Anatomy & Diagnostic Characters edit
In 1887, Harry Seeley divided Dinosauria into two clades: Ornithischia and Saurischia. Ornithischia is a strongly supported clade with an abundance of diagnostic characters (common traits). The two most notable traits are a "bird-like" hip and beak-like predentary structure though they shared other features as well.
"Bird-hip" edit
The ornithischian pelvis is "opisthopubic" meaning the pubis bone points down and towards the tail (posterior) parallel with the ischium with a forward-pointing process to support the abdomen.[dino hist] This results in a four-pronged pelvic structure. In contrast to this, the saurischian pelvis is "propubic" meaning the pubis points downward and toward the head (anterior), as in ancestral reptiles.
The opisthopubic pelvis independently evolved at least three times in dinosaurs (in ornithischians, birds and therizinosauroids).[encyclopedia 537-538] Some argue that the opisthopubic pelvis evolved a fourth time in the clade dromeosauridae but this is controversial as other authors argue that dromeosaurids are mesopubic.
Predentary edit
Ornithischians share a unique bone called the predentary.[dino hist] This unpaired bone was located at the front of the lower jaws and presumably aided ornithischians in clipping plant matter.
Other Characters edit
Beak attachment edit
Ornithischians had paired premaxillary bones. At the tip of the snout, these are toothless and roughened (presumably due to the attachment of a keratinous beak). [dino hist]
Palpebral Bone edit
Development of a narrow "eyebrow" bone across the outside of the orbital fenestra (eye socket). [dino hist]
Reduced Antorbital Opening edit
A reduced, or even closed-off, antorbital fenestra (the hole just before the eye socket). [dino hist] The purpose of the antorbital opening is unclear though researchers are leaning towards the "Pneumatic Hypothesis" which states that the opening is related to airflow. [pneumatic]
Jaw Joint edit
Ornithischian's jaw joints were lowered below the level of the teeth.[dino hist] Jaw joints at the same level of the teeth result in a slicing motion of the jaw - useful for animals who shred their food. Herbivores, however, often grind their food to make digestion easier. Jaw joints that are raised above or lowered below the level of the teeth result in teeth grinding against each other.
Cheek Teeth edit
"Leaf-shaped" cheek teeth. [dino hist]
Vertebral Adaptations edit
Ornithischian backbones were stiffened near the pelvis by the ossification of tendons above the sacrum. Additionally, Ornithischians had at least five sacral vertebrae (the sacrum attaches to the pelvis). [dino hist]