Mom, where do birds come from?

If you should ever get this question, the answer is rather short: “according to recent findings, birds are descended from maniraptoran theropod dinosaurs.” Makes sense, right?

On the 12th of December, Science published a special issue on bird evolution. One paper, by Xing Xu and colleagues, discussed the origin of birds based on current fossil evidence. I started reading it, but after a few paragraphs I got completely mind-boggled. Here is a brief discussion on what could be the most basal birds:

Suggested taxa include the probably flighted Archeopteryx and Rahovanis, the enigmatic scansoriopterygids, the four-winged Anchiornis and its kin, and the Gondwanan unenlagiids theropods.

What? Scansoriop-thingy’s? Unenlagiid? Clearly, I needed to brush up my knowledge of early bird fossils. Almost everybody knows that birds are actually small, feathered dinosaurs. But when you dive into the details, things get complicated rather quickly.

 

Some History

The idea that birds have evolved from dinosaurs was first proposed by Darwin’s Bulldog, Thomas Henry Huxley. Based on the Archeopteryx-fossils that were dug up in the late 1870s, he proposed an evolutionary link between birds and dinosaurs.

But in 1926, the Danish artist-turned-scientist Gerhard Heilmann, published a book entitled “The Origin of Birds.” Although the title was not very original, the contents were. He compared Archeopteryx and other birds with ancient reptiles and made an intriguing observation. Birds have fused clavicles (or collar bones) that are known as the furcula or “wishbone” to the more general public. You can also find clavicles in the ancient reptiles, but not in dinosaurs. Heilmann could not accept that the reptiles lost their clavicles as they evolved into dinosaurs, only to re-evolve them on the way to becoming birds. So, he proposed that a different set of ancient reptiles (which he named “thecodonts”) holds the ancestor of modern birds. However, new fossils have shown that dinosaurs do indeed possess clavicles. They are very delicate bones and don’t fossilize that easy. So, the case of the missing clavicles has been solved!

The Front Cover of "The Origin of Birds" by Heilmann, publised in 1926
The Front Cover of “The Origin of Birds” by Gerhard Heilmann, published in 1926

In the 1960s, John Ostrom described several fossils, such as Deinonychus (the Jurassic Park Velociraptors were inspired by this dinosaur). New insights led Ostrom to reject the “thecodont” hypothesis and revive the words of Huxley, namely that birds descended from dinosaurs. This was the beginning the Dinosaur Renaissance, which continues to this day.

 

Dino-divisions

The classification of dinosaurs is really not that complicated. When you take a closer look at the morphology of the hip, you can recognize two main types (I will not go into detail, just check the links if you want to know more). The Saurischia have a lizard-like hip, while the Ornithischia posses a bird-like hip. And in which group do the birds fit? Clearly, that would be the Ornithischia with their bird-like hips. But no, birds belong to the other group, the Saurischia. Ah, the logic of science!

 

An Overview of Dinosaur Taxonomy
An Overview of Dinosaur Taxonomy

 

The Saurischia are divided over two groups (called suborders): the Sauropodomorpha and the Theropoda. The former group houses some of the biggest animals that ever walked the Earth, such as the long-necked Diplocodus  that measured up to 33 meters in length. But because the birds belong to the latter group, let’s focus on the Theropoda. Animals that belong to this group share a couple of characteristics, such as hollow bones and a particular hand morphology. This suborder contains numerous tong-twisting infra-orders, divisions and families, for example Herrerasauria, Coelophysoidea, Carcharodontosauridae and Therizinosauridae. Within this collection of taxonomic jibber jabber, the birds belong to the following scheme:

  • Infraorder Tetanurae
    • Division Coelurosauria
      • Subdivision Maniraptoriformes

And so, we finally arrive at the Maniraptors. This taxonomic group contains all dinosaurs with a special kind of feather, namely the pennaceous feather, that you can find on most modern birds.

 

A Pennaceous Feather (from the Lab of Ornithology, Cornell University)
A Pennaceous Feather (from the Cornell Lab of Ornithology)

 

 

So to conclude our journey through the wonderful world of taxonomy. Birds are descended from maniraptoran theropod dinosaurs means that they possess hollow bones and a particular hand morphology (= theropod) in combination with pennaceous feathers (= maniraptoran). Now you can confidently answer the question “Where do birds come from?” Although you could also just respond with: “Eggs…”

 

More Gibberish

But what about the incomprehensible terms in the quotation at the top? Well, scansoriopterygids are members of a particular family, based on fossils from Liaoning in China. They are characterized by an extremely long third finger. So, basically little critters that are constantly flipping you off. I wonder why they went extinct…

A Member of the Scansoriopterygidae
A Member of the Scansoriopterygidae

 

References

Xu, X., Zhou, Z., Dudley, R., Mackem, S., Chuong, C., Erickson, G., & Varricchio, D. (2014). An integrative approach to understanding bird origins Science, 346 (6215), 1253293-1253293 DOI: 10.1126/science.1253293

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One thought on “Mom, where do birds come from?

  1. Have schools started to suggest the link between birds and dinosaurs to kids yet? This would be wonderful for creative and logical thinking and imagination regarding evolution. I find it odd that dinosaurs still seem to be introduced with the asteroid extinction story, whereas on the other hand kids are being told about Darwinism.

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