Velociraptor had feathers the Jurassic periods but their ancestors likely began to diverge or become different from dinosaurs anywhere from 227 to 174 MYA These ancestors were probably a group of theropod dinosaurs called coelurosaurs Theropods which include Tyrannosaurus rex and Velociraptor were carnivorous and bipedal meaning two footed which freed the front limbs for other uses including flight Coelurosaurs probably began decreasing in size approximately 200 MYA about 50 million years before A. lithographica appeared Small size was probably essential for flight A series of morphological or physical changes followed as theropods evolved into birds First long bones became hollow and the first foot digit lost its role in supporting weight Second the function of grasping hands improved with the evolution of a rotating wrist joint Third the coracoid bone of the shoulder and the sternum or breastbone expanded which helped support bigger stronger muscles and more feathers Fourth vaned feathers appeared arranged by precise
types and layers Fifth the trunk shortened and the tail became stiffer which improved balance and maneuverability Sixth changes in the shoulder joint the development of a wing and asymmetrical feathers and the reversal of the inner toe supported basic flight and perching These changes all occurred very early in bird evolution The sixth group of changes appears in A. lithographica Further refinements of physical structures lending themselves to powered flight and perching followed Beginning approximately 150 MYA the thorax or chest became deeper Bones of the shoulder and pelvis became strut shaped for better support Then new adaptations formed including a tendon for wing muscle attachment feathers to control airflow fan shaped tail feathers and a fully opposable inner toe for perching Next the shoulder blades fused to form the wishbone A deep ridge called a keel extended from the breastbone enabling huge chest muscles to develop which are essential for powered flight All of these adaptations were in place during the Cretaceous period 145-66 MYA but the explosion in bird diversity didn't begin until the end of that period shortly after an asteroid strike caused the extinction of the larger dinosaurs The few survivors of the asteroid served as the basis for the thousands of bird species to come For example a fossil of Vegavis iaai found in Antarctica in 2005 was dated to 67 MYA just before the asteroid hit Earth Very similar to the modern day duck V. iaai is considered to be part of today's line of birds
EVOLUTION SPOTLIGHT Feathers It's logical to assume that modern bird feathers their bases forming a tuft Stage three occurred evolved for flight But feathers existed long in two parts In part A a rachis or central before flight Many dinosaurs had feathers branch developed with a series of fused barbs They probably first served as insulation along its length In part B secondary barbs which is vital for temperature regulation in called barbules formed at right angles from warm blooded animals Later some dinosaurs the barbs Stages four and five represent more developed long feathers on their arms and modern feathers In stage four the vane or legs An oviraptorosaur fossil from China's main part of the feather closed as the barbs Gobi Desert was found hunched over a nest of and barbules hooked together In stage five eggs with its feathered arms spread over them the feather became asymmetrical that is the Many dinosaurs may have had feathers with rachis was no longer central but skewed to colors and patterns A 160 million year old one side This makes the feather much more Epidexipteryx fossil had long ribbonlike efficient for flight feathers that its Chinese discoverers think were used in mating displays Scientists think feathers evolved in five stages In stage one an unbranched hollow filament developed from a cavity surrounding a small bump in the epidermis or outer skin layer In stage two several filaments fused at
1. FILAMENT 2. TUFT 3. RACHIS FUSED BARBS AND BARBULES 4. BARBS AND BARBULES HOOK TOGETHER 5. ASYMMETRICAL FEATHER
