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Science Olympiad
Amphibia or Batrachomorpha
Tantalizing clues provide evidence that life started its invasion of land during 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 (450 mya) from England millipede-like trackways have been preserved (Palmer, 1999, p. 71). Evidence for land plants and primitive terrestrial ecosystems becomes more common in the Silurian.

Terrestrial Vertebrates

The 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 sister 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.

Two 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).

Living on Land

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 walking.

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).

Fish to Tetrapod

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 aquatic lobe-finned fish to fully four-legged tetrapods.

Prothero (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.

Panderichthyes from the Late Devonian had a flattened body with a straight tail. This "fishibian" 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 lobed fins.

Tiktaalik from the Late Devonian (375 million years ago) is nicknamed the "fishapod" by its discoverer 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 had lungs 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, 2008).

Acanthostega is a basal tetrapod from the Upper Devonian (365 million years ago) that shows characteristics intermediate 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 like a tetrapod. Studies of Acanthostega's limbs suggest that they were used for swimming or crawling along the bottom.

Ichthyostega from the Late Devonian is yet another basal tetrapod that represents a transition 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, but it also had the limbs and spine of a tetrapod. Acanthostega and Ichthyostega represent important transitional fossils linking fish and amphibians, but are not classified as amphibians.

Basal Tetrapods

Early 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 features associated 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 developed later.

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.


Traditionally, 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 bodies, large flat skulls, and short 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 during the Mesozoic and 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. The nectrideans (Order Nectridia) were aquatic organisms that had newt-like 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. 89-90).

Lissamphibians or Modern Amphibians

Lissamphibians (Infraclass 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 terrestrial life cycle.


The 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 amphibian-like 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).

Anthracosaurs (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 serve vertebrates well.

Science Olympiad Fossil Event

The 2016 Science Olympiad Fossil List includes the class Amphibia with the following genera listed: Acanthostega, Eryops, and Diplocaulus.


Benton, M.J. (2005) Vertebrate Palaeontology [3rd Edition]. Blackwell Publishing: Main, USA.

Kenrick P. and Davis, P. (2004). Fossil Plants. Smithsonian Books: Washington.

Palmer, D. (1999). Atlas of the Prehistoric World. New York: Discovery Books.

Prothero, D.R. (1998). Bringing Fossils to Life: An Introduction to Paleobiology. New York: McGraw-Hill.

Prothero, D.R. (2004). Bringing Fossils to Life: An Introduction to Paleobiology [2nd edition]. New York: McGraw-Hill.

Prothero, D.R. (2007). Evolution: What Fossils Say and Why It Matters. New York: Columbia University Press.

Tiktaalik roseae (2008):

Ridley, M. (2009). The Darwin Bicentennial Part II: Modern Darwins. National Geographic, February 2009, Vol. 215, No. 2.

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