What is a Transitional Fossil?
A transitional fossil is a fossil of an organism that has traits from multiple evolutionary stages. Proponents of creationism claim that “evolutionists have had over 140 years to find a transitional fossil and nothing approaching a conclusive transitionalform has ever been found”, despite the discovery of Archaeopteryx (a transitional form between maniraptoran dinosaurs and basal (primitive) birds, and among the best examples of evolution) only two years after Darwin published The Origin of Species. Creationists say that we never saw evolution happen, but transitional fossils are the next best thing.
What are Transitional Forms?
“A transitional form is an organism that has features intermediate of its ancestors and progeny. The term is most common in evolution to refer to organisms that show certain features (wings, feathers, gills and so on) partly in development. In theory, every fossil is a transitional form if it has descendants and each living creature is a transition between its parent and its offspring. However, evolution is about the features of populations rather than individuals; the transition at the species level can be too small in fossils; so the list below concentrates on broad transitional features and the genus or larger group.”
Transitions in vertebrates before the Cenozoic
Invertebrate to Vertebrate
- Unnamed Upper (U.) Pre-Cambrian chordate — First to bear a primitive notochord; archaetypical chordate.
- Pikaia gracilens — Middle (M.) 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 — primitive bony fishes.
- 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 or four legged animal from the Upper Devonian of Greenland, which has shed significant light on the derivation and early evolution of tetrapods. It had legs and feet but was aquatic, not an amphibian.
- Ichthyostega — like Acanthostega, another fishlike amphibian
- Hynerpeton — A little more advanced then Acanthostega and Ichtyostega
- Labyrinthodonts — still many fishlike features, but tailfins have disappeared
- Gars — Fish with vascularized swim bladders that can function as lungs
- Lungfish and Birchirs — fish that have lungs
Primitive to modern amphibians
- Dendrerpeton acadianum
- Archegosaurus decheni
- Eryops megacephalus
- Amphibamus lyelli
- Doleserpeton annectens
- Triadobatrachus — a primitive frog.
- Vieraella — an early modern frog
- Karaurus — a primitive salamander
Amphibian to reptile
Early reptile to diapsid
Early diapsid to turtle
- Pappochelys rosinae — diapsid skull with expanded ribs and fused gastralia
- Odontochelys semitestacea — secondary loss of temporal fenestrae, partial formation of a turtle shell, showing how the hard underbelly, or plastron, formed first.
- Deltavjatia vjatkensis
Early synapsid to mammal
- Protoclepsydrops haplous
- Adelobasileus cromptoni
- Morganucodon -- a transition between "proto mammals" and "true mammals".
- Steropodon galmani
- Vincelestes neuquenianus
- Pariadens kirklandi
Dinosaur to bird
- Kulindadromeus — A basal neornithischian (Ya know, Triceratops, Iguanodon, Hypsilophodon, and such) with feathers.
- Allosaurus — A large theropod with a wishbone.
- Aerosteon — A large theropod of the same lineage as the aforementioned Allosaurus that was discovered to have air sacs supplementing lungs, like modern birds.
- Compsognathus — A small coeleurosaur with a wishbone.
- Gigantoraptor — A large oviraptorosaur discovered brooding its nests in order to protect and incubate eggs.
- Mei — A troodont discovered sleeping with its head underneath its wing/
- Balaur — A large flightless bird.
- Sinosauropteryx — a basal coelurosaur discovered to be covered in feathers. It is also the first dinosaur to have its colour determined, thanks to preserved pigment structures in the feathers.
- Velociraptor — a very famous dromaeosaur discovered to have quill knobs on it's wrists. For SOME odd reason, sadly. everyone sees these things as mutant allosaur-looking... uh... things.
- Oviraptor — the first dinosaur discovered to
- 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.
- Hesperornis — A diving seabird with prominent teeth. It's also completely flightless.
- Ichthyornis — A flying seabird with prominent teeth.
- Columba — One of many typical modern birds.
Transitional mammalian fossils
- Purgatorius — the earliest primate-like organism
- Plesiadapis — Mammal closely related to primates.
- Carpolestes — Mammal closely related to primates
- Archicebus — First euprimate, or something very similar to it.
- Omomys — Tarsier-like primate
- Eosimias — Basal anthropoid
- Amphipithecus — Another basal anthropoid
- Apidium — The first, primitive monkey.
- Propliopithecus — Primitive New World Monkey
- Darwinius masillae — a link between earlier primates and later ones.
- Dryopithecus Primitive ape.
- Proconsul Primate that is closely related to apes.
- Sivapithecus Primate closely related to the ancestors of Orangutans
- Djebelemur First lemuriform primate.
- Cantius Extremely primitive prosimian from the Early Eocene of North America.
- Teilhardina First North American primate.
Non-human primate to human
- Sahelanthropus — possible candidate for last human-chimpanzee common ancestor; from placement of skull possibly walked upright.
- Orrorin — possible human ancestor, may have walked upright as shown by shape of femur.
- Australopithecus — a genus of bipedal apes
- Australopithecus sediba — advanced Australopithecus showing more human features
- 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 erectus — a transitional form from Australopithecus to later Homo (Latin for "human") species
- Homo heidelbergensis — a possible common ancestor of modern man and Homo neanderthalensis
- Homo neanderthalensis — Neanderthals likely interbred with modern humans.
- Homo sapiens idaltu archaic subspecies of modern human, possibly ancestral to Homo sapiens sapiens (modern humans).
- 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 (like Rhodocetus) to modern whales
- Dorudon — A small whale with vestigial hind flippers, close relative of Basilosaurus.
READ MORE: Whale Evolution
Transitional plant fossils
- Cooksonia — early vascular plant
- Archaeopteris — early tree
- Williamsonia — an early flowering plant ("stem angiosperm")
Misconception: “Gaps in the fossil record disprove evolution.”
Response: The fact that some transitional fossils are not preserved does not disprove evolution. Evolutionary biologists do not expect that all transitional forms will be found and realize that many species leave no fossils at all. Lots of organisms don’t fossilize well and the environmental conditions for forming good fossils are not that common. So, science actually predicts that for many evolutionary changes there will be gaps in the record.
READ ALSO: 10 Common Myths About Evolution
Also, scientists have found many transitional fossils. For example, there are fossils of transitional organisms between modern birds and their theropod dinosaur ancestors, and between whales and their terrestrial mammal ancestors.
Source: Wikipedia.com, NatGeo.com