don’t know.
So, keeping that in mind, look back at the abstract:
Here, for the first time to our knowledge, we apply a Bayesian phylogenetic approach to model the evolutionary dynamics of speciation and extinction through time in Mesozoic dinosaurs, properly taking account of previously ignored statistical violations.
Notice what the authors are doing: building a model of evolutionary dynamics using statistics. And, what would be the data set upon which this model is built and against which, presumably, its usefulness can be measured?
Fossils. And the problem is acknowledged, kinda, obliquely, in the article. Another researcher comments:
“The method used in [Sakomoto’s] study requires us to have an accurate dinosaur evolutionary tree, and although we are very happy with the major branches, the arrangements of the twigs at the end is still debated and constantly undergoing change with each new dinosaur discovered.”
Well, are you confident we’ve got an accurate dinosaur evolutionary tree? When considering it’s built on fossils? Are you worried only about the twigs?
Also note that some assumptions are made about what evolutionary dynamic *are* – we are presumably talking about survival of the fittest by means of natural selection, right? Yet:
Our results highlight that, despite some heterogeneity in speciation dynamics, dinosaurs showed a marked reduction in their ability to replace extinct species with new ones, making them vulnerable to extinction and unable to respond quickly to and recover from the final catastrophic event.
Ability? As in, to do something? Dinosaurs as species *trying* to do something yet lacking the “ability”? Not a very sciency way of talking, that. But characteristic of these sorts of discussions, in my experience.
One might wonder why this debate even exists, why dinosaur experts spend their time arguing over whether the asteroid took out a booming dinosaur dance party or merely obliterated the dregs of a dying empire. If so, you don’t understand competitive science very well. Turns out that the shakier the data, the more intense (and, often, vindictive) the arguments. Thus, while nuclear physicists have their disagreements, they tend to be (somewhat) more civil than, say, those of paleontologists. When you get to data-free zones such as sociology, the arguments become more brutal still, mostly because it’s a form of activism masquerading as a science where there is no information that can be used to settle a dispute – you either get it, or you don’t.
On that scale, this debate is a friendly chat – unlike the “wars” between O.C. Marsh and E.D. Cope. But there are other things going on as well, related to that whole Climate Change oddness with which we began this essay:
“This isn’t to say that the dinosaurs were going extinct before the asteroid impact, but they were getting more vulnerable and susceptible to mass extinction,” says Sakomoto. And that’s relevant to us in the modern world. “We are putting a lot of pressure on modern species, and extinctions are happening at an unprecedented rate. If some kind of catastrophe occurs, it might be even more damaging than what we’re observing right now.”
OK, so this is a cautionary tale: just as dinosaurs lost their ability to speciate (whatever the hell that even means) and thus were more vulnerable to giant rocks falling from the sky, we, too, are becoming more vulnerable to giant rocks falling from the sky because, um, we’re mean to the Gaia.
Something like that.
- This is all more or less informed speculation – but then again, so is just about anything one reads about the extinction of the dinosaurs.
- I write here as a non-mathematician, focusing on the conceptual problems inherent in applying mathematical tools to bits of information that are, in themselves, uncertain.
- One would think that such fossils skeletons were fairly certain – that they accurately represent the skeleton of the actual creature. History suggests maybe not.
- A little down and dirty math on data got through Wikipedia: mass of the earth is about 5.972E+21 tonnes; the mass of the crust is 2.18% of the total; mass of life on earth (biomass): 550,000,000,000 tonnes. This makes the mass of living stuff on earth about 0.0000004% of the mass of the crust. Say 10% of the mass of living things on earth die every year – seems reasonable, as the vast bulk of living things are tiny, short lived things like algae, bacteria and insects. Thus, if biomass is roughly stable over time, about 10% new life as measured by biomass would need to be added to balance things out. If life weren’t recycled – in other words, if dead things all got fossilized instead of rotting – then, after a bit under 3 billion years, the biomass + fossils and such would equal the mass of the earth’s crust. This is merely illustrative, like pointing out that, given sufficient churn in the world’s oceans, every glass of water you drink has passed through a dinosaur’s (and fish’s and bug’s and rat’s) bladder many times…