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Hypsilophodontidae Evolution / Skull Comparison

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Dinosaurs have always been famous for their glorious & glittering features. Ankylosaurs got armours, ceratopsians got frills, sauropods got big size and long necks, dilophosaurids oviraptorosaurs hadrosaurids proceratosaurids got crests, pachycephalosaurs got skull roofs, dromaeosaurs got sickle claws, tyrannosaurs got killer jaws etc...
But, there are some kind of dinosaurs that never aroused any interest. They have nothing in particular. They are not big, but not too small. They have no peculiar characteristics. They are neither primitive dinosaurs nor derivatives. They are the gray area of the dinosaur's popularity. The ignored ones. Since always, hypsilophodontids.
And that's not a good thing because hypsilophodontids are extremely important in order to well understand the origin of the main branches of Ornithischia, such as ornithopods, marginocephalians and thyreophorans. As you may know, they are not considered a natural group (or monophyletic). Ultimately, every single cladistic analysis found them to be paraphyletic or polyphyletic. Another thing that makes them foggy is the fact that any clear synapomorphy or symplesiomorphy has been established with clarity yet.
According to the broadest and more inclusive conception, hypsilophodontid-grade taxa are defined as all the neornithischians external both to Marginocephalia and Dryomorpha. That would include a lot of natural groups, such as Thescelosauridae, Jeholosauridae, Orodromidae and Rhabdodontidae. Due to linnean technicalities, this definition is considerable unacceptable.
That brought me to create another pseudo-definition.
Hypsilophodontids are all the neornithischians external to Marginocephalia and more basal than Thescelosauridae, Jeholosauridae, Orodromidae and Rhabdodontidae. If Hypsilophodon will turn out to be more derivate than any of these clades, a new "definition" will have to be given.
Anyway, as you can see in the cladogram zoomming the picture, my own analyses didn't found this grade to be polyphyletic. Whether Marginocephalia nests somewhere among these taxa is still uncertain, though.
Anyway, given exception to the node (Marginocephalia+Ornithopoda) which has a low Bremer support (2), relationships and evolution of basal ornithopods are well resolved (Bremer support for each node: 3/4) mostly thanks to the very high number of characters used (621) and the possibility to state a lot of clade characters as polymorphic. Relationships between Dashanpu (Shaximiao) hypsilophodontids are not well resolved but that doesn't matter much as some species are fragmentary/poorly overlappable/potential synonyms.
But now, let's look how basal ornithopods evolved within time...

Agilisaurus 
168 MYA (China) - 1.9 m (6.2 ft) long 
Highly primitive. Very long plesiomorphic tail* (about 50 caudal vertebrae, tail length = 65% body length). High number of teeth (about 70). 5 teeth per premaxilla. Third premaxillary tooth bigger, somewhat symplesiomorphycally with chaoyangsaurids or heterodontosaurids.
Hexinlusaurus 
- 167 MYA (China) - 1.9 m (6.2 ft) long
Primitive. Massive postorbital. Very high number of teeth (80-85). Premaxilla unknown. Number of sacral vertebrae = 5. Deltopectoral girdle homoplastically similar to dryosaurids.
Kulindadromeus 
- 161 MYA (Russia) - 1.5 m (4.9 ft) long
High number of teeth (about 80). Low number of caudal vertebrae (30/35). Low number of premaxillar teeth (3 per ramus).
Preserved integument parsimoniously attributed to all basal ornithopods.
Othnielosaurus 
- 151 MYA (North America) - 1.9 m (6.2 ft) long
Massive jugal. Very long palpebral. Premaxilla unknown. Number of sacral vertebrae = 6. Probably includes Drinker.
Hypsilophodon 
- 127 MYA (England) - 2.0 m (6.3 ft) long
Highly pneumatized skull (may be influenced by post-mortem bacteria). Number of teeth = 60/65. Number of premaxillar teeth = 5. Edentulous, pneumatized, rugose tip of premaxilla. Short palpebral. Highly placed infratemporal fenestra. Massive quadratojugal. Number of sacral vertebrae = 6. Number of caudal vertebrae = 48.
Gideonmantellia 
- 127 MYA (Spain) - 1.7 m (5.6 ft) long
Derivate, close to orodromids. No skull remains known. Massive caudo-femural musculature. Closely linked third and fourth metatarsal.** 

*This character compared at least another two times along Ornithopoda: among tenontosaurid-grade taxa (Leaellynasaura 71%, Tenontosaurus 62%) and within hadrosaurids (55-63%).
**This character compared at least another two times along basal ornithopods: it is present in thescelosaurid Parksosaurus and elasmarian Morrosaurus.

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AndreOF-Gallery's avatar
I finally could see the cladogram in the bottom, and I got surprised an want to ask you some questions:

1-Isaberrysaura as a Stem-Stegosaurian, I know it is was put as a basal Neornithischian, what are the reasons you found it as a Thyreophoran and a Stegosaur grade Thyreophoran?

2-What makes you think all the generally regarded as more basal thyreophorans (Scutellosaurus, Emausaurus, etc) in the Ankylosaur side of the tree instead of the more common basal position?

3-Why do you think Elasmaria is paraphyletic and what makes the 3 new groups together (that is the Macrogryphosaurus, Trinisaura and Gasparinisaura)?