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Introduction to Biogeography and Amniotes
BIOL 4270
Factors influencing tetrapod distributions
Biogeography: study of the spatial distribution of organisms through geological time. In other words, study of how and why plants and animals are where they are
Phylogeography: analysis of the relationship between population genetic structure and biogeography
Basic principles of biogeography• Species distributions are a result of
historical factors, including biotic factors such as extinction, speciation, dispersal, and ecological processes
• Distributions are also largely influenced by historic vicariance events, such as continental drift, glaciation, sea-level changes, degree of isolation, formation of mountain chains, etc. vicariance: process by which a species’
range is split by a physical barrier
Major themes in biogeography1) Classifying geographic regions based on
biotas2) Reconstructing the historical
development of lineages and biotas, including their origin, spread, and diversification
3) Explaining the differences in numbers as well as types of species among geographic areas, and along geographic gradients including those of area, isolation, latitude, elevation, and depth
4) Explaining geographic variation in the characteristics of individuals and populations of closely related species, including trends in morphology, behavior and demography
Some broad bioegeographic patterns
Spe
cies
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Species richness increases with the amount of solar energy and rainfall, but decreases with increased elevation
…which is exactly why we see high species richness in the tropics – lots of sun and rainfall
Spe
cies
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Complex Topography
Great geological age
Habitat size
Some broad bioegeographic patterns
British Columbia (944k sqkm)= 22 amphibians
18 reptiles 137
mammals 510 birdsEcuador (272k sqkm)= 492 amphibians
350 reptiles
317 mammals 1663 birds
So, Ecuador is ¼ the size of BC, but has 20x more amphibians and reptiles
So why does Ecuador have so many species?
1) Solar Energy, abundant rainfall leads to high productivity and large resource base
2) Longer periods of stability3) Warm temperature, high humidity
favorable for growth4) High competition, high niche
specialization due to predictability of environment
5) Pressure from parasites, disease6) Large geographic area
Terrestrial vertebrate species richness
Global amphibian diversity
How do we connect biogeography and current species distributions
with the past?• Phylogeography explains patterns of
population history across the globe, taking account of major regional differences in latitude, topography and ocean currents
• Based on comparison of genotype information (usually mtDNA) to build trees and haplotype networks. Those genetic data are then interpreted chronologically (when possible) and superimposed on geography
Phylogeographic studies can help us interpret current population genetic patterns in light of geographic distributions. In this case, the oak treehopper appears to have split from a common ancestor into eastern and western clades with subsequent divergence of the ozark and eastern clades
Phylogeography can also help us interpret historic patterns of dispersal and colonization
Amniote Origins
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
Late Cretaceous Extinction (65MYA)
Amniote Adapations
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
• Fig 26.2
Amniotic Egg
embryoMineralized shell
chorion
allantois
Yolk sac
amnion
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.
Amniotic Egg
embryoMineralized shell
chorion
allantois
Yolk sac
amnion
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
Amniotic EggRemember: 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 skinAmniote 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 chromatophores that 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
