Transitional Fossils
Every fossil is a transitional fossil.
Well-Documented Transitions
Fish to Amphibians (375-365 MYA)
- Tiktaalik (375 MYA)
- Ichthyostega (365 MYA)
- Acanthostega (365 MYA)
- Panderichthys (380 MYA)
- Eusthenopteron (385 MYA)
New Features:
- Development of limbs with digits
- Air breathing capability
- Mobile neck
- Robust ribs
Amphibians to Reptiles (340-310 MYA)
Key Fossils:
- Diadectes (300 MYA)
- Seymouria (280 MYA)
- Proterogyrinus (315 MYA)
- Westlothiana (338 MYA)
- Casineria (340 MYA)
- Hylonomus (315 MYA)
- Paleothyris (300 MYA)
New Features:
- Amniotic eggs
- Scales
- Water-tight skin
- More efficient lungs
- Improved locomotion
Reptiles to Mammals (260-200 MYA)
Key Fossils:
- Cynognathus (245 MYA)
- Thrinaxodon (250 MYA)
- Morganucodon (205 MYA)
- Probainognathus (235 MYA)
- Diarthrognathus (208 MYA)
- Megazostrodon (200 MYA)
- Sinoconodon (208 MYA)
- Adelobasileus (225 MYA)
- Hadrocodium (195 MYA)
New Features:
- Modified jaw joint
- Middle ear bones
- Specialized teeth
- Fur/hair development
- Endothermy (warm-bloodedness)
Dinosaurs to Birds (150-120 MYA)
Key Fossils:
- Archaeopteryx (150 MYA)
- Microraptor (120 MYA)
- Sinosauropteryx (124 MYA)
- Confuciusornis (125 MYA)
- Velociraptor (75 MYA)
- Anchiornis (160 MYA)
- Xiaotingia (155 MYA)
- Aurornis (160 MYA)
- Jeholornis (120 MYA)
New Features:
- Feathers
- Wings
- Hollow bones
- Wishbone
- Air sacs
- Modified wrist joint
Land Mammals to Whales (50-40 MYA)
Key Fossils:
- Pakicetus (50 MYA)
- Ambulocetus (48 MYA)
- Rodhocetus (46 MYA)
- Protocetus (45 MYA)
- Maiacetus (47 MYA)
- Kutchicetus (43 MYA)
- Basilosaurus (41 MYA)
- Dorudon (40 MYA)
- Aetiocetus (25 MYA)
New Features:
- Aquatic adaptations
- Modified limbs
- Streamlined body
- Tail flukes
- Blowhole development
- Underwater hearing
Objections
"There are no transitional fossils"
Thousands of transitional fossils have been discovered, including the ones above, showing clear evolutionary sequences. The fossil record contains numerous examples of intermediate forms with mixed characteristics of ancestral and descendant groups. Paleontologists have predicted the exact location and strata of transitional fossils based on the characteristics of the descendant and ancestral groups (see "Tiktaalik").
"The fossils are incomplete"
We have numerous well-preserved specimens that clearly show transitional features. Furthermore, different specimens often preserve different parts, allowing scientists to piece together a complete picture. The incompleteness of some fossils doesn't negate the patterns they reveal.
"Missing links still exist"
The term 'missing link' is misleading because evolution is a branching process, not a linear chain. Each new fossil discovery fills in our understanding, but also creates two new gaps on either side. Therefore, there will always be "missing links", even with a complete fossil record.