What Are Transitional Fossils
fossils are the fossilized remains of transitional forms of
that tangibly and demonstrably encode an evolutionary transition.
Thus, transitional fossils are characterized by their retention
of primitive (plesiomorphic) traits in contrast with their
recently evolved characteristics (the phenotype and genotype).
term "missing link" is a popular slang term for such
transitional forms, but is misleading. The term is particularly
used in popular media, but is inaccurate and confusing, partly
because it implies that there exists a single undiscovered fossil
that is needed to confirm the transition. In contrast, the continual
discovery of more and more transitional fossils is further refining
and validating evolutionary transitions. Transitional fossils
are numerous and varied throughout the tree of life, including
those between primates and early humans, contrary to the claims
of creationists who deny evolution.
theory considers all populations of organisms to be in transition,
whether changes be slow, as in genetic drift, or fast, as when
a changing environment imposes significant adaptive pressures.
A transitional form of life is one that demonstably illustrates
a particular intermediate evolutionary stage of change or adaptation.
Transitional fossils usually coexist with gaps in a sequence in
the fossil record. The probabilities of fossilization pretty much
precludes the discovery of detailed sequences of fossils spanning
millions of years. However, fine gradations of fossils between
species and genera are abundant in the fossil record, as are coarser
sequences between higher taxa.
transitionals between kingdoms and phyla:
The Cambrian fossils Halkiera and Wiwaxia have features that
connect them with each other and with the modern phyla of Mollusca, Brachiopoda,
In particular, one species of halkieriid has brachiopod-like
shells on the dorsal side at each end. This is seen also
immature stage of the living brachiopod species Neocrania.
It has setae identical in structure to polychaetes, a group
Wiwaxia and Halkiera have the same basic arrangement of hollow
sclerites, an arrangement that is similar to the chaetae
of polychaetes. The undersurface of Wiwaxia has a soft sole
like a mollusk's foot, and its jaw looks like a mollusk's
which are a group of primitive mollusks, have a soft body covered
with spicules similar to the sclerites of Wiwaxia (see Conway
and Precambrain fossils Anomalocaris and Opabinia are transitional
between arthropods and lobopods.
An ancestral echinoderm has been found that is intermediate between
modern echinoderms and other deuterostomes (Shu et al. 2004).
transitions between species and genera:
fossil record shows transitions between species of Phacops (a
trilobite; Phacops rana is the Pennsylvania state fossil; Eldredge
1972; 1974; Strapple 1978).
horns of titanotheres (extinct Cenozoic mammals) appear in progressively
larger sizes, from nothing to prominence. Other head and neck
features also evolved. These features are adaptations for head-on
ramming analogous to sheep behavior (Stanley 1974).
A gradual transitional fossil sequence connects the foraminifera
Globigerinoides trilobus and Orbulina universa (Pearson et al.
1997). O. universa, the later fossil, features a spherical test
surrounding a "Globigerinoides-like" shell, showing
that a feature was added, not lost. The evidence is seen in all
major tropical ocean basins. Several intermediate morphospecies
connect the two species, as may be seen in the figure included
in Lindsay (1997).
ancestry. There are many fossils of human ancestors, and the differences
between species are so gradual that it is not always clear where
to draw the lines between them.
Planktonic forminifera (Malmgren et al. 1984). This is an example
of punctuated gradualism. A ten-million-year foraminifera fossil
record shows long periods of stasis and other periods of relatively
rapid but still gradual morphologic change.
of the diatom Rhizosolenia are very common (they are mined as
diatomaceous earth), and they show a continuous record of almost
two million years which includes a record of a speciation event
(Miller 1999, 44-45).
Turkana mollusc species (Lewin 1981).
marine ostracodes (Cronin 1985).
Eocene primate genus Cantius (Gingerich 1976, 1980, 1983).
Scallops of the genus Chesapecten show gradual change in one "ear"
of their hinge over about 13 million years. The ribs also change
(Pojeta and Springer 2001; Ward and Blackwelder 1975).
(coiled oysters) become larger and broader but thinner and flatter
during the Early Jurassic (Hallam 1968).
following are fossil transitionals between families, orders, and
Human ancestry. Australopithecus, though its leg and pelvis bones
show it walked upright, had a bony ridge on the forearm, probably
vestigial, indicative of knuckle walking (Richmond and Strait
From dinosaur to bird
Haasiophis terrasanctus is a primitive marine snake with well-developed
hind limbs. Although other limbless snakes might be more ancestral,
this fossil shows a relationship of snakes with limbed ancestors
(Tchernov et al. 2000). Pachyrhachis is another snake with legs
that is related to Haasiophis (Caldwell and Lee 1997).
jaws of mososaurs are also intermediate between snakes and lizards.
Like the snake's stretchable jaws, they have highly flexible lower
jaws, but unlike snakes, they do not have highly flexible upper
jaws. Some other skull features of mososaurs are intermediate
between snakes and primitive lizards (Caldwell and Lee 1997; Lee
et al. 1999; Tchernov et al. 2000).
Transitions between mesonychids and whales.
Transitions between fish and tetrapods.
Transitions from condylarths (a kind of land mammal) to fully
aquatic modern manatees. In particular, Pezosiren portelli is
clearly a sirenian, but its hind limbs and pelvis are unreduced
(Domning 2001a, 2001b).
a Middle Devonian plant, was a precursor to seed plants. It
had all the qualities of seeds except a solid seed coat and
a system to guide pollen to the seed (Gerrienne et al. 2004).
bee, Melittosphex burmensis, from Early Cretaceous amber, has
primitive characteristics expected from a transition between
wasps and extant bees (Poinar and Danforth 2006).