Premiering back in 1980, Carl Sagan’s Cosmos led us on an enthralling tour of our universe and took us on a crash course of science. We watched as he strolled around the heavens and our planet as he guided us through the very essence of what it meant to be human in the natural order of things. Sagan was a wonderful guide who was clearly ahead of his time, highlighting such issues as the possibilities of nuclear Armageddon and global warming and there are many quotes by Sagan that are still cited today.
Chief amongst them and most often used by palaeontologists of today, is this one:
“The absence of evidence is not the evidence of absence.”
Most people would tend to agree with this, as would I, and yet should we? On the face of it this is a very strange concept since we are suggesting that we should accept certain issues as a given despite a lack of physical proof. In other words perhaps what we should actually be saying is:
“The absence of evidence is the evidence of absence.”
Let me explain what brought this on. The well documented, and almost universal, criticism of the makers of the already much hyped Jurassic Park 4 for not having feathered dromaeosaurids is scientifically well founded since we have extensive fossils now of these dinosaurs clearly displaying such coverage. It is bizarre that the producers have seen fit not to feather their raptors and their argument that it is best to maintain a form of continuum from the previous films is a weak excuse. Perhaps it is a blind to throw us off the scent and they will indeed unleash feathered dromaeosaurs but I fear not.
Then last week the trailer for the new Walking with Dinosaurs 3D movie went on line globally and generally met with a pretty good reaction. Scott Hartman, over at Skeletal Drawing, naturally enough featured a post about it since he was involved as one of the anatomy design team for the production. One of the comments on his blog mentioned the fact that there were no shaggy gorgosaurs in the trailer and Scott suggested that the movie world is not quite ready for that yet and I tend to agree with that statement.
In the comments section I pointed out the following:
“And yet, technically, the absence of shaggy gorgosaurs is correct since Tyrannosauridae is still fuzz free for now albeit phylogenetically likely. We cannot really moan about the film makers in this instance because they are depicting tyrannosaurids accurately - in as much as the fossil record permits.”
To which Scott replied:
”That doesn't necessarily follow - absence of evidence is not evidence of absence here. The phylogenetic data is really the only data we have, so it's what we should be relying on.”
Perfectly reasonable comment and yet my initial reaction was to be a little bemused. We can justly criticise film makers for not including feathered dromaeosaurs because there is ample physical proof and yet can we justify criticism of film makers for not including feathered tyrannosaurids when there is no actual physical evidence currently available?
The phrase itself demands closer scrutiny. In effect, and in the simplest of terms, the following shows a classic example:
Many people take glucosamine and chondroitin tablets to ward off the effects of osteoarthritis (OA) and yet there is no solid data that there is any form of relationship between the two ingredients and the prevention of OA. Therefore absence of evidence is clear but, in reality, those who manufacture or believe in the properties of these tablets would simply point out that just because there is no proof currently available it does not mean that the tablets do not work – therefore absence of evidence is not evidence of absence.
Now I confess that this has very little relevance in palaeontological terms but it does show the dichotomy and how easy it is to manipulate the lack of data into possibility – even probability. However, in the case of tyrannosaurids, we have a completely different picture. I suggested that since there is no physical proof of feathers or fuzz in tyrannosaurids then we should not necessarily take it as a given simply because, and as Scott pointed out, the phylogenetic data is highly indicative that they were.
So in this case, does the absence of physical evidence mean that there is no evidence for feathered tyrannosaurids at all? Well of course not – in fact virtually the whole coelurosaurian clade displays integument of one form of another and Tyrannosauridae is currently the last group devoid of physical evidence. Very important here – we are referring to tyrannosaur-ids and not tyrannosaur-oids of which there are indeed animals displaying integument of which the most famous in recent times is Yutyrannus (if, in fact, it is a tyrannosauroid).
So we have lots of phylogenetic data to support the probability of feathered tyrannosaurids but this still needs to be quantified and rigorously tested. When this is done then the dataset will provide a statistical probability of confidence in the hypothesis. If there is confidence in the dataset then you must turn the process on its head and try to prove that there is NOT a probability of feathered tyrannosaurids. This is an extremely important part of the process and lends substantial credence to the theory.
As a result, the phylogenetic signal is highly indicative that tyrannosaurids were indeed covered in some form of fuzz or feathered plumage. And yet for all that detail, all that probability – indeed all of this “evidence” – there is still no proof. And this is where we have to be a little careful since the available data is so compelling that we sometimes forget that physical evidence is required for confirmation.
|BHI 6230 (Wyrex) - No feathers here...|
The very essence of all science is based on this fundamental process and now the onus is on finding fossils of tyrannosaurids that may preserve impressions of fuzz/feathers. There are many specimens already in collections that need re-examining as well as those specimens that may still reside in their jackets and, of course, there are still many wonderful fossils out there awaiting discovery. And, if it exists, then we should be able to find the evidence since our detection techniques are much more advanced these days – especially with UV light.
But, for the sake of argument, what if we never find a feathered tyrannosaurid? What then? I mean we have the phylogenetic data but still no proof. So we will rerun our data, perhaps add more updated information and check and double check – but still no fossil. And then we will question ourselves again since we keep performing the science and keep coming out with the same results. And then if we still find no fossils then what? Perhaps we have to consider the possibility we may be wrong.
But this is unlikely to happen and we will continue to search for feathered tyrannosaurids because they must exist – the phylogenetic data insists it to be true. And this is the point – we must not then fall into the trap of “just because there are no feathered tyrannosaurids, this does not mean they did not exist”.
“Absence of evidence is not evidence of absence” must not become a given in science – not without the proof to back it up. Circumstantial evidence, no matter how strong, can lead to the wrong conclusion and we must all be aware of this. What this lack of feathered tyrannosaurid fossils does indicate, however, is that they are, if they exist, at the very least exceptionally rare but this, again, is not actually proof!
There are so many variables as well – sampling and preservational biases are probably the biggest issues here. It is also worth pointing out that the phylogenetic dataset also has vast gaps in it although there is still a wealth of relevant data available. As usual more fossils are required – especially when you consider how few tyrannosauroids are actually known throughout the Cretaceous.
So “absence of evidence is evidence of absence” is kind of true but, at the same time, is as equally fallible as the other. I accept the near certainty that tyrannosaurids had some form of integument but it is not enough to merely want to believe. In the end we have the data, we have the hypothesis and all we need now is the proof.