My, What Big Gastralia You Have, Nyctosaurus!

Little Red Riding Hood had it right.
Whenever something is out of the ordinary, like big teeth on ‘Grandma-ma’, we take notice.

Most pterosaurs have gracile gastralia, those boomerang-shaped bones that cover the belly of pterosaurs and many other reptiles. The most obvious exception is Nyctosaurus, in which the gastralia are quite robust and for good reason.

The various forces working on the gastralia of Nyctosaurus.

Figure 1. The various forces working on the gastralia of Nyctosaurus. Weight and long lever arm of the upper/anterior portion in red. The fulcrum at the lumbar/sacral interface above the lower/posterior portion in yellow. Ventral support of the fulcrum from the gastralia in blue.

While all pterosaurs had to support the long lever arm of their upper/anterior selves at the fulcrum of the lumbar/sacral series interface, perhaps no pterosaur had quite the imbalance of forces seen in Nyctosaurus, with its long metacarpals and rostrum. In any case, Nyctosaurus developed a strong ventral basket of robust gastralia to prevent rotation of the lower lumbar area, which would have acted to shorten the distance between the sternum and prepubes in Nyctosaurus. I learned about this mechanical problem while building a full scale Nyctosaurus model based on UNSM 93000 (skull largely unknown, Fig. 2).

Standing model of Nyctosaurus

Figure 2. Standing model of Nyctosaurus. This model slumped over its belly without substantial reinforcement in the gastralia.

Quadrupedal Pterosaurs Avoided This Problem
Whether you support yourself with long metacarpals, long antebrachia or long ski poles, the stress on the lower back goes away and you don’t need to develop such robust anti-rotational supports in your belly. And of course, the whole problem goes away when you are airborne and supporting yourself by lift from both wings and thighs.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

2 thoughts on “My, What Big Gastralia You Have, Nyctosaurus!

  1. (From a non- scientist and non- English-speaker)

    I do not write here to comment speci­fically about those gastralia (Nycto­saurus), but about something that i hope to have noticed right in its skeleton photo. I.e: the WRIST, or rather the meta­carpals, are mobile (mainly?) in a DORSALVENTRAL direction, rela­tive to the radius. Is that right?

    I ask because in near all man­-made pictures of pterosaurs i see, even the modern ones, and often also in their fossil, compressed (and often twisted) specimens, it looks as if they were mobile BACKFORTH in the same plane as the radius, which always seemed sort of very akward mecha­nically to me, for many reasons, espe­cially in not­-airborne stance.

    In the photo, instead, it looks that the radï are parallel to each other, opposing their ventral faces, and the two “V’s” formed by the meta­carpals and the first phalanges [of the two big fingers] CONVERGE ventrally. And probably, i suppose (if i may), those “V’s” could also diverge, dorsally.

    What i mean is that in airborne stance, for example, the meta­carpals and first phalanges could move (if i’m right) only up and down, or nearly so, relative to the rest of the wing. By the way, one effect of that would be changing the wing lift­-gene­rating CURVA­TURE at the leading edge (ana­logous to what bats do the other way around at the trailing edge, flexing their distal phalanges. Birds just cannot do it, their feathers’ curva­ture cannot be modu­lated).

    I hope i supposed rightly, since the forward­-pointing wrist angles i see in walking pterosaurs pictures give an idea of “broken arms”, if one imagines them mobile in a proximaldistal direction (rather than ventraldorsal).

    By the way, did anybody ever poit out, as it deserves, that pterosaurs wings, seen from in front [as if the animal was crashing into the observer], are basi­cally 4­-segmented, rather that 3-segmented as in birds and bats?

    In any case, thank You for all your great, careful work, and most of all for divulg­ating it in a blog. You’re right, Sir, peer­-reviewing is often a quagmire for new views; and, believe me, already in the 80’s, when i was just i boy with no edu­cation, the images of pterosaurs i found in books always looked VERY, un­toler­ably WRONG to me. The ones i see here are really a big breath of fresh air.

    MBL

    • Thank you for your kind comments. Mauro. From what I have heard and observed, in flight the metacarpals were relatively inflexible dorosventrally and only moderately flexible anteroposteriorly mainly due to the influence of the radius sliding slightly along the face of the ulna (forming a very tight parallelogram) in response to elbow flexion and extension. When you say the “radi are parallel to each other” I presume you mean the radius and ulna and yes they were appressed and parallel, unable to pronate or supinate, like the human forearm can do. The precise shape of the metacarpals and their articulations is best seen in 3D specimens from South America. It’s complex and the elements need to be manipulated with each other to determine their range of motion. I think, based on observation and the literature, that the wing finger was relatively stiff dorsoventrally, like a bird’s wing bones, while in flight configuration and flexible only in the plane of the wing, like a swing-wing airplane. The curvature of the pterosaur wing was increased by raising the elbow relative to the leading edge, something I think was built in to the architecture but is commonly overlooked. This afternoon you’ll see a fleshed-out Rhamphorhynchus sculpture I did that includes the raised elbow. Also visible right now at http://www.davidpetersstudio.com/sculpture.htm. If I did not answer your query fully, please feel free to send another query.

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