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The Tetrapterygidae: Four-Winged Dinosaurs


Carnoferox

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Screen-Shot-2017-03-25-at-6-00-11-PM-uy3k3bne3p.png  

Beebe's hypothetical stages of flight, with 1 being the Tetrapteryx. Fig. 11 from Beebe (1915).

In 1915, William Beebe proposed that birds had gone through a four-winged phase, with flight feathers on both the forelimbs and hindlimbs. He based this hypothesis off his personal study of specimens of Archaeopteryx, noticing the faint traces of feathers on the legs. While dismissed at the time, the presence of hindlimb feathers on Archaeopteryx would be confirmed in 2006. Beebe's Tetrapteryx would be mostly ignored for the rest of the 20th century, until the discovery of the four-winged dinosaur Microraptor in China in 2003. Further discoveries of other four-winged dinosaurs like Anchiornis, Pedopenna, and Changyuraptor have given new relevance to the idea of Tetrapteryx. Although Beebe was correct concerning both Archaeopteryx and Tetrapteryx, he wrongly considered the ancestors of birds to be "thecodonts" (an outdated term for basal archosaurs) instead of theropod dinosaurs.

In 2015, Sankhar Chatterjee revived the Tetrapteryx in the form of a new clade, the Tetrapterygidae, in the second edition of his book Rise of the Birds. Chatterjee included four genera of four-winged dinosaurs in this clade - Microraptor, Anchiornis, Xiaotingia, and Aurornis. The members of the Tetrapterygidae were grouped together only by a single shared synapomorphy, the flight feathers on the forelimbs and hindlimbs. Unfortunately, Chatterjee's proposed Tetrapterygidae is highly problematic. For starters, there is no genus Tetrapteryx after which to name the clade. According to ICZN Article 29.1a clade must be named after its designated type species; thus, Tetrapterygidae is an invalid name for Chatterjee's clade. A similar situation happened in 2003 when Alexander Kellner named a clade of anurognathid pterosaurs the Asiaticognathidae, which contained the genera Batrachognathus, Jeholopterus, and Dendrorhynchoides. When it was pointed out that there was no genus Asiaticognathus to properly named the clade after, Kellner amended it to the more appropriate Batrachognathinae. If Chatterjee wanted the Tetrapterygidae to remain valid, he would have to do as Kellner did and designate a type species to amend the name. 

Cladogram-Combined-jy8dsuul64.png  

Competing cladograms of the Paraves, courtesy of Cau et al. (2015) and Chatterjee (2015). Members of Chatterjee's Tetrapterygidae are marked in red.

Secondly, Chatterjee doesn't include many four-winged dinosaurs in his Tetrapterygidae. Although Microraptor is included, other microraptorians like Sinornithosaurus and Changyuraptor are not. Basal avialans also known to have four wings like Archaeopteryx and Pedopenna are similarly absent. The reason why Chatterjee did not place these genera in the Tetrapterygidae is unknown. Thirdly, Chatterjee's Tetrapterygidae is paraphyletic, meaning that it contains species that share a common ancestor, but not all of the species that share that same ancestor. On Cau's cladogram of the Paraves, notice the placement of the tetrapterygids on different branches. In Chatterjee's cladogram, these genera have all been grouped together. Although these genera do share a common ancestor (the basalmost eumaniraptoran), a natural clade containing them should contain all of the descendants of that ancestor. Thus in order to contain Aurornis, Microraptor, Anchiornis, and Xiaotingia, the Tetrapterygidae would also have to contain the entire Eumaniraptora. The paraphyletic nature of the Tetrapterygidae renders it an unnatural grouping and an invalid clade. Needless to say, because of its many crucial flaws Chatterjee's Tetrapterygidae was mostly ignored by the paleontological community.

Although Chatterjee's revival in the form of the Tetrapterygidae failed, the Tetrapteryx still has relevance in understand the evolution of flight. Four wings evolved independently as many as three different times, in basal eumaniraptorans (Aurornis), microraptorian dromaeosaurids (Microraptor, Sinornithosaurus, Changyuraptor), and basal avialans (Archaeopteryx, Pedopenna, Anchiornis, Xiaotingia). This paraphyletic evolution of four wings is why "Tetrapteryx" should be used to refer to the bodyplan as Beebe intended, rather than a clade as Chatterjee intended. 

References

 

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Interesting stuff, thank you for posting. 

Just to let you know, there's something up on the salamander thread, it keeps breaking down whenever I click on it. Might just be me, but thought you'd like to know.  

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40 minutes ago, oldrover said:

Just to let you know, there's something up on the salamander thread, it keeps breaking down whenever I click on it. Might just be me, but thought you'd like to know.  

That's weird. I'm having no problems with it.

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12 minutes ago, Carnivorfox said:

That's weird. I'm having no problems with it.

Still keeps breaking. And it's only that one thread. Shame, because I really like salamanders. 

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26 minutes ago, Carnivorfox said:

That's weird. I'm having no problems with it.

One of the images you've included in the other thread is bugged and seems to crash the browser.

I've removed it and it's working fine now.

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22 minutes ago, Saru said:

One of the images you've included in the other thread is bugged and seems to crash the browser.

I've removed it and it's working fine now.

Sorry about that, I didn't know it was causing problems.

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  • 5 months later...

The new basal paravian Serikornis sungei also displays the tetrapteryx bauplan, one of the earliest dinosaurs to do so. It has a transitional mixture of plumaceous and pennaceous feathers on its hindlimbs that has never been seen before. However, the lack of barbules on both its forelimb and hindlimb feathers indicates that Serikornis was not capable of powered flight. Serikornis would have been a mostly terrestrial animal that may have occasionally climbed trees. It suggests that the tetrapteryx bauplan did not originally evolve for flight, but instead for other purposes such as display and brooding. 

https://link.springer.com/epdf/10.1007/s00114-017-1496-y?author_access_token=qK5jILmlXqTUfzaXSeOT4fe4RwlQNchNByi7wbcMAY5XtMiIuzLHT0w7pfMEQCqN57cyEs2GIzoqs5Z9sbEt05ydpRV-wedb1KJ5MwJh8Kg2RuubzDV9r0AJl8jBoI_iqK1-9ikzq8p8bOLVXyh_UA%3D%3D

Edited by Carnoferox
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