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Re: [dinosaur] Halszkaraptor, new dromaeosaurid with amphibious ecomorphology from Late Cretaceous of Mongolia

One of the many (very many) interesting things about _Halszkaraptor_ is the use of the forelimbs in locomotion - in this case, wing-propelled underwater locomotion. The shortness of the forelimb is no surprise - this reduces the torque at the shoulder during underwater 'flight' (water being much denser and more viscous than air). One question is how this forelimb motion for underwater locomotion ('swim stroke') evolved. Modern wing-propelled diving birds either use their wings for both aerial and underwater locomotion (such as auks), or exclusively for underwater locomotion (such as penguins) - in both cases the birds are descended from volant ancestors. Penguins gave up aerial flight in order for the wing to become more specialized (and biomechanically efficient) for underwater locomotion. So in these birds the flight stroke was co-opted (exapted) for underwater diving. Did this happen in _Halszkaraptor_? If so, this would make aerial flight primitive for the entire Dromaeosauridae (or even Paraves). Under this scenario, the powered (flapping) flight abilities inferred for small-bodied, large-winged dromaeosaurids like _Microraptor_ (and maybe _Rahonavis_) are a retention of ancestral flight abilities - secondarily lost by most dromaeosaurids (including eudromaeosaurs).

An alternative hypothesis is that, among non-avialan theropods, the underwater locomotion evolved only in the halskaraptorine branch of the dromaeosaurids, and evolved independently of aerial flight in other paravians (including _Microraptor_ and avialans). Under this scenario, the swim stroke and the flight stroke were each derived independently from an incipient aerodynamic motion of the forelimb used by terrestrial paravians, perhaps originally used for leaping or maneuverability or courtship displays. _Halszkaraptor_ has large sternal plates (with some mediolateral expansion), but these are unfused and lack a keel. So it shows how a theropod forelimb can be readily converted into a thrust-generating wing (in this case used for underwater flight) without the need for 'advanced' and extensive pectoral musculature. It's entire likely that the same thing occurred in the origin of aerial flight in paravians, including birds, based on _Archaeopteryx_, _Confuciusornis_, etc.

On Thu, Dec 7, 2017 at 5:12 AM, Ben Creisler <bcreisler@gmail.com> wrote:

Ben Creisler

A new paper:

Andrea Cau, Vincent Beyrand, Dennis F. A. E. Voeten, Vincent Fernandez, Paul Tafforeau, Koen Stein, Rinchen Barsbold, Khishigjav Tsogtbaatar, Philip J. Currie & Pascal Godefroit (2017)
Synchrotron scanning reveals amphibious ecomorphology in a new clade of bird-like dinosaurs.Â
Nature (advance online publication)

Maniraptora includes birds and their closest relatives among theropod dinosaurs. During the Cretaceous period, several maniraptoran lineages diverged from the ancestral coelurosaurian bauplan and evolved novel ecomorphologies, including active flight, gigantism, cursoriality and herbivory. Propagation X-ray phase-contrast synchrotron microtomography of a well-preserved maniraptoran from Mongolia, still partially embedded in the rock matrix, revealed a mosaic of features, most of them absent among non-avian maniraptorans but shared by reptilian and avian groups with aquatic or semiaquatic ecologies. This new theropod, Halszkaraptor escuilliei gen. et sp. nov., is related to other enigmatic Late Cretaceous maniraptorans from Mongolia in a novel clade at the root of Dromaeosauridae. This lineage adds an amphibious ecomorphology to those evolved by maniraptorans: it acquired a predatory mode that relied mainly on neck hyperelongation for food procurement, it coupled the obligatory bipedalism of theropods with forelimb proportions that may support a swimming function, and it developed postural adaptations convergent with short-tailed birds.


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