Vertebrates Move onto Land - the Amphibians
The invasion of land by vertebrates happened only once and it was one of the key
episodes in the history of life.
1. What Was The Earth Like When Vertebrates Moved Onto Land?
A. Vertebrates moved onto land in the late Devonian.
- i. Climate - The Devonian climate was warm and uniform. The polar
ice caps were probably small or even nonexistent.
- ii. Aquatic - In the Devonian, life existed primarily in aquatic habitats
(both freshwater and marine). In this time period, the three groups of
jawed fish (Osteichthyes, Chondrichthyes, and Placoderms) flourished
and the jawless fish were declining in numbers. In freshwater pools,
lakes and streams, algae floating on the tops of the water was probably
thick because there was no shading from tree-like forms on land. The
bottom was probably a mat of decomposing algae, robbing most of the
water of dissolved oxygen.
- ii. Terrestrial - On land, few plants and invertebrates had merged fully
onto land. There had been little recent geologic activity that results in
the production of mountains and no glaciers to churn the soil so that
most of the land was barren rock. Most erosion was from running
water so that the soil that did form would settle along the bottom of
drainage channels, at the mouths of rivers, and along the shores of the
oceans. These narrow bands of soil probably created narrow "green
belts" of plants, insects and arachnids (spiders and scorpions). These
would be the habitats into which the first land vertebrates would
2. Fish Ancestors To The First Land Vertebrates
We have discussed the origin of all the major groups of fishes. Lets return to
the bony fish (Osteichthyes), because their role in evolutionary history isn't finished.
A. There are two groups of bony fishes:
- i. The Sarcopterygii (lobefin fish)
- ii. The Actinopterygii (fin-ray fish)
B. One of the first groups of fish to appear were the lobefin fish. There are three
groups of lobefin fish that are relevant to the story of the evolution of the
amphibians - Coelocanths, Lung-Fish, and Panderichtyids. Of these the
Panderichtyids are the ancestors to the amphibians. But they are all extinct. So we
look to their still-living relatives the Coelocanths and the Lung-Fish for clues about
the biology of the ancestors to amphibians.
- i. Coelocanths: These lobefin fish arose in the Devonian and
flourished throughout the Mesozoic. Scientists had generally
agreed that the coelocanth had gone extinct at the end of the
mesozoic (with the Dinosaurs). Imagine, then, the astonishment of
the scientific world when a coelocanth was found off the coast of
South Africa in 1938. An intensive search was begun in the
Comoro Islands (near Madagascar) where it was learned that native
fishermen occasionally caught them at great depths. Still another
coelocanth was not found until 1958. This coelocanth does not
have lungs or a swim bladder, but these were probably secondarily
lost as an adaptation to deep sea life. In 1987, coelocanths were
filmed swimming for the first time in their deep sea habitat. Their
film shows that coelocanths move their front and back legs similar to the way tetrapods do (land vertebrates that
walk on four legs).
- ii. Lungfish: The lungfish are represented today by just three
species (one in each of the southern continents). These fish have
nostrils that opened into the pharynx. A bi-lobed sac is attached to
the floor of the pharynx by a duct. The sac was highly vascularized
and the fish can use it for respiration by swallowing air into it.
- The Australian lungfish lives in stagnant shallow pools
where oxygen content of the water is poor. The lung enables them
to gulp in air from the surface. The other two species can live out of
water for long periods of time. The African species lives in streams
that dry during the dry season, with their mud baked hard by the
hot tropical sun. The fish burrows down at the approach of the dry
season and secretes a copious slime that is mixed with mud to form
a hard cocoon in which it sleeps until the rains return.
Fin-Ray Fish -
- iii. Panderichtyids: All extinct. These fish had a number of
- -- They are the only fish to have a frontal bone. This
becomes a shared derived character they share with terrestrial
- They were adapted for living in shallow, ponds in two ways:
- -- Body shaped like a crocodile ( dorsoventrally flattened -
not laterally flattened and straight tail without dorsal and
- -- Dorsally placed eyes (eyes on the top of the head). This
feature suggests aerial vision because it is seen in
mudskippers and crocodiles.
- This probably means that the pandericythids were living in shallow
ponds. What did we say before about shallow freshwater ponds in
the Devonian? That they were probably dominated by algae and
had low oxygen. This situation has led to some speculation about
the steps leading to moving on land (this is a story!! Its plausible,
but we are just guessing here):
- 1. When the pond becomes stagnant and O2 levels drop, the
gills are no of much use. These fish had a nostril that opened
into the pharynx and so probably had a bi-lobed sac attached
to the floor of the pharynx by a duct. The sac was highly
vascularized and the fish can use it for respiration by
swallowing sir into it. (e.g., like lungfish).
- 2. The bones of the fins of these fish were strengthened and
thickened to produce cylindrical lobe-shaped fins. These
allowed them to push along substrate for more rapid
locomotion in shallow water (like a boater may pole his craft
in very shallow water, rather than row it).
Swim bladder - Sharks (and their relatives) and Placoderms rely on movement
and oils glands to keep them afloat. One of the greatest advantages for the bony
fish is the advent of the swim bladder from the lung. Lungs develop into swim
bladder. Lighter skeleton. More flexible fins. Modified mouths - specialized
jaws in which the whole mouth can be pushed forwards in a sort of pouting
expression (feed with more precision).
A recent fossil humerous has been described from Devonian age rocks in Pennsylvania (about 370 million years ago). The humerus was thickly muscled for push-ups but it lacked the front-to-back motion essential for walking.
In the Late Devonian (about 363 million years ago), an animal called Icthyostega
appears in the fossil record.
The fossils are common in Greenland (which sat on the equator during
the Devonian). The animals are about a meter long and fairly robust. They have
conical teeth in both the upper and lower jaws. The body is dorsoventrally
flattened and the tail is long and fish-like. Fish scales covered the ventral surface
(stomach) of the animal and probably protected it from abrasion. But scales were
missing from the top part of the body.
This animal is, in many respects, a normal pandericthid fish in many respects
- 1. it has powerful limbs and feet. The hind limb of Ichthyostega matches
the pattern of all later tetrapods.
- 2. The vertebra have developed zygopophyses, bony interlocks that
prevent the backbones from twisting:
3. The rib cage is strengthened to keep internal organs from being crushed.
It is one of the first Amphibians - the first of the land vertebrates.
The essence of amphibians is that they can live in air or in water, and
most of them spend some time in both environments.
Living on Land - Problems to be solved (and how are the amphibians doing??)
- 1. Desiccation of body
- -- Fish scales don't protect the body much against desiccation.
Besides Amphibians lost those scales early in their evolution.
Amphibians never really solved the skin-permeability problem.
They are restricted today to moist habitats.
- 2. Desiccation of respiratory structures
- -- lungs inside body
- -- Stronger rib cage
- 3. Desiccation of gametes and young
- -- No special adaptation, return to water to mate
- 4. Stress of gravity
- -- Endoskeleton of bone and lobe fins modified to limbs. Also
changes in skeleton for land hunting - Shoulder free from skull to
give head more flexibility and arm greater lateral movement
(evolution of a neck).
- 5. Sense organs must be modified to work in air rather than in water.
- -- Lateral line which senses vibrations under water becomes lost and
an operculum overlaying the lower jaw is used to sense vibrations
- 6. Feeding - appearance of a tongue and a flexible head.
- 7. Excretion - largely ammonia washed away with lots of water.
Adaptive shift to live on land?
- 1. Old idea - ponds drying up
- 2. Basking - heat body for hunting
- 3. Reproduction - moving to small pools to lay eggs: The most
vulnerable part of the life cycle of a fish to predators is when it is an
egg or hatchling. It is advantageous, then, to lay eggs in shallow
protected pools without direct access to the sea or larger bodies of
fresh water. Hatchlings could feed on algae and plankton in
relative safety in these pools. Thus, natural selection would favor
the habit of making a short journey over land or very shallow water
to the pools. Because of this selection, the lobe-fin lung fish would
have become better and better adapted for scooting across land with
Why live on land?
- 1. more oxygen;
- 2. escape predators;
- 3. escape competition for food
The biology and ecology of late Paleozoic amphibians reflects different
adaptations to different habitats and different ways of life - some were
predominately terrestrial, some predominately aquatic, and many spending
approximately equal time in both environments. The exact pattern of their
evolution is uncertain and our understanding will undoubtedly change as time,
and more specimens are found.
Living amphibians (salamanders, newts, toads and frogs) are all small
bodied and soft-skinned, and quite unlike the paleozoic amphibians. They
evolved quite late in amphibian history - in the Jurassic, and although their
biology is fascinating, they are no guide to the origin and paleobiology of other