Major Subgroups in the Phylum

Used with permission from T. Holtz Jr. (http://www.geol.umd.edu/~tholtz/G104/10417what.htm)

MAJOR SUBGROUPS

SAURISCHIA ("Lizard hips")

• Saurischian dinosaurs have long necks, in which the cervical vertebrae (neck bones) are extremely long in some forms
• The hand has an enlarged thumb claw
• The second finger is the longest in the hand
• Some dinosaurs within this clade remained carnivores (meat eaters) like their ancestors, while others became herbivores (plant eaters)
• The most basal known saurischian dinosaur is Eoraptor (see above figure)

SAUROPODOMORPHA ("sauropod form")

• These dinosaurs have very large nostrils
• Extremely long necks with extra cervical vertebrae
• Shortened fingers, and the first toe on each foot was longer than the rest
• Leaf- or peg-shaped teeth designed for eating vegetation
• The earliest sauropodomorphs were both bipedal and quadrupedal, but later forms were extremely large and too heavy to walk on two limbs so they were purely quadrupedal

THEROPODA ("beast foot")

• Theropods were almost exclusively carnivorous
• They all had hollow limb bones, which lightened the skeleton
• The skull was large in comparison to their sauropodomorph relatives
• Teeth were sharp, pointed, and had serrated edges for cutting flesh
• Large functional hands with long fingers
• Every member of this clade walked on two legs
• The earliest known, true theropod may be Herrerasaurus from the Triassic of Argentina

ORNITHISCHIA ("bird hips")

• All known forms are herbivorous, with leaf-shaped teath
• A predentary bone at the front of the lower jaw that forms a beak
• The most primitive form is Pisanosaurus from the Triassic of Argentina

THYREOPHORA ("shield bearer")

• These are quadrupedal animals that are covered in dermal armour in the form of spikes, shields, spines, and scutes
• The tibia (shin bone) is shorter than the fibula (thigh bone)
• Some used their armour in defence, and others in offense or attack

ORNITHOPODA ("bird foot")

• These dinosaurs had extremely specialized jaws, some with thousands of teeth
• Special jaw articulation that allowed for 'chewing' of the vegetation that they ate
• Some forms has extravagant head ornamentation

MARGINOCEPHALIA ("ridge head")

• The more primitive forms of Marginocephalia were bipedal, but later forms were all quadrupedal
• A large shelf of bone extended over the back of the skull

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Author: Emma Schachner
Last updated: 13 November 2004
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