The anurognathids – a review

Assuming things go to plan (so no chance there then) I’ll soon have to bury myself properly into the (small) world of anurognathids. I’m looking forward to it as they are a truly fascinating group of pterosaurs. Of course every clade no matter how big or small has some unusual trait that will make it interesting to an expert in the field, if only as a quirk – pteranodontids have a great fossil record, azhdarchids are huge, dimorphodontids have their place in history, dsungaripterids have their great skulls and the anurognathids? Well, they have everything.

It is always easy to say a fossil group is really weird and then back it up with just one little fact or odd snipet of information, but the anurognathids really are strange. They are very, very rare in the fossil record, yet the speciemns we have are among the best of any pterosaurs, their soft tissu preservation is exceptional, they are easily the smallest pterosaurs known, probably the only ones that even vagely qualify as insectivores, have a bizarrely large inferred ghost lineage / stratigraphic position, a unique skull and a very interesting phylogenetic position, all crammed into just a few tiny bits of rock scattered across Eurasia.

They are therefore genuinely deserving of the term ‘interesting’ even among the pterosaurs, and since I need to brush up a bit on them I’ll let you lot (well, both of you, might be more accurate) bear the brunt of the draft. This is not a formal review really, just a few notes strung together with little thought and even less research (so about the same as normal then), but I hope it will be interesting. It is hard to know what to deal with first as obviosuly all of the interesting features are rather intimately linked – describe the morphology first and you don’t know what I am talking about, do the taxonomy first and there is no framework for where the differences in morphology fit etc. I guess I’ll start with a very broad overview and then plunge into the rest. Once again I am cursed by the lack of figures here, but hopefully interest can be sustained and you will get something worthwhile from this.

Anurognathids (the ‘frog-jaws’) are small (0.5 m wingspan) ‘rhamphorhyncoid’ pterosaurs that are known from the Late Jurassic and Early Cretaceous of Europe and Asia. They are characterised by short, broad and tall skulls, huge orbits, short and broad wings, very short metacarpals, large manual claws, a large pedal digit V, and a reduced tail. They are generally interpreted as crepuscular hunters of flying insects. They might represent the most basal clade of pterosaurs, but their stratigraphic position and unusual lifestlye makes interpretatins difficult.b I guess the taxonomy should come up next, and it won’t take too long as there are just 4 monospecific genera – Anurognathus, Batrachognathus, Dendrorhyncoides and Jeholopterus.

Anurognathus is of course the name bearing taxon and the holotype was for a very long time the only anurognathid known which was readily accessible, which was a same because it is in rotten condition (see image below). It was described in 1923 by Doederlein from the Solnhofen limestines for Germany. Most of the bones were missing from the slab just leaving cavities (there was no counterplate) and calcite crystals obscured much of the rest. I have had several good looks at this specimen and it is very poor, though it does at least give you a reasonable idea of the shape of the skull as it is preserved in a rather unusual lateral view when all the others are in dorsal / ventral view. Only last year a second and incredibly well preserved Anurognathus specimen was described by Chris Bennett. It is tiny, just 20-25 cm in wingspan (as opposed to about 50 in the holotype) but immaculately preserved and has added much to our knowledge of them. Not least under UV light (in yet another example of the superb work by Helmut Tischlinger – more on him at some point) we are able to see details of the wing membranes, and yes, the actually muscles of the legs and arms.
Anuroganthus, with permission BSPG.

Historically, Batrachognathus came next from the Upper Jurassic of Kazakhstan being described in 1948 by Ryabinin. Sadly (for this review) I don’t have any of the original description, just notes in other papers that comment on it, but it appears to be known from two specimens, one of which includes a decent skull and a manus (but not much else) and can be unambiguously attributed to the anurognathids, though the other is undescribed. Still, while it is interesting, it adds little that cannot be gleaned from the other specimens.

Again, that was about it for a long time, until like a good bus, two cropped up at once, and if you know anything about Mesozoic palaeontology you will know that if you want to find something weird and or rare you should look in China. Dendrorhynchoides turned up first in 1998 from the Early Cretaceous and is a nice complete pterosaur which I was recently able to take a look at. It is really pretty good as pterosaurs go, and with lots of soft tissue preservation of the wings and other associated membranes. The outstanding feature is the long tail that is clearly visible, but on closer examination it does look rather like a fake. It *could* be real, but I doubt it sadly as it would make for a very interesting aspect of this particular taxon.

We had to wait just four more years for another anurognathid, and Jeholopterus was described in 2002, with a second specimen following in the same year. Sadly I have seen neither as although both specimens are housed in Chinese collections, they are currently on loan in Tokyo as part of a pterosaur exhibition, so I will have to go back to the descriptions. Jeholopterus made the news for its exceptional preservation of both the wings and the ‘fur’ (pterosaur fuzz that is not hair, and not protofeathers, but well, something similar at least) that surrounded the animal, most notably on the head. While there were already a number of fuzzy pterosaurs known (particularly Sordes) this was the first time they were seen in large numbers, attached to the body of the animal (rather than having ‘floated off’) and clearly distinct from the actinofibrils on the wing. It is a little bigger than the others, coming in closer to 60-ish cm in wingspan.

That is it in terms of basic taxonomy. Four monospecific genera, each known from only 1 or 2 specimens, mostly in pretty good shape and with soft-tissues known for most of them too which is nice, all floating around on the Jurassic / Cretaceous boundary. This last point is especially interesting – anurognathids are ‘rhamphorhyncoids’ – basal pterosaurs, which are supposed to have gone extinct at the end of the Jurassic and yet (assuming the dates are right, there are some problems with dating in the Jehol) at least one survived into the earliest Cretaceous. Anurognathids are really small, far smaller than pretty much any other pterosaur (at adult), even the small Pterodactylus was closer to 1 m at adult than 50 cm or even less in the others. This kind of tiny size (and it should be added, very weak skull) and great age makes they especially bad candidates for preservation and recovery so in fact it is a genuine surprise that we have so many of such great condition compared to say something nice and big and robust and recent like Quetzalcoatlus.

However, we would still expect them to have been around much earlier than the Late Jurassic – it would be odd indeed if new rhamphorhyncoid clades were arising just as the rest of the clade were dying out and the pterodactyloids were taking over. It is perhaps no surprise that we don’t have any older anurognathids, but at the same time, surely the pterosaur fossil record, while capricious and highly incomplete, should offer some bits of older anurognathids? And this brings us to the next point – their systematic affiliation. There is still some disagreement over the exact placing of the anuroganthids within pterosauria (different analyses say slightly different things) but one thing is clear, they are among the earliest of pterosaurs, and their evolutionary history almost certainly dates back to the Triassic. In other words, anurognathids have a massive ghost lineage, having first appeared in the Late Triassic, but with no actual record of them until the Late Jurassic, some tens of millions of years later. Their exact phylogenetic position is actually pretty important too, normally systematicists don’t get too worried if two clades regularlay swap positions in different phylogenetic analyses, but here there remains the tantalising possibility that anurogantids are actually the most primitive pterosaurs – they are the most basal pterosaurs.

If that is true, then of course they become even more important as they will be the best source of information to link them to clades outside of the Pterosauria and try to establish with greater accuracy the ancestry of the pterosaurs. Anurognathids have some unusual features which suggest they are basal to other pterosaurs, notably an unusual joint between the manus and wing finger, a short manus, a very short and simple pteroid, and a rather elongate fifth pedal digit. Preservation of the wings suggest the main wing was unusually broad and there was a large uropatagium. Between them, these suggest a primitive flier which focused on large wings to generate lift and could not fold the wings as effectively as other pterosaurs, and had not yet elongated the manus and other bones to produced a longer wing. However, in the best traditions of evolution we are of course dealing with a group that has also had the best part of 70 or 80 million years to evolve independently of their origins, so it is no surprise that they have some very derived features too. Foremost amongst these is the reduced tail (these are the only short-tailed rhamphorhyncoids) which looks very pterodactyloid-like.

Their lifestyle is also unique compared to other pterosaurs and can be seen as a very derived and specialised mode of living, or a primitive one that anurognathids adopted early and successfully and thus were able to maintain despite competition from other pterosaurs and later birds and their apparent primitive nature. Anuroganthids were, in all probability, crepuscular (or possibly even nocturnal) insect hunters. While we have (not surprisingly) no direct evidence for this, the circumstantial and inferred evidence is pretty good and it is worth running over it.

First off the skull shape: if the name hadn’t given it away already, they cranium is very frog-like – wide jaws with a deep and rounded skull as well as small, well-spaced, peg-like teeth. In other words something well suited to making quick snaps that cover a large area and with teeth that are good at holding, but not much else. There is even a series of tiny pits around the rim of the upper ‘lip’ that has been suggested would hold hairs to increase the catchment area of the mouth when snapping at insects, something also seen in nightjars and whip-poor-wills. The orbits and sclerotic rings are massive which suggests very latrge eyes, and of course very good eyesight to go with it, suggesting that unlike other pterosaurs they were living in low light conditions. The wings are short and broad which would give them good manoeuvrability in following flying prey, and being short would help them navigate dense environments such as woodland (just look at wrens and magpies). In fact their small size overall favours navigating in tight spaces and of course if you are going to eat insects, it is hard to get much to eat if you are very large (again, look at modern birds and bats). Finally, there are of course plenty of flying insects around at the time and pretty much no competition to feed on them so it would have been a successful clade indeed that could move into this niche. In fact the idea that anurognathids are basal and insect feeders makes a lot of sense form a lot of different angles.

Early arboreal proto-pterosaurs would have been looking for a rich source of unexploited food and adapting from a gliding to flying lifestyle would have opened up a huge niche of available resources. At the same time, with flight still developing, they would have been vulnerable to arboreal and terrestrial predators, so a nocturnal or crepuscular lifestyle might have kept them safe. Adopting this lifestyle early would have given them access to new foods while keeping them relatively safe themselves. I would suggest that they did this so effectively that they were able to stay in this niche pretty much indefinately with other pterosaurs adapting into new realms, but the anurognathids continuing to plug away at catching insects. It might also explain why they left so few fossils – if they were hunting in primarily terrestrial environments, they were not living in areas where fossilisation occurs often and thus simply didn’t enter the fossil record. They probably spent their time clambering around on tree trunks or in small hollows, holding on with their big hand claws and wide-spread toes. It is all speculation of course, but it does come together into a pretty convincing picture, based on the limited evidence available.

Finally, it is worth spending a little more time on the wings and fur of the anurognathids, since they are exceptionally well preserved (if rather cryptic thanks to the unusual preservation in Jeholopterus at least) and contain much that is interesting when considering pterosaurs in general. As I have brought up before pterosaur wings are very unusual biological structures that, while quite well understood, still have much that requires further investigation. At the very least in terms of shape, it is clear that anurognathids (well, certainly Jeholopterus & Dendrorhyncoides) had a truly broad wing, that is one that reached from the tip of the fourth finger to the ankle, and did not have a steeply concave trailing edge. This is interesting in general as it is not seen in other pterosaurs even where ankle attachments are known, the wing can still be quite narrow, and of course gains potentially even more significance if anurognathids are basal, as it would suggest such a wing planform is primitive for pterosaurs. Also known in the tow Chinese taxa is a clear and extensive uropatagium (rear wing) between the legs and attaching to the 5th digit on the foot. This particular bit of tissue has long been reconstructed in pterosaurs and there are lots of decent fragments known (having no actinofibrils, it tended to disintegrate and move easily from its presumed positioning most specimens) but nothing so clear-cut (if still a bit hazy) as can be seen in these specimens.

The ‘fur’ too is far better preserved here (in Jeholopterus especially) than in other pterosaurs (again, mostly through lack of movement) and can be clearly seen to stick tight to the skull. Other furry pterosaurs tend to only show the fur as part of the body or neck and it does not reach the head, implying a vulture-like arrangement of fur. Here though the whole body including the head and neck were clearly furred. Not only that, but the individual fibres were quite thick, both in their actual structure and distribution – they were densely packed on the body and would have made the living animal furry rather than perhaps bristly.

Well, that is about if for anurognathids. That is much more than I had intended to write, not through lack of interest, just because I didn’t think it would take more than a thousand words, and I have been trying to keep the damn thing down. Still, I hope it provides a decent, if brief, review of this most unusual clade of the generally unusual pterosaurs. If this goes down well, I might have to consider taking a stab at some of the other clades or species, though these are in a way exceptionally easy as there are so few of them, and all are in great condition and mostly described both recently and thoroughly, I really would hate to try and do this for the ctenochasmatids.

This is a revised Mk.1 post, to see the original with comments etc., go here.

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