A few selected transitional fossils
Introduction: All species undergo gradual change over time, but in the fossil record we find evidence of some changes that are particularly striking. This website is dedicated to some of these so-called transitional fossils.
Warning 1: The images are only artist's conceptions and might contain errors; so I keep a page with links to photos or diagrams of the fossils themselves.
Warning 2: When a fossil is called "transitional" between two types of animal, that means it shows some of the traits of both, but it does not mean it links those animals by direct descent. Evolution is a branching process - by which we mean that species often split in two. Therefore:
"Because evolution is a branching process that produces a complex bush pattern of related species rather than a linear process that produces a ladder-like progression, and because of the incompleteness of the fossil record, it is unlikely that any particular form represented in the fossil record is a direct ancestor of any other."
— Rusty Cashman / Wikipedia
In short, transitional fossils are best thought of as being close relatives of the species which actually link two groups. They may have lived at the same time as those actual links, or they may not have (this confuses many people). As long as these problems are borne in mind, transitional fossils give a rough indication of what evolutionary changes were occurring. But don't be misled into thinking that fossils are the only evidence for evolution. They're not even the strongest evidence for evolution.
Apes - humans
Most ape-like at the top (though in a technical sense, humans are still considered apes). Images and diagrams of the fossils here.
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Ardipithecus ramidus ~4.4 million years ago
Ardipithecus ramidus had a brain the size of a chimp's, but probably walked upright on the ground, while still able to go on all fours in the trees, where it would find its opposable big toe useful (Gibbons, 2009).
Australopithecus afarensis ~3.6 mya
Australopithecus afarensis was a more advanced walker, with nongrasping feet (White et al, 2009), but it still had the brain size of a chimpanzee (Dawkins, 2009). Probably not a direct ancestor of modern humans (Rak et al, 2007).
Australopithecus africanus ~3 mya
Similar.
Homo habilis ~2 mya?
Homo habilis had a brain about 50% bigger than a chimp's. The fossils are found with a variety of stone tools; this is the earliest human which we're sure used tools (Coyne, 2009).
Homo erectus ~1 mya
A tool-maker, Homo erectus had a brain size of about 1,000 cc, still smaller than our own (Dawkins, 2009).
Homo heidelbergensis ~0.5 mya
Homo heidelbergensis had a brain size approaching our own, and shows a mix of Homo erectus and modern human features (Coyne, 2009).
Fish - tetrapods
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Eusthenopteron ~385 million years ago
A pelagic fish, Eusthenopteron is probably representative of the group from which tetrapods evolved. It had a tetrapod-like skull and spine (Prothero, 2007).
Panderichthys ~385 mya
Panderichthys had a tetrapod-like braincase and tetrapod-like teeth, and had also lost its dorsal and anal fins (Prothero, 2007).
Tiktaalik ~375 mya
Though still a water-dweller, Tiktaalik had fins that were halfway towards being feet, and ears capable of hearing in air or water (Prothero, 2007). It was capable of crawling around in very shallow water, and it had a neck, unlike fish but like tetrapods (Coyne, 2009).
Ventastega ~365 mya
The bones of Ventastega are intermediate between Tiktaalik and Acanthostega (Ahlberg et al, 2008). Sadly, the fossil is incomplete and we can't see its fins/feet.
Acanthostega ~365 mya
Possessing four definite legs, Acanthostega was presumably capable of movement over land (Coyne, 2009), though the legs were still better suited for crawling along the bottom of the water (Prothero, 2007). Its tail was still adapted for propulsion through water...
Dinosaurs - birds
Most dinosaur-like at the top. Images and diagrams of the fossils here.
Anchiornis ~155 million years ago
Although many feathered dinosaurs are known, Anchiornis is the first to be found that probably predates Archaeopteryx. The feathers were "not obviously flight-adapted" (Hu et al, 2009).
Archaeopteryx ~145 mya
The famous Archaeopteryx had feathers and was probably capable of at least gliding, but it also had dinosaur-like teeth, claws, and a long bony tail. Its skeleton was "almost identical to that of some theropod dinosaurs" (Coyne, 2009). Precisely how closely related it is to the main line of bird evolution remains the subject of controversy (Xu et al, 2011).
Confuciusornis ~125 mya
Confuciusornis had a bird-like tail and a pygostyle, which is a feature of modern birds. It retained dinosaur-like claws (Prothero, 2007). It had strong shoulder bones, but was probably not capable of true flapping flight (Senter, 2006). It may have glided. It is the earliest known bird with a toothless beak, but other lineages continued to have teeth for a long time.
Sinornis ~110 mya?
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One will occasionally encounter vague suggestions that birds evolved from (non-dinosaur) reptiles. John Ruben and colleagues are the main supporters of this idea, but it remains very much a minority view.
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Synapsids - mammals
Strictly speaking, the group that gave rise to mammals were not true reptiles (though they were closely related). Therefore, there is no transition from reptiles to mammals, but rather from synapsids to mammals.
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Indohyus ~48 million years ago
Although only a cousin species of the ancestor of whales, Indohyus had bones denser than normal mammals, indicating it was partially aquatic: heavy bones are good ballast (Thewissen et al, 2009). Its ears shared a feature with modern whales: a thickened wall of bone which assists in underwater hearing; non-cetaceans don't have this (Thewissen et al, 2009).
Pakicetus ~52 mya
Perhaps the actual ancestor, Pakicetus was probably semi-aquatic; like Indohyus, it had dense bones for ballast (Thewissen et al, 2009). Its body was "wolf-like" but the skull had eye sockets adapted for looking upwards, presumably at objects floating above it (Thewissen et al, 2009). Although initially known from just a skull, many more bones were found later ...
The nostrils of Rodhocetus have started to move backwards (towards the blowhole position) and the skeleton indicates a much stronger swimmer (Coyne, 2009). On land it would struggle, moving "somewhat like a modern eared seal or sea lion" (Gingerich et al, 2001). Its teeth were simpler than its predecessors (Futuyma, 2005), a trend that continued to the present.
Maiacetus ~47 mya
Seems similar to Rodhocetus. One fossil was found with what appeared to be a foetus, in a position indicating head-first birth (Gingerich et al, 2009) unlike modern whales.
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Whales evolved relatively quickly. As Coyne (2009) explains, "adapting to life at sea did not require the evolution of any brand-new features - only modifications of old ones". Thewissen et al (2009) give a good overview of whale evolution, which is freely available.
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Protohorses - horses
Least horse-like at the top. Images and diagrams of the fossils here.
With horse evolution, it's particularly important to bear the warning at the top in mind: we have many fossils, and the known family tree is very bushy, not a straight line. At one time, 13 different genera of horses existed simultaneously (Raven et al, 2008). Regardless, there are definite trends over time towards larger body size; larger, ridged teeth suitable for grazing; longer limbs; and reduction of side toes (Raven et al, 2008). These trends were not absolute, however.
Hyracotherium ~60 million years ago
A cousin species of the ancestor of horses. The forelimb of Hyracotherium had four toes (Raven et al, 2008).
Protorohippus ~50 mya
Bigger. The forelimb had four toes.
Mesohippus ~35 mya
Bigger. The forelimb had three toes (Raven et al, 2008).
Miohippus ~35 mya
The skull and snout of Miohippus are becoming more horse-like (Prothero, 2007).
Parahippus ~23 mya
The skeleton of Parahippus was more adapted to long-distance running, for escaping predators in an open environment (Evans, 1992)
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Miscellaneous
A few other transitional fossils of interest. This is, obviously, not a sequence. Images and diagrams of the fossils here.
Aardonyx, a proto-sauropod dinosaur that, though bipedal, could probably also walk on all fours (Yates et al, 2009). Contrary to what you might expect, in this case bipeds evolved to become quadrupeds.
Amphistium, an early flatfish, with eyes intermediate in position between an ordinary fish and a modern flatfish (Friedman, 2008).
Claudiosaurus, an early relative of marine reptiles like plesiosaurs, but the limbs are not very specialised for swimming (Prothero, 2007).
Darwinopterus, a pterosaur, has the advanced skull and neck of the
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Other
This website mentions many of the more famous transitional fossils, but these represent only a tiny fraction of what we know. We have good fossil evidence for a great many other transitions, some of which might be added here in the future.
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References
Articles behind a paywall are accessible for free from some universities and libraries. In any case, you can usually get at least an abstract.
Ahlberg PE, Clack JA, Luksevics E, Blom H, Zupins I (2008) Ventastega curonica and the origin of tetrapod morphology. Nature 453(7199): 1199–1204 (paywall, figures).
Anderson JS, Reisz RR, Scott D, Frobisch NB, Sumida SS (2008) A stem batrachian from the Early Permian of Texas and the origin of frogs and salamanders. Nature 453(7194): 515-518 (paywall, figures).
Apesteguia S, Hussam Z (2006) A Cretaceous terrestrial snake with robust hindlimbs and a sacrum. Nature 440(7087): 1037-1040 (paywall, figures).
Benton MJ (2005) Vertebrate Palaeontology, 3rd edition. Oxford: Blackwell Publishing.
Clack JA (2002) An early tetrapod from 'Romer's Gap'. Nature 418(6893): 72-76 (paywall, figures).
Clack JA (2005) Getting a Leg Up on Land. Scientific American 293(6): 100-107 (online).
Coyne JA (2009) Why Evolution is True. Oxford: Oxford University Press.
Dawkins R (2009) The Greatest Show on Earth: The Evidence for Evolution. London: Bantam Press.
Evans WE (1992) Horse Breeding and Management. New York: Elsevier Science.
Florida Museum of Natural History (date unknown) Dinohippus (online).
Forster CA, Chiappe LM, Krause DW, Sampson SD (1996) The first Cretaceous bird from Madagascar. Nature 382(6591): 532-534 (paywall).
Friedman M (2008) The evolutionary origin of flatfish asymmetry. Nature 454(7201): 209-212 (paywall, figures).
Futuyma DJ (2005) Evolution. Sunderland: Sinauer.
Gibbons A (2009) A New Kind of Ancestor: Ardipithecus Unveiled. Science 326(5949): 36-40 (free reg required).
Gingerich PD, Arif M, Clyde WC (1995) New Archaeocetes (Mammalia, Cetacea) from the Middle Eocene Domanda formation of the Sulaiman range, Pun Jab (Pakistan). Contributions from the Museum of Paleontology, The University of Michigan 29(11): 291-330 (online).
Gingerich PD, Ul-Haq M, Zalmout IS, Khan IH, Malkani MS (2001) Origin of Whales from Early Artiodactyls: Hands and Feet of Eocene Protocetidae from Pakistan. Science 293(5538): 2239-2242 (paywall, data).
Gingerich PD, Ul-Haq M, Von Koenigswald W, Sanders WJ, Smith BH, Zalmout IS (2009) New Protocetid Whale from the Middle Eocene of Pakistan: Birth on Land, Precocial Development, and Sexual Dimorphism. PLoS ONE 4(2): e4366 (online).
Hu D, Hou L, Zhang L, Xu X (2009) A pre-Archaeopteryx troodontid theropod from China with long feathers on the metatarsus. Nature 461(7264): 640-643 (paywall, figures).
Jenkins FA, Walsh DM (1993) An Early Jurassic caecilian with limbs. Nature 365(6443): 246-250 (paywall).
Lu J, Unwin DM, Jin X, Liu Y, Ji Q (2009) Evidence for modular evolution in a long-tailed pterosaur with a pterodactyloid skull. Proceedings of the Royal Society B (online).
Macdonald DW (ed.) (2009) The Encyclopedia of Mammals, 3rd edition. Oxford: Oxford University Press.
MacFadden BJ (1986) Fossil Horses from "Eohippus" (Hyracotherium) to Equus: Scaling, Cope's Law, and the Evolution of Body Size. Paleobiology 12(4): 355-369 (paywall).
Niedzwiedzki G, Szrek P, Narkiewicz K, Narkiewicz M, Ahlberg PE (2010) Tetrapod trackways from the early Middle Devonian period of Poland. Nature 463(7277): 43-48 (paywall, figures).
Prothero DR (2007) Evolution: What the Fossils Say and Why It Matters. New York: Columbia University Press.
Prothero DR (2008) What missing link? New Scientist 197(2645): 35-41 (paywall).
Rak Y, Ginzburg A, Geffen E (2007) Gorilla-like anatomy on Australopithecus afarensis mandibles suggests Au. afarensis link to robust australopiths. Proceedings of the National Academy of Sciences 104(16): 6568-6572 (online).
Raven PH, Johnson GB, Losos JB, Mason KA, Singer SR (2008) Biology, 8th edition. New York: McGraw Hill.
Ridley M (2004) Evolution, 3rd edition. Oxford: Blackwell Publishing.
Romer AS (1969) Cynodont Reptile with Incipient Mammalian Jaw Articulation. Science 166(3907): 881-882 (paywall).
Senter P (2006) Scapular orientation in theropods and basal birds, and the origin of flapping flight. Acta Palaeontologica Polonica 51(2): 305-313 (online).
Thewissen JGM, Williams EM, Roe LJ, Hussain ST (2001) Skeletons of terrestrial cetaceans and the relationship of whales to artiodactyls. Nature 413(6853): 277-281 (paywall, figures).
Thewissen JGM, Cooper LN, George JC, Bajpai S (2009) From Land to Water: the Origin of Whales, Dolphins, and Porpoises. Evo Edu Outreach 2(2): 272-288 (online).
Thewissen JGM, McLellan WA (2009) Maiacetus: displaced fetus or last meal? PLoS ONE (online).
Van Valen L (1968) Monophyly or Diphyly in the Origin of Whales. Evolution 22(1): 37-41 (paywall).
White TD, Asfaw B, Beyene Y, Haile-Selassie Y, Lovejoy CO, Suwa G, WoldeGabriel G (2009) Ardipithecus ramidus and the Paleobiology of Early Hominids. Science 326(5949): 75-86 (free reg required).
Xu X, You H, Du K Han F (2011) An Archaeopteryx-like theropod from China and the origin of Avialae. Nature 475: 465-470 (paywall).
Yates AM, Bonnan MF, Neveling J, Anusuya C, Blackbeard MG (2009) A new transitional sauropodomorph dinosaur from the Early Jurassic of South Africa and the evolution of sauropod feeding and quadrupedalism. Proceedings of the Royal Society B (online).
Zimmer C (1998) At the Water's Edge: Macroevolution and the Transformation of Life. New York: Free Press.
Last site update: 2013-12-02
