List of transitional fossils

Possibly the best known of all transitional fossils, the Berlin specimen of Archaeopteryx lithographica

This is a tentative list of transitional fossils (fossil remains of a creature that exhibits primitive traits in comparison with more derived organisms to which it is related). 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 lines. These changes often represent major changes in anatomy, related to mode of life, like the acquisition of feathered wings for an aerial lifestyle in birds, or legs in the fish/tetrapod transition. As noted already by Darwin, the fossil record is incomplete.

Ideally, this list would only recursively include 'true' transitionals, fossils representing ancestral species from which later groups evolved, but most if not all, of the fossils shown here represent extinct side branches, more or less closely related to the true ancestor. They will all include details unique to their own line as well. Fossils having relatively few such traits are termed "transitional", while those with a host of traits found neither in the ancestral or derived group are called "intermediate". Since all species will always be subject to natural selection, the very term "transitional fossil" is essentially a misconception. It is however a commonly used term and a useful concept in evolutionary biology. The fossils listed represent significant steps in the evolution of major features in various lines and therefore fit the common usage of the phrase.

Nautiloids to Ammonoids

The Nautiloids → Ammonoids Evolutionary Series
Appearance	Taxa	Relationships	Status	Description	Image
>500 Ma	Subclass: Nautiloidea
390 Ma	Order: Bactritida	Member of the Nautiloids. Direct ancestor of the ammonoids.
370 Ma	Subclass: Ammonoidea	Direct descendants of Bactirida.

Cephalopods

The Cephalopod Evolutionary Series
Appearance	Taxa	Relationships	Status	Description	Image
296 Ma	Genus Pohlsepia			The earliest described octopod.
164 Ma	Genus: Proteroctopus			A primitive octopod.
165–164 Ma	Genus: Vampyronassa			An early Vampyromorphida.
89 - 71 Ma	Genus: Palaeoctopus			A primitive octopod.

Evolution of insects

The Insect Evolutionary Series
Appearance	Taxa	Relationships	Status	Description	Image
400 Ma	Genus: Rhyniognatha			The world’s oldest known insect.
400 Ma	Genus: Rhyniella			Early springtail.
300 Ma	Genus: Archimylacris			An ancestral to cockroaches, mantids and termites.
316.5 Ma	Genus: Aphthoroblattina			A primitive cockroach.
140 Ma	Genus: Archaeolepis			The earliest known Lepidopteran.
92 Ma	Genus: Melittosphex			The oldest known species of bee.
80 Ma	Genus: Sphecomyrma			The earliest known species of ant.
56 - 34 Ma	Genus: Eophyllium			First leaf insect from the fossil record.

Evolution of spiders

The Spider Evolutionary Series
Appearance	Taxa	Relationships	Status	Description	Image
390 Ma	Genus: Attercopus			Previously thought to be the world's oldest spider.
165 Ma	Genus Eoplectreurys			The oldest known haplogyne spider.

Invertebrates to Fish

This list is incomplete; you can help by expanding it.

The Invertebrates → Fish Evolutionary Series
Appearance	Taxa	Relationships	Status	Description	Image
??? Ma	Genus: Pikaia			Lancelet-like in appearance. Oldest known ancestor of modern vertebrates Vertebrate characters Very primitive proto-notochord.
??? Ma	Class: Conodont	Had fin rays, chevron-shaped muscles and a notochord.
530 Ma	Genus: Haikouichthys			Appears to have a cranium, thus being a craniat.
480 to 470 Ma	Genus: Arandaspis		Jawless fish	A well armoured jawless fish, resembling a large tadpole in life
??? Ma	Genus: Birkenia		An anaspid, ancestral to the jawed vertebrates,	An unarmored, scaly jawless fish
419 Ma	Genus: Guiyu		Oldest known bony fish

Chondrichthyes

This list is incomplete; you can help by expanding it.

The Chondrichthyes Evolutionary Series
Appearance	Taxa	Relationships	Status	Description	Image
370 Ma	Genus: Cladoselache			An early primitive shark.
70 - 65 Ma	Genus: Dalpiazia			An early Sawfish
99 – 65 Ma	Genus: Cyclobatis			An early stingray-like skate.

Bony Fish

This list is incomplete; you can help by expanding it.

The Bony Fish Evolutionary Series
Appearance	Taxa	Relationships	Status	Description	Image
420 Ma	Genus: Adreolepis			The earliest-known Actinopterygiian.
??? Ma	Genus: Amphistium			An early relative of the Flatfish, one eye had already migrated towards the body midline.
48 – 37 Ma	Genus: Eobothus			The earliest known true flatfish
183.7–125.0 Ma	Genus: Leptolepis			One of the first teleosts.
99 – 93 Ma	Genus: Aguillavus			The oldest known eel.
13 Ma	Genus: Hippocampus sarmaticus			One of the oldest known seahorse.
13 Ma	Genus: Hippocampus slovenicus			One of the oldest known seahorse.
83 - 70 Ma	Genus: Nardovelifer			The oldest known lamprid fish
56 - 34 Ma	Genus: Eomola			A primitive sunfish
58 - 55 Ma	Genus: Corydoras revelatus			The oldest known member of the catfish family Callichthyidae.
56 - 34 Ma	Genus: Ruffoichthys			A primitive rabbitfish.
48 - 37 Ma	Genus: Palaeoperca			A primitive perch
58 - 55 Ma	Genus: Trachicaranx			A primitive pomfret
48 - 40 Ma	Genus: Histionotophorus			An early handfish
48 - 40 Ma	Genus: Eolactoria			The oldest known ostraciid boxfish
48 - 40 Ma	Genus: Proaracana			The oldest known aracanid boxfish
48 - 40 Ma	Genus: Gazolaichthys			A basal surgeonfish
48 - 40 Ma	Genus: Psettopsis			A primitive monodactylid moonyfish
48 - 40 Ma	Genus: Pasaichthys			A primitive monodactylid moonyfish
48 - 40 Ma	Genus: Eozanclus			A short-snouted ancestor of the modern Moorish Idol.
83 - 65 Ma	Genus: Cretatriacanthus			A primitive Tetraodontidae
83 - 65 Ma	Genus: Nardoichthys			A primitive Perciforme
58 - 55 Ma	Genus: Protozeus			A primitive Zeidae
58 - 55 Ma	Genus: Archaeozeus			A primitive Zeidae
??? Ma	Genus: Cooyoo			A primitive Ichthyodectidae
65 Ma	Genus: Protriacanthus			A primitive tetraodontid

Fish to Tetrapods

The Fish → Tetrapods Evolutionary Series
Appearance	Taxa	Relationships	Status	Description	Image
416-359 Ma	Genus: Osteolepis	An early member of the Tetrapodomorpha, the piscine line leading to tetrapods, Osteolepis is generalised enough to give a fair approximation of the common ancestor of tetrapods and lungfish.	Fish	A small to medium sized sarcopterygian fish with internal nostrils and pectoral fins stiffened by bony components broadly homologous to the humerus and radius/ulna found in tetrapods.
385 Ma	Genus: Eusthenopteron	Belonging to the family Tristichopteridae, a family that form a sister group to Panderichthys and the tetrapods.	Though not on the evolutionary path to tetrapods, Eusthenopteron is of fairly general build and is very well known, serving as an iconic model organism in tetrapod evolution.	A medium sized, mainly pelagic fish, Eusthenopteron mainly use the pectoral and pelvic fins for navigation, and the tail for propulsion. The fin was of diphycercal, foreshadowing the straightening of the spine and the evolution of a contiguous fin in fish like Panderichthys
380 Ma	Genus: Panderichthys	Very close to the origin of tetrapods, a "fishapod".	Fish	A large, predatory shallow water fish. As common in shallow water fish, the pectoral and pelvic fins was flexible and paddle-like for propulsion. The dorsal and anal fins are lost, the tail fin contiguous. The spiracles were short and wide, indication large amount of oxygen were taken up by the lungs rather than through the gills.
375 Ma	Genus: Tiktaalik	A "fishapod" more tetrapod-like than Panderichthys.	A fish, transitional between fish and the early, fish-like labyrinthodonts.	"Fish" with stout, fleshy pectoral fins with a joint between the innermost and the two next bony elements, corresponding to the elbow in higher tetrapods. The cleithrum bone was free of the skull, functioning as anchoring for the pectoral fins, and at the same time allowing for movement of the neck.
368 Ma	Genus: Elginerpeton		A fairly fragmentary find, Elginerpeton straddles the fish/tetrapod divide with a mosaic of features resembling 'Panderichthys, Ichthyostega and Hynerpeton. Probably one of the "fishapods".
365 Ma	Genus: Ventastega	Morphologically midway between Tiktaalik and Acanthostega/Ichthyostega.	Possibly oldest animal to have feet rather than fins.	A large, dorso-ventrally flattened predatory fish with a well armoured labyrinthodont-like skull. While the fins themselves has not been found, the shoulder girdle is essentially similar to that of Acanthostega, indicating it too had feet rather than fins.
365 Ma	Genus: Acanthostega		Together with Ichthyostega the sole early labyrinthodont known from fairly complete skeletons. It is the oldest animal known to have feet rather than fins, thus making it a true tetrapod.	First known animal with toes rather than fins. The feet was broad and paddle-like, adapted for movement in water. It retained functional gills in adulthood, behind a fleshy operculum.
365 Ma	Genus: Ichthyostega	Fairly closely related to Acanthostega. It possibly represent an early (and ultimately unsuccessful) line adapted to moving on land by inchworm-like movements.	Together with Acanthostega the sole early labyrinthodont known from fairly complete skeletons.	Early labyrinthodont with polydactylous, paddle-like feet and reinforced vertebrae and neural spines. It probably spent time on land, yet retained gills and a tail with fin rayes.
360 Ma	Genus: Hynerpeton			A large, basically salamander-like creature. The shoulder girdle was powerful, indicating it was a competent walker.
??? Ma	Genus: Tulerpeton			A large animal with paddle-like six-toed feet. It did however not have gills in adulthood, and is thus the oldest labyrinthodont known to depend entirely on breathing with its lungs.
359 - 345 Ma	Genus: Pederpes	Hailing from the fossil-poor Romer's Gap, Pederpes may be ancestral to the higher labyrinthodonts.	Intermediate between the earlier Ichthyostegalian and the later, more advanced labyrinthodonts.	Despite an extra toe on the forelimbs, Pederpes had limbs that terminated in feet adapted primarely for walking rather than paddles for combined swimming and walking like the earlier groups.
295 Ma	Genus: Eryops	The Temnospondyli are derived paleozoic amphibians, possibly ancestral to modern amphibians	A "classical" temnospondyl, an advanced labyrinthodont group.	One of the best known labyrinthodonts, Eryops combines the large, flat skull and short limbs typical of the group.

Labyrinthodonts to Modern amphibians

The Labyrinthodontia → Lissamphibia Evolutionary Series
Appearance	Taxa	Relationships	Status	Description	Image
290 Ma	Genus: Gerobatrachus	Colloquially referred to as a "frogamander" due to this taxon being both chronologically and morphologically basal to both anurans and salamanders	One of the first transitional fossils towards modern amphibians (Lissamphibia).	Plesiomorphic traits Backbone with intermediate characteristics Retains a fully developed tail Derived traits Bears a large space for a tympanic ear Ankle bones are fused together like in salamanders Lightly built wide skull as in frogs
250 Ma	Genus: Triadobatrachus	Intermediate between generalized amphibians and derived frogs	Early "almost frog" transitional amphibian	Plesiomorphic traits Possessed short limbs and therefore was unable to hop, unlike all extant anurans Retains fourteen vertebra unlike modern frogs who have four to nine vertebra Tibia and fibula are not fused into a tibiofibula Derived traits Skull resembles that of modern anuran skull with a latticework of thin bones in skull
190 Ma	Genus: Prosalirus	Another transitional form which could be properly classified as a frog	An intermediate form which may replace Triadobatrachus as the "ultimate" ancestor of anurans	Plesiomorphic traits Still possess relatively short limbs Derived traits Tail is greatly reduced Does not have greatly enlarged legs, but shows some adaptations for hopping, such as a three-pronged pelvis
213-188 Ma	Genus: Vieraella	A derived fossil frog completing the series of transitional fossils between early amphibians and modern anurans	The oldest "true" frog	Plesiomorphic traits Retains ten presacral vertebra Derived traits Hind legs are adapted for hopping
210 Ma	Genus: Eocaecilia	Intermediate between basal amphibians and caecilians	An early caecilian	Plesiomorphic traits Bears three-toed vestigial limbs The size of the orbits indicates well developed eyes and suggest a non-subterranean lifestyle Derived traits The body has been adapted to a sort of serpentine shape

Amphibians to Amniotes (early reptiles)

The Amphibians → Reptiles Evolutionary Series
Appearance	Taxa	Relationships	Status	Description	Image
326 - 318 Ma	Genus: Proterogyrinus	One of the early reptile-like amphibians	Amphibian	A large, somewhat lizard-like labyrinthodont with a deep skull, laterally placed eyes and five digits to each foot.
??? Ma	Genus: Limnoscelis	The order Diadectomorpha is the sister group of the amniotes.	The Limnoscelis was originally described as a "cotylosaur" (early reptiles) together with the other diadectomorphans. Today the large-bodied diadectomorphs are thought to have had a larval stage, falling close to, but just outside the amphibian/reptile divide.	A large, predatory reptile-like amphibian. The limbs are extremely heavily built, indicating it fed on slow moving prey.
??? Ma	Genus: Tseajaia	Uncertain phylogeny, possibly a Seymouriamorph or Diadectomorph	Amphibian	A medium sized, probably herbivorious animal
350 Ma	Genus: Westlothiana	Uncertain phylogenetic position. Westlothiana may be a small-bodied diadectopmorph, falling just outside the amphibian/reptile divide	Originally described as the first reptile, it is now considered an advanced reptile-like amphibian.	Small, probably insectovorious animal. The body and tail was long, the limbs small, somewhat like a modern skink.
320-305 Ma	Genus: Solenodonsaurus	Possibly allied to the Diadectomorpha, or belinging to a sister group to Diadectomorpha and Amniota	Likely an amphibian	Smallish, likely carnivorious.
340 Ma	Genus: Casineria	The fragmentary nature of the fossil (it lacks a cranium) makes an exact phylogenetic position hard to establish.	Possibly the first animal with an amniote egg, and thus the first reptile.	Small lizard-like animal, the first known tetrapod to possess claws, indicating it has reptilian type skin with scutes.
315 Ma	Genus: Hylonomus	One of several small, basal reptile genera	Reptile	An early anapsid reptile, considered to be ancestral to both the synapsid and sauropsid lines, and thus the oldest representative of the crown group amniotes.
312 - 304 Ma	Genus: Paleothyris	One of several small, basal reptile genera	Reptile (most likely a sauropsid)	An early anapsid reptile. In phylogenetic analysis it falls on the sauropsid side, it is thus likely a progenitor of the diapsids

Turtles

The Turtle Evolutionary Series
Appearance	Taxa	Relationships	Status	Description	Image
220 Ma	Genus: Odontochelys			The oldest known turtle.
210 Ma	Genus: Proganochelys
164 Ma	Genus: Eileanchelys			An evolutionary bridge between early land turtles and sea turtles.

From Lizards to Snakes

The Lizard → Snake Evolutionary Series
Appearance	Taxa	Relationships	Status	Description	Image
92 Ma	Genus: Eupodophis			A transitional form between Cretaceous lizards and limbless snakes retaining distinct, if non-functional, legs.
90 Ma	Genus: Najash			A basal snake with two hind-limbs.

Lizards

This list is incomplete; you can help by expanding it.

The Lizard Evolutionary Series
Appearance	Taxa	Relationships	Status	Description	Image
61 - 58 Ma	Genus: Anqingosaurus			The earliest known chameleon.
92 Ma	Genus: Dallasaurus			A basal mosasauroid from the Upper Cretaceous of North America.
71 - 82 Ma	Genus: Palaeosaniwa			One of the earliest Varanoidea.
146–100 Ma	Genus: Gangiguana			An primitive iguanid
97–100 Ma	Genus: Cretaceogekko			The oldest known gecko

Pterosaurs

Rhamphorhynchoidea → Pterodactyloidea Evolutionary Series
Appearance	Taxa	Relationships	Status	Description	Image
160 Ma	Genus: Darwinopterus			Basal to both rhamphorhynchoids and pterodactyloids
160 Ma	Genus Pterorhynchus

Archosaurs to Dinosaurs

This list is incomplete; you can help by expanding it.

The Archosauria → Dinosauria Series
Appearance	Taxa	Relationships	Status	Description	Image
??? Ma	Genus: Proterosuchus			The oldest known archosaur.
??? Ma	Genus: Marasuchus
??? Ma	Genus: Asilisaurus
??? Ma	Genus: Spondylosoma
228 Ma	Genus: Eoraptor

Dinosauria

This list is incomplete; you can help by expanding it.

The Dinosauria Evolutionary Series
Appearance	Taxa	Relationships	Status	Description	Image
228 to 216.5 Ma	Genus: Pisanosaurus			The oldest known ornithischian.
216–200 Ma	Genus: Thecodontosaurus			The most primitive well-known representative of the sauropodomorph dinosaurs.
??? Ma	Genus: Huayangosaurus			The oldest and most primitive known stegosaur.
??? Ma	Genus: Stenopelix			A basal pachycephalosaur from the Barremian Stage of the Cretaceous.
160 Ma	Genus: Yinlong			A genus of basal ceratopsian dinosaur from the Late Jurassic Period of central Asia.
160 Ma	Genus: Guanlong			A genus of proceratosaurid tyrannosauroid dinosaur, one of the earliest known examples of the lineage.
126 Ma	Genus: Falcarius			An early genus of therizinosaur
208–194 Ma	Genus: Scelidosaurus			One of the most primitive thyreophorans.
130–125 Ma	Genus: Probactrosaurus			A possible ancestor of the duck-billed dinosaurs.
??? Ma	Genus: Pelecanimimus			A primitive (basal) ornithomimosaur.

Dinosaurs to birds

Further information: Origin of birds

Further information: Evolution of birds

The Dinosaurs → birds Evolutionary Series
Appearance	Taxa	Relationships	Status	Description	Image
168-140 Ma	Genus: Pedopenna			The find is represented only by a hind leg, but one that is very bird-like. It belonged to a small maniraptoran dinosaur with long, pennaceous feathers on its hind legs and (in all likelihood) arms.
155 Ma	Genus: Anchiornis	Although once classified as a bird, Anchiornis is now considered a basal troodontid which bears pennaceous, symmetrical feathers on all four limbs.		Plesiomorphic traits Wings symmetrical and rounded, probably not used for flight or even sustained gliding Long legs similar to that of other troodontids Derived traits Wrists more similar to aves than other theropods Like dromaeosaurs and birds, and unlike troodontids, Anchiornis had arms nearly the same length as the hind legs Bore primary and secondary pennaceous wings on both arms, legs, toes, and wrist
164-158 Ma	Genus: Scansoriopteryx	A small arboreal dinosaur with long arms with wing feathers, Scansoriopteryx could climb well and possibly glide	Maniraptoran dinosaur, representative of the family Scansoriopterygidae	Plesiomorphic traits Forward pointing pubis Long tail Derived traits Arms longer than hind legs, adapted for climbing Curved climbing claws on all digits Foot with grasping ability
150–145 Ma	Genus: Archaeopteryx	Known for its mosaic of avian and theropod characteristics Archaeopteryx is both the first primitive bird in the fossil record and one of the first transitional fossils discovered.	Traditionally seen as the first proper bird, though it is not directly ancestral to modern birds. An excellent intermediate form between dinosaurs and birds. Capable of gliding, but lacking alula and keel, it could likely not sustain powered flight.	Plesiomorphic traits Slower dinosaur-like growth rate No keel Spine attaches to the back end of the skull rather than the base Forelimbs have three unfused, clawed fingers, no alula Maxilla and premaxilla bore unserrated teeth Moderately long tail Derived traits Fully developed asymmetrical flight feathers Fused furcula from two joined clavicles Backward and elongated pubis similar to maniraptors, but not found in more primitive theropods
120 Ma	Genus: Confuciusornis	Found in the famous Liaoning province Confuciusornis is the first primitive bird with a pygostyle.	With its short tail and toothless beak, Confuciusornis is very modern looking compared to Archaeopteryx. The toothless beak is however a case of convergent evolution, as more advanced birds retained teeth, illustration the sometimes confusing mosaic evolution of the dinosaur-bird transition.	Plesiomorphic traits Skull more primitive than Archaeopteryx Retained unfused clawed digits, no alula Sideways-facing glenoid joint Derived traits Short tail with fused vertebrae at the end (pygostyle) Larger sternum with a low keel Unlike other early birds Confuciusornis had a toothless beak
146 Ma-100 Ma	Genus: Sinosauropteryx	Small theropod discovered in China that has traces of small feathers that resembles fur.		derived traits The specimen has proto-feathers that demonstrates a preserved colouration of red and white.
115 Ma	Genus: Eoalulavis	Primitive bird and possibly a descendant of "urvogels" like Archaeopteryx. First bird to possess an alula.		Derived traits First digit bearing an alula rather than claw
93.5-75 Ma	Genus: Ichthyornis	Considered a close relative to the ancestor to modern birds	A flying bird found in several epochs in the late Cretaceous which still bore teeth, but in most respects very similar to Neornithes.	Plesiomorphic traits Numerous sharp teeth in much of the beak Derived traits Fused bones (metacarpals) II & III of the hand Rigid ribcage with a well developed carina No functional claws on the hand Short childhood with distinct adult stage.

Bird Evolution

This list is incomplete; you can help by expanding it.

The Bird Evolutionary Series
Appearance	Taxa	Relationships	Status	Description	Image
60-58 Ma	Genus: Waimanu			The earliest-known Penguin.
??? Ma	Genus: Elornis			An early flamingo.
??? Ma	Genus: Colymboides			An early gaviiform.
55-48 Ma	Genus: Mopsitta			An early psittacine.
??? Ma	Genus: Masillaraptor			An basal falconiform.
50 Ma	Genus: Primapus			An early apodiform.

Synapsid ("mammal-like reptiles") to mammals

The Synapsids → Mammals Evolutionary Series
Appearance	Taxa	Relationships	Status	Description	Image
??? Ma	Genus: Protoclepsydrops
306 Ma	Genus: Archaeothyris
??? Ma	Genus: Clepsydrops
265 Ma	Genus: Dimetrodon
??? Ma	Genus: Procynosuchus
248-245 Ma	Genus: Thrinaxodon
205 Ma	Genus: Morganucodon
125 Ma	Genus: Yanoconodon		An early crown group mammal.

Evolution of mammals

The Mammal Evolutionary Series
Appearance	Taxa	Relationships	Status	Description	Image
100–104 Ma	Genus: Kollikodon			The earliest-known monotreme to date.
125 Ma	Genus: Sinodelphys			The oldest metatherian known.
?? Ma	Genus: Djarthia			The earliest-known marsupial.
125 Ma	Genus: Eomaia			The oldest known eutherian.
63-50 Ma	Genus: Eritherium			The earliest known proboscidean.
60-55 Ma	Genus: Miacis			The ancestor of the modern Order Carnivora.
15.97–11.61 Ma	Genus: Heteroprox			The earliest known cervid.
20-18 Ma	Genus: Eotragus			The earliest known bovid.
45-40 Ma	Genus: Protylopus			The oldest camel known, it was also the smallest.
??? Ma	Genus: Hyrachyus			Suspected to be the ancestor of modern tapirs and rhinoceroses.
55.4—48.6 Ma	Genus: Heptodon			Suspected to be the ancestor of modern tapirs.
38—33.9 Ma	Genus: Hesperocyon			The earliest of the canids.
??? Ma	Genus: Eurymylus			The earliest of the lagomorphs.
52.5 Ma	Genus: Onychonycteris			One of the most primitive of the two oldest known monospecific genera of bat.
2 Ma	Genus: Ailuropoda microta			The earliest known ancestor of the Giant Panda.
63 - 61.7Ma	Genus: Purgatorius			Believed to be the earliest example of a primate or a proto-primate, a primatomorph precursor to the Plesiadapiformes.
12.5-8.5 Ma	Genus: Sivapithecus			This genus may have been the ancestor to the modern orangutans.
16 - 8 Ma	Genus: Kenyapotamus			An possible ancestor of living hippopotamids.
?? Ma	Genus: Eomanis			The earliest known true (and scaled) pangolin.

Early Artiodactylans to whales (Evolution of whales)

The whale Evolutionary Series
Appearance	Taxa	Relationships	Status	Description	Image
55.8 ± 0.2 - 33.9 ± 0.1 Ma	Genus: Pakicetus
50 Ma	Genus: Ambulocetus
46 Ma	Genus: Kutchicetus
47 Ma	Genus: Artiocetus
41-33 Ma	Genus: Dorudon
25 Ma	Genus: Aetiocetus
40-34 Ma	Genus: Basilosaurus
8-15 Ma	Genus: Eurhinodelphis
26 Ma	Genus: Mammalodon

Evolution of sirenians

The Sirenia Evolutionary Series
Appearance	Taxa	Relationships	Status	Description	Image
50 Ma	Genus: Pezosiren			A primitive sirenian.
40 Ma	Genus: Prorastomus
??? Ma	Genus: Protosiren
48.6–33.9 Ma	Genus: Eotheroides			An evolutionary bridge between primitive land-dwelling sirenians to aquatic sirenians
??? Ma	Genus: Halitherium

Evolution of the Pinnipeds

The Pinniped Evolutionary Series
Appearance	Taxa	Relationships	Status	Description	Image
21 to 24 Ma	Genus: Puijila			The oldest known pinniped.
??? Ma	Genus: Potamotherium			A very basal pinniped.
24-22 Ma	Genus: Enaliarctos			An early seal, but with more primitive skull and feet.

Evolution of the horse

The Hyracotherium → Equus Evolutionary Series
Appearance	Taxa	Relationships	Status	Description	Image
60-45 Ma	Genus: Hyracotherium
40-30 Ma	Genus: Mesohippus
20 Ma	Genus: Parahippus
17-11 Ma	Genus: Merychippus
12 Ma	Genus: Pliohippus
1.8-0 Ma	Genus: Equus

Human evolution

Further information: Timeline of human evolution

The Human Evolutionary Series
Appearance	Taxa	Relationships	Status	Description	Image
36-32 Ma	Genus Apidium	The oldest primitive monkey known in the fossil record, dating back before the split between Old and New world monkeys.	Basal to both Old and New world monkeys.	Plesiomorphic traits Smaller canines than later monkeys such as Parapithecus Retains some post-cranial characters seen in prosimians Derived traits Fused mandibular symphysis Scapula similar to modern squirrel monkeys Low rounded molar cusps rather than high cusps as is seen in tarsiers and strepsirrhine
33 Ma	Genus Aegyptopithecus	A Miocene monkey which bridges the gap between the Eocene ancestors of Old world monkeys and Miocene ancestor of Hominoidae.	Tentatively positioned transitional form prior to the Old world monkey/ape split.	Plesiomorphic traits Retained auditory features similar to Old world monkeys Incapable of true brachiation unlike extant apes Reduced capitular tail, but was proportionally smaller than Apidium Derived traits Ape-like teeth including broad, flat incisors and sexually dimorphic canines A low sagittal keel and strong temporalis muscles Increased size in the visual cortex
27-14 Ma	Genus Proconsul	This primate has very ape-like features like its teeth, but much of its post-cranial remains are more similar to monkeys.	Universally accepted to be intermediate between 'ape-like monkeys' such as Aegyptopithecus and later apes including hominids.	Plesiomorphic traits Monkey-like wrist Narrow, monkey-like illium Derived traits Completely lacked a fully formed tail 5-Y pattern on lower molar cusps as also seen in hominoids
13 Ma	Genus: Pierolapithecus	A European ape which is considered to be the predecessor of the great apes.	Some objections have been raised to this fossils status due to its location in Spain, but Pierolapithecus is likely a transitional taxon between generalized apes and the lineage which led to great apes.	Pleisomorphic traits Relatively short fingers and walked in a similar quadrupedal fashion like baboons Lacks adaptations for both gibbon-style brachaition as well as derived knuckle-walking like in chimpanzee's and gorilla's Derived traits Flat, wider rib cage like great apes for tree-climbing The clavicle is large and similar to modern chimps suggesting a dorsally positioned scapula
4.4 Ma	Genus: Ardipithecus	A woodland hominid adapted to quadruped arboreal locamotion, but also for bipedalism.	Intermediate between the last common ancestor of chimps and humans, and the australopithecines.	Plesiomorphic traits Brains smaller than later hominids ranging from about 300-350 cc Foot thumb is not retracted into the foot as a 'big toe' Phalanges are more heavily curved than in Australopithecus Derived traits Reduced size in canines, however still retained dimorphic characters Hind leg dominant, bipedal locomotion while walking, however were quadrupedal while climbing trees
4.4-2.0 Ma	Genus: Australopithecus	First known genus of fully bipedal apes which are probably ancestral to robust australopiths and the genus Homo	Intermediate between extinct quadrupedal and bipedal apes. While the relationship between some species are being revised, Australopithecus afarensis is considered to be, by most experts, the ancestor to all later hominids.	Plesiomorphic traits Some species retain a sagittal crest Curved phalanges, indicating semi-arboreal lifestyle Semisectorial premolar is present Prognathic face to varying degrees Derived traits Fully bipedal as indicated by many features including the knee joint, hips, lumbar curve in the spine, position of the foramen magnum, and feet Increase in brain size ranging from about 375-500 cc Development of a parabolic jaw
2.5-1.5 Ma	Species: Homo habilis	An early human which is the morphological link between australopithecines and later human species.	Perfect intermediate between early hominids and later humans, possibly ancestral to modern humans.	Plesiomorphic traits Pronounced brow ridge Foramen magnum is not positioned as anteriorly like in modern humans, giving a slightly semi-erect appearance Although reduced in size the teeth are still fairly large Derived traits Increase brain size ranging from 510-800 cc Face is slightly prognathic, but at a much steeper angle Bulge in the Broca area, possibly the first hominid to use rudimentary speech Associated with the first use of stone tools
2.0-1.0 Ma	Species: Homo erectus	Very successful hominid, which was probably ancestral to both modern humans and neanderthals. Probably the first hominid to leave and successfully colonize territories outside of Africa.	Ancestral to modern humans and neanderthals.	Plesiomorphic traits Still retains a heavy brow ridge and nuchal torus Lacked the complexity of modern human language, but does show increase in the Broca area Thicker bones and larger teeth than modern humans Derived traits Rounder and larger brain (about 900-1100 cc) than H. habilis Face is orthognathic compared to H. habilis Probably lived in bands and was an active group hunter Associated with advanced stone tools and possibly the first hominid to use and produce fire
500 Ka-recent	Species 'Archaic' sapiens	Archaic sapiens were the immediate ancestors of modern humans which evidently displaced the neanderthals in Europe and the island 'hobbits' of southeast Asia. 'Archaic' sapiens evolved from H. erectus about half a million years ago but still retains some primitive characteristics such as relatively thick bones and molars larger than modern humans.	Ancestral to modern humans.

See also

• Chimpanzee genome project — Genes of the Chromosome 2 fusion site
• List of fossil sites (with link directory)
• List of human evolution fossils
• Transitional fossil

External links

• Vuletic.com, Section V: Paleontology – Transitional fossils between every animal group
• Palaeos.com, Palaeos vertebrates starting with lobe-finned fish (very comprehensive)
• Talk.origins.org, FAQ: Transitional vertebrate fossils
• (A few) transitional fossils