Lab 4: Archosauria Info Flashcards
Evolution of Crocodilians and Birds
Archosauria
- With adoption of cladistic classification, Class Aves is now considered by some to be an artificial, obsolete grouping of vertebrates.
Crocodilians and birds are two surviving lineages of an extremely successful group of amniotes known as the Archosauria (ruling reptiles).
Extinct members of this group included the bird-hip dinosaurs (Ornithischia) and reptile-hip dinosaurs (Saurischia) and the Pterosaurs (“flying reptiles”).
Evolution of Crocodilians and Birds
diapsid skull, antorbital fenestra, and bipedalism
Ornithischia, Saurischia, and Pterosaurs
These animals share a diapsid skull, with two openings in the temporal region, which is also seen in a modified form in lizards and snakes.
In addition, archosaurs have another opening, the antorbital fenestra, in the skull between the eye sockets and the snout.
The antorbital fenestra is secondarily lost in modern crocodilians.
The hind legs are longer than the forelegs reflecting a tendency towards bipedalism (walking erect on the two hind limbs).
Evolution of Crocodilians and Birds
Crocodylomorpha
The oldest Crocodylomorpha are known from about 230 million years ago.
This group of archosaurs was once more diverse both morphologically and ecologically than are surviving forms.
Some extinct crocodilians were bipedal and completely terrestrial.
However, the semiaquatic lifestyle of living crocodilians is a very ancient one within this lineage.
Evolution of Crocodilians and Birds
Archaeopteryx
The oldest undisputed bird ancestor is Archaeopteryx, a crow-sized animal that is 150 million years old.
With its teeth in sockets, mobile clawed fingers, and long bony tail, Archaeopteryx very much resembled its near relatives, carnivorous theropod dinosaurs (a subgroup of the reptile-hip dinosaurs).
In many of its features (unfused bones, inflated brain case, large eye sockets and simple tooth shape) Archaeopteryx resembles a juvenile theropod, indicating a neotenic origin from this group.
However, it bore completely modern feathers and was probably capable of weak flapping flight.
Recently discovered bird-like theropods from China indicate that feathers are a dinosaurian character (probably for thermoregulation) and not unique to Aves.
Some authorities think that Archaeopteryx was a side-branch in avian evolution and that true birds diverged from other archosaurs much earlier.
Evolution of Crocodilians and Birds
endotherms
Our knowledge of the early history of birds is growing; a variety of toothed birds, terrestrial and aquatic, have been described from the Mesozoic.
The bones of some of the Cretaceous birds display growth rings, indicating that they were not true endotherms and were thus incapable of sustaining a consistently high body temperature year-round.
Evolution of Crocodilians and Birds
By the end of the Cretaceous period, 66 million years ago, representatives of living orders of birds had already evolved.
By the end of the Cretaceous period, 66 million years ago, representatives of living orders of birds had already evolved.
The flightless ratites (Superorder Palaeognathae) represent the most primitive living birds.
Research using DNA – DNA hybridization techniques indicate that waterfowl (Anseriformes) and terrestrial fowl (Galliformes) form the next most ancient group.
The perching or songbirds (Passeriformes) were the last to evolve; the diversification of passerines paralleled the diversification of their food sources: insects and the flower, fruit, and seed of flowering plants.
Characteristics of Birds – Weight-reducing adaptations
pneumatization
Because of the limitations placed on their body form by the demands of flight, birds are a very uniform group of vertebrates morphologically.
Skeletal parts are thin and often fused. Many bones are hollow (pneumatization).
Some portions of the bird skeleton are proportionately lighter than in mammals; other portions (leg bones) are more robust.
a. Sacral vertebrae are fused to each other and to the pelvic girdle = synsacrum (are similar to fusing of lower back in frogs called Eurostyle, so these are separate)
b. Skull completely fused; jaws lack teeth.
c. Overall reduction of size and number of skeletal elements.
Phones are somewhat hollow. Balance between lighter skeleton with less energy needed to get airborne versus skeleton becoming too weak. Therefore, bones fuse together = extra strength, and support or sheet like —> less 3-D dense
Legs are involved with takeoff.
See fusion of bones in four limbs, more distally down the wing at the wrist and hand bones
Characteristics of Birds – Weight-reducing adaptations
- Feathers increase surface area of forelimbs, to generate lift, without significantly increasing weight.
- Reduction in number of skin glands, usually only the uropygial gland is present. This gland secretes oils to maintain and waterproof feathers; located on the lower back.
- Air sacs, part of birds’ unique respiratory system, are a series of thin-walled structures through which air flows during respiration. With the lungs, the air sacs allow air to flow along a one-way route so that newly inhaled air does not mix with syale air in the system. They also reduce overall body density.
- Loss of the urinary bladder in association with the excretion of uric acid.
- Females possess only a single, functional ovary and oviduct.
- Marked reduction in size of gonads during non-breeding season.
- Embryonic development within an egg outside of the female’s body.
- Rapid and efficient utilization of small amounts of high-energy food.
Integument – feathers
Feathers are the only truly unique feature found in living birds and no other extant vertebrates.
Feathers are highly modified epidermal scales.
Birds have more typical epidermal scales on their legs and feet.
Feathers develop from follicles in the skin and are generally arranged in tracts which are separated by areas of unfeathered skin.
A typical feather consists of a long, tapering central shaft with two main parts; the calamus or quill (hollow, cylindrical basal portion. Part that goes into integumentary) and the rachis (solid, squarish distal portion).
On each side of the rachis are a row of short parallel branches, or barbs, collectively form the vane.
On the barbs are smaller branches called barbules, which have hooklets that interlock with the barbules of adjacent barbs to stiffen the vane.
Hooklets he,p make relatively solid surface
Integument – feathers
There are five commonly recognized types of feathers: contour feathers (includes the flight feathers), semiplumes, down feathers, bristles, and filoplumes
Contour feathers
• Flight, aerodynamics, streamlining
• Insulation
• Waterproofing
• Courtship and displays
Semiplumes (shape, tail)
• Insulation
• Fill out contour of body for aerodynamics and streamlining • Buoyancy in waterbird
Down feathers (some shape)
• Insulation
Bristles (narrow and fine)
• Protection around the eyes and nostrils from foreign particles
• Tactile sense
• Possibly aids in the aerial capture of small flying prey
Filoplumes
• Sensory structures – aid in the operation of the other feathers
• Decorative function, i.e., Peacock tail feathers
Bare patches on body = useful in fruiting more direct insulation/heat to eggs
Integument – feathers
Plumage coloration
The extraordinary variation of plumage coloration is due to two main factors: pigments and structural color.
• Pigments – There are three principal types of pigments in feathers: melanins, the most common pigment, produces black, grays, and browns; carotenoids, produce the intense reds and yellows; and porphyrins, produce a range of reds, browns and greens.
• Structural color – resulting from a complex pattern of refraction (breaking up) and reflection of white light in the cell membranes of the barbs and barbules of each feather. It produces white, blue, green and iridescent colors.
Pigments can provide extra strength, specifically melanin birds that do long migrations have a black wing tips
Integument – feathers
Uses of color and polymorphism
Color plays many important roles in the life of birds. Appendix B defines major types of coloration and their functions.
Polymorphism. In some bird species, individuals within the same population can display different adult color forms or morphs, e.g., snow goose with white and blue morphs. Typically, adults of different morphs interbreed freely.
Integument – feathers
Moulting
Since feathers are subject to constant wear and deterioration, they are not permanent structures and are regularly shed and replaced in a process called molting.
The exact schedule and number of molts varies for each bird species, depending on breeding cycle, habitat, and whether the species is migratory or sedentary.
Most species of birds go through a quick series of different plumages in their first months of life, then cycle between a winter, or basic, plumage worn for most of the year and a breeding (nuptial), or alternate, plumage worn only during the spring and summer.
For most species the late summer post-nuptial molt is the most important because during this molt the bird replaces all of its body and flight feathers.
Sensory Systems and Communication
Vision.
Birds depend on their eyes more than on the other senses; they are perhaps more fully visual than are any other vertebrate.
The visual acuity of hawks and eagles is roughly three times better than human vision.
The eyes of birds are extremely large.
Their shape is not spherical, the lens and cornea bulge forward in front of the posterior chamber.
The sclerotic ring, a ring of bony plates, maintains their form.
i. Color vision has been demonstrated in diurnal birds. Species have two to four cone types. It is now known that many birds can see ultraviolet light.
ii. Birds commonly have two foveae in their retina. A fovea is a pit in the retina that contains extremely high concentrations of light-sensitive receptor cells. The presence of two foveae may compensate for a lack of binocular vision in most bird’s field of vision.
Sensory Systems and Communication
Hearing
The ear of a bird is that of a typical diapsid vertebrate and not highly modified like the ear of a mammal.
Birds have a single sound-transmitting bone, the columella in the middle ear (vs. 3 in mammals) and the cochlea (the fluid-filled tube of the inner ear that contains the sensory cells) is a straight tube (vs. highly coiled in mammals).
The hearing of most birds is similar in range and acuity to that of humans.
Owls, however, have an exceptional capacity to locate the source of very weak sounds, a capacity that they use to capture small mammals at low light levels.
