Origins of the Tetrapods To find the earliest “AMPHIBIANS,” we can look to freshwater sedimentary deposits in Greenland and Russia, which date to the late DEVONIAN period, about 360 million years before the present— (Note that these areas were close to the equator at that time, and the Atlantic Ocean did not exist!)

Acanthostega (and the closely related Ichthyostega) probably lived in shallow freshwater habitats

Skeletal reconstruction of Acanthostega

Most Acanthostega specimens came from one site representing a point bar in an active river channel, where a series of burial events took place, resulting in deposition of several well-articulated specimens. These are interpreted as having been deposited in a flash-flood. The good preservation of these specimens suggests that they had not been carried far, and thus that the animals inhabited these channels. A few isolated holoptychiid and lungfish scales and bones were also found in this lens. In other localities yielding Acanthostega material, Ichthyostega, Holoptychius, Eusthenodon, lungfish and placoderms also occur (Bendix-Almgreen, Clack and Olsen 1990).

Acanthostega is interpreted as a primarily if not entirely aquatic animal, based on the form of the limb joints and digits, the extensive tail fin, notochordal vertebrae, … fish-like dentition retaining large vomerine fangs, lateral line organ embedded in bone, small naris, large stapes, and possibly functional spiracle. It also retains a number of primitive features independent of its aquatic life, such as the notochordal braincase, form of the fenestra vestibulae, persistent embryonic braincase fissures, fish-like occiput, anocleithrum, form of the scapulocoracoid-cleithral complex, relative lengths of radius and ulna, retention of dermal fin rays and supraneural spines. The large number of digits fits the hypothesis that early in limb evolution, digit number was not fixed. All of these characters suggest that not only was Acanthostega aquatic, but that it was primitively so, and not derived from a more terrestrial forebear. Its structure supports the idea that limbs with digits evolved for use in water, only later to be used on land, rather than the more conventional view that it was among sarcopterygian fishes that excursions over land first began (Clack 1997, Clack and Coates 1995, Coates and Clack 1995).

Ichthyostega was a slightly more robust animal, and may have been a bit more terrestrial than Acanothostega, but…

Ichthyostega has traditionally been considered as fully capable of terrestrial life, but this is uncertain… and it almost certainly had an aquatic larval stage. Zdenek Burian

Another conception: Acanthostega and Ichthyostega

The next slide is a chart showing the geological placement of Ichthyostega, Acanothostega, and their descendants (which we will discuss later)…

Derivation of tetrapods from sarcoptery- gians; for detatils see du/books/g ee/shubin1. jpeg. du/books/g ee/shubin1. jpeg Shubin, et.al

We can be certain, based on morphology, fossils, and molecular data, that tetrapods evolved within the bony fish (Osteichthyes), and specifically the so- called lobe-fin fish (Sarcopterygia). But which of these is closest to Acanthostega and later tetrapods? Here’s one view, accepted by most paleontologists: (this group often called “Rhipidistia”)

The Sarcopterygian fish and all the early “amphibian” groups share a number of connecting characters, but foremost of these is the possession of labyrinthodont teeth, with odd infoldings of enamel: This discovery clearly linked the sarcopterygian fish, Ichthyostega, Acanothostega, and many later amphibian groups such as the TEMNOSPONDYLS and the ANTHRACOSAURS (more on these later).

Fossil lobe-fins first appear in the Lower Devonian, and diversified into several major groups by the Middle Devonian. Lobe-fin diversity remained high during the Upper Devonian, Carboniferous and Lower Permian, but declined significantly thereafter.

Note: The sarcopterygian fish group closest to tetrapods has been variously referred to by several taxonomic names, which keep changing as new data makes old names obsolete. These include: Crossopterygians Rhipidisteans Osteolepiforms Osteolepimorphs

Today the osteolepiform “rhipidisteans” are represented only by the deep ocean fish called Latimeria that are occasionally caught off the African coast (Comoros Islands) and recently in Indonesia.

Coelacanths are known from the fossil record dating back over 360 million years, with a peak in abundance about 240 million years ago. Before 1938 they were believed to have become extinct approximately 80 million years ago, when they disappeared from the fossil record. How could Coelacanths disappear for over 80 million years and then turn up alive and well in the twentieth century? The answer seems to be that the Coelacanths from the fossil record lived in environments favoring fossilisation. Modern Coelacanths, both in the Comoros and Sulawesi were found in environments that do not favor fossil formation. They inhabit caves and overhangs in near vertical marine reefs, at about 200 m depth, off newly formed volcanic islands. Latimeria is a member of the coelacanth group of sarcopterygians…

BUT-- Latimeria is probably not a close relative of the earliest tetrapods. It has obvious specializations for life in a deep marine environment, and fossil evidence suggests that early tetrapods inhabited shallow freshwater environments…

The other living sarcopterygian group is the lungfish (DIPNOI). There are three genera of extant lungfishes in Africa, South America, and Australia, and lots of fossil genera going back to the Paleozoic… Lepidosiren (South America)

Not surprisingly, when modern taxonomists look at the structure and genetics of the LIVING sarcopterygians and other vertebrates, they decide that lungfish are the closest SISTER GROUP to the tetrapods. Of course, we cannot get blood samples or study the soft tissues of extinct osteolepiforms (=osteolepimorphs), so they will be “out of the running… but once fossil studies are included, it becomes clear that it is more likely that the osteolepiforms are the closest sister group to the tetrapods…

BUT WHY SHOULD “FISH” COME OUT OF THE WATER AT ALL??? To live on land requires bracing against gravity, and… A change in breathing mechanism, and… New feeding strategies… New hearing strategies… And… many more adaptations! isn’t it easier just to stay in water??

Of course, some living fish do have adaptations for leaving the water, or at least surviving when the water disappears… … like the mudskipper fish of brackish mangrove swamps in Africa, Asia, and Australia, which can leave the water to feed and chase potential mates.

Or the lungfish, which can bury in the mud and survive for months if their lake dries up. They have both gills and lungs for respiration…

Today’s amphibians, with their transformation from gilledfish-like larvae to lung-breathing adult, show that this water to land movement must still have some benefits…

Did the lobe-fin fish leave water to seek out new habitats during seasonal drought? Or were they exploiting new food sources on land, using their strong fins and “lungs?”

This might lead to selection for stronger fins, and eventually feet… Note that the precursors of the foot bones and digits and other skeletal features are found in the osteolepiform fish…

FEET:

OR THE SKULL:

But it seems doubtful that osteolepiform fish would actually leave the water with any frequency. The strong “lobe-fins” were just as likely to have been used to scuttle along on the bottom of rivers and lakes, and the lungs would help in stagnant water situations… Model of Eusthenopteron

So… it seems that the first real “feet” were probably adaptations to scuttling along the bottom in weedy, shallow water, and the first tetrapod animals were aquatic animals pre-adapted to movement onto land… maybe for some of the same reasons that “fish” move onto land today… Elginerpeton, an early tetrapod that was probably largely aquatic. Natural History Museum, London.