Cartoon reasoning about dinosaurs 
In a recent XKCD cartoon (which I've also embedded below), Randall Munroe juxtaposes two claims:

1. "By any reasonable definition, T. Rex is more closely related to sparrows than to Stegosaurus."

2. "Birds aren't descended from dinosaurs, they are dinosaurs."

As far as I know, claim 1 represents what is true of the phylogeny. The picture below the claim nicely illustrates some of the reasoning for it.

The cartoon suggests without quite saying that claim 1 is a reason to accept claim 2. But I just don't see how the inference is supposed to work. Moreover, claim 2 is basically false.

Either 'dinosaur' is meant to be a scientific term or it is not.

If it is not a scientific term, but rather just a bit of popular language, then claim 2 is obviously false. If you tell someone that you have eaten dinosaur meat, you have said something false even though you have eaten fowl. And so on.

We could, of course, change how we use the word 'dinosaur'. Perhaps it will catch on by the year 2063 to call chickens dinosaurs, and then you can say that you 'have eaten dinosaur'. But this would make "Birds are dinosaurs" true in a cheap way. It wouldn't be because of claim 1, but instead because of how we decided to use language.

So suppose that 'dinosaur' is supposed to be a scientific term. Consider the kinds of phenomena which scientists try to describe or explain using the word. Claim 2 is only justified if there is some descriptive or explanatory leverage to be gained.

Paleontologists were excited about dinosaurs in the late 19th century because there were fossilized skeletons of these giant things that didn't seem to be around anywhere. Scientists wondered: Are there any of these around in unexplored corners of the world? If not, what happened to them?

So a scientist might try to explain why dinosaurs went extinct. It would be no answer to say that they didn't go extinct - look, a chicken! The giant things are gone, and the question of how and why can be expressed using the word 'dinosaur'. If we were to start counting birds as dinosaurs, then we have to introduce new verbiage to express that question.



I suppose I should add: People sometimes claim that any legitimate biological categories must be based on phylogeny, and that might underwrite the move from claim 1 to claim 2. But that won't help here. First, those people are wrong. Lots of legitimate biological terms aren't phylogenetic; e.g. 'predator'. Second, claim 2 suggests that if only the relationship were distant enough birds could be descended from dinosaurs without actually being dinosaurs. The people who insist that all biological categories must be clades, however, are forced to say that any creatures descended from members of some group are thereby members of that group. So claim 2 looks at best misleading.

It may be that those people have already had enough of an influence on usage that "Birds are dinosaurs" counts as a true utterance, at least given the usage of people who read XKCD. But then we're just in a case like the hypothetical 2063.

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