Bvert-Lec4 slide

What is a Tetrapod?

• All living tetrapods possess (or their recent ancestors possessed) four identifying traits, making the group distinct.

• The evolution of these features requires synthesizing information from both the fossil record and developmental genetics.

• Example: Snakes are classified as tetrapods because their not-so-distant ancestors had four legs. Modern snakes have secondarily lost these limbs. Tetrapodophis, a fossil snake, provides evidence with its functioning legs.

Tetrapod Origins

Timeline and Transitional Species

• Lobe-finned fish: Appeared around $390 \text{-} 380 \text{ million years ago}$.

• Earliest known tetrapods: Discovered from approximately $363 \text{ million years ago}$.

• This period represents a critical time for transitional species.

Key Fossils

Eusthenopteron: A Tetrapod-Like Fish

• Age: Circa $380 \text{ MYA}$ (Millions of Years Ago).

• A sarcopterygian (lobe-finned) fish exhibiting tetrapod-like characteristics.

• Tetrapod-Like Features:

  • Lobed fins containing homologous bones to tetrapod limbs, specifically a humerus, radius, and ulna.

• Fish-Like Features:

  • Body covered in scales.

  • Possessed a fishy cranium.

  • Had gills for aquatic respiration.

  • Lacked robust ribs with strong vertebral connections, common in land-dwelling tetrapods.

Acanthostega: A Fish-Like Tetrapod

• Age: An early, true tetrapod from approximately $~365 \text{ MYA}$.

• Fish-Like Features:

  • Retained a tail fin with bony rays.

  • Its skeleton was still largely adapted for an aquatic lifestyle.

• Tetrapod-Like Features:

  • Developed arm bones with distal digits (fingers/toes).

  • Possessed a robust pelvis and femur, providing stronger support.

  • Lost scales on its back and sides.

  • Developed ribs strongly connected to the vertebral column, providing better support and aiding respiration on land.

• Acanthostega represents a significant step in the origin of tetrapods, occurring after the origin of vertebrates, the ostracoderm explosion, the origin of jaws, and the origin of chondrichthyes and osteichthyes.

The Search for a "Missing Link"

• The question arose whether transitional fossils existed between Eusthenopteron (lobe-finned fish) and Acanthostega (early tetrapod).

The Discovery of Tiktaalik (The "Fishapod")

• Discovery: In $2004$, Neil Shubin and colleagues discovered Tiktaalik roseae.

• Methodology: Researchers targeted rocks approximately $375 \text{ MYA}$ that were in the right age range, specifically focusing on Ellesmere Island in the Canadian Arctic, a site mapped by geologists.

• Tiktaalik is considered a remarkable transitional fossil, bridging the gap between fish and tetrapods.

• Fish-Like Features:

  • Presence of scales.

  • Retained fins.

  • Possessed gills.

• Tetrapod-Like Features:

  • Developed a distinct neck.

  • Had robust ribs.

  • Featured a flattened head, suggesting a more dorsal-ventral compression typical of shallow water or land-dwelling animals.

  • Fins contained tetrapod-like wrist bones.

  • Possessed primitive ears.

• It was not a true tetrapod but shared many advanced features.

• Habitat: Lived in ancient tropical river systems.

How Did Tiktaalik Move?

• Tiktaalik was primarily aquatic but had the ability to move on land.

• Propulsion: Its front fin/shoulder structure suggests it could both swim and prop itself up (similar to a push-up position).

• Environment: Likely lived in shallow water, using strong front fins to push along the bottom and stabilize itself in moving currents.

• Limitations: Its limited range of motion prevented it from swinging its fins forward for walking in the manner of a modern amphibian.

• Hypothesis: The ability to occasionally move on land might have provided advantages, such as escaping predators or accessing new food sources in changing aquatic environments.

Phylogeny of Extant and Extinct Tetrapods (Conceptual Flow)

• $420 \text{ - } 360 \text{ Million Years Ago}$: The period of major evolutionary transitions.

• Ray-finned fish: Possess bony skeletons and lungs (though lungs were later lost in many living fish).

• Coelacanth & Lungfish: Represent sarcopterygian (lobe-finned) fish.

• Lobe fins: Characterized by a single large bone in the fins attached to the shoulder and pelvic girdle.

• Eusthenopteron (Tetrapod-like Fish): Exhibits the single large limb bone articulating to two smaller bones, and a nostril connected to the mouth.

• Panderichthys: Another intermediate form.

• Tiktaalik ("Fishapod"): Shows weight-bearing elbows, bending wrists, and a neck.

• Acanthostega: The earliest true tetrapod with forelimbs and hindlimbs possessing digits, pelvis attached to the rest of the skeleton, and loss of thick bony scales covering the back and flanks.

• Ichthyostega: Further development of limbs.

• Tulerpeton: Gills lost, tail fin reduced.

• Living Tetrapods: Characterized by wrists making contact along the entire span of the hand, ankles making contact across the entire span of the foot, and each hind limb having two thin shin bones (tibia and fibula).

• Key Tetrapod Feature: Five digits, with wrists and ankles turned forward for walking on land, and loss of external fin rays.

Trackway Fossils and an Alternative History

• Controversial Discovery: Footprint trackways from a Polish fossil site suggest an earlier origin for tetrapods, potentially pushing it back by approximately $~18 \text{ million years}$.

• Evidence: Footprints from $395 \text{ MYA}$ show digits and match the expected movement patterns of early true tetrapods, with no signs of the body being dragged.

• Lack of Skeletal Evidence: No skeletal remains have yet corroborated this hypothesis.

• Implication: This suggests true tetrapods might have coexisted with "fishapods" like Tiktaalik for over $10 \text{ million years}$.

Paradigms of Tetrapod Origin

• Previous Paradigm: True tetrapods evolved much later than "fishapods" (e.g., Tiktaalik, Acanthostega). The sequence was: Origin of Vertebrates $\rightarrow$ Ostracoderm Explosion $\rightarrow$ Origin of Jaws $\rightarrow$ Origin of Chondrichthyes and Osteichthyes $\rightarrow$ Origin of Tetrapods (True Tetrapods appearing after "fishapods").

• New Paradigm (with trackways): Tetrapods and "fishapods" may have originated around the same time, with all lineages coexisting for millions of years. This implies a much deeper history for true tetrapods.

The Origin of Limbs

Basic Anatomy

• Richard Owen (Mid-$1800$s): Noticed that land-dwelling vertebrates (tetrapods) shared a common general pattern in their limbs.

• Homologous Elements: These limb elements (e.g., humerus, radius, ulna, carpals, phalanges) are homologous across all tetrapods that possess limbs, indicating a common ancestry.

• EvoDevo: Evolutionary developmental biology studies provide insights into how these structures evolved.

EvoDevo: Genetics of Limb Development

Zone of Polarizing Activity (ZPA)

• Function: The ZPA is crucial for regulating the anteroposterior (front-to-back) patterning and asymmetry of developing limbs.

• Saunders and Zwilling ($1950$s) Experiment: Grafting an extra ZPA onto a developing chick embryo limb bud resulted in duplicated, mirror-image distal limb elements (e.g., two sets of digits facing each other).

• Interpretation: This indicated that gene products (likely proteins) diffuse outwards from the ZPA, and their concentration or distance from the ZPA influences the type and size of the digit that develops.

Sonic Hedgehog (SHH) Protein

• Gene Driver: The Sonic Hedgehog (SHH) protein is expressed in the ZPA.

• Digit Differentiation: The thumb is never exposed to SHH and receives its developmental cues from other proteins. Meanwhile, the distance from the ZPA (and thus the concentration of SHH) causes other digits to differentiate.

• Experimental Trigger: Adding retinoic acid to a specific part of a developing limb can trigger SHH activity, mimicking ZPA function.

• Conservation: Further experiments revealed that the same set of genes is turned on during limb development in all tetrapods, suggesting a highly conserved genetic toolkit.

• Question: Do these genes exist and function in non-tetrapods?

SHH Activity in Non-Tetrapod Vertebrates

• Early Belief: Previously, it was thought that sharks and rays, which have fin rays instead of true limbs, lacked SHH activity in their appendicular development.

• New Discoveries: Recent experiments have shown that SHH homologs (evolutionarily related genes) are indeed active during the late limb development stages in sharks and rays.

• **Experiment (Dahn et al., $2007$ *Nature*):

  • When retinoic acid was added to the anterior end of developing stingray fins, it caused the same mirror-image duplication of fin rays observed in chick embryos.

• Conclusion: While sharks and rays do not possess true limbs, their fins respond to the same developmental signals (like SHH pathways) in the same way as tetrapod limbs.

Key Point on Limb Evolution

• Many of the limb transformations observed from fish to tetrapods involve only subtle shifts in the timing and location of gene activity.

• Crucially, novel genes did not need to evolve to accomplish these major morphological changes; rather, existing gene regulatory networks were repurposed and modified.

The Tetrapods

Early Tetrapods

• Various groups of non-amniote tetrapods emerged since the late-Devonian period.

• Their diversity "exploded" towards the end of the Paleozoic, around $~300 \text{ MYA}$.

• These groups included both terrestrial and aquatic species, some exhibiting very reptile-like characteristics.

• Examples: Pantylus, Diplocaulus, Diadectes, Gerrothorax.

• This period marked the peak diversity of non-amniote tetrapods.

Challenges to Life on Land

Locomotion

• Body Support: On land, body weight must be supported against gravity by the limbs, pelvic girdle, and pectoral girdle.

• Early Gaits: Many early tetrapods and modern amphibians employ sprawling gaits, often dragging their bodies along the ground due to less efficient limb placement.

• Evolutionary Trend: In amniotes (reptiles, birds, mammals), there has been an evolutionary trend towards more efficient semi-erect and erect gaits, which lift the body higher off the ground.

Avoiding Desiccation (Water Loss)

• Adaptations: Evolution of mucous glands and the stratum corneum (a protective outer layer of dead, keratinized cells) helped to prevent water loss from the skin.

Reproduction

• Vulnerability: Early tetrapod eggs (and modern amphibian eggs) are not waterproof, making them susceptible to desiccation.

• Requirement: All amphibians today still need to seek out water or moist terrestrial areas for oviposition (laying eggs).

Origin of Modern Amphibians

• Fossil Record: The first fossils of the three major extant groups of amphibians (Anura - frogs/toads, Caudata - salamanders/newts, Caecilians) appeared in the early Mesozoic Era.

• Ancestry Debate: These modern amphibian groups evolved from one of the diverse non-amniote tetrapod lineages, while others went extinct. However, there is ongoing disagreement about which specific extinct group represents the closest relatives to modern amphibians.

• The next class will delve into the diversity and adaptations of living amphibians.

Reading Guide

• For further study, focus on Chapter $5$: Amphibians, specifically the "Introduction" and "Evolution" sections.

• Note that sections on developmental genetics are not directly covered in the textbook; rely on these slides for that information.