clues provide evidence that life started its invasion of
the Ordovician. A variety of spores (reproductive
structures from primitive land plants) make their first appearance
in the mid-Ordovician (470 mya). One category of spore, the
tetrads, possesses a mark that indicates cell division
by meiosis. Along
side these spores are bits and pieces of plant tissues. (Kenrick & Davis,
2004, pp. 19-21). Plant debris becomes more common during
the late Ordovician and Silurian. In late Ordovician rock
mya) from England millipede-like trackways have been preserved
1999, p. 71). Evidence for land plants and primitive terrestrial
ecosystems becomes more common in the Silurian. |
first terrestrial vertebrates or tetrapods (Superclass
Tetrapoda) appear during the Devonian period. Tetrapods
include vertebrates with four toe-bearing legs, or descendants
of such a vertebrate. The word tetrapod is used to refer
to vertebrates other than fish. Traditionally, the first tetrapods
were regarded as amphibians; however, early tetrapod groups
represent multiple evolutionary lines. The class Amphibia
is now reserved for living amphibians and their extinct
groups. Some use Batrachomorpha to group living amphibians
with extinct sister groups. In this article we will explore
challenges to living on land, transitional fossils linking
fish to tetrapods, basal tetrapods, and finally amphibians.
hypotheses have been put forward to explain the tetrapod
invasion of land. First, the movement to land during the
Devonian could have been due to seasonal draughts and escape
from drying pools. Second, early tetrapods were carnivores
and the land represented a habitat with an untapped food
supply of arthropods (Benton, 2005, p. 75).
The invasion of land really represents a move from living in water to living
in an atmosphere and requires adaptations to address respiration, locomotion,
structural support, sensing the environment, and desiccation.
The problem of gas exchange or respiration in the atmosphere was solved by the
evolution of lungs. Lungs possess vascularized internal folds and pouches that
must remain moist. Present day lungfishes and, it is assumed, extinct osteolepiforms
had functional lungs. Basal tetrapods probably had similar lungs. Aquatic fish
can gulp air and force it into the lungs by diving downward. Tetrapods, such
as salamanders, force air into the lungs through costal ventilation (expansion
and contraction of ribs and costal muscles). Frogs use buccal pumping, which
utilizes the mouth and throat to force air into the lungs.
The most advanced osteolepiforms already had the bony supports needed to form
tetrapod limbs. However, to support the body on land requires modified pelvic
and pectoral girdles as well as a stronger more articulated vertebral column.
Several evolutionary trends were critical in constructing the tetrapod form.
The pelvic girdle became enlarged and connected to the vertebral column. The
pectoral girdle became detached from the skull and fused to the vertebral column.
Vertebrae became locked together and developed spines for stronger muscle and
tendon attachments. These changes allowed the upper body to be propped up for
breathing, the head to turn, and created a rigid structure strong enough to support
Osteolepiforms and early tetrapods had lateral lines for detecting vibrations
in the water. Lateral lines do not work on land. The main bone associated with
hearing in modern amphibians and reptiles is the stapes. Early tetrapods possessed
the stapes, but it was too heavy to detect high frequency sounds. Eyesight is
more important on land than in ponds, so it is not surprising that early tetrapods
had larger eyes than their predecessors (Benton, 2005, p. 78).
In the air water evaporates quickly, so desiccation is a problem for terrestrial
organisms. Early tetrapods may have needed to stay close to sources of water.
Most modern amphibians must return to water to reproduce. The eggs and larval
stages of modern amphibians require an aquatic environment. Although fossil tadpoles
are rare, sufficient specimens have been found in Carboniferous and Permian rocks
to confirm that at least some basal tetrapod life cycles were like that of modern
amphibians (Benton, 2005, p. 79).
The first terrestrial
vertebrates or tetrapods (Superclass Tetrapoda) appear during
the Devonian period. Fossil evidence suggests that lobefinned
fish, such as Eusthenopteron (Order Osteolepiforemes)
and Panderichthyes (Order Panderichthyida)
are among the closest sarcopterygian relatives of tetrapods
(Benton, 2005, p. 80). In fact, Eusthenopteron, Panderichthyes, Tiktaalik, Acanthostega,
and Ichthyostega make up a series
of fossils used to model the transition from
fish to fully four-legged
(2007) explores the transitional features of these fossils
and how they help us to better understand
the evolution of tetrapods from lobefinned fish (pp. 222-230). Eusthenopteron is
a Devonian aged lobefinned fish. The pelvic and pectoral
bones in this fish are homologous to tetrapod limb bones.
The skull bones of Eusthenopteron are the same
as early tetrapods.
the Late Devonian had a flattened body with a straight
had a skull and brain case similar to early tetrapods
with upward facing eyes. In fact, the skull of Panderichthyes was
classified as a tetrapod until its fishlike body was
discovered. Panderichthyes had
labyrinthodont-like teeth, which characterize later tetrapods. Panderichthyes possessed
both lungs and gills. This fish had lost its dorsal
and anal fins, but retained pectoral and pelvic foot-like
the Late Devonian (375 million years ago) is nicknamed
the "fishapod" by
Neil Shubin. Tiktaalik
roseae like other fish had gills, scales,
and fins. However, the fins were weight bearing possessing
both wrist bones and finger-like bones. Tiktaalik also
possessed tetrapod characteristics. The head of Tiktaalik ("large
freshwater fish"-taken from the Inuktitut language)
was flat with eyes positioned on the top of the skull (Ridley,
2009, p. 70). The pectoral girdle was separate from the
skull forming a neck and allowing the head to turn. The
ribs were designed to support the body and allow breathing.
The fact that Tiktaalik had spiracles on its
skull and a more robust ribcage indicates this organism
as well as gills. The appearance of tetrapod characteristics
in a fish that existed 12 million years before the
first tetrapod is very significant (Tiktaalik website,
a basal tetrapod from the Upper Devonian (365 million
years ago) that shows characteristics
between lobe-finned fish and the first fully terrestrial
tetrapods. Acanthostega had gills, fins,
and a lateral line for sensing vibrations, but possessed
lungs, spine, and limbs
tetrapod. Studies of Acanthostega's limbs
suggest that they were used for swimming or crawling
the Late Devonian is yet another basal tetrapod that represents
between fish and amphibians. Ichthyostega possessed
limbs that were longer and more tetrapod-like than Acanthostega.
Ichthyostega still possessed a tail, gill
slits, and a lateral line for sensing movement,
limbs and spine of a tetrapod. Acanthostega and Ichthyostega represent
important transitional fossils linking fish and amphibians,
but are not classified as amphibians.
amphibian-like fossils are referred to
as basal tetrapods. Acanthostega and Ichthyostega from
Greenland are the best known Devonian-aged basal tetrapods. Ichthyostega and Acanthostega measured
around 1 meter in length. As we have already mentioned, both
had fully developed limbs and limb girdles, but still retained
with their aquatic life. These animals had a gill apparatus
as shown by their opercular bones. Flanges on their ribs indicate
they also had lungs. They possessed a strong fin tail used
for swimming. Evidence of lateral lines can be seen on their
skulls and a possible otic notch on the back of the skull may
indicate the presence of an eardrum (Prothero, 1998, p. 362).
Interestingly, it was recently discovered that Acanthostega had
eight toes and Ichthyostega had seven. Evidently,
the standard pentadactyl (five-fingered) condition in tetrapods
Acanthostega and Ichthyostega are found in sedimentary
deposits that indicate they lived in meandering rivers that
flowed through forests of lycopods and ferns (Benton, 2005,
pp. 82-85). They probably lived in water choked with vegetation.
Being able to swim and step over vegetation would be a real
adaptive advantage. Basal tetrapods and amphibians underwent
a great adaptive radiation during the Carboniferous.
the term amphibian has been used to refer to all tetrapods
that are not amniotes (reptiles, birds, and mammals).
However, it is now clear that this is a paraphyletic
term. The class Amphibia now refers to present-day amphibians
and their extinct sister groups.
Temnospondyls (Order Temnospondyli) are primitive amphibians
that dominated the terrestrial and freshwater habitats of
the Carboniferous. Temnospondyli,
Ichthyostegalia, and Anthracosauria use to be
grouped together as Labyrinthodonts. Labyrinthodtia is now
an obsolete term. In general, temnospondyls had long
large flat skulls,
legs. Eryops and Edops from
the Lower Permian of Texas were up to 2 meters in length,
making them some of
the largest predators outside Dimetrodon. Eryops and Edops probably
lived their lives like crocodiles lurking in the streams
and moving into and out of water. Temnospondyls declined
went extinct in the Cretaceous.
Lepospondyls (Superorder Lepospondyli) are early amphibians
that range from the Carboniferous to the Permian. Microsaurs
and nectrideans are the best-known lepospondyls. Microsaurs
(Order Microsauria) were the largest group of lepospondyls
and had a body form reminiscent of salamanders or lizards.
Most microsaurs were terrestrial feeding on arthropods
although, some became secondarily adapted to aquatic environments.
nectrideans (Order Nectridia) were aquatic organisms that
bodies with long tails. Their heads were equipped with
horn-like structures that grew as the animal aged. Diplocaulus is
a well-known nectridean with a “boomerang” shaped
skull. Biomechanical studies on models of a Diplocaulus head
provide evidence that it acted as a hydrofoil, providing
lift. Diplocaulus swam
in streams and lakes feeding on fish (Benson, 2005, pp.
Lissamphibians or Modern Amphibians
Lissamphibia) or the modern amphibians include the extinct
albanerpetontids and the living anurans
(frogs and toads), the urodeles (newts and salamanders), and
the gymnophionans (caecilians). Modern amphibians are represented
by roughly 4000 species. Albanerpetontids range from the Jurassic
to the Miocene and are very much like salamanders. Frogs and
toads (Order Anura) have a skeletal structure modified for
jumping. Triadobatrachus from the Lower Triassic of
Madagascar is the oldest known frog. Prosalirus is
the earliest known jumping frog from the Early Jurassic of
the South-West USA.
Salamanders and newts (Order Urodela) first appear in the Mid-Jurassic.
They have elongate bodies with four short walking limbs and
a flattened tail for swimming. The caecilians (Order Gymnophiona)
look somewhat like earthworms. They have lost their limbs and
burrow through leaf litter, soil, and swim in ponds. Eocaecilia is
the earliest known caecilian from the Early Jurassic of Arizona,
USA. Eocaecilia has the typical features of caecilians,
but also has small, reduced legs. Based upon tooth structure,
modern amphibians have their origins among the temnospondyls
(Benton, 2005, pp. 102-103).
A Double Life
Amphibia means, “double life” and refers to a life
cycle that includes an aquatic existence and a terrestrial
existence. Most amphibians must return to the water to reproduce.
The tadpole or larval stage has fish-like characteristics with
a caudal fin, lateral line, gills, a two-chambered heart and
a single-loop circulatory system. Through metamorphosis the
adult form gains limbs, a tympanic membrane, lungs, and a three-chambered
heart with a double-loop circulatory system. Sometime in the
Late Devonian or Early Carboniferous reptile-like amphibians
gave rise to amniotes
that evolved a fully
superorder Reptiliomorpha includes reptile-like amphibians
that range from the Early Carboniferous to the Early Triassic
as well as the amniotes that evolved from them. The
classification of tetrapods that have both reptile and
characteristics continues to be debated. The line between
amphibians and amniotes is blurred among advanced reptiliomorphs.
However, many paleontologists regard some reptile-like
amphibian groups as tetrapod evolutionary lines that branched
long after the amniotes split off (Prothero, 2004, p. 378).
(Order Anthracosauria), seymouriamorphs (Order Seymouriamorpha),
and diadectomorphs (Order Diadectomorpha) are well known
reptiliomorphs. Diadectimorphs are very close to the origin
of amniotes. Diadectes from the Early Permian
of Western USA and Germany is a massively built reptiliomorph
that represents one of the first terrestrial vertebrate
herbivores. Diadectes had a reptile-like skeleton
with massive limb girdles, short limbs, and heavy vertebrae.
At up to 3 meters in length, Diadectes represents
one of the first fully terrestrial tetrapods to attain
a large size. Diadectes skull was amphibian-like.
The front of the jaw had eight peg-like incisors for clipping
vegetation and rows of blunt cheek teeth for grinding (Benton,
2005, p. 101). Diadectes also possessed an otic
notch, like other Paleozoic amphibians. The tympanum of Diadectes was
ossified. Reptile-like amphibians would go extinct in the
Early Triassic, but one of their evolutionary lines, the
amniote clade, would live on.
The first amniotes were reptiles and
they would lay eggs that did not need to hatch in water, eggs that protected
and nourished the embryo as it developed into a fully terrestrial
hatchling. As the climate became dryer, this adaptation would
Olympiad Fossil Event
2016 Science Olympiad Fossil List includes the class Amphibia with the following genera listed: Acanthostega, Eryops, and Diplocaulus.