List of transitional fossils

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

This is a tentative partial list of transitional fossils (fossil remains of groups that exhibits both "primitive" and derived traits). 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.

Almost all of the transitional forms in this list do not actually represent ancestors of any living group or other transitional forms. 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.^[1][2] 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 (in this case it is a grandparent). 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.^[3]

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 member of the 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			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.
52 Ma	Genus: Protoclaviger			Transitional fossil myrmecophile (social parasite of ant colonies) of the rove beetle subfamily Pselaphinae.

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

The Invertebrates → Fish Evolutionary Series
Appearance	Taxa	Relationships	Status	Description	Image
523 Ma	Genus: Pikaia			Lancelet-like in appearance. Oldest known ancestor of modern vertebrates Vertebrate characters Very primitive proto-notochord.
504 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.[4]
480 to 470 Ma	Genus: Arandaspis		Jawless fish	A well armoured jawless fish, resembling a large tadpole in life
422–412 Ma	Genus: Birkenia		An anapsid, ancestral to the jawed vertebrates,[5]	An unarmored, scaly jawless fish
419 Ma	Genus: Guiyu		Oldest known bony fish[6]

Chondrichthyes

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

The Bony Fish Evolutionary Series
Appearance	Taxa	Relationships	Status	Description	Image
420 Ma	Genus: Andreolepis			The earliest-known Actinopterygiian.
??? Ma	Genus: Amphistium			An early relative of the flatfishes, 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: Anguillavus			The oldest known eel.
13 Ma	Genus: Hippocampus sarmaticus			One of the oldest known seahorses.
13 Ma	Genus: Hippocampus slovenicus			One of the oldest known seahorses.
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 member of the Tetraodontidae
83–65 Ma	Genus: Nardoichthys			A primitive Perciforme
58–55 Ma	Genus: Protozeus			A primitive member of the Zeidae
58–55 Ma	Genus: Archaeozeus			A primitive member of the Zeidae
??? Ma	Genus: Cooyoo			A primitive member of the 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.[7]	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.[7]
385 Ma	Genus: Eusthenopteron	Belonging to the family Tristichopteridae, a family that form a sister group to Panderichthys and the tetrapods.[7]	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.[8]	A medium-sized, mainly pelagic fish, Eusthenopteron mainly use the pectoral and pelvic fins for navigation, and the tail for propulsion.[8] 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" elpistostegalian.[7]	Fish	A large, predatory shallow water fish. As common in shallow water fish, the pectoral and pelvic fins were flexible and paddle-like for propulsion.[9] The dorsal and anal fins are lost, the tail fin contiguous.[10] The spiracles were short and wide, indication large amount of oxygen were taken up by the lungs rather than through the gills.[11]
375 Ma	Genus: Tiktaalik	A "fishapod" more tetrapod-like than Panderichthys.[7]	A fish, transitional between fish and the early, fish-like labyrinthodonts.[12][13]	"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.[13][14]
368 Ma	Genus: Elginerpeton	Analysis of the cranial material shows it was more advanced than Tiktaalik, and together with Obruchevichthys form a sister group to the higher tetrapods.[15]	A fairly fragmentary find, Elginerpeton straddles the fish/tetrapod divide with a mosaic of features resembling Panderichthys, Ichthyostega and Hynerpeton.[15] Probably one of the "fishapods".[16]	Though fragmentary, the find includes a shoulder blade (Cleitrum bone) as well as elements of the limbs, which shows it had comparable limbs Ichthyostega and Hynerpeton, indicating feet rather than fins.
365 Ma	Genus: Ventastega	Known only from fragmentary remains, mostly a lower jaw, Ventastega is morphologically midway between Tiktaalik and Acanthostega/Ichthyostega.[17]	Possibly oldest animal to have feet rather than fins.[17]	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.[17]
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 and the oldest known unquestionable ichthyostegalian.[18]	First known animal with toes rather than fins. The feet were broad and paddle-like, adapted for movement in water.[19] 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	While known only from fragmentary remains, it is more advanced than Ichthyostega.	Early labyrinthodont amphibian	A large, basically salamander-like creature. The shoulder girdle was powerful, indicating it was a competent walker.[20]
??? Ma	Genus: Tulerpeton		An advanced ichthyostegalian, it straddle the divide between the fish-like Devonian forms and the more advanced Carboniferous amphibians. It has been suggested it is an early reptil-like amphibian.[21]	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.[22]
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 primarily for walking rather than paddles for combined swimming and walking like the earlier groups.[23]
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.

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).[24]	Primitive 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[24]
250 Ma	Genus: Triadobatrachus	Intermediate between generalized amphibians and derived frogs	Early "almost frog" transitional amphibian	Primitive 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	Primitive 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[25]	Primitive 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	Primitive 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

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[26][27]	Amphibian	A medium-sized, probably herbivorous 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 belonging to a sister group to Diadectomorpha and Amniota[28]	Likely an amphibian[28]	Smallish, likely carnivorous.[29]
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 amniote and thus the lattest common ancestor to both Synapsids and sauropsids.	Small lizard-like animal, the first known tetrapod to possess claws, indicating it has amniote type skin with scutes.[30]
315 Ma	Genus: Hylonomus	One of several small, basal reptile genera	Reptile	once thought to be the common ancestor of both synapsids and sauropsids, Hylonomus is now considered a eureptilan creature nested inside sauropsida.
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
240 Ma	Genus: Pappochelys			Reptile closely related to turtles.
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.[31]
90 Ma	Genus: Najash			A basal snake with two hind-limbs.

Lizards

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: 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 [32][33]

Archosaurs to dinosaurs

The Archosauria → Dinosauria Series
Appearance	Taxa	Relationships	Status	Description	Image
??? Ma	Genus: Archosaurus			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: Marasuchus
??? Ma	Genus: Asilisaurus		The oldest known animal on the dinosaur/pterosaur side of the archosaurian tree (the Ornithodira), dating to about 245 million years ago.[34]	A small, lightly built animal. It had a fairly long neck (contrary to the short necked relatives of crocodiles), but ran on all four legs.
??? Ma	Genus: Spondylosoma		Known from a somewhat fragmentary find, Spondylosoma was possibly an early dinosaur, or near dinosaur.[35] It has however also been classified as a rauisuchian.[36]
228 Ma	Genus: Eoraptor		A very early representative of the sauropod stem line or perhaps even the Saurischia as a whole.[37][38][39]	A small (1 meter, ~ 10 kg) 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

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.
160 Ma	Genus: Huayangosaurus			The oldest and most primitive known stegosaur.
90 Ma	Genus: Wannanosaurus			A basal pachycephalosaur from the late 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.
95 Ma	Genus: Protohadros			A possible ancestor of the duck-billed dinosaurs.		120 Ma	Genus: Pelecanimimus			A primitive (basal) ornithomimosaur.

Dinosaurs to birds

The Dinosaurs → Birds Evolutionary Series
Appearance	Taxa	Relationships	Status	Description	Image
152–151 Ma	Genus: Juravenator			Primitive traits Undifferentiated hind digits displaying no specialties for climbing Spine attaches to the back end of the skull rather than the base Moderately long, bony tail Derived traits Basic proto-feathers cover parts of the body for insulation
168–152 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.
161–151 Ma	Genus: Anchiornis	Basal troodontid	Although once classified as a bird, Anchiornis is now considered a basal troodontid which bears pennaceous, symmetrical feathers on all four limbs.	Primitive traits Wings symmetrical and rounded, probably not used for flight but instead insulation, mating displays, and gliding Long legs overall morphology similar to that of other troodontids Spine attaches to the back end of the skull rather than the base Moderately long, bony tail Derived traits Flexible wrists which are more similar to aves than other theropods Like birds and unlike troodontids, Anchiornis had arms nearly the same length as the hind legs Bore primary and secondary pennaceous symmetrical wings on both arms, legs, toes, and wrist
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.[40] An excellent intermediate form between dinosaurs and birds. Capable of gliding, but lacking alula and keel, it could likely not sustain powered flight.	Primitive 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, bony 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.	Primitive traits 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 primitive keel Unlike other early birds Confuciusornis had a toothless beak
115 Ma	Genus: Eoalulavis	Primitive bird and possibly a descendant of "urvogels" like Archaeopteryx. First bird to possess an alula.		Plesiomophic traits Two unfused, functional digits remain on second and third digit 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.	Primitive 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.[41]

Bird evolution

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			A 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		Known from very fragmentary finds, Protoclepsydrops may be the earliest synapsid (mammal-like reptile)	A low-slung, lizard-like animal of moderate size.
306 Ma	Genus: Archaeothyris		The oldest undisputed synapsid (mammal-like reptile)	Primitive traits A relatively flat, reptile-like skull Typically reptilian sprawling gait Generally lizard-like proportions with a dorso-ventrally flattened body Derived traits Temporal opening low on the side of the skull roof, between the zygomatic bone and the elements above. Tendency to enlarged forward teeth on the maxilla
297 Ma	Genus: Haptodus	A primitive member of the Sphenacodontidae, or possibly just outside the group.[42][43]	A pelycosaur-grade synapsid	Derived traits Two or three moderately large canine-like teeth about a third down the maxilla.[44] Dentary bone the largest element of the lower jaw[43] The skull deeper than in Archaeothyris
265 Ma	Genus: Dimetrodon	An advanced member of the family Sphenacodontidae, from which the therapsids (advanced synapsids) evolved	A pelycosaur-grade synapsid. At up to 4 meters, Dimetrodon was one of the largest animals of its time. The distinct sail of the back makes it the most recognized synapsid known	Primitive traits Cold blooded metabolism dependent of external heat source (hence the "sail")[45] Sprawling gait No secondary palate No enlarged side teeth in the lower jaw Derived traits Distinctly elongated 2nd and 3rd tooth on the maxilla, corresponding to the canine in mammals. The first canine generally longer than the second.[46] Skull deep and narrow Body overall deeper than in earlier forms
267 Ma	Genus: Biarmosuchus		A primitive therapsid. About the size of a large dog, Biarmosuchus was a lightly built and likely fairly agile animal for its size.[47]	Primitive traits No respiratory turbinates indicate limited overall oxygen consumption and hence bradymetaboliic metabolism[48] Sprawling legs, but the legs longer and more slender than in pelycosaurs[47] Long pelycosaur-like tail Derived traits A single canine as the first tooth on the maxilla, all other maxillary teeth small Tendency for an enlarged caninelike tooth on the dentary Internal nostrils covered by a partial fleshy palate[49] Enlarged temporal opening giving more powerful bite
247–237 Ma	Genus: Cynognathus	An advanced synapsid	All species of Cynognathus were rather heavyset carnivores about a meter in length and with a sprawling gait and heavy jaws.	Primitive traits No bony palate No differentiated cheek teeth Derived traits Teeth clearly differentiated into incisors, canines and cheek teeth in both upper and lower jaws Cheek teeth with multiple cusps
248–245 Ma	Genus: Thrinaxodon	A small bodied relative of the larger Cynognathus.	An advanced cynodont, just on the reptilian side of the reptile/mammal divide. Ranging from badger to marten-sized, it was a burrower of very mammal-like habit.	Primitive traits While the dentary dominated the lower jaw, the hinge was between the articulare and quadrate.[50] Teeth even at very young age with no occlusion, indicating no or limited lactation and hence slow growth. No Harderian gland, indicating lack of fur and hence limited enothermy.[51] May have had whiskers Derived traits Well developed respiratory turbinates and palate, indicating homeothermy Generally mammal-like dentition. Mammal-like ecology: burrowing and small size Animals of different sizes found together, indicating post-hatching parental care.
205 Ma	Genus: Morganucodon	A smaller, more shrew-like relative of Thrinaxodon and Sinoconodon	An early mammal, possibly representing the earliest lactating animals, but outside the crown group (a mammaliform)	primitive traits Semi-sprawling gait Articular and quadrate bones still forming a small jaw articulation, though main joint being the between the dentary and squamosal bone Large number of teeth Advanced traits Only two sets of teeth with full occlusion. No teeth in infancy, indication lactation Short mammalian-type lifespan Presence of Harderian gland, indication pelage and hence endothermy
125 Ma	Genus: Yanoconodon	One of the Triconodonts	An early crown group mammal.	Primitive traits Long body with 26 lumbar and thoracic vertebrae (only 20 in modern mammals) Lumbar vertebrae with ribs Articular and quadrate bones still attached to lower jaw via Meckel's cartilage (the evolution of the ammalian ear ossicles have taken place separately in monotremes and therians) Advanced traits Small, very lightly built Borrowing Insectivorious[52]

Evolution of mammals

The Mammal Evolutionary Series
Appearance	Taxa	Relationships	Status	Description	Image
100–104 Ma	Genus: Kollikodon			The earliest known monotreme.
125 Ma	Genus: Sinodelphys			The oldest metatherian known.
?? Ma	Genus: Djarthia			The earliest-known marsupial.
164–165 Ma	Genus: Juramaia			The oldest known eutherian[53]
63-50 Ma	Genus: Eritherium			The earliest known proboscidean.
60–55 Ma	Genus: Miacis			The possible 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 known canid.
??? Ma	Genus: Eurymylus			The earliest known lagomorph.
52.5 Ma	Genus: Onychonycteris			One of the two oldest known monospecific genera of bat.
2 Ma	Species: Ailuropoda microta			The earliest known member of the giant panda clade.
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

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 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

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.	Primitive 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 Hominoidea.	Tentatively positioned transitional form prior to the Old World monkey/ape split.	Primitive 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 exhibits 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.	Primitive 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 locomotion, but also for bipedalism.	Intermediate between the last common ancestor of chimps and humans, and the australopithecines.	Primitive 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.	Primitive 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
2.3–1.4 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.	Primitive 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.	Primitive 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 900–1,100 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 Homo rhodesiensis	Homo rhodesiensis was the immediate ancestor of modern humans which evidently displaced the neanderthals in Europe and the island 'hobbits' of southeast Asia. H. rhodesiensis evolved from Homo 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.	Primitive traits Large teeth Heavy brow ridge Extremely robust build in most groups Derived traits Rounder, less broad based cranium Larger brain size, approaching (and sometimes exceeding) modern values

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

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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