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Prominent Examples of Transitional Fossils




Archaeopteryx is a genus of theropod dinosaur closely related to the birds. Since the late 19th century, it has been accepted by palaeontologists, and celebrated in lay reference works, as being the oldest known bird, though a study in 2011 has cast doubt on this assessment, suggesting instead that it is a non-avialan dinosaur closely related to the origin of birds.

It lived in what is now southern Germany in the Late Jurassic period around 150 million years ago, when Europe was an archipelago in a shallow warm tropical sea, much closer to the equator than it is now. Similar in shape to a European Magpie, with the largest individuals possibly attaining the size of a raven, Archaeopteryx could grow to about 0.5 metres (1.6 ft) in length. Despite its small size, broad wings, and inferred ability to fly or glide, Archaeopteryx has more in common with other small Mesozoic dinosaurs than it does with modern birds. In particular, it shares the following features with the deinonychosaurs (dromaeosaurs and troodontids): jaws with sharp teeth, three fingers with claws, a long bony tail, hyperextensible second toes (“killing claw”), feathers (which suggest homeothermy), and various skeletal features. These features make Archaeopteryx a clear candidate for a transitional fossil between dinosaurs and birds,[15] making it important in the study both of dinosaurs and of the origin of birds.

The first complete specimen was announced in 1861, and ten more Archaeopteryx fossils have been found since then. Most of the eleven known fossils include impressions of feathers—among the oldest direct evidence of such structures. Moreover, because these feathers take the advanced form of flight feathers, Archaeopteryx fossils are evidence that feathers began to evolve before the Late Jurassic.

Australopithecus afarensis

The hominid Australopithecus afarensis represents an evolutionary transition between modern bipedal humans and their quadrupedal ape ancestors. A number of traits of the A. afarensis skeleton strongly reflect bipedalism, to the extent that some researchers have suggested that bipedality evolved long before A. afarensis. In overall anatomy, the pelvis is far more human-like than ape-like. The iliac blades are short and wide, the sacrum is wide and positioned directly behind the hip joint, and there is clear evidence of a strong attachment for the knee extensors, implying an upright posture.

While the pelvis is not entirely like that of a human (being markedly wide, or flared, with laterally orientated iliac blades), these features point to a structure radically remodelled to accommodate a significant degree of bipedalism. The femur angles in toward the knee from the hip. This trait allows the foot to fall closer to the midline of the body, and strongly indicates habitual bipedal locomotion. Present-day humans, orangutans and spider monkeys possess this same feature. The feet feature adducted big toes, making it difficult if not impossible to grasp branches with the hindlimbs. Besides locomotion, A. afarensis also had a slightly larger brain than a modern chimpanzee[18] (the closest living relative of humans) and had teeth that were more human than ape-like.


Australopithecus afarensis


Pakicetids, Ambulocetus

The cetaceans (whales, dolphins and porpoises) are marine mammal descendants of land mammals. The pakicetids are an extinct family of hoofed mammals that are the earliest whales, whose closest sister group is Indohyus from family Raoellidae. They lived in the early Eocene, around 53 million years ago. Their fossils were first discovered in North Pakistan in 1979, at a river not far from the shores of former Tethys Sea. Pakicetids could hear under water, using enhanced bone conduction, rather than depending on tympanic membrane like most land mammals. This arrangement does not give directional hearing under water.

Ambulocetus natans, which lived about 49 million years ago, was discovered in Pakistan in 1994. It was probably amphibious, and resembled the crocodile in appearance. In the Eocene, ambulocetids inhabited the bays and estuaries of the Tethys Ocean in northern Pakistan. The fossils of ambulocetids are always found in near-shore shallow marine deposits associated with abundant marine plant fossils and littoral molluscs. Although they are found only in marine deposits, their oxygen isotope values indicate that they consumed water with a range of degrees of salinity, some specimens showing no evidence of sea water consumption and others none of fresh water consumption at the time when their teeth were fossilized. It is clear that ambulocetids tolerated a wide range of salt concentrations. Their diet probably included land animals that approached water for drinking, or freshwater aquatic organisms that lived in the river. Hence, ambulocetids represent the transition phase of cetacean ancestors between freshwater and marine habitat.





Tiktaalik is a genus of extinct sarcopterygian (lobe-finned fish) from the late Devonian period, with many features akin to those of tetrapods (four-legged animals). It is one of several lines of ancient sarcopterygians to develop adaptations to the oxygen-poor shallow water habitats of its time—adaptations that led to the evolution of tetrapods.[28] Well-preserved fossils were found in 2004 on Ellesmere Island in Nunavut, Canada.

Tiktaalik lived approximately 375 million years ago. Paleontologists suggest that it is representative of the transition between non-tetrapod vertebrates such as Panderichthys, known from fossils 380 million years old, and early tetrapods such as Acanthostega and Ichthyostega, known from fossils about 365 million years old. Its mixture of primitive fish and derived tetrapod characteristics led one of its discoverers, Neil Shubin, to characterize Tiktaalik as a “fishapod”. Unlike many previous, more fish-like transitional fossils, the “fins” of Tiktaalik have basic wrist bones and simple rays reminiscent of fingers. They may have been weight bearing. Like all modern tetrapods, it had rib bones, a mobile neck with a separate pectoral girdle, and lungs, though it had the gills, scales, and fins of a fish.



Tetrapod footprints found in Poland and reported in Nature in January 2010 were “securely dated” at 10 million years older than the oldest known elpistostegids (of which Tiktaalik is an example), implying that animals like Tiktaalik, possessing features that evolved around 400 million years ago, were “late-surviving relics rather than direct transitional forms, and they highlight just how little we know of the earliest history of land vertebrates”


Pleuronectiformes (flatfish) are an order of ray-finned fish. The most obvious characteristic of the modern flatfish is their asymmetry, with both eyes on the same side of the head in the adult fish. In some families the eyes are always on the right side of the body (dextral or right-eyed flatfish) and in others they are always on the left (sinistral or left-eyed flatfish). The primitive spiny turbots include equal numbers of right- and left-eyed individuals, and are generally less asymmetrical than the other families. Other distinguishing features of the order are the presence of protrusible eyes, another adaptation to living on the seabed (benthos), and the extension of the dorsal fin onto the head.




Amphistium is a 50-million-year-old fossil fish which has been identified as an early relative of the flatfish, and as a transitional fossil[35] In Amphistium, the transition from the typical symmetric head of a vertebrate is incomplete, with one eye placed near the top-center of the head. Paleontologists concluded that “the change happened gradually, in a way consistent with evolution via natural selection—not suddenly, as researchers once had little choice but to believe.”

Amphistium is among the many fossil fish species known from the Monte Bolca Lagerstätte of Lutetian Italy. Heteronectes is a related, and very similar fossil from slightly earlier strata of France.


A middle Devonian precursor to seed plants has been identified from Belgium, predating the earliest seed plants by about 20 million years. Runcaria, small and radially symmetrical, is an integumented megasporangium surrounded by a cupule. The megasporangium bears an unopened distal extension protruding above the multilobed integument. It is suspected that the extension was involved in anemophilous pollination. Runcaria sheds new light on the sequence of character acquisition leading to the seed, having all the qualities of seed plants except for a solid seed coat and a system to guide the pollen to the seed.

Complete list of transitional fossils can be found here



Xing Xu, Hailu You, Kai Du and Fenglu Han (28 July 2011). “An Archaeopteryx-like theropod from China and the origin of Avialae”. Nature 475 (7357): 465–470. doi:10.1038/nature10288. PMID 21796204.

Erickson, Gregory M.; Rauhut, Oliver W. M., Zhou, Zhonghe, Turner, Alan H, Inouye, Brian D. Hu, Dongyu, Norell, Mark A. (2009). Desalle, Robert. ed. “Was Dinosaurian Physiology Inherited by Birds? Reconciling Slow Growth in Archaeopteryx”. PLoS ONE 4 (10): e7390. Bibcode 2009PLoSO…4.7390E. doi:10.1371/journal.pone.0007390. PMC 2756958. PMID 19816582. Retrieved 2009-10-25.

Yalden D.W. (1984). “What size was Archaeopteryx?”. Zoological Journal of the Linnean Society 82 (1–2): 177–188. doi:10.1111/j.1096-3642.1984.tb00541.x.

Archaeopteryx: An Early Bird. University of California, Berkeley Museum of Paleontology. Retrieved 2006-10-18

Wellnhofer, P. (2004). “The Plumage of Archaeopteryx”. In Currie PJ, Koppelhus EB, Shugar MA, Wright JL. Feathered Dragons. Indiana University Press. pp. 282–300. ISBN 0-253-34373-9.

Lovejoy, C. Owen (1988). “Evolution of Human walking”. Scientific American 259 (5): 82–89. doi:10.1038/scientificamerican1188-118.

“Australopithecus afarensis”. Smithsonian Institution. Retrieved May 06, 2012-2014.

White, T.D. , Suwa, G., Simpson, S., Asfaw, B. (January 2000). “Jaws and teeth of Australopithecus afarensis from Maka, Middle Awash, Ethiopia”. American Journal of Physical Anthropology 111 (1): 45–68. doi:10.1002/(SICI)1096-8644(200001)111:1<45::AID-AJPA4>3.0.CO;2-I. PMID 10618588.

Northeastern Ohio Universities Colleges of Medicine and Pharmacy (2007, December 21). “Whales Descended From Tiny Deer-like Ancestors”. ScienceDaily. Retrieved 2007-12-21.

Philip D. Gingerich, D. E. Russell (1981). “Pakicetus inachus, a new archaeocete (Mammalia, Cetacea) from the early-middle Eocene Kuldana Formation of Kohat (Pakistan)”. Univ. Mich. Contr. Mus. Paleont 25: 235–246.

Castro, E. Huber, Peter, Michael (2003). Marine Biology (4 ed). McGraw-Hill.

Nummela, Sirpa; Thewissen, J. G. M., Bajpai, Sunil, Hussain, S. Taseer, Kumar, Kishor (11 August 2004). “Eocene evolution of whale hearing”. Nature 430 (7001): 776–778. Bibcode 2004Natur.430..776N. doi:10.1038/nature02720. PMID 15306808.

J. G. M. Thewissen, E. M. Williams, L. J. Roe and S. T. Hussain (2001). “Skeletons of terrestrial cetaceans and the relationship of whales to artiodactyls”. Nature 413 (6853): 277–281. doi:10.1038/35095005. PMID 11565023.

Thewissen, J. G. M.; Williams, E. M. (1 November 2002). “The Early Radiations of Cetacea (Mammalia): Evolutionary Pattern and Developmental Correlations”. Annual Review of Ecology and Systematics 33 (1): 73–90. doi:10.1146/annurev.ecolsys.33.020602.095426.

Thewissen, J. G. M.; Bajpai, Sunil (1 January 2001). “Whale Origins as a Poster Child for Macroevolution”. BioScience 51 (12): 1037. doi:10.1641/0006-3568(2001)051[1037:WOAAPC]2.0.CO;2. ISSN 0006-3568.

Edward B. Daeschler, Neil H. Shubin and Farish A. Jenkins, Jr (6 April 2006). “A Devonian tetrapod-like fish and the evolution of the tetrapod body plan”. Nature 440 (7085): 757–763. Bibcode 2006Natur.440..757D. doi:10.1038/nature04639. PMID 16598249.

Jennifer A. Clack (21 November 2005). “Getting a Leg Up on Land”. Scientific American.

Easton, John (2008-10-23). “Tiktaalik’s internal anatomy explains evolutionary shift from water to land”. University of Chicago Chronicle (University of Chicago) Vol. 28 (Issue 3). Retrieved 19 April 2012-2014.

John Noble Wilford, The New York Times, Scientists Call Fish Fossil the Missing Link, Apr. 5, 2006.

Shubin, Neil (2008). Your Inner Fish. Pantheon. ISBN 978-0-375-42447-2.

Niedzwiedzki, G., Szrek, P., Narkiewicz, K., Narkiewicz, M and Ahlberg, P., Nature 463(7227):43–48, 2010, Tetrapod trackways from the early Middle Devonian period of Poland, 7 January 2010.

Editor’s summary: Four feet in the past: trackways pre-date earliest body fossils. Nature 463.

Chapleau, Francois & Amaoka, Kunio (1998). Paxton, J.R. & Eschmeyer, W.N.. ed. Encyclopedia of Fishes. San Diego: Academic Press. ISBN 0-12-547665-5.

Minard, Anne (July 9, 2008). “Odd Fish Find Contradicts Intelligent-Design Argument”. National Geographic. Retrieved 2008-07-17.

Matt Friedman (2008-07-10). “The evolutionary origin of flatfish asymmetry”. Nature 454 (7201): 209–212. Bibcode 2008Natur.454..209F. doi:10.1038/nature07108. PMID 18615083.

Gerrienne, P., Meyer-Berthaud, B., Fairon-Demaret, M., Streel, M., and Steemans, P. (2011). “Runcaria, a Middle Devonian Seed Plant Precursor”. Science Magazine (American Association for the Advancement of Science) 306 (5697): 856–858. Bibcode 2004Sci…306..856G. doi:10.1126/science.1102491. PMID 15514154. Retrieved March 22, 2011.