Transitional fossils

Hesperocyon is an extinct genus of canids belonging to the subfamily Hesperocyoninae of the canid family that was endemic to North America, ranging from southern Canada to Colorado. It appeared during the Uintan age, –Bridgerian age (NALMA) of the Mid-Eocene– 42.5 Ma to 31.0 Ma. (AEO). Hesperocyon existed for approximately .

Sinodelphys is an extinct mammal from the Early Cretaceous, estimated to be 125 million years old. It was discovered and described in 2003 in rocks of the Yixian Formation in Liaoning Province, China, by a team of scientists including Zhe-Xi Luo and John Wible. While initially suggested to be the oldest known metatherian, later studies interpreted it as a eutherian.

Eritherium is an extinct genus of early Proboscidea found in the Ouled Abdoun basin (early Thanetian age), Morocco. It lived about 60 million years ago. It was first named by Emmanuel Gheerbrant in 2009 and the type species is Eritherium azzouzorum. Eritherium is the oldest, smallest and most primitive known elephant relative.

Elginerpeton is a genus of stegocephalian (stem-tetrapod), the fossils of which were recovered from Scat Craig, Morayshire in the UK, from rocks dating to the late Devonian Period (Late Frasnian stage, 375 million years ago). The type species is Elginerpeton pancheni.

Paleothyris is an extinct genus of small reptiliomorph which lived in the Moscovian (Carboniferous) age of the Late Carboniferous in Nova Scotia.

Vieraella is an extinct genus of frogs from the Lower Jurassic (Early Pliensbachian to Toarcian) Roca Blanca Formation of Argentina, and one of the oldest true frogs known. This genus is known by a single exceptionally well-preserved specimen, P.V.L. 2188, with at least eight presacrals vertebrae, free ribs, ulna and radius not fused, bony skull with some discoglossid characters.

Pezosiren portelli, also known as the "walking manatee", is a basal sirenian from the early Eocene of Jamaica, 50 million years ago. The type specimen is represented by a Jamaican fossil skeleton, described in 2001 by Daryl Domning, a marine mammal paleontologist at Howard University in Washington, DC. It is believed to have had a hippopotamus-like amphibious lifestyle, and is considered a transitional form between land and sea mammals.

Aetiocetus is a genus of extinct basal mysticete, or baleen whale that lived , in the Oligocene in the North Pacific ocean, around Japan, Mexico, and Oregon. It was first described by Douglas Emlong in 1966 and currently contains known four species, A. cotylalveus, A. polydentatus, A. tomitai, and A. weltoni. These whales are remarkable for their retention of teeth and presence of nutrient foramina, indicating that they possessed baleen. Thus, Aetiocetus represents the transition from teeth to baleen in Oligocene mysticetes. Baleen is a highly derived character, or synapomorphy, of mysticetes,

Ventastega (Venta referring to the Venta River at the Ketleri Formation where Ventastega was discovered) is an extinct genus of stem tetrapod that lived during the Upper Fammenian of the Late Devonian, approximately 372.2 to 358.9 million years ago. Only one species is known that belongs in the genus, Ventastega curonica, which was described in 1996 after fossils were discovered in 1933 and mistakenly associated with a fish called Polyplocodus wenjukovi. ‘Curonica’ in the species name refers to Curonia, the Latin name for Kurzeme, a region in western Latvia. Ventastega curonica was discovered

Prosalirus is an extinct genus of primitive frog known from the Early Jurassic of North America. It contains a single species, P. bitis, known from the Kayenta Formation of Arizona.

Kadanuumuu ("Big Man" in the Afar language) is the nickname of KSD-VP-1/1, a 3.58-million-year-old partial Australopithecus afarensis fossil discovered in the Afar Region of Ethiopia in 2005 by a team led by Yohannes Haile-Selassie, curator of physical anthropology at the Cleveland Museum of Natural History. Based on skeletal analysis, the fossil is believed to conclusively show that the species was fully bipedal.

Megapiranha is an extinct serrasalmid characin fish from the Late Miocene (8–10 million years ago) Ituzaingó Formation of Argentina, described in 2009. The type species is M. paranensis. It is thought to have been about in length and in weight. The holotype consists only of premaxillae and a zigzag tooth row; the rest of its body is unknown. This dentition is reminiscent of both the double-row seen in pacus, and the single row seen in the teeth of modern piranhas, suggesting that M. paranensis is a transitional form. Its bite force is estimated between .

Ocepeia is an extinct genus of afrotherian mammal that lived in present-day Morocco during the middle Paleocene epoch, approximately 60 million years ago. First named and described in 2001, the type species is O. daouiensis from the Selandian stage of Morocco's Ouled Abdoun Basin. A second, larger species, O. grandis, is known from the Thanetian, a slightly younger stage in the same area. In life, the two species are estimated to have weighed about and , respectively, and are believed to have been specialized leaf-eaters. The fossil skulls of Ocepeia are the oldest known afrotherian

Eotragus is an extinct genus of early bovid. Members of this genus had a wide range inhabiting Europe, Africa, and Asia during the Miocene around 20-18 million years ago. It is related to the modern nilgai and four-horned antelope. It was small and probably lived in woodland environments.

Protoclepsydrops is an extinct genus of early synapsids, found in Joggins, Nova Scotia. The name means 'first Clepsydrops', and refers to it being the predecessor of the other early synapsid Clepsydrops.

Clepsydrops is an extinct genus of primitive synapsids from the early Late Carboniferous that was related to Archaeothyris. The name means 'hour-glass appearance' (Greek klepsydra = "hourglass" + Greek ops = "eye, face, appearance").

left|thumb|Drawing of skeleton Protosiren is an extinct early genus of the order Sirenia. Protosiren existed throughout the Lutetian to Priabonian stages of the Middle Eocene. Fossils have been found in the far-flung locations like the United States (South Carolina, North Carolina and Florida), Africa (Egypt), Europe (France, Germany and Hungary) and Asia (India and Pakistan).

Protylopus is an extinct genus of camel that lived during middle to late Eocene some 50-40 million years ago in North America.

Psarolepis (; psārolepis, from Greek ψαρός 'speckled' and λεπίς 'scale') is a genus of extinct bony fish which lived around 397 to 418 million years ago (Pridoli to Lochkovian stages). Fossils of Psarolepis have been found mainly in South China and described by paleontologist Xiaobo Yu in 1998. It is not known certainly in which group Psarolepis belongs, but paleontologists agree that it probably is a basal genus and seems to be close to the common ancestor of lobe-finned and ray-finned fishes. In 2001, paleontologist John A. Long compared Psarolepis with onychodontiform fishes and refer to th

Parmastega is an extinct genus of stem tetrapod from the Devonian, dated to the earliest Famennian age (about 372 million years ago), in contrast to later fossils known from late Famennian (365–359 million years ago). These later fossils are considerably younger, by about 30 million years, than the earliest recorded tetrapod footprints, which presented a mystery that Parmastega's more recently described morphology from a three dimensional reconstruction has helped provide insight on.

Pappochelys (παπποχέλυς [πάππος (grandfather) + χέλυς (turtle)] meaning "grandfather turtle" in Greek) is an extinct genus of diapsid reptile possibly related to turtles. The genus contains only one species, Pappochelys rosinae, from the Middle Triassic of Germany, which was named by paleontologists and Hans-Dieter Sues in 2015. The discovery of Pappochelys provides strong support for the placement of turtles within Diapsida, a hypothesis that has long been suggested by molecular data, but never previously by the fossil record. It is morphologically intermediate between the definite stem-turtl

Pierolapithecus catalaunicus is an extinct species of primate which lived around 12.5-13 million years ago during the Miocene in what is now Hostalets de Pierola, Catalonia, Spain. Some researchers believe that it is a candidate for common ancestor to the great ape clade, or is at least closer than any previous fossil discovery. Others suggest it being a pongine, or a dryopith. On 16 October 2023, scientists reported the facial reconstruction of the great ape.

Entelognathus primordialis (“primordial complete jaw”) is an early placoderm from the late Silurian (Ludlow epoch) of Qujing, Yunnan, 419 million years ago. A team led by Min Zhu of the Academy of Sciences' Institute of Vertebrate Paleontology and Paleoanthropology in Beijing discovered the intact, articulated fossil in rock formations at Xiaoxiang reservoir.

Solenodonsaurus ("single-tooth lizard") is an extinct genus of reptiliomorphs that lived in what is now Czech Republic, during the Westphalian stage.

Promissum is an extinct genus of conodonts, primitive chordates, that lived during the Upper Ordovician period.

Amphistium paradoxum (from , 'on both sides', 'sail', and 'extraordinary'), the only species classified under the genus Amphistium and the family Amphistiidae, is a fossil fish which has been identified as a Paleogene relative of the flatfish, and as a transitional fossil. In a typical modern flatfish, the head is asymmetric with both eyes on one side of the head. In Amphistium, the transition from the typical symmetric head of a vertebrate is incomplete, with one eye placed near the top of the head.

Saurophthirus is an extinct genus of giant stem-group flea, and the only member of the family Saurophthiridae. The type species, S. longipes is found in early Cretaceous strata of Baissa, Siberia. Two other species S. exquisitus and S. laevigatus are from the Lower Cretaceous Yixian Formation of China.

Tseajaia is an extinct genus of diadectomorph tetrapod from the Early Permian of western North America. The skeleton is that of a medium-sized, rather advanced reptile-like animal. In life it was about long and may have looked vaguely like an iguana. The dentition was somewhat blunt, indicating herbivory or possibly omnivory. It contains a single known species, Tseajaia campi.

Fossilized mandible

Brasilodon ("tooth from Brazil") is an extinct genus of small, mammal-like cynodonts that lived in what is now Brazil during the Norian age of the Late Triassic epoch, about 225.42 million years ago. While no complete skeletons have been found, the length of Brasilodon has been estimated at . Its dentition shows that it was most likely an insectivore. The genus is monotypic, containing only the species B. quadrangularis. Brasilodon belongs to the family Brasilodontidae, whose members were some of the closest relatives of mammals, the only cynodonts alive today. Two other brasilodontid genera,

Semantoridae is an extinct family of stem-pinnipeds with fossils found in France, Kazakhstan, and Canada, dating back to various points in time in the Miocene epoch. Based on their overall anatomy semantorids were not marine specialists, as their elongated bodies, a long tail and robust limbs suggest they were freshwater animals not unlike otters. Indeed, at least some taxa such as Semantor and Potamotherium were initially classified as mustelids closely related to otters.

Heteroprox is an extinct genus of deer from the Miocene of Europe.

Parictis is an extinct arctoid belonging to the family Subparictidae.

Gephyrostegus is a genus of extinct gephyrostegid reptiliomorph amphibian. It was a small animal at 22 cm snout-vent length, of generally lizard-like build and presumably habit. It had large eyes and a large number of small, pointed teeth, indicating it was an active insectivorous hunter. The remains have been found in Nýřany, Czech Republic, dating from around 310 million years ago (upper Carboniferous).

The Bactritida are a small order of more or less straight-shelled (orthoconic) cephalopods that first appeared during the Emsian stage of the Devonian period (407 million years ago) with questionable origins in the Pragian stage before 409 million years ago, and persisted until the Carnian pluvial event in the upper middle Carnian stage of the Triassic period (231 million years ago). They are considered ancestors of the ammonoids, as well as of the coleoids (octopus, squid, cuttlefish, and the extinct belemnites).

Eorhynchochelys (meaning "dawn-beaked turtle" in Greek) is an extinct genus of stem-turtle from the Late Triassic Xiaowa Formation (or Wayao Member of the Falang Formation) of southwestern China.

Haasiophis, consisting of the sole species Haasiophis terrasanctus, is an extinct genus of snakes with hind limbs. It is one of three genera of Cenomanian snakes known to have possessed hindlimbs.

Rhyniognatha is an extinct genus of arthropod of disputed placement. It has been considered in some analyses as the oldest insect known, as well as possibly being a flying insect. Rhyniognatha is known from a partial head with preserved mouthparts from the Early Devonian aged Rhynie chert around 400 million years ago, when Earth’s first terrestrial ecosystems were being formed. The type, and only species is R. hirsti, which was named and described in 1928. Other analyses have interpreted the specimen as a myriapod.

Elpistostege is an extinct genus of finned tetrapodomorphs that lived during the Frasnian age of the Late Devonian epoch. Its only known species, E. watsoni, was first described in 1938 by the British palaeontologist Thomas Stanley Westoll, based on a single partial skull roof discovered at the Escuminac Formation in Quebec, Canada.

Rhyniella is a genus of fossil springtails (Collembola) from the Rhynie chert, which formed during the Pragian stage of the Early Devonian. One species has been described, Rhyniella praecursor. For some time it was believed to be the only hexapod from the Early Devonian ( )

Eotheroides is an extinct genus of Eocene sirenian. It is an early member of the family Dugongidae, which includes the extant dugong. Fossils have been found from Egypt, India, and Madagascar. Eotheroides was first described by Richard Owen in 1875 under the name Eotherium, which was replaced by the current name in 1899.

Archimylacris (meaning "primitive Mylacris", in reference to another species of Carboniferous cockroach) is an extinct genus of cockroach-like blattopterans, a group of insects ancestral to cockroaches, mantids, and termites.

Obruchevichthys is an extinct genus of stem-tetrapod from Latvia during the Late Devonian. When the jawbone, the only known fossil of this creature, was uncovered in Latvia, it was mistaken as a lobe-fin fish. However, when it was analyzed, it proved to hold many similarities to Elginerpeton, from Scotland. It was then declared belonging to the earliest group of tetrapods.

Antennacanthopodia is a small lobopodian from the Chengjiang biota that dates to about 520 million years ago (Cambrian Stage 3). It is similar to the extant Onychophora (velvet worm) and is the only widely accepted stem-onychophoran lobopodian from the Cambrian period. Antennacanthopodia had nine pairs of stubby legs, a pair of potential ocelli, and two pairs of antennae. The first pair of antennae were much longer than the second and are still present in modern onychophorans. The identity of the smaller antennae are less clear, but they might be homologous with either the slime papillae or on

Andreolepis is an extinct genus of prehistoric fish, which lived around 420 million years ago. It was described by Walter Gross in 1968 based on scales found in the Hemse Formation in Gotland, Sweden. It is placed in the monogeneric family Andreolepididae and is generally regarded as a primitive member of the class Actinopterygii based on its ganoid scale structure; however some new research regards it as a stem group of osteichthyans.

Prohesperocyon ("before Hesperocyon") is an extinct genus of canid, endemic to North America appearing during the Late Eocene around 36.6 mya (AEO).

Abdalodon is an extinct genus of late Permian cynodonts, known by its only species A. diastematicus. Abdalodon, along with the genus Charassognathus, belongs to the clade Charassognathidae. This clade represents the earliest known cynodonts, and is the first known radiation of this group during the Permian.

Heteronectes is a fossil fish which has been identified as a primitive flatfish, dating to the early Eocene (Lutetian stage) of France. The genus contains a single species H. chaneti.

Livoniana is a genus of prehistoric tetrapodomorphs which lived during the Devonian period (Givetian - Frasnian stages, about 374 to 391 million years ago).

Tristichopterus is an extinct genus of Devonian eotetrapodiform fish in the family Tristichopteridae. The type species, T. elatus, was first described by Philip Grey Egerton in 1861 from remains of Givetian age found in Scotland. In 2013, a second species, Eusthenopteron kurshi was attributed to the genus based on remains from Latvia, originally assigned to Eusthenopteron, but this has not been universally recognised, With a maximum length of sixty centimetres, is the smallest genus in the family. Tristichopterus was thought by Egerton to be unique for its time period as a fish with ossified v

Archaeolepis mane is amongst the earliest undisputed lepidopteran fossils. It dates from the Lower Jurassic (ca ). It was found in the Charmouth Mudstone Formation, Dorset, United Kingdom.

Jozaria is an extinct genus of stem perissodactyl from the Early to Middle Eocene of the Kuldana Formation of Kohat, Pakistan. It and other anthracobunids were formerly classified with proboscideans.

thumb|Comparison of isolated tarsals of Djarthia murgonensis with an extant australidelphian marsupial and an extant non-australidelphian ('ameridelphian') marsupial