Hmm, time to get back and make some replies/rebuttals.
First, I'd like to point out that my annoyance is primarily with the wording of this comic: stating "by any reasonable definition" is extremely disingenuous in this case (and an example of scientific weasel-wording) when, by almost every real biologist's metric, T. rex is more similar to a Stegosaurus. (But someone who did a bit of research first would find that there are plenty of other dinosaurs less related to a Tyrannosaur than today's birds are, such as any mid-to-late Cretaceous ornithischian).
tigerhawkvok wrote:Relatedness in biology is based on "shared derived characteristics" (changes), or "synapomorphies".
Nope, this is wrong. Relatedness in biology is, in fact, based on genetic similarity (search wikipedia for "relatedness"; this forum isn't letting me post a link for some reason). If you take a class on population genetics, you'll learn that the biggest indicator of this is usually evolutionary distance (measured in millions of years). Synapomorphies, on the other hand, are traits that are determined to be the result of relatedness, and are used to establish cladistic hierarchies far more often than actual direct similarity.
tigerhawkvok wrote:During that period from 230mya -> 150ish mya, the saurischian/theropod lineage accumulated a bunch of changes that *all of them share* that were never accumulated in the ornithischian lineage.
Bingo! But you appear to have missed the point of my argument. The therapods got a good 80 million year run to evolve new traits not found in ornithischians. Of course, in the period from 160 mya to the present, birds got twice as long to evolve traits not found in Tyrannosaurs, so I don't see how this could be an argument for more similarity between T. rex and modern birds (extinct birds, maybe).
tigerhawkvok wrote:we have birds sharing with _T. rex_ that it doesn't share with _Stegosaurus_:
Lacrimal exposed on dorsal skull roof, cervicals 3-6 longer than axis...
Oh boy, a bunch of anatomical differences. But how many things do birds not
share with Tyrannosaurs, and how would you quantify this? (I don't have any paleontology books sitting on my desk, but here's a few off the top of my head: hollow bones, lack of teeth, existence of a beak, extra neck vertebrae, fused collarbones... in short, some pretty darn significant differences)
tigerhawkvok wrote:The temporal problem becomes clear when you realize that the distance from us to Compsognathus (us->basal amniotes = +340 MY, basal amniote -> Chasmatosuchus, basal psuedosuchian = +90 MY = ~430 MY) is about the same temporal distance from us to Dimetrodon, also a synapsid like us ( ~ 380 MY temporal distance), which is clearly false.
Two things. First,
we are a lot more similar to a Dimetrodon than to a Compsognathus; this is because that extra fifty million years
is actually quite a lot of time, in which Compsognathus diverged quite significantly from the common ancestor we share with it.Second,
, the Compsognathus passed through a significant extinction event that the Dimorphodon did not (the T-J extinction), and extinctions are often followed by greater changes in both morphological and genetic variation. Considering that birds also went through more significant extinctions in their split from Tyrannosaurs than Stegosaurs did, this also supports my original point.
Eebster the Great wrote:Your theorycrafting won't convince me as much as actual fossil evidence. The fact of the matter is that morphologically, birds and T. rex are much more similar than T. rex and Stegosaurus.
I believe you're the one presenting theorycrafting as evidence. Maybe T. rex is more morphologically similar to a modern bird than a Stegosaurus, but that's not how any biologist would define "relatedness." Besides, all we have access to is fossilized bones (and a few feathers); bone structure is often only a part of morphological variation (and is scored very arbitrarily), so we can't really use this as our best estimate of relatedness when other more reliable metrics exist.
Copper Bezel wrote:A person is more closely related to a grandfather than to an aunt, right? (Not a perfect analogy, since the relatedness meant and the source of new genetic information are different, but they're both still there.)
You're on the right track; they're actually pretty similar concepts of relatedness. However
, since you have two
grandparents in common with your aunt, this means that you both have the same relatedness (but you're less related to your cousin).
Someday, I may have children, and now I'm wondering how hard I'll have to resist the urge to teach them that whales are the world's largest fish and giraffes the tallest amphibians.