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发表于 2012-6-7 19:51:31
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Boreopterid forelimbs are overdeveloped in the manner typical of ornithocheiroids and, with the exception of the wing finger, they are proportionally very similar to those of ornithocheirids. The scapulocoracoids are reinforced, blocky and large, anchoring onto broad sterna with pronounced cristospines. Their humeri possess elongate, warped deltopectoral crests and stout shafts. The radius and ulna are similarly both robust and around a third longer than the humerus, but the carpals are poorly known thanks to poor preservation and incomplete fusion. The pteroid is relatively slender and around 40 per cent of the forearm length. It isn’t clear if, as in ornithocheirids, some of the metacarpals had lost contact with the carpals as this region is obscured in both known boreopterid specimens. Their wing fingers are proportionally more like that of istiodactylids than ornithocheirids, occupying half of the wing length compared to the 60 per cent seen in ornithocheirids. Their non-flight fingers are small but capped with relatively robust claws. Boreopterid pelvic girdles are poorly known, but their hindlimbs are very diminutive. They are generally similar in length and proportions to those of ornithocheirids, but boreopterid feet are seriously titchy, their maximum length being an embarrassing 10 per cent of total leg length.
: E) I$ `" _" [5 ]Locomotion
- e/ d0 k! C- n; D# H9 z0 KWith perhaps one tiny exception, diddly-squat has been said in print about the biomechanics and functional biology of boreopterids, including their locomotory abilities. Their proportional similarity to ornithocheirids suggests that some generalities about ornithocheirid locomotion will fit boreopterids nicely, however. For instance, the elongate, warped deltopectoral crest and reinforced scapulae that we see in boreopterids probably signifies frequent launching from deep water, just as it does in ornithocheirids (Habib and Cunningham 2010). Similarly, the tiny hindlimbs of boreopterids dictate that their wings bore a correspondingly narrow chord. Their shorter wing fingers would make the wing somewhat shorter and lower aspect than those of ornithocheirids, however, possibly reflecting a preference for inland habits where short wings are beneficial for takeoff (Rayner 1988). The finding of all (two) boreopterid specimens in a freshwater deposit ties in nicely with this idea, but, of course, a much larger sample size is needed to ascertain a depositional bias in the boreopterid record.
+ @; {9 \" k- Z# E3 m) `The miniscule feet of boreopterids suggest that they didn’t worry too much about moving around on the ground, and their heavily-skewed limb proportions would have probably limited them to shuffling or bounding around in the fashion we described for ornithocheirids in the previous chapter. We may assume, given their apparent adaptations for taking off from water, that they were at least stable when swimming or floating and, indeed, their probable adaptations for aquatic feeding suggest they should be at home in aquatic settings.
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The only comments about boreopterid dietary preferences I know of stem from Wang and Zhou (2006), who suggested that boreopterids habitually dined on fish. This makes some sense: the elongate jaws and necks of boreopterids are well suited to being plunged into deep into water to gather fish or other nektonic critters, but exactly how they would’ve apprehended their prey hasn’t been established. Boreopterid teeth are the sticking point as they look so slender and delicate that it’s easy to imagine feisty prey damaging their teeth without too much hassle. Their eyes, too, are pretty tiny, which doesn’t really suit a predator out to spot prey from a distance. As a consequence, boreopterids may have been restricted to using their jaws as a cage to trap numerous, small swimming critters in one go. The close spacing and extremely long nature of their teeth is certainly consistent with this, and we can imagine boreopterids combing the water for food before lifting their heads to strain consumables from the water through their tightly-packed teeth. They seem to possess the necks necessary for this task, too, with the large and complex nature of their cervical vertebrae indicating a powerfully muscled neck. This doesn’t sound a million miles away from the dabbling that we see in modern ducks and geese, but boreopterids would be ill-suited to surviving on the types of vegetative matter that these birds eat: their torsos are not large enough house large, plant-digesting guts for that task. Such a foraging strategy would probably require the feeding boreopterid to alight on the water surface or stand in shallow water to be successful, but this brings their stunted feet and hindlimbs into question. Perhaps they preferred rather still waters where they didn’t need large, flipper-like feet to punt themselves around or, maybe they were able to use their forelimbs to shunt themselves about while their bodies were buoyed up by water. 2 K3 S2 C7 A3 [7 f( G
This new clade of pterosaurs clearly has a lot left to tell, then, and we can only hope that new discoveries revealing more details of their evolutionary history and distribution are not too far away. Hopefully, in years to come they will be as well documented as the next group we’re going to meet: known from well over 1000 specimens and the subject of study for almost 150 years, they can boast numerous bone-by-bone monographic descriptions and buttloads of research into all manners of their palaeobiology. They are also the last clade of ornithocheiroids on our tour and, with their iconic skulls shapes, are perhaps the most famous pterosaurs in the world. Next stop, then, the Pteranodontia.
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发表于 2012-6-7 19:31:46
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发表于 2012-6-8 06:47:33
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