Was Archaeopteryx a bird?

[beaglelady]

Coursera has a good course on the dino-to-bird transition:

Paleontology: Theropod Dinosaurs and the Origin of Birds

The instructor is Phillip Currie, of the University of Alberta.

[Andy7 (Andy)]

This is interesting stuff from the humour thread .

There is also Dr. Steve Brusatte’s (Stephen Brusatte, Paleontology Research) book “The Rise and Fall of the Dinosaurs”, chapter 8, “Dinosaurs Take Flight”, for a good exposition for non-biologists like me.

Somewhere within this flock of paravian species lies the line between non-bird and bird. As with the division between non-dinosaur and dinosaur, way back in the Triassic, this distinction is blurry. And it’s getting less distinct with each new fossil from Liaoning. Truthfully, it’s just a matter of semantics: today’s paleontologists define a bird as anything that falls into the group that includes Huxley’s Archaeopteryx, modern birds, and all descendants of their Jurassic common ancestor. It’s more historical convention than a reflection of any biological distinction.

[T_aquaticus]

@Mervin_Bitikofer

Help me understand this. Because it sounds to me like you’re saying that if ‘X’ is a bird, then we are obliged to include all the common ancestors preceding ‘X’ in what we call ‘birds’ as well.

If you say that species X and Y are birds then you also have to include the common ancestor of X and Y and all descendants of that common ancestor in the bird group.

@Mervin_Bitikofer

Or if we only go back to the ‘crown group’ (which I gather is what you’re saying), then doesn’t that chosen ‘crown group’ represent an arbitrary cutoff (or cuttoff of convenience according to fossils that have been found)?

The common ancestor of X and Y and all descendants of that common ancestor is objective and is not arbitrary. The hard part is determining how species are related to one another, both extant and extinct.

In comparison, how species are split up or grouped into genera, families, orders, etc. within Linnaean taxonomy is arbitrary. There is nothing stopping us from lumping humans and chimps into the same genus, or all apes into the same genus.

[Mervin_Bitikofer]

@T_aquaticus

If you say that species X and Y are birds then you also have to include the common ancestor of X and Y and all descendants of that common ancestor in the bird group.

Okay - but then there must be something very basic I’m missing here. Because we could say that some protazoan from 4 billion years ago could be the common ancestor of all birds (and everything else too), right? So now what I hear you saying is that the common ancestor (single-celled organism) must now be considered a bird - because it is a common ancestor of both X and Y after all.

[T_aquaticus]

@Mervin_Bitikofer

Because we could say that some protazoan from 4 billion years ago could be the common ancestor of all birds (and everything else too)

It’s the “everything else too” that is confusing you. We are talking about the common ancestor of birds, not the common ancestor of all vertebrates. For cladistics, you go back to the common ancestor of the group, and no farther. If you go back farther than the common ancestor of all birds then you are no longer talking about the bird clade. That would be a larger clade, such as the dinosaur clade or the amniote clade.