A transitional fossil is any fossilized remains of a life form that
exhibits traits common to both an ancestral group and its derived
descendant group. This is especially important where the
descendant group is sharply differentiated by gross anatomy and mode
of living from the ancestral group. These fossils serve as a reminder
that taxonomic divisions are human constructs that have been imposed
in hindsight on a continuum of variation. Because of the
incompleteness of the fossil record, there is usually no way to know
exactly how close a transitional fossil is to the point of divergence.
Therefore, it cannot be assumed that transitional fossils are direct
ancestors of more recent groups, though they are frequently used as
models for such ancestors.
In 1859, when Charles Darwin's
On the Origin of Species
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 that
followed from his theory of descent with modification through natural
Archaeopteryx was discovered just two
years later, in 1861, and represents a classic transitional form
between earlier, non-avian dinosaurs and birds. Many more transitional
fossils have been discovered since then, and there is now abundant
evidence of how all classes of vertebrates are related, including many
transitional fossils. Specific examples of class-level
transitions are: tetrapods and fish, birds and dinosaurs, and mammals
and "mammal-like reptiles".
The term "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 refers to a pre-evolutionary view of nature.
1 Evolutionary and phylogenetic taxonomy
1.1 Transitions in phylogenetic nomenclature
1.2 Transitional versus ancestral
2 Prominent examples
2.2 Australopithecus afarensis
2.3 Pakicetids, Ambulocetus
4.2 The rise of plants
5.1 Missing links
5.2 Punctuated equilibrium
6 See also
9 External links
Evolutionary and phylogenetic taxonomy
Transitions in phylogenetic nomenclature
Main article: Phylogenetic nomenclature
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, having a mixture of
characteristics from inside and outside the newly branched
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, and only these are given phylogenetic names. While in
traditional classification tetrapods and fish are seen as two
different groups, phylogenetically tetrapods are considered a branch
of fish. Thus, with cladistics there is no longer a transition between
established groups, and the term "transitional fossils" is a misnomer.
Differentiation occurs within groups, represented as branches in the
In a cladistic context, transitional organisms can be seen as
representing early examples of a branch, where not all of the traits
typical of the previously known descendants on that branch have yet
evolved. Such early representatives of a group are usually
termed "basal taxa" or "sister taxa," depending on whether
the fossil organism belongs to the daughter clade or not.
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
of 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 microfossils, 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.
Further information: List of transitional fossils
Main article: Origin of birds
Archaeopteryx is one of the most famous transitional fossils and
gives evidence for the evolution of birds from theropod dinosaurs.
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
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
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.
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
Australopithecus afarensis and
See also: List of human evolution fossils
A. afarensis - walking posture.
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
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 (the closest living relative of humans)
and had teeth that were more human than ape-like.
Evolution of cetaceans
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Reconstruction of PakicetusSkeleton 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
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 the former Tethys
Sea.[page needed] Pakicetids could hear under
water, using enhanced bone conduction, rather than depending on
tympanic membranes 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 looked
like a crocodile. 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.
Evolution of tetrapods
Tiktaalik roseae had spiracles (air holes) above the eyes.
Life restoration of
Tiktaalik is a genus of extinct sarcopterygian (lobe-finned fish) from
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
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."
Flatfish § Evolution
Modern flatfish are asymmetrical, with both eyes on the same side 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 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
Amphistium is among the many fossil fish species known from the Monte
Heteronectes is a related, and
very similar fossil from slightly earlier strata of
Devonian fossil plant
Runcaria resembles a seed but lacks a
solid seed coat and means to guide pollen.
Evolution of plants
Evolution of plants § Seeds
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
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.
See also: Taphonomy
Not every transitional form appears in the fossil record, because the
fossil record is not complete. Organisms are only rarely preserved as
fossils in the best of circumstances, and only a fraction of such
fossils have been discovered. 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, suggesting that the number of species known through
fossils must be far less than 1% of all the species that have ever
Because of the specialized and rare circumstances required for a
biological structure to fossilize, logic dictates that known fossils
represent only a small percentage of all life-forms that ever
existed—and that each discovery represents only a snapshot of
evolution. The transition itself can only be illustrated and
corroborated by transitional fossils, which 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, the brachiopods and some groups of
History of paleontology
History of paleontology and Timeline of paleontology
A historic 1904 reconstruction of Archæopteryx
Reconstruction of Rhynia
The idea that animal and plant species were not constant, but changed
over time, was suggested as far back as the 18th century.
Darwin's On the Origin of Species, published in 1859, gave it a firm
scientific basis. A weakness of Darwin's work, however, was the lack
of palaeontological evidence, as pointed out by Darwin himself. While
it is easy to imagine natural selection producing the variation seen
within genera and families, the transmutation between the higher
categories was harder to imagine. The dramatic find of the London
Archaeopteryx in 1861, only two years after the
publication of Darwin's work, offered for the first time a link
between the class of the highly derived birds, and that of the more
primitive reptiles. In a letter to Darwin, the
Hugh Falconer wrote:
Had the Solnhofen quarries been commissioned—by august command—to
turn out a strange being à la Darwin—it could not have executed the
behest more handsomely—than in the Archaeopteryx.
Thus, transitional fossils like
Archaeopteryx came to be seen as not
only corroborating Darwin's theory, but as icons of evolution in their
own right. For example, the Swedish encyclopedic
Nordisk familjebok of 1904 showed an inaccurate
Archaeopteryx reconstruction (see illustration) of the fossil, "ett af
de betydelsefullaste paleontologiska fynd, som någonsin gjorts" ("one
of the most significant paleontological discoveries ever
The rise of plants
Transitional fossils are not only those of animals. With the
increasing mapping of the divisions of plants at the beginning of the
20th century, the search began for the ancestor of the vascular
plants. In 1917,
Robert Kidston and
William Henry Lang found the
remains of an extremely primitive plant in the
Rhynie chert in
Aberdeenshire, Scotland, and named it Rhynia.
Rhynia plant was small and stick-like, with simple dichotomously
branching stems without leaves, each tipped by a sporangium. The
simple form echoes that of the sporophyte of mosses, and it has been
Rhynia had an alternation of generations, with a
corresponding gametophyte in the form of crowded tufts of diminutive
stems only a few millimetres in height.
Rhynia thus falls
midway between mosses and early vascular plants like ferns and
clubmosses. From a carpet of moss-like gametophytes, the larger Rhynia
sporophytes grew much like simple clubmosses, spreading by means of
horizontal growing stems growing rhizoids that anchored the plant to
the substrate. The unusual mix of moss-like and vascular traits and
the extreme structural simplicity of the plant had huge implications
for botanical understanding.
The human pedigree back to amoeba shown as a reinterpreted chain of
being with living and fossil animals. From G. Avery's critique of
Ernst Haeckel, 1873.
Java Man" or Pithecanthropus erectus (now Homo erectus), the
original "missing link" found in
Java in 1891–92.
The term "missing link" refers back to the originally static
pre-evolutionary concept of the great chain of being, a deist idea
that all existence is linked, from the lowest dirt, through the living
kingdoms to angels and finally to God. The idea of all
living things being linked through some sort of transmutation process
predates Darwin's theory of evolution. Jean-Baptiste Lamarck
envisioned that life is generated in the form of the simplest
creatures constantly, and then strive towards complexity and
perfection (i.e. humans) through a series of lower forms.
In his view, lower animals were simply newcomers on the evolutionary
After On the Origin of Species, the idea of "lower animals"
representing earlier stages in evolution lingered, as demonstrated in
Ernst Haeckel's figure of the human pedigree. While the
vertebrates were then seen as forming a sort of evolutionary sequence,
the various classes were distinct, the undiscovered intermediate forms
being called "missing links."
The term was first used in a scientific context by
Charles Lyell in
the third edition (1851) of his book Elements of Geology in relation
to missing parts of the geological column, but it 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 it
was generally thought that the end of the last glacial period marked
the first appearance of humanity, but Lyell drew on new findings in
his Antiquity of Man to put the origin of human beings much further
back in the deep geological past. Lyell wrote that it remained a
profound mystery how the huge gulf between man and beast could be
bridged. Lyell's vivid writing fired the public
imagination, inspiring Jules Verne's Journey to the Center of the
Earth (1864) 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
Garden of Eden shown in the 1863
The idea of a "missing link" between humans and so-called "lower"
animals remains lodged in the public imagination. The
search for a fossil showing transitional traits between apes and
humans, however, was fruitless until the young Dutch geologist Eugène
Dubois found a skullcap, a molar and a femur on the banks of Solo
Java in 1891. The find combined a low, ape-like skull roof with
a brain estimated at around 1000 cc, midway between that of a
chimpanzee and an adult human. The single molar was larger than any
modern human tooth, but the femur was long and straight, with a knee
angle showing that "
Java Man" had walked upright. Given
the name Pithecanthropus erectus ("erect ape-man"), it became the
first in what is now a long list of human evolution fossils. At the
time it was hailed by many as the "missing link," helping set the term
as primarily used for human fossils, though it is sometimes used for
other intermediates, like the dinosaur-bird intermediary
Sudden jumps with apparent gaps in the fossil record have been used
as evidence for punctuated equilibrium. Such jumps can be explained
either by macromutation or simply by relatively rapid episodes of
gradual evolution by natural selection, since a period of say 10,000
years barely registers in the fossil record.
"Missing link" is still a popular term, well recognized by the public
and often used in the popular media. It is, however,
avoided in the scientific press, as it relates to the concept of the
great chain of being and to the notion of simple organisms being
primitive versions of complex ones, both of which have been discarded
in biology. In any case, the term itself is misleading, as
any known transitional fossil, like
Java Man, is no longer missing.
While each find will give rise to new gaps in the evolutionary story
on each side, the discovery of more and more transitional fossils
continues to add to our knowledge of evolutionary
Main article: Punctuated equilibrium
The theory of punctuated equilibrium developed by Stephen Jay Gould
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
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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
Evidence of common descent
List of transitional fossils
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