Birds as glorified reptiles

Abhi giriM.Sc. –I / Sem –II

Class Aves – Birds Birds (class Aves)

are archosaurs but almost every feature of their reptilian anatomy has undergone modification in their adaptation to flight.

Classification The first

classification of birds was developed by Francis Willoughby and John Ray in their 1676

Most taxonomist classify the nearly 9,000 species of Class Aves into 27 orders

To classify birds into orders and families taxonomists most often use morphological evidence from beaks, feet, plumage, bone structure, and musculature.

Characteristics Feathers- composed of keratin,

essential for flight, insulate body Wings- modified forelimbs, covered by

feathers Lightweight, rigid skeleton- bones

are thin & hollow Endothermic metabolism- high body

temperature Unique respiratory system- rapid

metabolism, lungs connected to air sacs, oxygen-rich

Beak- no teeth, horny sheath made of keratin

Oviparity- lay amniotic eggs- eggs are incubated by both parents

Bird evolution

• Birds arose in the Jurassic period• Birds evolved from

reptiles.• Thomas H. Huxley:

Birds are “merely glorified reptiles”.

• J.Z. Young described birds as ‘Masters of air’.

Birds and Reptile Homologies Scales- birds’ feet. Yolked, polar eggs Nucleated red blood cells. In

mammals the red blood cells lack nuclei.

A single middle ear bone: the stapes. Mammals have three.

The lower jaws (mandibles) have five or six bones on each side.

The Connecting Link!

Archaeopteryx• Fossil found in

Bavaria in 1861 dated at 135 to 155 mya.

• Clearly showed: Wing bones, Flight feathers, Pairs of feathers attached to each vertebra of the tail

• Immensely important for the theory of evolution.–Found only two

years after Darwin published Origin of Species (1859)

Archaeopteryx Archaeopteryx was a crow-sized, bipedal

“reptile” with a blunt snout and many small, reptilian teeth.

–Feathers on wings and tail–A strong-running terrestrial “bird” that

could leap into trees, jump among branches and make short flights.

–Capable of gliding, not long sustained flight.

–Strong, curved claws, like those of perching birds.

–Could not launch from the ground because it lacked the principal muscles that lift the wing rapidly in the recovery stroke.

–Vanes were asymmetrical, like that of strong fliers.

Birds are Dinosaurs

Evolution of Flight

Two Competing Theories1. Arboreal Theory-

Started from trees down

2. Cursorial Theory- Started from ground up

The Arboreal Theory

originally proposed in 1880 by Othniel C. Marsh

suggest that the ancestors of Archaeopteryx lived in trees and glided into flapping flight.

Arboreal Theory Evolution of flight started

with the parachuting and gliding from elevated perches.

The extensions of the bones of the forelimb enhanced by elongated (flight) feathers enabled the ancestors of Archaeopteryx to parachute and glide between trees.

The favored theory for many years.

The Cursorial Theory Proposed by Samuel Wendell

Williston in 1879 It states that “Flight evolved in

running bipeds through a series of short jumps”

Cursorial Theorysuggest that these ancestors used

their long, powerful legs to run fast with their arms outstretched, and were at some point lifted up by air currents and carried into flapping flight

As the length of the jumps extended, the wings were used not only for thrust but also for stability, and eventually eliminated the gliding intermediate.

Flight would be a logical extension of the first small jumps by this little dinosaur.

Evolution of Flight

What were the changes that lead to the glorification of

birds over reptiles ?

Amazing Physiology Higher metabolism &

temperature regulation Red fibers of avian flight muscles have an extraordinary capacity for sustained work. – can also produce heat by shivering. EndothermyTo help conserve body heat, birds fluff out there feathers to insulation. – Enables birds to inhabit both cold and hot climate

Incredible Circulatory & Respiratory Systems

The highly adapted respiratory system of birds is adapted for the high metabolic demands of flight.

Circulatory system is also well developed with complete separation of oxygenated & Deoxygenated blood.

Circulatory System

Birds have a four-chambered heart.– Separate systemic and

respiratory circulations. Fast heartbeat – faster in

smaller birds. – Hummingbird-600 times a minute Red blood cells are nucleated

and biconvex.– Mammals are enucleated and

biconcave.

Respiratory System Air follows a one way

journey. Do not have Diaphragm.

– Allows birds to fly at high altitude

– The finest branches of the bronchi are developed as tubelike parabronchi through which air can flow continuously – instead of ending in saclike alveoli as in mammals.

– The result is that there is an almost continuous stream of oxygenated air passing through the highly vascularized parabronchi.

– Air flows - posterior air sacs - to the lung - anterior air sacs and out.

Bird Brains Large well,

developed brains 6 to 11 times larger than that of similarly sized reptiles• Bird brains exhibit

functional lateralization, with left hemispheric dominance associated with learning and innovation in vocal repertoires.

Bird Neural Systems Highly developed neural systems

and acute senses mediate feats of communication and navigation. • Birds (esp. song birds) have the

greatest sound-producing capabilities of all vertebrates.

• Birds can navigate using patterns of the Earth’s magnetism, celestial cues, and perhaps polarized light.

• Birds can see into the near-ultraviolet and can hear infrasound's-sounds below the range of human hearing.

Nervous System

Birds have well developed cerebral hemispheres, cerebellum (important for coordinating movement & balance), and optic lobes.

Reproduction

Much elaborate for mating purposes

behavior include nest building, dancing, singing & colorful displays Requires dedicated

parental care. Most birds form

monogamous pairs, though many engage in additional sexual liaisons.

Skeleton Modern birds are

toothless.– Instead they have a

keratinized beak. Most birds have

kinetic skulls.– They have a wide

gape.– Upper jaw is

attached loosely increasing the gape.

Skeleton System All birds that can

fly have a large, thin Heel on their sternum that provides area for the large flight muscles to attach.

Skeleton The sternum supports the large breast

muscles The humerus, ulna, and radius, along

with the pectoral girdle and the sternum, support the wing.

Bones are strong but hollow hence light weight filled with air allowing increased respiration efficiency

copyright cmassengale

Efficient Digestive & Excretory system

Digestive system has a Crop which stores food

The avian excretory system is also efficient and light weight

Digestive and Excretory system Food passes from the

mouth cavity straight to the esophagus.

Enlargement of the esophagus called the crop stores and moistens food.

In the first chamber, The proventriculus, gastric fluids begin breaking down the food.

Then passes through the gizzard, a muscular organ that kneads and crushes the food

Excretory System Urine is formed in large, paired

metanephric kidneys. Shows two specialization not found in

reptiles 1- Uriniferous tubules are relatively longer 2- U-shaped piece called the loop of henle is usually inserted in the middle.

– There is no urinary bladder.– Nitrogenous wastes are secreted as

uric acid rather than urea.– Bird kidneys can only concentrate

solutes to 4-8 times that of blood concentration.

Excretory System

Some birds, including marine birds, have a salt gland to help rid the body of excess salts.– Salt solution is

excreted from the nostrils.

Senses Birds usually have poor sense

of smell & taste.– Some, carnivores, waterfowl,

flightless birds have well developed sense of smell & taste.

Birds have the keenest eyesight in the animal kingdom and also very good hearing.– A hawk can clearly see a

crouching rabbit a mile away!

Flight

To fly, birds must generate lift forces greater than their own mass and they must provide propulsion to move forward.

Bird wings are designed to provide lift.

Flight - Wings are Specialized for Particular Kinds of Flight

Elliptical wings are good for maneuvering in forests.

High speed wings are used by birds that feed during flight or that make long migrations.

Dynamic soaring wings are used by oceanic birds that exploit the reliable sea winds.

High lift wings are found in predators that carry heavy loads. Soaring over land with variable air currents.

Migration

Many species of birds undergo long migrations using well established routes.– Some species

make the trip quickly, others stop along the way to feed.

– Often, they follow landmarks such as rivers and coastlines.

Migration

Birds navigate using a number of cues:– Visual cues – landmarks.– Accurate sense of time.– Some may use the Earth’s magnetic

field.– Celestial cues – sun by day, stars at

night.

And thus is the end of our tale

References A Life of Vertebrate by J.Z.Young. Rise of Birds by Dr. Shankar

Chatterjee. http://

people.eku.edu/ritchisong/554notes2.html.

http://originoflight.bio54.html scienceblogs.com/

tetrapodzoology/.../luis-chiappes-glorified-dinosaurs/