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The birth of Stegosauria

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Still thrilled due to Lingwulong, I felt the needing to fill my free time with another fascinating issue regarding the early-middle Jurassic evolutionary boom. This issue is the birth of Stegosauria which is currently deeply linked to Isaberrysaura mollensis, published a year ago. Initially classified in Neornithischia by original authors (Salgado and colleagues), so many other pros are finding it to be a stegosaur (don't wanna be sassy but... guess who found it to be the basalmost stegosaur before them all?...). Tbh, even before/during its coding process, it was pretty clear that stegosaurian features exceed neornithischian features. On top of that, Salgado and colleagues didn't describe postcranial remains at all (what many of you don't know is that many postcranial clues are actually available consulting the codings in the paper's supplementary info) and provided a skeletal drawing which is pretty fallacious as it is outlined by a neornithischian, bipedal cursorial-like silhouette. I remember someone justified that saying that traits coded for pectoral girdle are discrepant in relation to basal stegosaurs and adjacent to neornithischians, unfortunately it happens the same adjacency is found in some obv graviportal stegosaurs too (Gigantspinosaurus, Kentrosaurus, Miragaia). That may mean Huayangosaurids are more likely an instance of reversal regarding scapular anatomy, anyway still irrelevant for quadrupedal-or-not image. (Note: Chungkingosaurus is a wildcard taxon, only 2 more steps are required to place it as sister to Tuojiangosaurus thus removing it from Huayangosauridae and other positions inside Stegosauria as plausible as well). Another reason why Isaberrysaura's skeletal is fallacious is that any dorsal plates are illustrated since they are not recovered, but you know the number one rule of taphonomy: absence of evidence is not evidence of absence. Last but not least, six cervicals are found in Isaberrysaura, but being disarticulated from the dorsal series, it can't be clear whether that is the actual cervical count or (as it probably is) just a part of it. (Cervical count is an easy parameter in order to deduce a stegosaur's grade of evolution).  

Neverthless, there are so many feasible alternatives regarding the position of Isaberrysaura in the tree, here you are some examples:

- Sister to Laquintasaura+Scutellosaurus (+3 steps)
- Basalmost Thyreophora (+4 steps)
- Sister to Huayangosaurus (+4 steps)
- Sister to Eurypoda (+4 steps)
- Sister to Jiangjunosaurus+Gigantspinosaurus (+5 steps)
- Some positions inside Heterodontosauridae (from +7 to +13 steps)
- Basalmost Ankylosauria (+9 steps)
- Basalmost Neornithischia (+9 steps)
- Sister to Dacentrurinae+Stegosaurinae (+9 steps)
- Sister to Emausaurus (+12 steps)
- Sister to Lesothosaurus (+15 steps)
- Sister to Chilesaurus [wtf] (+23 steps)
- Sister to Hippodraco [even more wtf] (+26 steps)
- Sister to Genasauria (+34 steps)
- Sister to Koreanosaurus (+39 steps).

So yes, its assumed position is far from being sure.
I'll patiently wait for an Isaberrysaura monography.

Currently enjoying Huayangosaurus' position which has always been solid.


..Ops! I forgot one thing: have you ever notice how insanely big is Isaberrysaura considering its primitive status? Skull length equal to 52 cm (20-21 inches) is very close to Stegosaurus itself. Pretty impressive for an almost non-stegosaur.
I think there is a sitgma in most paleoartists minds (potentially including mine) when it comes to groups as Stegosauria, like "the more evolved it is the bigger/heavier/more-quadrupedal it should be". Unfortunately, evolution doesn't follow such elementary "unilateral" stigmas.


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HewyToonmore's avatar
Was Huayangosaurus ever found in South America?
vasix's avatar
Isaberrysaura would either be a very large scelidosaur with that skull or an average-sized stegosaur really.
Artapon's avatar
are these the ancestors of the stegosaurus?
Dennonyx's avatar
The term "ancestor" is deceptive.

They are the furthest relatives of Stegosaurus that are still more related to Stegosaurus rather than to Ankylosaurus.
Megasupream's avatar
I wouldn't use the term "ancestor" if it's not a descendant. For example, your great grandmother's brother isnt an ancestor of you.
JarHead981's avatar
PrimordialProse's avatar
Ooh, nice! I had a feeling it was a stegosaur based on the skull
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