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What Are Transitional Fossils

Transitional fossils are the fossilized remains of transitional forms of life that tangibly and demonstrably encode an evolutionary transition. Thus, transitional fossils are characterized by their retention of primitive (plesiomorphic) traits in contrast with their more recently evolved characteristics (the phenotype and genotype).

The term "missing link" is a popular slang term for such transitional forms, but is misleading. The term is particularly used in popular media, but is inaccurate and confusing, partly because it implies that there exists a single undiscovered fossil that is needed to confirm the transition. In contrast, the continual discovery of more and more transitional fossils is further refining and validating evolutionary transitions. Transitional fossils are numerous and varied throughout the tree of life, including those between primates and early humans, contrary to the claims of creationists who deny evolution.

Evolutionary theory considers all populations of organisms to be in transition, whether changes be slow, as in genetic drift, or fast, as when a changing environment imposes significant adaptive pressures. A transitional form of life is one that demonstably illustrates a particular intermediate evolutionary stage of change or adaptation.

Transitional fossils usually coexist with gaps in a sequence in the fossil record. The probabilities of fossilization pretty much precludes the discovery of detailed sequences of fossils spanning millions of years. However, fine gradations of fossils between species and genera are abundant in the fossil record, as are coarser sequences between higher taxa.

Fossil transitionals between kingdoms and phyla:

The Cambrian fossils Halkiera and Wiwaxia have features that connect them with each other and with the modern phyla of Mollusca, Brachiopoda, and Annelida. In particular, one species of halkieriid has brachiopod-like shells on the dorsal side at each end. This is seen also in an immature stage of the living brachiopod species Neocrania. It has setae identical in structure to polychaetes, a group of annelids. Wiwaxia and Halkiera have the same basic arrangement of hollow sclerites, an arrangement that is similar to the chaetae arrangement of polychaetes. The undersurface of Wiwaxia has a soft sole like a mollusk's foot, and its jaw looks like a mollusk's mouth. Aplacophorans, which are a group of primitive mollusks, have a soft body covered with spicules similar to the sclerites of Wiwaxia (see Conway Morris 1998, 185-195).

Cambrian and Precambrain fossils Anomalocaris and Opabinia are transitional between arthropods and lobopods.

Tentative List of Transitional Fossils

Invertebrate to Vertebrate

Unnamed Upper (U.) Pre-Cambrian chordate — First to bear a primitive notochord; archaetypical chordate.
Pikaia gracilens — Middle (M.) from Burgess Shale - a Cambrian chordate with lancelet-like morphology.
Haikouella— Lower (L.) Cambrian chordate, first to bear a skull; archaetypical craniate.
Haikouichthys— L. Cambrian quasi-vertebrate, intermediate in developing a vertebral column; archaetypical vertebrate.
Conodonts— U. Cambrian to Triassic quasi-vertebrates with spinal cord; “bug-eyed lampreys”.
Myllokunmingia— L. Cambrian vertebrate with primitive spinal column; oldest true crown-group vertebrate.
Arandaspis— L. Ordovician vertebrate, armoured jawless fish (ostracoderm), oldest known vertebrate with hard parts known
from (mostly) complete fossils.

Jawless Fish to Jawed Vertebrate

Birkenia— Silurian primitive, jawless fish, a typical member of the Anaspida
Cephalaspis— Silurian armoured jawless fish, archaetypical member of the “Osteostraca,” sister group to all jawed
vertebrates.
Shuyu— Silurian to Devonian, armoured jawless fish belonging to Galeaspida, related to Osteostraca. Internal cranial
anatomy very similar to the anatomy seen in basal jawed vertebrates. This similarity is directly implied with the translation
of its name, “Dawn Fish,” with the implication that it represents the “dawn of jawed vertebrates.”

Acanthodian to shark

Ptomacanthus— sharklike fish, originally described as an acanthodian fish: brain anatomy demonstrates that it is an
intermediate between acanthodians and sharks.
Cladoselache— primitive/basal shark.
Tristychius— another sharklike fish.
Ctenacanthus— primitive/basal shark.
Paleospinax— sharklike jaw, primitive teeth.
Spathobatis— Ray-like fish.
Protospinax— Ancestral to both sharks and skates.

Primitive jawed fish to bony fish

Acanthodians— superficially similar to early bony fishes, and some have been identified as being the ancestors of sharks.
Palaeoniscoids — the first primitive bony fishes - see example from Bear Gulch
Canobius, Aeduella— palaeoniscoids with more advanced jaws.
Parasemionotus— combination of modern cheeks with more primitive features, like lungs
Oreochima— first teleost fish
Leptolepids— vaguely herring-like ancestors of modern teleost fish. Lung modified into swim bladder.
Amphistium and Heteronectes— percomorphs that demonstrate the transition of the eye location of flatfishes.

Fish to amphibian

Paleoniscoids— both ancestral to modern fish and land vertebrates.
Osteolepis— modified limb bones, amphibian like skull and teeth.
Kenichthys— shows the position of exhaling nostrils moving from front to fish to throat in tetrapods in its halfway point, in
the teeth
Eusthenopteron, Sterropterygion— fin bones similarly structured to amphibian feet, but no toes yet, and still fishlike bodily
proportions.
Panderichthys, Elpistostege— tetrapod-like bodily proportions.
Obruchevichthys— fragmented skeleton with intermediate characteristics, possible first tetrapod.
Tiktaalik— a fish with developing legs. Also appearance of ribs and neck.
Acanthostega gunnari— famous intermediate fossil. most primitive fossil that is known to be a tetrapod
Ichthyostega— like Acanthostega, another fishlike amphibian
Hynerpeton— A little more advanced then Acanthostega and Ichtyostega
Labyrinthodonts— still many fishlike features, but tailfins have disappeared
Lungfish—A fish-that has lungs.

Primitive to modern amphibians

Temnospondyls
Dendrerpeton acadianum
Archegosaurus decheni
Eryops megacephalus
Trematops
Amphibamus lyelli
Doleserpeton annectens
Triadobatrachus— primitive frog
Vieraella
Karaurus— primitive salamander

Amphibian to reptile

Proterogyrinus
Limnoscelis
Tseajaia
Solenodonsaurus
Hylonomus
Paleothyris

Early reptile to turtle

Captorhinus
Scutosaurus
Odontochelys Semitestacea— partial formation of a turtle shell, showing how the hard underbelly, or plastron, formed first.
Deltavjatia vjatkensis
Proganochelys

Early reptile to diapsid (dinosaurs and modern reptiles except for turtle)

Hylonomus
Paleothyris
Petrolacosaurus
Araeoscelis
Apsisaurus
Claudiosaurus
Planocephalosaurus
Protorosaurus
Prolacerta
Proterosuchus
Hyperodapedon
Trilophosaurus

Reptile to mammal

Paleothyris
Protoclepsydrops haplous
Clepsydrops
Archaeothyris
Varanops
Haptodus
Dimetrodon
Sphenacodon
Biarmosuchia
Procynosuchus
Dvinia
Thrinaxodon
Cynognathus
Diademodon
Probelesodon
Probainognathus
Exaeretodon
Oligokyphus
Kayentatherium
Pachygenelus
Diarthrognathus
Adelobasileus cromptoni
Sinoconodon
Kuehneotherium
Eozostrodon
Morganucodon— a transition between “mammal-like reptiles” and “true mammals”.
Haldanodon
Peramus
Endotherium
Kielantherium
Aegialodon
Steropodon galmani
Vincelestes neuquenianus
Pariadens kirklandi
Kennalestes
Asioryctes
Cimolestes
Procerberus
Gypsonictops

Dinosaur to bird

Allosaurus—A large therapod with a wishbone
Coelophysis
Compsognathus—A small coeleosaur with a wishbone
Eoraptor
Epidendrosaurus
Herrerasaurus
Ceratosaurus
Compsognathus
Sinosauropteryx
Protarchaeopteryx
Caudipteryx
Velociraptor
Deinonychus
Oviraptor
Sinovenator
Beipiaosaurus
Lisboasaurus
Sinornithosaurus
Microraptor— a feathered bird with distinctly dinosaurian characteristics, such as its tail.
Xiaotingia— slightly earlier than Archaeopteryx, slightly more like a dinosaur and less like a bird
Archaeopteryx— the famous bird-with-teeth.
Rahonavis
Confuciusornis
Sinornis
Patagopteryx
Ambiortus
Hesperornis
Apsaravis
Ichthyornis
Columba— One of many typical modern birds

Primates

Darwinius masillae— a link between earlier primates and later ones.

Non-human primate to human

Sahelanthropus tchadensis— One of the oldest known species in the human family tree. Lived around 6.5-7 million years
ago, one of the earliest bi-pedal fossils.
Orrorin tugenensis— Over 20 fossils of the 6 million year old species have been found and show very human-like
morphology.
Ardipithecus ramidus— 4.5 million year old species showing bi-pedal adaptations and opposable thumbs.
Australopithecus— a genus of bipedal apes
Australopithecus anamensis
Australopithecus afarensis
Kenyanthropus platyops— 3.5 million year old himinim fossil, most likely a subspecies of Australopithecus
Australopithecus africanus
Australopithecus garhi
Australopithecus sediba— advanced australopithecus showing more human features
Australopithecus aethiopicus
Australopithecus robustus
Australopithecus boisei
Homo habilis— a transitional form from Australopithecus to later Homo
Homo rudolfensis— a type of Homo habilis or a different species
Homo ergaster— a form of Homo erectus or a distinct species
Homo georgicus— a form of Homo Erectus or a distinct species
Homo erectus— a transitional form from Australopithecus to later Homo (Latin for humans) species
Homo antecessor— is an extinct human species, or subspecies, dating from 1.2 million to 800,000 years ago
Homo heidelbergensis— A possible common ancestor of modern man and homo neanderthalensis
Homo neanderthalensis— They may or may not have done Humpy bumpy with modern humans.
Homo floresiensis— Extinct Homo species, living 94,000-12,000 years ago, used tools, bipedal, very human-like
Cro-magnon— considered early modern human and perhaps as smart as we are

Cetaceans

Indohyus— a vaguely chevrotain-like or raccoon-like aquatic artiodactyl ungulate with an inner ear identical to that of
whales.
Ambulocetus— an early whale that looks like a mammalian version of a crocodile
Pakicetus— an early, semi-aquatic whale, a superficially wolf-like animal believed to be a direct ancestor of modern whales.
Rhodocetus— An early whale with comparatively large hindlegs: not only represents a transition between semi-aquatic
whales, like Ambulocetus, and obligately aquatic whales, like Basilosaurus.
Basilosaurus— A large, elongated whale with vestigial hind flippers: transition from early marine whales (likeRhodocetus) to
modern whales
Dorudon— A small whale with vestigial hind flippers, close relative of Basilosaurus.

List reference:  Eclectic Irony

Wikipedia has an extensive section listing notable transitional fossil sequences.