Tropy Palaeo-Cliché
There are plenty of palaeoart examples of Rhamphorhynchus skim-feeding in the style of the extant tern-like bird, Rynchops. It's understandable - after all, Rhamphorhynchus is a seagoing pterosaur with a mouthful of forward-pointing teeth, occasionally preserved with the remains of its fishy meals within it. Factor that stuff together, and it's easy to imagine Rhamphorhynchus zipping along just above the surface of some shallow Jurassic sea, thrusting forward with its mandible slicing the water's surface, and snatching morsels of food as it finds them.
Humphries and Chums' 'Just Say No!' Campaign
In a 2007 paper investigating the possibilities of pterosaurs engaging in skim-feeding, Humphries et al found few adaptations towards this method of prey-capture, with the skull lacking the types of reinforcement seen in Rynchops. Read the paper here. Despite the refutation of the idea, it's a persistent one in palaeoart, probably in part because it makes for attractive images. Thanks to Humphries et al, this is probably as close as I dare get to showing a rhamph skimming:
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| Rhamphorhynchus experiments with skim-feeding, remembers why it doesn't. (Copyright © 2015 Gareth Monger) |
References:
Humphries S, Bonser RHC, Witton MP, Martill DM (2007) Did Pterosaurs Feed by Skimming? Physical Modelling and Anatomical Evaluation of an Unusual Feeding Method. PLoS Biol 5(8): e204. doi:10.1371/journal.pbio.0050204
That doesn't mean that it didn't dip its beak in the water the moment it saw a fish
ReplyDeleteNo, it doesn't, but that's very different to keeping part of your mandible under the surface for protracted periods of time whilst flying. Take a look at the paper linked above, if you haven't already - it's pretty interesting. :)
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ReplyDeleteHumphries et al. only tested on a jaw now described as Banguela oberlii; their model principally concerned with whether the tip of the jaw of Thalassodromeus could skim. Ultimately, they cannot have answered this point. But in the interim they also pointed out, as has Witton on his blog more extensively, the absence of a large suite of cranial attributes used by skimmers to skim. Problematically, some of these adaptations are found in other birds, principally gulls (Bock, 1960), in which many of the features are related to stabilization of the mandible against the compression forces at the tip, translation into the skull, and dissipating into the tissues of the neck. We could not say, as may be implied, that the cranial adaptations in skimmers are all required, when if you remove one or two skimming would cease. As this suggests that trait for success might not be selected for via behavioral exaptation rather than prexisting and thus permissive of behavior that is "discovered," it leads into problem areas of hypothesizing large complexes of (very expensive) form leading to function, rather than the other way around (that functionl behavior leads to adaptation of complexes to support it).
ReplyDeleteWe would thus assume early skimmers were less well-adapted, and less well-suited, to skimming, and this suggests there were cranial adaptations leading to skimming that skimmers developed more of. Finding these "core, intrinsic" features is possible if we try to reverse engineer a model fo the skimming apparatus in full, but that's a very complex thing to do and I don't think anyone (except maybe me?) is interested in seeing it done.
My personal feeling with regards to pterosaurs should be held off for a while, following more extensive analysis. However, I think at least for rhamphos you're on point (and the previous post, linked therein). To the short of it, Rhampho has a different sort of mandibular adductor system than a typical biter might, more conducive to closing the jaw under different oriented forces.
There are a lot of errors in this comment. Our 2007 paper modelled three jaws (Rynchops, Banguela and a 2 m wingspan Tupuxuara) and extrapolated results of the former to model pterosaur species with similar jaw shapes to test their skim feeding energetics. We also ran different numbers to account for variation in body masses. None of our pterosaur models met requirements for skim-feeding, and even the relative streamlining of the Banguela jaw made no difference to this. Contrary to this comment, we note that skimming (that is, skimming without feeding, as practised in terns) might be possible in species with less than 2 m wingspans without strong specialisation, suggesting that size alone has some bearing the evolution of modern skimming behaviour. The suggestion that these results have no longer have bearing on Thalassodromeus is not true: the jaw tip of Banguela remains a good proxy for the shape in Thalassodromeus, and it's power requirements vastly exceed those permissible for skim-feeding. Our models could be two of three times more effective at water trawling and skim-feeding would still be unobtainable for this animal. Our energetic work was bolstered by discussions of comparative anatomy presented in the same paper, not afterwards, in which found that appropriate adaptations in mandible shape, jaw muscle size, skull reinforcement, neck morphology and so on were absent in pterosaurs. We are not looking at animals with one or two 'missing' skim-feeding adaptations, but species which have no functional signals pertaining to skim-feeding habits whatsoever.
DeleteThe suggestion that these points have only been followed in blog posts is also erroneous. Darren Naish and I discussed azhdarchid skim-feeding and a similar foraging hypothesis at length across two papers (2008 and 2013). Both include a lot of comparative anatomy, and in the latter we demonstrated that the drag on pterosaur jaws from trawling mandibles was sufficient to snap azhdarchid neck vertebrae! Other authors (e.g. Chatterjee and Templin 2004; Osi et al. 2010) have added similar points to this discussion. The point is that investigations on pterosaur skim-feeding have been quite a bit more extensive, and properly published, than implied here. The possibility of a known pterosaur being a skim-feeder, or even a proto-skim-feeder, is not supported by these tests.