What are the key features of dinosaurs though, exactly?
Kind of a long story and it’s a fluctuating list. Here is a list from Nesbitt:
in the skull, a supratemporal fossa (excavation) is present in front of the supratemporal fenestra, the main opening in the rear skull roof
epipophyses, obliquely backward pointing processes on the rear top corners, present in the anterior (front) neck vertebrae behind the atlas and axis, the first two neck vertebrae
apex of deltopectoral crest (a projection on which the deltopectoral muscles attach) located at or more than 30% down the length of the humerus (upper arm bone)
radius, a lower arm bone, shorter than 80% of humerus length
fourth trochanter (projection where the caudofemoralis muscle attaches on the inner rear shaft) on the femur (thighbone) is a sharp flange
fourth trochanter asymmetrical, with distal, lower, margin forming a steeper angle to the shaft
on the astragalus and calcaneum, upper ankle bones, the proximal articular facet, the top connecting surface, for the fibula occupies less than 30% of the transverse width of the element
exoccipitals (bones at the back of the skull) do not meet along the midline on the floor of the endocranial cavity, the inner space of the braincase
in the pelvis, the proximal articular surfaces of the ischium with the ilium and the pubis are separated by a large concave surface (on the upper side of the ischium a part of the open hip joint is located between the contacts with the pubic bone and the ilium)
cnemial crest on the tibia (protruding part of the top surface of the shinbone) arcs anterolaterally (curves to the front and the outer side)
distinct proximodistally oriented (vertical) ridge present on the posterior face of the distal end of the tibia (the rear surface of the lower end of the shinbone)
concave articular surface for the fibula of the calcaneum (the top surface of the calcaneum, where it touches the fibula, has a hollow profile)