Amphibians, Reptiles, and Mammals

Land Organisms: Amphibians, Reptiles, and Mammals

Introduction

  • Focus on amphibians, reptiles, and mammals (excluding primates and humans).

  • Life originated in the water, leading to marine representatives in almost every phylum.

  • The Cambrian explosion occurred in the water, with all phyla developing there first.

  • Marine diversity is high in terms of phyla, families, classes, and orders.

  • Insects have significantly diversified on land, potentially evening out the total number of species between land and water.

Species Discovery

  • New species are continuously being discovered.

  • Hundreds of new species are found annually.

  • Dozens of new genera are identified each year.

  • The discovery of completely new phyla is unlikely, but possible.

Evolution

  • All organisms evolved from prokaryotic single-celled organisms.

  • Early animals moved onto land and diverged into amphibians, reptiles, and mammals.

  • Mammals further diverged into different groups, including primates.

Amphibians: Key Characteristics

  • Possess legs (though some have lost them).

  • Have lungs as adults (exceptions exist).

  • Exception: Axolotls, a group of adults, do not have lungs.

  • Cutaneous respiration: Absorb oxygen through their skin in addition to lungs or gills.

  • This is common for amphibians because they are still in the water.

  • Evolutionarily advantageous to reduce the need to surface for air.

  • Some species hibernate by burrowing into mud and absorbing oxygen through their skin.

  • Partially divided heart; pulmonary circulation.

  • Approximately 6,150 known species.

  • New organisms, especially frogs, are regularly discovered in rainforests.

Main Amphibian Groups

  • Salamanders and newts.

  • Frogs and toads.

  • Coelicans (legless amphibians): worm-like and segmented.

Amphibian Life Stages

  • Two distinct life stages: fully aquatic and terrestrial.

  • Aquatic phase with gills followed by metamorphosis into an adult phase.

  • Some species brood their young in their mouth or on their back.

  • Some species have bubbles in their skin where eggs sit, eventually rupturing to release the young.

  • One Australian species swallows eggs, with tadpoles developing in their stomach.

Amphibian Extinction

  • Highly susceptible to pollution and climate change, serving as indicator species.

  • Porous skin absorbs water and oxygen, also drawing in chemicals.

  • Leads to mutations, especially affecting Hox genes, resulting in multiple or missing legs.

  • Citric funguses can enter through the skin.

  • Evolution can't keep up with the rate of synthetic chemical production in recent years.

  • For adaptation to occur, some individuals must possess traits like tougher or less porous skin.

Amniotic Transition

  • Climate change led to a transition from amphibians to amniotes.

  • Hotter, more arid conditions favored adaptations for drier environments.

  • Amniotes evolved to deal with drier conditions.

Amniotes Characteristics

  • Drier skin with scales to prevent water loss.

  • More robust pelvic girdle for spending time out of water.

  • Legs positioned underneath the body for better support.

  • Four-chambered heart for better blood pumping to support muscles.

  • Thoracic breathing using muscles to draw in more oxygen.

  • Internal fertilization: Sperm fertilizes eggs internally.

  • Amniotic egg: Shell prevents water evaporation.

  • Leathery shell in reptiles, calcified shell later on.

  • Yolk sac: Provides greater development within the egg before hatching.

  • More yolk allows for greater development within the egg before hatching.

  • More developed hatchlings are less susceptible to predation.

  • Birds are born warm-blooded but lose heat easily due to small size and lack of fat reserves.

  • Internalization of the larval phase occurs within the egg.

  • Embryos have chordate characteristics inside the egg until hatching.

Amniote Evolution

  • Ancestral amniotes diverged into reptiles and synapsids.

  • Synapsids led to mammals.

  • Turtles' placement is uncertain but are known to be one of the first.

Reptile Groups

  • Extant reptiles: tuatara, lizards, snakes, turtles, crocodiles.

  • Extinct groups: dinosaurs and pterodactyls.

Tuatara

  • Called a "living fossil" due to minimal evolutionary change.

  • Bony extrusions on their back.

  • Only found in New Zealand.

Reptile Characteristics

  • Scales: Waterproof barrier for living in drier conditions.

  • Scales modified into feathers evolutionarily.

  • Shelled eggs on land.

  • Ectothermic (cold-blooded) in general.

  • Some lineages were partially or fully warm-blooded, such as dinosaurs and pterodactyls.

  • Muscle structure infers speed and endothermy in major predatory dinosaurs.

Feather Evolution

  • Feathers likely evolved initially for insulation rather than flight.

  • Early birds evolved from raptors and were partially endothermic.

  • Splitting scales increased surface area for insulation.

Dinosaurs

  • Many were agile, fast-moving, and showed parental care.

  • Many were endothermic.

  • Birds are considered reptiles.

  • Early birds had teeth on their wingtips.

  • New species found resemble an "ostrich chicken crocodile thing."

Bird Characteristics

  • First birds appeared about 50 million years ago.

  • Key features: Feathers for insulation and flight, keel sternum for flight muscle attachment.

  • Birds lost teeth.

  • Birds gained endothermy.

  • Lightness of the skeleton.

  • Fusion of wrist bones.

  • Honeycombed, hollow skeleton.

  • Evolution focused on lighter body structure.

  • Birds without flight have solid bone structures for strength.

  • Approximately 8,500 known species adapted worldwide.

Mammals

  • Key characteristics: Hair and milk production.

  • Hair present at some stage of life (even in dolphins).

  • Females produce milk via mammary glands.

  • Approximately 5,000 species.

  • High metabolic rate.

  • Larger brain.

  • Differentiated teeth.

  • Early mammals were likely small, maneuverable, and lived in burrows.

  • Expanded around the early Cretaceous period.

Mammalian Lineages

  • Monotremes: Egg-laying mammals with no nipples (milk secreted through pores).

  • Australian echidna and platypus.

  • Platypus has electroreception abilities in its bill.

  • Marsupials: Have a placenta, but it doesn't provide as many nutrients, and they don't use it for very long. Early embryo gets nutrition from the mother through the placenta and then crawls up into the pouch.

  • Possums, kangaroos, koalas.

  • Embryo crawls into a pouch, latches onto a nipple, and develops there.

  • May be advantageous in harsh environments for controlled development.

  • Placental Mammals (Eutherians): Possess a full placenta for providing nutrients throughout development.

  • More developed young are born.

Exam Information

  • No Heidi Weinberg calculations on the exam.

  • Bring a scantron.

  • Exam includes short answer questions.

This note covers land organisms, focusing on amphibians, reptiles, and mammals (excluding primates and humans). It begins by highlighting that life originated in water, leading to marine representatives across various phyla, and notes the high marine diversity versus the diversification of insects on land. The note discusses species discovery rates and the evolutionary path from prokaryotic single-celled organisms to amphibians, reptiles, and mammals. Key characteristics of amphibians, their main groups, life stages, and susceptibility to extinction due to pollution and climate change are detailed. The transition from amphibians to amniotes is explained in the context of climate change, along with characteristics of amniotes, their evolution into reptiles and synapsids, and the various reptile groups. Furthermore, the note explores the evolution of feathers, dinosaur characteristics, bird features, and mammalian characteristics, lineages, and concludes with exam information.