Origin and Evolution of Vertebrates
The clades to which the Dinosauria belong, and the shared, derived
characters associated with each are listed below.
1. Chordata
bilateral symmetry, segmented muscles, notocords & nerve cord, gill slits
fossil chordates are known from the Cambrian (520 Ma)
2. Vertebrata
Big head with brain, bone around head, more developed senses
3. Gnathostoma
true jaws
4. Tetrapoda
Four weight-bearing limbs, neck, loss of gills, lungs
5. Amniota
Amniotic egg, thick skin, distinctive skulls
Skulls: anapsid, synapsid, and diapsid patterns (euryapsid = derived diapsid)
Holes also called fenestrae
Why holes? To lighten skulls and provide muscle attachment.
6. Synapsida
One opening in skull; lower: anapsid, synapsid, and diapsid patterns
7. Reptilia
Distinctive neck vertebrae and skull roof and braincase bone arrangement.
8. Diapsida
Two openings behind the eye, in the temporal region
9. Archosauria
Teeth in sockets, antorbital and mandibular fenestrae, rotating ankles
10. Ornithodira
Erect stance
Advanced mesotarsal ankle: astragalus much larger than calcaneum; both
rigidly attached to one another and to the tibia; AM ankle has single hinge
between the ankle and the rest of the foot; allows little twisting of the ankle
when walking; swings backwards and forwards only; enables speedy
movement
11. Pterosauria
Elongate fourth finger (for wing support)
Many other flight specializations
12. Dinosauria
3+ sacral vertebrae
Perforate acetabulum
Asymmetric hand
Etc..