Overview
This lecture explores Tiktaalik, a 375-million-year-old fossil that reveals key steps in the evolutionary transition from fish to four-legged land vertebrates (tetrapods), highlighting its anatomical features and significance as a transitional form.
Discovery and Significance of Tiktaalik
- Tiktaalik was discovered in Arctic Canada and is about 375 million years old.
- It is technically a fish with scales and gills, but has a flat, crocodile-like head and unique fins.
- Tiktaalik’s fins have both ray bones for swimming and sturdy bones for propping up its body.
- The combination of fish and tetrapod features makes Tiktaalik an important transitional form in vertebrate evolution.
- Its discovery confirmed hypotheses about the origin of terrestrial vertebrates.
Evolutionary Importance
- Tiktaalik shows the evolutionary transition from aquatic fish to four-legged vertebrates (tetrapods).
- It had both fish traits (fins, scales, gills) and tetrapod traits (wrist bones, neck, shoulders, thick ribs).
- Transitional forms like Tiktaalik help trace the stepwise evolution of new adaptations.
- The term “missing link” is misleading; evolution is understood through multiple transitional forms, not a single organism.
Tiktaalik’s Anatomy and Adaptations
- Tiktaalik’s fins allowed movement in shallow water and partial support of its body.
- The skull of Tiktaalik shows a mix of fish-like joints and more rigid, tetrapod-like traits.
- Tiktaalik had a mobile neck, unlike most fish, aiding in prey capture in shallow environments.
- The hyomandibula bone in Tiktaalik is intermediate between fish (jaw support) and tetrapods (hearing function).
Ongoing Research and New Evidence
- Footprint fossils nearly 400 million years old suggest tetrapods may have evolved earlier than Tiktaalik.
- If confirmed, Tiktaalik and early tetrapods like Acanthostega have “ghost lineages” not yet found in the fossil record.
- Tiktaalik’s status as a transitional form remains, but evolutionary timelines may need adjustment.
Key Terms & Definitions
- Fossil — The preserved remains or traces of an organism from the past.
- Vertebrate — An animal with a backbone or spinal column.
- Clade — A group of organisms believed to have evolved from a common ancestor.
- Transitional form — A species that exhibits traits common to both ancestral and derived groups.
- Adaptation — A trait that increases an organism’s fitness in its environment.
- Tetrapod — A vertebrate with four limbs.
- Hyomandibula — A bone in fish aiding jaw support, which became the stapes (middle ear bone) in tetrapods.
- Ghost lineage — An inferred evolutionary lineage not directly preserved in the fossil record.
Action Items / Next Steps
- Read the provided links for more about Tiktaalik and the fish-to-tetrapod transition.
- Review discussion questions to test understanding of transitional forms and evolutionary concepts.
- Watch the recommended “Fish with Fingers” documentary for comparison with Acanthostega.
- Review how fossils are formed and how scientists interpret the fossil record.