Transitional fossil

"Missing link" redirects here. For other uses, see Missing Link.

Archaeopteryx lithographica is one of the most famous transitional fossils and shows the evolution of birds from theropod dinosaurs

A transitional fossil is any fossilized remains of a life form that exhibits characteristics of two distinct taxonomic groups. It is the fossil of an organism near the branching point where major individual lineages (clades) diverge. It will have characteristics typical of organisms on both sides of the split, but because of the incompleteness of the fossil record, there is usually no way to know exactly how close it is to the point of divergence. Transitional fossils serve as a reminder that taxonomic divisions are human constructs that have been imposed in hindsight on a continuum of variation.

In 1859, when Charles Darwin's On the Origin of Species was first published, the fossil record was poorly known. Darwin described the perceived lack of transitional fossils as "the most obvious and gravest objection which can be urged against my theory", but explained it by relating it to the extreme imperfection of the geological record. He noted the limited collections available at that time, but described the available information as showing patterns which followed from his theory of descent with modification through natural selection. Indeed, Archaeopteryx was discovered just two years later, in 1861, and represents a classic transitional form between dinosaurs and birds. Many more transitional fossils have been discovered since then, and there is now considered to be abundant evidence of how all classes of vertebrates are related, much of it in the form of transitional fossils. Specific examples include humans and other primates, tetrapods and fish, and birds and dinosaurs.

The phrase "missing link" has been used extensively in popular writings on human evolution to refer to a perceived gap in the hominid evolutionary record. It is most commonly used to refer to any new transitional fossil finds. Scientists, however, do not use the term, as it is misleading and inaccurate.

Evolutionary taxonomy and cladistics

Cladistics

Main article: Cladistics

See also: Evolutionary taxonomy

Traditional spindle diagram showing the vertebrates classes "budding" off from each other. Transitional fossils typically represent animals from near the branching points

In evolutionary taxonomy, the prevailing form of taxonomy during much of the 20th century and still used in non-specialist textbooks, taxa based on morphological similarity are often drawn as "bubbles" or "spindles" branching off from each other, forming evolutionary trees. Transitional forms are seen as falling between the various groups in terms of anatomy, and are placed near the branching points.

With the establishment of cladistics in the 1990s, relationships commonly came to be expressed in cladograms that illustrate the branching of the evolutionary lineages in stick-like figures. The different so-called "natural" or "monophyletic" groups form nested units. While tetrapods and fishes are seen as two different groups in traditional classification, tetrapods are but a branch of fishes in cladistics. With cladistics there is thus no longer a transition between established groups, and the term transitional fossils is cladistically speaking a misnomer. The differentiation occurs within groups, represented as branches in the cladogram. In this context, transitional organisms can be seen as representing early examples of a branch, where not all of the traits typical of the living descendants have yet evolved.

Transitional versus ancestral

A source of confusion is the notion that a transitional form between two different taxonomic groups must be a direct ancestor of one or both groups. The difficulty is exacerbated by the fact that one of the goals of evolutionary taxonomy is to identify taxa that were ancestors of other taxa. However, it is almost impossible to be sure that any form represented in the fossil record is a direct ancestor of any other. In fact, because evolution is a branching process that produces a complex bush pattern of related species rather than a linear process producing a ladder-like progression, and because of the incompleteness the fossil record, it is unlikely that any particular form represented in the fossil record is a direct ancestor of any other. Cladistics deemphasizes the concept of one taxonomic group being an ancestor of another, and instead emphasizes the identification of sister taxa that share a more recent common ancestor with one another than they do with other groups. There are a few exceptional cases, such as some marine plankton micro-fossils, where the fossil record is complete enough to suggest with confidence that certain fossils represent a population that was actually ancestral to a later population of a different species. But in general, transitional fossils are considered to have features that illustrate the transitional anatomical features of actual common ancestors of different taxa, rather than to be actual ancestors.

Comparison with intermediate forms

The terms transitional and intermediate are for the most part used as synonyms; however, a distinction between the two can be made:

• Transitional can be used for a form whose unique derived traits are shared by its more derived relative, with no significant exceptions. In other words, a transitional organism is morphologically close to the common ancestor it shares with its more derived relative.
• Intermediate can be used for a form that has a large number of uniquely derived traits not possessed by its derived relative.

According to this definition, Archaeopteryx is transitional, since all of its derived traits are also possessed by birds (its more derived relatives), whereas the platypus is intermediate, because it retains certain reptilian traits no longer found in modern mammals and possesses derived traits apt for a highly specialized aquatic animal.

Following this definition, all living organisms are in fact to be regarded as intermediate forms when they are compared to some other related life-form. Indeed there are many species alive today that can be considered to be intermediate between two or more groups.

Examples

Main article: List of transitional fossils

The London specimen of Archaeopteryx, discovered only two years after the publication of On the Origin of Species

Archaeopteryx

Main article: Archaeopteryx

See also: Origin of Birds

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, making it important in the study both of dinosaurs and of the origin of birds.

In 1861, the first complete specimen was announced. Over the years, ten more Archaeopteryx fossils have been found. Most of the eleven known fossils include impressions of feathers—among the oldest direct evidence of such structures. Moreover, because these feathers are of an advanced form (flight feathers), these fossils are evidence that the evolution of feathers began before the Late Jurassic.

Ambulocetus

Main article: Ambulocetus

See also: Evolution of cetaceans

Reconstruction of Pakicetus

Reconstruction of Ambulocetus natans

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 roseae

Main article: Tiktaalik roseae

See also: Tetrapods#Evolution

Life restoration of Tiktaalik roseae

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. 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".

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 were "late-surviving relics rather than direct transitional forms, and they highlight just how little we know of the earliest history of land vertebrates" possessing features that actually evolved around 400 million years ago.

Amphistium

Main article: Amphistium

See also: Flatfish#Origins

Modern flatfish are asymmetrical, with both eyes on the same side of the head

Fossil of Amphistium with one eye at the top-center of the head

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 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.

Runcaria

Main article: Evolutionary history of plants#Seeds

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.

Limitations of the fossil record

Not every transitional form appears in the fossil record, because the fossil record is nowhere near complete. Organisms are only rarely preserved as fossils in the best of circumstances, and only a fraction of such fossils have been discovered. The paleontologist Donald Prothero noted that this is illustrated by the fact that the number of species known through the fossil record was less than 5% of the number of known living species, which suggests that the number of species known through fossils must be far less than 1% of all the species that have ever lived.

Because of the specialized and rare circumstances required for a biological structure to fossilize, only a very small percentage of all life-forms that have ever existed can be expected to be represented in discoveries, and each discovery represents only a snapshot of the process of evolution. The transition itself can only be illustrated and corroborated by transitional fossils, but it will never demonstrate an exact half-way point between clearly divergent forms.

The fossil record is very uneven and, with few exceptions, is heavily slanted toward organisms with hard parts, leaving most groups of soft-bodied organisms with little to no fossil record. The groups considered to have a good fossil record, including a number of transitional fossils between traditional groups, are the vertebrates, the echinoderms, brachiopods and some groups of arthropods.

Missing links

A popular term used to designate transitional forms is "missing links". It was first used by Charles Lyell in the third edition (1851) of his book Elements of Geology, but was popularized in its present meaning by its appearance on page xi of his book Geological Evidences of the Antiquity of Man of 1863. By that time geologists had abandoned a literal biblical account and it was generally thought that the end of the last glacial period marked the first appearance of humanity, a view Lyell's Elements presented. His Antiquity of Man drew on new findings to put the origin of human beings much further back in the deep geological past. Lyell's vivid writing fired the public imagination, inspiring Jules Verne's Journey to the Center of the Earth and Louis Figuier's 1867 second edition of La Terre avant le déluge ("Earth before the Flood"), which included dramatic illustrations of savage men and women wearing animal skins and wielding stone axes, in place of the Garden of Eden shown in the 1863 edition.

The idea of a "missing link" between humans and so-called "lower" animals remains lodged in the public imagination. The concept has been fueled by the successive discoveries of Australopithecus africanus (Taung Child), Australopithecus sediba, Homo erectus (Peking Man, Java Man, Turkana boy), and other Hominina fossils. The term is often used in the popular media but is avoided in the scientific press, as it relates to the links in the great chain of being, a static pre-evolutionary concept now abandoned. In reality, the discovery of more and more transitional fossils continues to add to our knowledge of evolutionary transitions.

Punctuated equilibrium

Main article: Punctuated equilibrium

The theory of punctuated equilibrium developed by Stephen Jay Gould and Niles Eldredge and first presented in 1972 is often mistakenly drawn into the discussion of transitional fossils. This theory, however, pertains only to well-documented transitions within taxa or between closely related taxa over a geologically short period of time. These transitions, usually traceable in the same geological outcrop, often show small jumps in morphology between extended periods of morphological stability. To explain these jumps, Gould and Eldredge envisaged comparatively long periods of genetic stability separated by periods of rapid evolution. Gould made the following observation concerning creationist misuse of his work to deny the existence of transitional fossils:

"Since we proposed punctuated equilibria to explain trends, it is infuriating to be quoted again and again by creationists – whether through design or stupidity, I do not know – as admitting that the fossil record includes no transitional forms. The punctuations occur at the level of species; directional trends (on the staircase model) are rife at the higher level of transitions within major groups."

—Stephen Jay Gould, The Panda's Thumb

See also

• Evidence of common descent

References

• Browne, E. Janet (2002). Charles Darwin: vol. 2 The Power of Place. London: Jonathan Cape. ISBN 0-7126-6837-3 
• Darwin, Charles (1859). On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life (1st ed.). London: John Murray. http://darwin-online.org.uk/content/frameset?itemID=F373&viewtype=text&pageseq=1 
• Prothero, Donald R. (2007). Evolution: What the Fossils Say and Why it Matters. Columbia University Press. ISBN 978-0-231-13962-5 

External links

• Transitional vertebrate fossils FAQ, at the TalkOrigins Archive.
• University of Chicago website dedicated to tikaalik rosae
• Timeline of Whale Evolution - Smithsonian Ocean Portal
• Cetacean Paleobiology – University of Bristol
• Evolution of Whales segment from the Whales Tohorā Exhibition Minisite of the Museum of New Zealand Te Papa Tongarewa
• Are Birds Really Dinosaurs? – University of California Museum of Paleontology.