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Introduction to Biogeography and Amniotes. BIOL 4270. Biogeography of Ratite Birds. Biogeography of Ratite Birds. Biogeography of Ratite Birds.
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By understanding where the major glacial refugia were (based on where the ice sheets extended), we can begin to investigate how species recolonized Europe after the loss of the ice sheets.
Construct a phylogeny to determine the relationships among populations across the study area (in this case Europe) Map the different phylogenetic groups in relation to geographic location Based on the current relationships among populations, we can estimate the order of divergence (who diverged from whom) If we combine the relationships among species, orders of divergence, and geographic location of current populations, we can estimate the movement of populations from the refugia and across the mainland
Amniote origins and nonavian reptiles “Enclosing the pond” • In Amphibians, life and reproduction is intimately tied to water • Shell-less eggs, thin, moist skin, and (usually) gilled larvae all depend on water. • How to cope? Answer: Enclose those pond-conditions within an egg.
Once the ties to aquatic reproduction were cut, conquest of land truly began. During the early Paleozoic, a group of tetrapods employing this reproductive tactic arose from a monophyletic assemblage called Amniota. By the end of the Paleozoic, multiple lineages had already diverged giving rise to all nonavian reptiles, birds, and mammals.
Evolution of Amniotes Extant amniotes evolved from a lineage of small, lizard like forms that retained the anapsid skull pattern of early tetrapods Early amniotes evolved an amniotic egg, allowing them to exploit drier habitats than their ancestors
Amniote skull morphology Anapsid: no openings in the temporal area of the skull behind the orbit (opening for the eye). Temporal region completely roofed by dermal bones. Ancestral form. Present in turtles, but likely through independent evolution from the ancestral form.
Amniote skull morphology Diapsid: two temporal openings: one located low over the cheeks, and a second positioned above the lower pair in the roof of the skull. Characterize all birds, lizards, and snakes
Amniote skull morphology Synapsid: Single pair of temporal openings located low on the cheeks and bordered by a bony arch. Present in mammals
Who cares why they have different skulls? • Temporal openings are occupied by large muscles that elevate the lower jaw • Changes in jaw musculature might reflect shift from suction feeding in aquatic vertebrates to terrestrial feeding (requires larger muscles for more static pressure) • Amniotes have much more variation in feeding biology than anamniotes
Amniotic Egg The embryo develops within the amnion and is cushioned by amniotic fluid. Food is provided by yolk from the yolk sac and metabolic wastes are deposited within the allantois. As development proceeds, the allantois fuses with the chorion, a membrane lying against the surface of the shell; both membranes are supplied with blood vessels that assist in the exchange of oxygen and carbon dioxide across the porous shell. embryo Mineralized shell chorion allantois Yolk sac amnion
Amniotic Egg How did the amniote egg evolve? - One hypothesis: 1st step was replacement of the jelly layer in anamniote eggs (which limits larval size and development speed). Shells provide better support and O2 transport. Shell calcium can also be dissolved and reabsorbed by developing embryo embryo Mineralized shell chorion allantois Yolk sac amnion
Amniotic Egg Remember: all amniotes lack gilled larvae and have internal fertilization. Thus, no need for aquatic environments during reproduction, but internal fertilization is required because sperm cannot penetrate the shell. In most amniotes, a copulatory organ is used (except tuataras and most birds).
Thicker and more waterproof skin Amniote skin tends to be thick, highly keretanized, and has low water permeability Keratin can be modified to create scales, hair, feathers, and claws; provides protection from trauma and hydrophobic lipids in the skin limit water loss Scales are not homologous to fish scales – those are mostly bony, dermal structures
In crocodilians, scales remain throughout life and grow gradually to replace wear In lizards and snakes, a new, keratinized epidermis grows beneath the old, which is shed at intervals Turtles add new layers of keratin under old layers of the plate-like scutes(modified scales)
Why? Because crocodilians have bony plates called osteoderms located in the dermis that contain chromatophoresthat give lizards and snakes their colourful hues. It is also the layer that is converted to boots.
Rib ventilation of the lungs Amniote lungs are much better developed than those of amphibians; amniote lungs have much more surface area and different ventilation mechanisms Why? Amniotes have higher metabolic demands and poor cutaneous respiration Amniotes draw air into their lungs (aspiration) by expanding the thoracic cavity using rib muscles or pulling the liver posterior.
There are always exceptions…. Sea snakes primarily use cutaneous respiration The Fitzroy River turtle is also known as the “bum breathing turtle” by locals
Stronger jaws Most fish jaws are designed for suction and quick closure. Once prey are seized, little force can be applied Skeleton and muscles of the jaws of early tetrapods were adapted to seize prey. Expansion of the jaw musculature (esp. into temporal openings) provided a better mechanical advantage.
High-pressure cardiovascular systems All amniotes have functionally separate circulations. Mammals, birds and crocodilians have 2 completely separated ventricles; other nonavian reptiles have partitioned chambers. Higher blood pressure is adaptive for active terrestrial organisms because of higher metabolic needs and because the heart must overcome gravity to pump “uphill”
Water-conserving nitrogen excretion Amphibians excrete waste as ammonia or urea, but ammonia is toxic and requires a very dilute solution (doesn’t work so well for animals in dry, terrestrial habitats) Mammals excrete waste as urea, which is concentrated in the kidneys, reducing water loss through excretion Birds and nonavian reptiles excrete uric acid. They aren’t able to concentrate urine in the kidneys. Use urinary bladder to reabsorb water (and salts) and void a semisolid mass
Expanded brain and sensory organs Relatively large cerebrum and cerebellum in all amniotes, esp. birds and mammals Important for integrating sensory information, control of muscles during locomotion Vision in nonavian reptiles and birds is particularly good – many birds can see in UV, while some lizards and snakes can detect UV and infrared Birds generally have poor smell, but smell is highly developed in snakes and lizards; olfaction is assisted by Jacobson’s organs – olfactory chambers in the roof of the mouth
