When did Pakicetus and Basilosaurus Diverge?

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On a related note, I was wondering about something. How is the time to the most recent common ancestor of Pakicetus and Basilosaurus determined? It seems to me there’s quite a lot of wiggleroom concerning the date at which that common ancestor could have existed.

The cladogram below implies that there’s 15 million years from the common ancestor to Basilosaurus, so where does Bechly get the 5 million year interval from?

I can just move the positions of those nodes further back(the relative positions nor the timings of the fossils themselves don’t change), and then there’s over 20 million years to the most recent common ancestor:
Whale evolution nodes

It seems to me you need some sort of model of rate of morphological evolution to say where the nodes in the tree should go, since we (as far as we know) don’t necessarily have fossils of the actual common ancestors.

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This is a very good point.

And I like your figure. (see this @art?)

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Excellent question. TimeTree.org doesn’t have any details for Pakicetus or basilosaurus, so it can’t be there. So he probably shouldn’t be using that site to get comparative divergence times for other species.

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I mean the simple answer of course is it’s all based on contextual radiometric or stratigraphic dating of fossils.

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Bechly assumes the fossil record is complete and that Pakicetus is ancestral to Basilosaurus. He’s just counting the time gap between them.

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It’s probably related to this:

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ROFL. But that’s stupid. The timing aspect of his challenge is built on an extremely dubious premise then.

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That would explain the dates of fossils, not necessarily when (within some possibly rather large interval of time) their common ancestors lived.

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This surprises you?

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There’s methods in paleontology where fossil dates are used to calibrate branch points. I don’t build morphological based phylogenies however so I couldn’t do the details more justice than that.

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That is stupid.

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Simply speaking, the branch point must be at least as old as any fossil descended from it. Fossils can only constrain the minimum age of a node, not the maximum age. You can also attach an age distribution to the node constraint, often an exponential decay of probability as the node gets older than the fossil. People often use Bayesian methods (e.g., BEAST) to get age calibrations for molecular trees with nodes anchored by fossils.

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Seriously? That’s bonkers.

But if he actually is taking the time gap between the species existence and not the time back to their common ancestor, then any two extant species will have a time gap of zero and can be used to meet his challenge.

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Of course, the whole challenge is based on on the extremely dubious premise that there no conceivable reason evolutionary change might have occurred more rapidly at various times in the past, so…

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I can see how this makes sense as a rough approximation across the fossil record.

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