Animals: Non-Mammalian Tetrapods Flashcards
Which lineage did tetrapods evolve from?
Tetrapods are the third surviving lineage of lobe-fins.
- Tetrapods are vertebrates with two pairs of sturdy, skeleton-supporting limbs with feet and digits.
- Shared derived trait of clade Tetrapods.
- tetra = four, pod = foot
- Tetrapods evolved from lungfish-like ancestors during the late Devonian period.
What are the three parts to tetrapod limbs
Tetrapod limbs are three-parted:
1. Upper, single long bone hinged to;
2. A pair of parallel long bones;
3. Terminal hand/foot with jointed digits.
Tetrapod origins
The first tetrapods appeared around 365 mya during the Devonian Period, coinciding with vertebrates’ colonization of land.
- This transition followed the earlier colonization of land by plants and invertebrates, which occurred approximately 450 to 500 mya.
What factors drove natural selection to favour adaptations in vertebrates for life on land?
Why did vertebrates evolve for life on land?
Name and explain all three
Early colonization by other organisms
- Plants and invertebrates colonized land ~100 million years before tetrapods, providing abundant food resources and forming stable terrestrial ecosystems.
- The diversification of land plants, including lycophyte forests during the Devonian and Carboniferous periods, created vegetation that supported terrestrial food webs.
Environmental changes
- Continental drift during the Devonian period formed new landmasses and altered coastlines.
- These changes created diverse habitats, such as shallow freshwater environments, swampy regions, and brackish water zones enabling the transition of vertebrates from water to land.
Ecological pressures
- Competition and predation in aquatic environments pushed vertebrates to explore the land, where they initially faced less competition and fewer predators.
- On land, early tetrapods exploited new ecological niches, such as feeding on terrestrial invertebrates and plants unavailable to aquatic competitors.
Challenges of terrestrial life for aquatic-adapted organisms
Name and explain all seven
Gravity: terrestrial animals require supporting tissues to counteract the effects of gravity. − Tetrapods evolved strong endoskeletons that supported body weight and facilitated movement on land.
Water conservation: to avoid desiccation, terrestrial animals needed protective skin or coats, including mechanisms to protect embryos.
Reproduction: spawning and external fertilization are effective in water but unsuitable for air.
- Terrestrial animals adapted their reproductive strategies accordingly
Gas exchange: gills desiccate upon exposure to air.
- Terrestrial animals evolved internal lungs for gas exchange.
Locomotion: the lateral side-to-side swimming motion was inefficient on land, leading to the development of specialized locomotor adaptations.
Sensory perception: light, sound, and smell transmit differently in the air than in water.
- Some aquatic senses, like electrical sense and lateral line, do not function in the air.
Temperature regulation: temperature fluctuations are more pronounced in air than in water, requiring adaptations for thermoregulation
Adaptations critical for the tetrapod transition to land
Name all 5
Adaptations critical for the transition to land evolved before tetrapods left the water.
Vertebral column: Developed early in clade Vertebrata for structural support.
Mineralized skeleton & paired lateral fins: Enabled better mobility in clade Gnathostomes.
Simple lungs: Evolved in clade Osteichthyans as gas-filled outgrowths of the gut for respiration.
Bony, muscular pectoral/pelvic fins: Lobe-finned fishes had robust fins, the structural precursors to tetrapod limbs.
Internal nostrils: Connected external nostrils to the mouth in early aquatic tetrapods, enabling breathing without opening the mouth.
- Nostrils function only in olfactory responses (smell) in bony fish; paired nostrils are not connected to the mouth or gills.
- In early tetrapod evolution, posterior nostrils shifted internally, connecting the external/anterior nostrils to the buccal cavity (inside of the mouth) facilitating breathing at the water surface without necessitating the opening of the mouth.
The Tiktaalik discovery
Evolution of Tetrapods
Discovered in Ellesmere Island in 2006, Tiktaalik (375 mya) is a remarkable transitional fossil that shows a blend of fish and tetrapod characteristics.
- Fish traits include scales, lobe fins lacking digits (w/ fin rays), and both gills and lungs.
- Tetrapod features are evident in its neck structure, rib configuration, limb skeleton, flattened skull, and eyes positioned atop the head.
What are the three traits that link this organism to fish and tetrapods?
Tiktaalik: transitional fossil linking fishes and tetrapods
Neck development: The separation of the shoulder (pectoral girdle)
from the skull allowed independent head movement.
- Loss of the operculum (bony gill covering) further enhanced neck mobility, although Tiktaalik still retained gills.
“Fishapod” (1⁄2-fish, 1⁄2-tetrapod) limbs:
- Tiktaalik had shoulder, elbow, and proto-wrist joints, resembling tetrapod limb structures, but lacked digits and retained fin rays.
- It could likely prop itself on its fins but wasn’t adapted for full terrestrial locomotion.
Pelvis structure: Tiktaalik’s pelvis supported a range of movements, from swimming to limited weight-bearing, showing early adaptations towards terrestrial life.
- However, Tiktaalik’s pelvis was still fish-like:
- Composed of a single bone, unlike the three-part pelvis of tetrapods.
- Not attached to the vertebral column, limiting its weight-bearing capacity.
Describe structure and habitat
Acanthostega
Early tetrapods
Acanthostega (365 mya), a later evolving transitional tetrapod.
Limbs and digits: Acanthostega had limbs with numerous (>5) bony digits (without fin rays).
Pelvis and weight-bearing: The pelvis was weakly attached to the vertebral column, allowing limited weight-bearing through the hind legs but not fully adapted for terrestrial support.
Tail and propulsion: Long fin-rays surrounded the tail, likely aiding in aquatic propulsion.
Joint structure: The knee and ankle joints were not yet developed for weight-bearing functions, indicating Acanthostega was not fully terrestrial.
Respiratory system: It had both internal gills and lungs, but short ribs made it unable to support its chest outside of water.
Habitat: Acanthostega was primarily aquatic, likely inhabiting weed-filled swamps, where its limbs with webbed digits helped with paddling or grasping aquatic plants
Shared derived traits of Tetrapods
Name all five
- Four limbs with feet and digits: Limbs adapted for locomotion and weight-bearing on terrestrial surfaces.
- Distinct neck: Enables independent head movement, improving sensory perception and maneuverability.
- Fusion of pelvic girdle to vertebral column: Provides stability and support for terrestrial locomotion.
- Absence of gills (in most species): Reflects a reliance on lungs for respiration, although some aquatic tetrapods retain gills.
- Ears for airborne sounds: Adaptation to detect auditory stimuli in terrestrial environments.
What are the two groups of extant tetrapods?
Amphibia and Amniota
Ichthyostega
Early Amphibians
Ichthyostega (365 mya), an extinct semi- terrestrial tetrapod with an amphibian-like body plan.
- Primitive fish-like traits: tail with fin rays, gills, >5 digits.
- Extant tetrapods have at most 5 digits.
- Tetrapod traits: amphibian-like skull; pelvic girdle attached to the vertebral column (with a three-part pelvis); weight-bearing knee and ankle joints; primarily relied on lungs for respiration; employed limbs for locomotion and a tail for balance.
Extant amphibians
Extant amphibians (class Amphibia) form a monophyletic group of terrestrial or freshwater tetrapods.
- ~6,150 species, divided into three clades: Frogs, Salamanders, Caecilians
- Their sizes range from minuscule frogs (measuring a few mm) to the Chinese giant salamander (exceeding one meter in length)
What are the three clades of amphibians?
- Frogs
- Salamanders
- Caecilians
Amphibian Skin
Amphibians have thin, water- and gas-permeable, moist skin.
- Amphibian skin is “naked”: no scales, little keratin (fibrous structural proteins).
- Amphibian skin is permeable to water; it can absorb and lose water directly through the thin skin.
- Due to their permeable skin, amphibians are susceptible to desiccation and typically inhabit moist habitats.
- Amphibians are susceptible to salt (no marine amphibians).
- Amphibian skin contains mucous glands that produce secretions to maintain moisture.
- Certain glands secrete distasteful or toxic substances for defence.
- Amphibians use their skin as a supplementary respiratory surface to complement gas exchange by the lungs.
- Some small terrestrial salamanders and frogs lack lungs and rely entirely on their skin for gas exchange.
Are amphibians ecto- or exothermic?
Amphibians are ectothermic organisms, relying on external heat sources rather than metabolic heat production to regulate body temperature.
- They exhibit a low metabolic rate, leading to reduced food and energy requirements.
- Amphibians require a stable temperature environment or use behavioural thermoregulation (seeking out heat sources or sheltering from excessive heat)
How do amphibians ventalate their lungs?
Amphibians use a buccal pump mechanism to ventilate lungs (“cheek breathing”).
- Air is drawn into the oral/throat cavity through the nostrils or mouth.
- Next, the mouth and nostrils are sealed and the oral/throat cavity contracts to force air into the lungs.
Amphibian reproduction
Amphibians reproduce in water or moist habitats.
- Nearly all amphibians spawn in water (oviparous).
- Male and female individuals pair, but fertilization is external in most species.
- Produce large numbers of eggs; fertilized eggs are rarely cared for by adults.
- Eggs hatch quickly into swimming larvae, e.g. tadpoles.
Eggs lack protective shells and extraembryonic membranes.
- Eggs are surrounded by a desiccation-sensitive gelatinous coating.
- Eggs must be laid in water or at least in moist habitats.
- Amphibian eggs and embryos are very sensitive to salt.
How many chambers are in an amphibian heart?
Amphibians possess a three-chambered heart
- two atria
- one ventricle.
Amphibian eardrums
Amphibians have external eardrums.
- The eardrum transmits sound vibrations through a single bone, the stapes, to the inner ear.
- But only frogs communicate through sound.
When do amphibians have gills
Amphibians only have gills in aquatic larval stages.
- Some aquatic salamanders retain gills into adulthood.
Amphibian Life Cycle
Amphibian means “both ways of life.”
- Refers to the metamorphosis of aquatic larvae into terrestrial adults.
- Most amphibians fertilize eggs in water that develop into herbivorous aquatic larvae (tadpoles) with gills and no limbs.
- Larvae undergo metamorphosis to become terrestrial adults with legs, lungs, external eardrums, and adaptations for carnivory.
- The amphibian life cycle is dependent on water, involving external aquatic fertilization, eggs and embryos that are sensitive to desiccation, and indirect development through aquatic larval stages.
- Amphibians are not fully adapted to life on land
Structure, Ecological role, Development, and Habitat
Salamanders
Amphibians
Salamanders (Order Urodela; ~550 species) are amphibians with tails in the adult stage (uro = tail).
- Superficially lizard-like appearance: slender bodies, blunt snouts, and short laterally extended legs.
- Salamanders walk, not jump, using side- to-side lateral movements.
- Salamanders are predators of invertebrates as larvae and adults
Salamanders typically lay eggs in water and have aquatic larvae, but there’s considerable variation in their life cycles.
Larvae have external gills; otherwise, they resemble adults with the presence of legs.
- Both larvae and adults have tails.
- Paedomorphosis, the retention of juvenile features in sexually mature organisms, is common in aquatic species.
Some species of salamanders remain aquatic throughout their lives, while others transition to terrestrial habitats, but require damp environments to keep their skin moist.
Development and Structure
Frogs
Amphibians
Development:
- Transform from larval tadpoles with tails to adults without tails.
Structure:
- Frogs are characterized by their long, powerful hind legs, which are modified for jumping, although they lack legs in early larval development stages.
- Frogs have well-developed ears.
- The large circular eardrum lies on the surface of the head just behind the eye.
- Most frogs communicate with each other through vocal sounds using well- developed vocal cords.
