Ahhh, I’ve finally got round to writing part 2 of my project introduction. As a great man once said, hold onto your butts… We’re going straight in to look at Completeness.
So I’ve chatted for a bit about what pterosaurs are and why they’re cool. But what’s this about completeness? To answer this, we first need to talk about diversity and bias.
Diversity is the number of species of one particular genus that exist at any one time. In the modern world, we can assess this by just looking in the wild and counting the number of species of an animal. However, for looking at groups which became extinct millions of years ago we have to use the fossilized remains that we find in the field averaged over the period of time during which they were alive. When you plot the number of species found at particular times over the entire time of their existence you generate a diversity curve like this one below:
This is a famous diagram within the world of palaeontology, generated by Jack Sepkoski in 1981, and is rather intuitively referred to as Sepksoki’s Diversity Curve. It depicts diversity of the major marine fauna from the Cambrian all the way up to the tertiary, showing the number of phyletic families throughout time. The rises and falls in the graph indicate periods of high and low diversity, and as such a higher and lower number of species. As expected, you can see the number of species growing throughout time with a few plateaus of stability. What’s also nice is that the big mass extinctions (extinctions which killed off whole families of organisms) can be recognised by the large dips on the graph. This is a really interesting piece of data to look at for many reasons, and people are still puzzling over what exactly it shows today, approximately 30 years after it was first published. I think it’s a fascinating piece of information, and I’ll likely have a post sometime in the future talking a bit more about it. Watch this space…
So diversity seems fairly easy to discuss. There are however some fairly major problems with the idea of using fossilized evidence as the basis for diversity of extinct creatures. These problems are also pretty difficult to get around as they all stem from the same basic issue: bias.
Bias is something that needs to be considered extremely carefully when it comes to the fossil record. In essence, it’s the idea that there’s always going to be some pre-existing disposition for some features to be highlighted compared to others. For instance, imagine if an alien was sent on a mission to find life on other planets, and landed on earth for the total of 10 minutes in the Sahara desert. This alien finds one insect and decides that’s the sum total of life on earth, and then flies off after his allotted time. Obviously his conclusions are very wrong, but appear correct to him given the parameters with which he was forced to work within. This is like us looking into the past – even counting for the millions of fossils that have been found around the world, we’re really only been given a tiny tiny snapshot into the prehistoric world, and because of that we’re never going to have conclusions that are entirely accurate. However, if we know where this bias comes from, we can try to limit it and at least keep it in mind when we’re trying to get to the bottom of life in the past.
Within palaeontology, bias really comes in two forms that overlap to some extent: natural and man-made. Natural bias is due to factors outside of our control; some examples would be erosion removing potential fossil content, the environment at the time not being suitable to fossilize creatures, and removal of key features from a fossil at a later date. One especially interesting one is the idea of bias within the organisms themselves – for instance, animals consisting of mainly hard-parts, such as bones, are much more likely to be preserved than soft-bodied animals. We find far more ammonites within marine deposits than we do jelly fish or other soft bodied sea creatures, but this is unlikely to be a fair representation of species numbers at that point in time.
It’s also important to remember that bias can effect things positively and negatively. Imagine the same alien scenario I wrote about above, but instead of the Sahara desert, the alien lands in the middle of New York. He’ll come away saying that the planet is entirely populated by humans and a few other animals, and then scoot off. He’ll get a lot of data on us, but again it’ll be somewhat of an unfair sample, especially when you’re considering the whole scope of space and time. This happens within the fossil record as deposits known as Lagerstatten. This nicely sounding word means “the mother load” in German, and are deposits which contain exquisitely preserved specimens, often with details such as skin or feathers which we would not normally find. Many of these Lagerstatten deposits have provided us with an exceptional window into past life which we previously wouldn’t have observed, giving incredibly detailed accounts of numerous species. Famous examples include the Burgess Shales from Canada which have highlighted a huge variety of life during the Cambrian, and the recent numerous areas in China which have provided a wealth of information on feathered dinosaurs. However, these deposits can cause a bit of a nightmare when attempting to assess diversity. Due to the high quality preservation of lagerstatten, numerous new species often of the same family can often be classified within a single deposit. When this is mapped out on a diversity curve, this will show an apparent peak of diversity. However, this is unlikely to be the case; it is far more likely that we are actually being given just a more accurate view of diversity at the time, which is portrayed as an increase. We need to be careful when integrating these kinds of deposit into analyses to make sure what we’re saying about a region is actually true.
And to some degree, that’s where my project comes in. There are ways and means to remove some degree of bias from results (I’ll get to those in a later post), and these are regularly used in diversity studies. However, something that’s been a bit ignored has been the actual completeness of the fossils themselves. Incomplete specimens can be a nightmare when it comes to unravelling diversity; if you’re trying to figure out global patterns and all the records from one time consist of fragments of bone or limited fossil content, it’s going to be pretty difficult to create an accurate representation of what’s going on. This particularly plagues pterosaurs, as by nature their skeletons were pretty fragile to account for the whole flying thing.
My aim is to map pterosaur completeness through time by using a clever new method developed partly by my supervisor Dr. Phil Mannion, called the Character Completeness Metric, or CCM for short, which uses cladistic characters to measure the completeness of a specimen. As I mentioned in my last post here, palaeontologists can construct cladograms to see the relationships between different species. In order to construct one of these, we need to look at features within the organisms which are both shared and unique traits – from these “characters”, we can see how closely species sit next to each other, and what evolved in what order. A set of characters can as such be generated to apply to a whole group to see the relationships between them; some examples for pterosaurs are the length of different bones in the wings, the number and shape of vertebrae, and the measurements and placements of different bones in the skull. These characters can then be marked as being present or not present within a specimen, and this data makes up a character set. Character sets are normally pretty big – mine’s currently sitting at a whopping 183 character in total. Ideally, we need complete specimens to assess relationships using characters – it’s pretty hard to identify relationships between species when half the things you’re comparing against just aren’t there to begin with. Because of this, the CCM measures the percentage of characters which you can actually look at within a specimen – say if your specimen has 40 measurable characters out of 100 present, it would have a CCM value of 40%. This is much better than the previous methods of measuring completeness, which were pretty arbitrary in what they were looking at, and had really coarse and ambiguous values.
With the CCM values of all the available pterosaur species, I can then visualise this value through time, and apply numerous tests to see if anything correlates to potentially show bias- I can look at the number of pterosaur bearing formations to see if there’s a strong bias in collecting species relating to diversity, compare the results I gain to those accumulated previously for sauropods and birds, and check values against currently produced diversity, sea level curves and possibly latitudes. There’s lots to do, but it should be pretty interesting!
Whole aim of a scientific project is to produce an outcome. Scientists need results! It’s the only way we can really check if what we’re doing is accurate or even worth our time. So what are my results going to show? And are they going to make a difference?
At this stage, I really don’t know. Previous research has had fairly mixed views on bias and measuring diversity within pterosaurs; some say that current methods have helped alleviate bias to a degree where we’re getting a reasonable representation of pterosaur diversity, and others think it’s all a load of rubbish and it’s a pointless exercise. My results could go either way, and only time can really tell.
Something I haven’t really touched on that could be interesting is how my work could potentially effect future research. This project is quite nice in that it’s actively setting up stuff for other people – with a good view on how bias and completeness effects the fossil record of pterosaurs, others can approach the subject from different angles, and produce work which will hopefully take all these things into account. One area it could help out is assessing what processes were going on in the extinction of pterosaurs and the rise of birds; did pterosaurs die out naturally and birds take their place, or did they actively compete against them and come out more successful? As it is in the world of science, only (lots more) time (and research) will tell.
Where do we go from here?
Well, I think this marks the end of my overly wrought out introduction to my project (especially as it’s quite solidly underway at this point in time). I hope I’ve shed a bit of light on what I’m doing – I’ll try and clarify anything that people aren’t to sure of, and again, if I’ve made mistakes please let me know! Over the coming weeks, my aim to to publish a couple of blogs a week just giving a general update on progress and what I’m up to, as well as linking to other science news and blogs that I find interesting.
Wish me luck – there’s a lot left to do!
Today’s post title comes from this great song.