List of transitional fossils
This is a tentative partial list of transitional fossils. The fossils are listed in series, showing the transition from one group to another, representing significant steps in the evolution of major features in various lineages. These changes often represent major changes in morphology and anatomy, related to mode of life, like the acquisition of feathered wings for an aerial lifestyle in birds, or limbs in the fish/tetrapod transition onto land.
Darwin noted that transitional forms could be considered common ancestors, direct ancestors or collateral ancestors of living or extinct groups, but believed that finding actual common or direct ancestors linking different groups was unlikely. Collateral ancestors are relatives like cousins in genealogies in which they are not in your direct line of descent but do share a common ancestor. This kind of thinking can be extended to groups of life. For instance, the well-known Archaeopteryx is a transitional form between non-avian dinosaurs and birds, but it is not the most recent common ancestor of all birds nor is it a direct ancestor of any species of bird alive today. Rather, it is considered an extinct close evolutionary "cousin" to the direct ancestors. This may not always be the case, though, as some fossil species are proposed to be directly ancestral to others, like how Australopithecus anamensis is most likely to be ancestral to Australopithecus afarensis.
Nautiloids to ammonoids
Cephalopods
Evolution of insects
| Appearance | Taxa | Relationships | Status | Description | Location | Image |
411 Ma | Genus:
| The world's oldest known insect. | ||||
411 Ma | Genus:
| Early springtail. | ||||
300 Ma | Genus:
| Ancestral to cockroaches, mantids and termites. | ||||
316.5 Ma | Genus:
| A primitive cockroach. | ||||
140 Ma | Genus:
| The earliest known Lepidopteran. | ||||
92 Ma | Genus:
| The oldest known species of bee. | ||||
80 Ma | Genus:
| The earliest known species of ant. | ||||
| 56–34 Ma | Genus:
| First leaf insect from the fossil record. | ||||
52 Ma | Genus:
| Transitional fossil myrmecophile of the rove beetle subfamily Pselaphinae. |
Evolution of spiders
Invertebrates to fish
[Chondrichthyes]
Bony fish
Fish to tetrapods
Amphibians to [amniotes]
Turtles
From lizards to snakes
Lizards
Pterosaurs
Archosaurs to dinosaurs
| Appearance | Taxa | Relationships | Status | Description | Image |
| 259-252 Ma | Genus:
| The oldest known archosaur, Archosaurus was one of the largest land reptiles during the Late Permian, about the size of to today's Komodo dragons. It looked somewhat crocodile-like, with sprawling legs, long jaws, powerful neck muscles and a long tail. A distinct proterosuchid trait is the peculiar hook-shaped mouth. | |||
| ??? Ma | Genus:
| ||||
| ??? Ma | Genus:
| The oldest known animal on the dinosaur/pterosaur side of the archosaurian tree, dating to about 245 million years ago. | A small, lightly built animal. It had a fairly long neck, but ran on all four legs. | ||
| ??? Ma | Genus:
| Known from a somewhat fragmentary find, Spondylosoma was possibly an early dinosaur, or near dinosaur. It has however also been classified as a rauisuchian. | |||
| 228 Ma | Genus:
| A very early representative of the sauropod stem line or perhaps even the Saurischia as a whole. | A small bipedal carnivore with numerous sharp teeth. It was a swift digigrade runner. The forelimbs were half the length of the hindlimbs and the hands had five fingers |
Dinosauria
| Appearance | Taxa | Relationships | Status | Description | Image |
| 228 to 216.5 Ma | Genus:
| The oldest known ornithischian. | |||
| 216–200 Ma | Genus:
| The most primitive well-known representative of the sauropodomorph dinosaurs. | |||
| 160 Ma | Genus:
| The oldest and most primitive known stegosaur. | |||
| 90 Ma | Genus:
| A basal pachycephalosaur from the late Cretaceous. | |||
| 160 Ma | Genus:
| A genus of basal ceratopsian dinosaur from the Late Jurassic Period of central Asia. | |||
| 160 Ma | Genus:
| A genus of proceratosaurid tyrannosauroid dinosaur, one of the earliest known examples of the lineage. | |||
| 126 Ma | Genus:
| An early genus of therizinosaur | |||
| 208–194 Ma | Genus:
| One of the most primitive thyreophorans. | |||
| 95 Ma | Genus:
| A possible ancestor of the duck-billed dinosaurs. | - | ||
| 120 Ma | Genus:
| A primitive ornithomimosaur. |