Where is the pterosaurian 5th finger?

Anyone who has breezed their way through the more pterosaurian sections of this blog (or browsed much pterosaur stuff at all) will know that the huge finger on the pterosaur hand that supports the main wing is the fourth (ring) finger. What you may not have asked yourself is how we *know* it’s the fourth finger. Since pterosaur ancestors presumably had the more normal five fingers like all but the most basal or derived tetrapods, how do we know that the remaining fingers and 1-4 and not 2-5?

There are several lines of evidence available which push us towards the answer to this problem, and indeed still others we can’t use but would be of practical help to similar problems in other animal groups and so I will include them here as part of the general problem. The key issue of course is being able to track the individual digits and the bones that make them up through the fossil record. First though, it’s worth running through a few bits of hand terminology to avoid confusion.

The pterosaur forelimb. Black - humerus, yellow - radius & ulna, orange - pteroid, blue - metacarpals, white - phlanges of digit IV (4th phalanx not shown).

The long bones that lie between the small wrist bones and the fingers (and make up the palm of the hand in humans) are called the metacarpals. The fingers (or digits) themselves are numbered from I to V in Roman numerals, with I being the thumb and V the little finger (pinkie). The thumb is often called the pollex by the way. The individual bones of the digits are called the phalanges (singular is phalanx) and are themselves numbered from the metacarpals outwards. Thus a human pollex then phalanges I-1 and then I-2 sitting on top of metacarpal I. The last phalanx on the digit if claw shaped / supported a keratin sheath is called an ungual.

Now we have that out of the way we can start our quest to track down the missing manual pterosaurian digit. However, the first two are not really relevant to pterosaurs, so let’s get them out of the way first.

The morphology of digit I. Digit I in tetrapods is often highly specialised in both shape and function (the primate pollex being a great example). Therefore its loss or retention should be pretty obvious. If you find a fossil ape with only four fingers on the hand it should be pretty obvious if the thumb is missing or if it was digit V that went, since you can’t really confuse it with any other digit – colobus monkeys are a good example of this, so take a look at one the next time you are in a zoo. This doesn’t work for pterosaurs since their ancestors didn’t really have a specialised digit I for us to look for.

Secondly we can look at embryological data. Embryos often begin to develop some anatomical features like digits that are then reabsorbed later on. So looking at enough embryos you can see all five digits start to form (condense) and then one or more may be lost. Since you can see which are lost directly through observation it should be quite obvious which digit was lost. You don’t need living members of a group to test this, close relatives or descendents should be fine, but pterosaurs don’t have any, so again this is ruled out (and is the source of much contention in the bird-theropod debate).

Next we have the issue of digit size. In reptiles at least, digit IV is the biggest (the longest and also often the most robust) of the five digits. That makes it relatively easy to spot and adds another reference point as with the pollex (and furthermore V is typically the shorted and smallest). In terms of pterosaurs this is obviously a strong indicator that the wing finger is indeed digit IV since not only is it really big, but also digit III is similar to both digits I and II. It is therefore most parsimonious to assume that digit IV started as the biggest and got bigger, and III stayed the same size, than V became super sized and IV shrank to look like II and II.

The number of phalanges present in each digit is also important. While the number can vary between and even within clades it is actually fairly conservative and thus can be tracked across lineages. If pollexes always have 3 phalanges, but the other digits have 5 and your new fossil with just 4 digits has a digit I with 3 phalanges, you can be fairly sure that the pollex has been retained and digit 5 was lost. In reptiles, this is actually really useful as there are different numbers of phalanges in most of the digits (unlike in humans) giving you a whole set to check and cross reference. Reptiles typically have a 2-3-4-5-3 pattern (that is 2 phalanges on the pollex, 3 on digit II etc.) so four of the digits normally have a characteristic number of phalanges. In pterosaurs, the basic formula is one of 2-3-4-4-X (the X denotes there are none present) so it is easy to see how this more closely matches a pattern of I-IV than of II-V. When you add to that the fact that there is no ungual on the pterosaur digit IV and thus it originally would have had 5 phalanges and not 4, it becomes an even better match.

By now you may haven noticed that I have only been talking about the loss of a single digit and only refer to either I or V being lost. But could say, digit II or III be lost first and muck up at least some of these assumptions about digit loss? Well, actually no. The reduction pattern seen in tetrapods is actually incredibly conservative and tied to the development of tetrapod embryos. Digit V is almost invariably the first to go and then digit I next. After that it gets a bit complex, but the first two are pretty much universal (more on this at the appropriate time with respect to birds and dinosaurs). In terms of pterosaurs, with only one digit missing, we would expect it to be number V and not I.

So if we add all of that together, we can be really pretty confident that the pterosaurian wing finger is indeed digit IV. It has the right number of phalanges, it is the biggest on the finger and we would expect V to be lost before I. If it were a digit V we would have to have had IV shrink in size, have different numbers of phalanges on pretty much every digit, have an odd reversal in digit I being lost first and have the digit V ungual change to look like the other phalanges. It’s pretty clear therefore which option we should pick!

There is therefore quite a lot of underlying evidence pointing to this single conclusion, something I think if often overlooked. You simply can’t always get all of this information across for even a relatively simple and unimportant point like this in pretty much any forum and it can lead to people thinking that the scientist has over-extrapolated or even been somewhat ‘creative’ in the interpretation. However, take on something complex like whether or not you think a species is herbivorous, and the lines of evidence get more numerous and more complex – teasing that out into a soundbite comes out as “because of it’s teeth” which is often very unconvincing and can lead to the classic challenge of ignorance “how do they know that?”.

Hmmm, I’m drifting off again into one of my rants, so I’ll cut it there. In short, pterosaurs have 4 fingers, and the big one is the 4^th one, which means that their wedding rings would be really expensive if it wasn’t for the fact that you would not be able to get it on over the wing membrane.