RE: Pachycephalosaurus, Dracorex, and Stygimoloch

[Shadowknight1]

It may be dumb of me to ask, but why are Dracorex and Stygimoloch still treated as their own genus when it sounds to me like most scientists are agreeing that they're both juveniles of Pachycephalosaurus?

[Halichoeres]

I'm not sure how unanimous that is. My impression is that the evidence is circumstantial and the remains only moderately good. But even if it's true that they are juvenile Pachys, their morphology is different enough that they're still interesting, and it's easier to remember than, say, "juvenile stage 1" and "juvenile stage 2." For what it's worth, it's unusual in amniotes to see something as elaborate as Stygimoloch's horns grow and then be resorbed.

[Shadowknight1]

I'm not sure how unanimous that is. My impression is that the evidence is circumstantial and the remains only moderately good. But even if it's true that they are juvenile Pachys, their morphology is different enough that they're still interesting, and it's easier to remember than, say, "juvenile stage 1" and "juvenile stage 2." For what it's worth, it's unusual in amniotes to see something as elaborate as Stygimoloch's horns grow and then be resorbed.

This is the thing that constantly confused me about that.  I could understand a frill growing larger, so I don't have a problem with the whole Torosaurus/Triceratops thing as much, but losing the horns completely from Stygi to Pachy?  Seemed a little odd to me.

[Mini Minmi]

From what I understood, it was deemed plausible that stygimoloch's horns would disappear based on triceratops' horns changing direction as they age as well as the tiny triangles around the frill (you'll have to forgive me the name eludes me at the moment) disappearing between sub-adult and full grown adult size. The adults seem to show an almost smooth frill. But still, stygimoloch's horns seem much larger so it's tougher to believe.

[Cretaceous Crab]

I'm not sure how unanimous that is. My impression is that the evidence is circumstantial and the remains only moderately good. But even if it's true that they are juvenile Pachys, their morphology is different enough that they're still interesting, and it's easier to remember than, say, "juvenile stage 1" and "juvenile stage 2." For what it's worth, it's unusual in amniotes to see something as elaborate as Stygimoloch's horns grow and then be resorbed.

Unusual but not unheard of.  One species that comes readily to mind is the common snapping turtle (Chelydra serpentina). Hatchlings and juveniles have heavily ridged carapaces, which often leads to them being mistaken for Alligator snapping turtles. The ridges recede as the animal matures, and  a grown common snapper will have a smooth shell.

[Halichoeres]

I'm not sure how unanimous that is. My impression is that the evidence is circumstantial and the remains only moderately good. But even if it's true that they are juvenile Pachys, their morphology is different enough that they're still interesting, and it's easier to remember than, say, "juvenile stage 1" and "juvenile stage 2." For what it's worth, it's unusual in amniotes to see something as elaborate as Stygimoloch's horns grow and then be resorbed.

Unusual but not unheard of.  One species that comes readily to mind is the common snapping turtle (Chelydra serpentina). Hatchlings and juveniles have heavily ridged carapaces, which often leads to them being mistaken for Alligator snapping turtles. The ridges recede as the animal matures, and  a grown common snapper will have a smooth shell.

That's an interesting example. As you say, not unheard of. And of course quite common in amphibians and fishes.

[Cretaceous Crab]

Unusual but not unheard of.  One species that comes readily to mind is the common snapping turtle (Chelydra serpentina). Hatchlings and juveniles have heavily ridged carapaces, which often leads to them being mistaken for Alligator snapping turtles. The ridges recede as the animal matures, and  a grown common snapper will have a smooth shell.

That's an interesting example. As you say, not unheard of. And of course quite common in amphibians and fishes.

And we already know that tyrannosaurus and ceratopsians went through some pretty hardcore changes as well. So I don't feel its that much of a stretch from a Stygi or Dracorex to go to a Pachy.

[Halichoeres]

Unusual but not unheard of.  One species that comes readily to mind is the common snapping turtle (Chelydra serpentina). Hatchlings and juveniles have heavily ridged carapaces, which often leads to them being mistaken for Alligator snapping turtles. The ridges recede as the animal matures, and  a grown common snapper will have a smooth shell.

That's an interesting example. As you say, not unheard of. And of course quite common in amphibians and fishes.

And we already know that tyrannosaurus and ceratopsians went through some pretty hardcore changes as well. So I don't feel its that much of a stretch from a Stygi or Dracorex to go to a Pachy.

They did, but none seem to involve resorption. All the changes in Triceratops and Tyrannosaurus seem to be explainable in terms of allometry. Even the comparatively long arms of Nanotyrannus, if that proves to be a juvenile T. rex, weren't larger in absolute terms than those of an adult. That's the part that I'm stuck on. I'm not sure that snapping turtle shells aren't allometry, either, or how much of the difference is only in the keratinous layer of the shell (maybe you can shed light on that). And of course we have no data on the extent of keratin extensions of pachycephalosaur horns, as far as I know.