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Ornithischia Intro, Stegosauria

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Ornithischia Intro, Stegosauria 1 Ornithischia Intro, Stegosauria I. ORNITHISCHIA - highly derived (appeared after Saurischia) & highly diverse A. Shared derived characters: 1. back-prong on pubis 2. long, thin forward process on ilium 3. at least five sacral vertebrae 4. ossified tendons around sacrum Tendon is connective tissue that connects muscle to bone (aka sinew) for further strengthening. Why would this group need further strengthening in its sacrum? 5. all herbivores 6. predentary = unpaired, scoop-shaped bone at front of lower jaw 7. toothless tip of snout with "beak" 8. small, triangular teeth 9. jaw joint set below level of the upper tooth row B. Main Groups of Ornithischians 1. Fabrosaurids: most primitive ornithischians a. Pisanosaurus: upper Triassic: poorly known, lacks clear ornithischian characters b. Lesothosaurus: lower Jurassic: least derived, well known ornithischian 2. Genasauria Shared derived characters: Reduced mandibular foramen, muscular cheeks a. Cerapoda ("horned foot") Have a gap (diastema) between premaxilla and maxilla 5 or fewer teeth on premaxilla i. Ornithopoda ii. Marginocephalia: skull bone shelf extends behind the skull Two main groups - Pachycephalosaurs and Ceratopsians b. Thyreophora ("shield bearer") - named by Baron Franz Nopsca in 1915 Shared derived characters: Dermal armor (rows of keeled scutes on dorsal surface) i. Several less derived taxa: Scutellosaurus (small biped), Scelidosaurus (larger quadruped) ii. Eurypoda ("broad foot") Shared derived characters: enlarged anterior ilium, short ilium behind hip socket stocky foot/hand bones new bones around the eye Two main groups - Stegosauria and Ankylosauria 2 II. STEGOSAURIA ("roofed reptile") A. What? Shared Derived Traits: Rows of osteoderms over the body that developed into spines and plates Parascapular spines = over shoulder blade, spines on tail Loss of ossified tendon Body sloped forwards due to short front legs (not in most primitive ones) Very tall neural arches on thoracic vertebrae (WHY? - more room for viscera) Feet with broad hooves Solid box-like skulls (defense, defense, defense) Other features Five toes on front feet, three to four toes on back feet 3-9 meters long 300-6500 kg (to over 7 tons) B. When? early Jurassic to early Late Cretaceous Late Jurassic was heyday of Stegosauria Ultimately 11+ genera C. Where? Cosmopolitan - earliest = in China and western Europe D. Who? Many specimens in poor shape = difficult to classify group Only a couple of Chinese stegos + Stegosaurus have relatively complete skeletons Most derived stegos have plates above all vertebrae + large nares Huayangosaurus: basal stegosaur (14 feet, Middle Jurassic), legs more equal in size Kentrosaurus - Tanzania, about 17 feet long, 2 tons Stegosaurus: largest and best-known member of clade, thanks to the Morrison Fm. Elongated snout End of snout covered by a horny beak Teeth are deeply set in from sides of face, suggesting cheeks Many Chinese forms Only one known South American stegosaur E. Physiology and Lifestyles : LIMITED INFO! A very successful group despite their bad wrap as idiotic microcephalics Present for over 100 million years Lived in floodplains + coastal areas +? Osteoderms/Plates: what is the deal? Some enlarged as spikes 3 All our embedded in the skin Plates probably related to thermoregulation + intimidation Mobile or immobile??? Probably evolved as midline armor, since Huayangosaurus (primitive) has midline spikes Shapes and pattern of spikes and plates species-specific Plates vascularized, for heat control = solar panels + swamp coolers (rapid pumping of blood could cool dinos) Plates in paired, staggered rows. No dorsal plates in juveniles Habitats: floodplains and coastal plains Gait/Speed Not built to burn rubber, but like elephants were probably able to move quickly in bursts Hind limbs able to outrun forelimbs at high speeds - bipedal? Trackways suggest quadrupeds, not bipeds. Speed? 7km/hr max? Feeding Cropped plants with sharp, toothless beaks Tiny teeth and weak jaw muscles Gastrolith users? No evidence yet. Narrow heads, short legs = low browsers or, did they rear up at feed on higher parts of trees? (Hindlimbs supported 80% of wt.). Flexible tail could be third leg in tripod position Brains Brains rot after death and burial, brain info comes from natural or lab-made endocasts Brain in verts. are proportional to body size, but brains grow in size at slower rate than bodies Only Kentrosaurus and Stegosaurs brain casts available - WHY? These relatively long, slightly bent, small (3 oz), with large olfactory bulbs, typically reptilian - as big as 3 walnuts Why small brains? They relied on armor rather than wily evasion to escape consumption Unhurried lifestyle and uncomplicated range of behaviors Large expansion of spinal cord in hip region = 20x size of brain DID NOT house a rear brain, but some other types of soft tissues Behavior Some sexual dimorphism Mass accumulation of Kentrosaurus in Tanzania is hard to interpret! Defense .
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