Lecture on Transitional Fossils

Overview

• Transitional fossils illustrate evolutionary links between different groups.
• Key transitions include:
  • Fish to tetrapods
  • Theropod reptiles to birds
  • Synapsids to mammals
  • Land mammals to whales

Fish to Tetrapods

• Devonian Period:
  • Various groups of fishes existed: jawless fishes, cartilaginous fishes, ray-finned fishes, and lobe-finned fishes.
• Lobe-finned fishes:
  • More closely related to tetrapods than other fishes.
  • Featured fleshy, bony lobes pre-adapted for limb development.
• Transitional Fossils:
  • Eusthenopteron (lobe-finned fish) shows pectoral/pelvic girdles, bony limb structures resembling tetrapod limbs.
  • Tiktaalik shows intermediate limb structures with bones homologous to tetrapods.
  • Ichthyostega (early tetrapod) shares homologous bones with lobe-finned fish ancestors.

Theropod Reptiles to Birds

• Amniotes:
  • Include reptiles, birds, and mammals.
  • Birds evolved from theropod dinosaurs.
• Dinosaurs:
  • Birds are descendants of dinosaurs; not all dinosaurs went extinct.
  • Birds form monophyletic group with dinosaurs.
• Transitional Fossils:
  • Archaeopteryx from the Jurassic period had bird-like features including feathers but shared many characteristics with theropod dinosaurs.
  • Feathers have been found in other dinosaurs, suggesting they were more common and not exclusive to modern birds.
• Bird Characteristics:
  • Modern birds have specialized structures absent in Archaeopteryx, like a keeled sternum and specific hand bone modifications.
  • Some feathered dinosaurs had feathers on all four limbs.