Dinosaurs and Bird Evolution

Dinosaurs are Birds

• Evolution is a branching process, with features added, lost, or modified incrementally.
• Most species go extinct, limiting our understanding of evolutionary history to surviving and fossilized species.
• Archaeopteryx exhibits bird-like flight capabilities yet retains theropod features like a long bony tail and hand claws.
• Bird bones are fragile and often poorly preserved.
• The Jehol biota preserves feathered non-avian theropods as well as birds.
• Birds are theropods belonging to the maniraptoran group, closely related to anchiornithids, troodontids, and dromaeosaurs.
• Flight evolution likely involved a combination of ground-up running and tree-down gliding.
• Birds experienced a significant evolutionary radiation after the Cretaceous extinction.
• During the Cenozoic Era, the now-extinct Gatornithiformes and phorusrhacids likely were predatory birds.

Posture

• Sprawling posture is observed in crocodilians, lizards, tuataras, and amphibians.
• Mammals and birds have a vertically aligned limbs.
• Early dinosaur depictions wrongly showed sprawling postures.
• Early dinosaur reconstructions incorrectly showed dragging tails.
• Some early illustrations were anatomically unsound.
• The dinosaur renaissance began with John Ostrom's study of Deinonychus, highlighting its active, upright posture supported by a slender build, ossified tail tendons, and a sickle claw.
• Ossified tendons stiffened the tails of some ornithopods and ankylosaurs.
• Sauropods, theropods, and stegosaurs lacked ossified tendons, allowing greater tail movement.
• Trackways indicate dinosaurs generally held their tails horizontally without dragging them.
• Most dinosaur trackways suggest an upright posture.
• Sauropods like Diplodocus had tall hips and held their necks horizontally, while Brachiosaurus had taller shoulders than hips and held their necks upright.

Cold-blooded or Warm?

• Overlap issues make "warm-blooded" and "cold-blooded" terms confusing.
• Endotherms internally regulate temperature; ectotherms rely on external heat.
• Poikilotherms' body temperatures vary, while homeotherms' stay relatively constant.
• Modern endotherms: mammals and birds and modern ectotherms: reptiles, amphibians, fish, and invertebrates.
• Endothermy enables sustained activity but requires significantly more energy than ectothermy.
• Dinosaurs were initially assumed to be ectotherms.
• Ostrom's Deinonychus discovery prompted reconsideration of dinosaur metabolism toward endothermy.
• Upright postures support higher energy levels associated with endothermy, especially in bipeds.
• Trackways indicate slower paces, which is expected as animals don't often move at top speed.
• Insulating feathers in dinosaurs and similar structures in pterosaurs suggests heat conservation adaptations.
• Predator-prey ratios suggest many prey animals for fewer predators which suggests endothermy.
• Dinosaurs were found in high latitudes suggesting warmer poles, since ectothermic crocodilians and lizards were also present.
• Bone microstructure, nasal turbinates, lungs and heart anatomy, and bone chemistry have been used to determine whether dinosaurs were endotherms, with limited and conflicting results.
• It is believed some dinosaurs were likely endothermic homeotherms or gigantothermic homeotherms.

Parenting

• Reptilian and avian eggshells have calcium carbonate to prevent drying out.
• Birds’ and crocodilians’ eggs are hard due to a thick layer, but turtles and squamates have soft, leathery eggs.
• Squamates and turtles provide minimal parental care, while crocodilians offer some protection. Birds give extensive care to their young.
• Eggs are difficult to fossilize as they are hollow and thin-shelled.
• Dromaeosaur theropod eggs were found to be blue.
• Dinosaur eggs were typically laid in shallow excavations.
• Oviraptors have been found preserved on top of nests which suggest parental care.
• Adult and juvenile skeletons found together show parental care.
• CAT scans are hard to interpret because there is little difference in density between and eggshell and surrounding rock.
• Raman microscopy and microscopic examination of dinosaur eggshells reveal that some were highly calcified, like birds and crocodilians, while others were leathery, like turtles and lizards.
• Pterosaurs likely had leathery, soft eggshells, too.
• Hard eggshells evolved multiple times within dinosaurs and parental care varied.

What to Feed a Dinosaur

• Tooth shape and function in modern animals provide evidence for dinosaur diets.
• The coronoid process of the jaw is used to determine the ability to chew.
• Many dinosaurs were herbivorous, including stegosaurs, ankylosaurs, pachycephalosaurs, ornithopods, ceratopsians, and sauropods.
• Stegosaurs, ankylosaurs, pachycephalosaurs, and early ornithopods had flattened, serrated teeth, suggesting little chewing.
• Ceratopsians and advanced ornithopods had dental batteries for shearing and grinding, continually shedding and regrowing teeth.
• Sauropods had odd, peg-shaped teeth, adapted for browsing and branch stripping.
• Plant-eating dinosaurs used fermentation to digest food. Stegosaurs, ankylosaurs, pachycephalosaurs, ceratopsians, hadrosaurs, and sauropods did this.
• Many herbivorous dinosaurs had a keratin beak.
• Ornithomimids, oviraptors and birds are theropods containing keratin beaks.
• Theropod teeth varied based on prey.
• Dromaeosaurs and troodontids had narrow serrated teeth.
• Tyrannosaurids had serrated bulky teeth for crushing through bone, while spinosaurs had conical teeth for seizing fish.
• Toothlessness and keratin beaks evolved separately three times in ornithomimids, oviraptors, and birds.

Behavior

• Dinosaur behavior is inferred from anatomy/modern animals.
• CAT scans estimate brain lobe sizes (eyesight, smell, hearing) and head orientation.
• CAT scans show T. rex had binocular vision, rapid eye movement, a strong sense of smell, and good hearing.
• Hadrosaur crests likely made low trumpeting sounds, for species identification, display, and mating.
• Horns and frills of ceratopsians likely served same purpose.
• Organisms can have multiple functions.
• Ceratopsian horn and frill variation comes from species differences and variation within.
• Mass burials suggest herding by hadrosaurs, ceratopsians and Sauropods.
• How sex worked in dinosaurs is unknown.
• A Psittacosaurus specimen preserves a cloaca.
• How it likely worked can be inferred from birds/crocodilians.
• Face biting occurred with theropods which indicates mating behaviors.
• Fossilized behaviors include dinosaurs fighting, dinosaurs sitting on nests.
• Fossils need killed/buried quickly to stay locked.
• The meaning behvaioral fossils may be unique.

Life as Giants

• Sauropods were the largest land animals, but the blue whale is larger.
• Gigantic size evolved independently 36+ times, approaching land animal limit.
• It is unknown why large size was selected for in sauropods.
• Sauropods' size made burial limited, and most are known from few bones.
• Body mass estimated from scale models/limb size relationships.
• Sauropods had different food digestion forms.
• Digestion primarily took place as hind-gut fermentation.
• Sauropods had two primary forms
• Macronarians -- tall necks/shoulders -- browsed in trees -- spatula-shaped teeth -- tall skulls
• Diplodocids -- Flattened skulls -- Pencil-shaped teeth -- browsed along the ground -- tall hips
• Tooth wear patterns support browsing (diplodocids teeth had fine wear, but macronarian teeth had coarse wear).
• Nutritious Mesozoic foods= horsetails, Araucaria, ginkgos, tree ferns
• Poor foods= most ferns, podocarp conifers like cypress/redwood, cycads
• Weight Adaptations of sauropods: pillar-shaped legs, dense bones, simple joints, and small head.
• Sauropods likely had 4-chambered hearts and giraffe-like blood pressure adaptations.
• Sauropods had continuous airflow breathing.
• There are complex set of features that enabled Sauropods to become giants.
• Their ancestral traits: Clutch of eggs + lack of food chewing
• Derived traits: fast growth, respiration + slower metabolism rate.