LIZARD

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Temperature Relationships in Animals -The lizard’s mode of life is energetically cheap -By not generating heat internally, lizards can devote up to 50% of food energy to reproduction and growth, vs. 1-2% in birds and mammals -And the three-chambered heart is advantageous. Blood coming back to the heart from the body and the lungs remains largely separate in the ventricle, but when the lizard is warming itself, it can shunt blood directly back to the body, bypassing the lungs.

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Temperature Relationships in Animals: Summary -Most fish: Cold-blooded, poikilothermic, ectotherms -Lizards & Snakes: Warm-blooded (often), homeothermic (often), ectotherms (typically) -Birds & Mammals: Warm-blooded, homeothermic, endotherms – helps to permit an active lifestyle in a range of environmental conditions, but at an energetic cost. -Dinosaurs? Modeled after lizards and other ecothermic reptiles?

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Diversification of Diapsids -Amniotes began to diversify in the Permian Period, along with gynmosperms. -Synapsid lineage actually became the predominant group of large land vertebrates in the Permian (more later) -Diapsids diversified in a number of directions in Mesozoic Era

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Diversification of Diapsids -Lepidosaurs included the aquatic Plesiousaurs and Ichthyosaurs, and the Squamates (including lizards and snakes). -Squamate-type diapsids appeared in the late Permian, Ichthyosaurs in the Triassic, and Plesiosaurs in the Jurassic

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Diversification of Diapsids -The Archosaurian lineage includes crocodiles, pterosaurs, dinosaurs, and the only living group of dinosaurs, the birds. -From its inception, this lineage showed adaptations for terrestrial mobility

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Archosaurs -Fosssils of the earliest archosaurs showed signs of asymmetry in the lengths of fore- and hind-limbs – suggesting that they may have utilized bipedal locomotion sometimes. Some lizards do this today.

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Archosaurs -The earliest true dinosaurs appeared in the Triassic period. Had: -Clear asymmetry between fore- and hind-limbs -Changes in hip structure -Weight-reducing skeletal features -Suggesting bipedal locomotion with limbs tucked under body, and a highly mobile life style Herrerasaurus

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Archosaurs -Both the Ornithischian and Saurischian lineages of dinosaurs were mobile -Ornisthsichians (“bird-hipped,” although they don’t include the birds!) included duck-billed dinosaurs and armored dinosaurs like Triceratops

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Archosaurs -The Saurischian dinosaurs (“lizard-hipped, although they do include the birds) included bipedal, obviously highly mobile carnivorous forms, covering a range of sizes -Tails, with overlapping vertebrae and a strong dorsal ligament, served as counterweights for the body. Bones were hollow and light, like birds.

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Archosaurs -Another lineage of Saurischians included the massive, secondarily-quadrupedal forms. Reached weights of 100,000 pounds or more! -Old idea: They were so massive that they could barely support their own bodies

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Archosaurs -Newer conception: Mobility on land. Footprints were just a couple of feet apart, and bones also showed weight-saving features.

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Archosaurs -So it is now pretty well established that dinosaurs may have been highly active animals, not sluggish lizard-like animals. -What were their temperature relationships? Some have argued for endothermy in dinosaurs, but there are several possibilities: -Warm temperatures in the Mesozoic era may have kept dinosaurs warm. -The largest dinosaurs may have had “inertial endothermy” (“gigantothermy”) – With their huge body masses, heat would have dissipated slowly (small surface to volume ratio), and perhaps they would have retained body heat from the previous day’s activities overnight, allowing them to resume activity in the morning. -But what about small, active dinosaurs?

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Archosaurs -Some recent fossils of bipedal saurischians seem to show impressions of downy feathers. -So, with insulation, were they endothermic? -There is evidence pro and con. They lack “turbinate” bones found in the noses of mammals and birds (used to condense water from exhaled breath), but they may well have had insulation, and they were most probably very active animals. Stay tuned.

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Archosaurs -Although there are a few holdouts, most paleontologists now regard birds as members of a clade of small, fast-moving saurischian dinosaurs. -Impressions of feathers associated with non-avian dinosaurs supports the origin of feathers as insulation, only later to acquire a function in flight. Feathers are homologous to reptilian scales. Fuzz of possible downy feathers on a small, ground-running dinosaur

Slide 15: 

Archosaurs -Archaeopteryx is clearly a bird, but shows a number of “reptilian” characters as well.