Given the sheer number of pterosaur-based posts on here, I really should have got around to this issue sooner. It was alluded to very briefly in the pterosaur basics post but really deserves a more thorough examination, if only at a still relatively superficial level (since I don’t want to get bogged down in anatomical details). If nothing else, it is certainly worth mentioning a few misconceptions and reversal that occur between the two clades, including one hugely significant one that almost everyone seems to ignore. Let’s take a look:
Pterosaurs as a whole are really quite conservative, which is perhaps no surprise given the extreme evolutionary pressures that come with powered flight. However, the rhamphorhynchoids and pterodactyloids are nevertheless quite different and in life could probably be told apart by even a casual observer from quite a difference. Those who have read the short post on monophyly and paraphyly will remember that rhamphorhynchoids are a paraphyletic group (they have a common ancestor, but as a clade do not include the pterodactyloids). Simply for convenience I will leave out the quote marks round the name.
Many of the more obvious differences can be seen on this little figure kindly provided by my student Ross Elgin and pseudo-student Edina Prondvai (heavily reworked and electronicified from two originals by Peter Wellnhofer) with a rhamphorhynchoid on the left and pterodactyloid on the right (in fact they are actually Rhamphorhynchus and Pterodactylus). I’ve not added labels or arros to the figure as it should be pretty obvious from the descriptions which parts I am referring to (though you can’t see all of them on this figure like the noratium).
Taking them in order:
Skull length: the skull is bigger in pterodactyloids (bigger than the body) but not in rhamphorhynchoids.
NAOF: the Naso-anorbital fenestra (commonly and understandably reduced to NAOF) is a feature of pterodactyloids where the naris and antorbital fenestra fuse into a single opening, but kept separate in rhamphorhynchoids.
Teeth: not entirely diagnostic, but only rhamphorhynchoids have multi-cusped teeth, or heterodont teeth (i.e. multiple kinds in the jaw) and a great many pterodactyloids have no teeth and those are always simple and of one type.
Crests: also not entirely diagnostic, by pterodactyloid crests are typically much larger, and many extend off the back of the skull which you do not see in rhamphorhynchoids.
Neck length: in the rhamphorhynchoids the neck is also shorter than the body (or at least the dorsal series – the vertebrae between the neck and the pelvis) but in pterodactyloids the neck is longer.
Cervical ribs: the rhamphorhynchoids have large cervical ribs (as the name suggests little ribs that attach to the neck vertebrae, common in many reptiles) but in the pterodactyloids these are either severely reduced or absent.
Notarium: this is a special fusion of vertebrae around the shoulder that effectively functions like a second pelvis, but to support the arms in flight rather than the legs on the ground and is only seen in pterodactyloids.
Tail: the tail is short in pterodactyloids and long in the rhamphorhynchoids. There are a couple of exceptions though, as the anuroganthids are rhamphorhynchoids with short tails, and the pterodactyloid Pteranodon has a long one. This is not quite the same thing however, as in fact Pteranodon has a short tail really, just with a set of long extensions coming off the last bone.
Wrist: in rhamphorhynchoids, the wrist is pretty much the shortest part of the wing, but is greatly elongated in the pterodactyloids and is as long or longer than one or two of the distal wing bones.
Pteroid: the pteroid is longer and often more complex in pterodactyloids, in the rhamphorhynchoids, it remains a simple, stubby spike-like thing.
Fifth toe: the fifth toe of a rhamphorhynchoid consists of at least two bones and is typically long and sticks out at an angle from the rest of the foot, in pterodactyloids it is strongly reduced or even lost and does not protrude.
So there you have it, the next time (and I’m sure it is a regular event) you are faced with a pterosaur fossil and you don’t know if it’s a rhamphorhynchoid or pterodactyloid, then this guide should set you on the straight and narrow. The good news is of course that a lot of these characters can be determined from just a few bits of bone and while I have written them in comparative terms (comparing rahmphorhynchoids and pterodactyloids, or one part of the body to another), they are often diagnostic in isolation too. So you can avoid any unpleasantness with identifications with the minimum of fuss and even the minimum of preserved bone. It should also be noted that while these basic structural differences would also be apparent in life so too would several other such as tail vanes and the uropatagium, which will of course be of interest to Ropen hunters.