Early Tetrapods

The discovery of Ichthyostega in 1932 pushed back the known origin of tetrapods to the Late Devonian. Prior to that, the earliest known tetrapods were the much more derived and amphibianlike specimens from the Carboniferous. The discovery of Ichthyostega provided some excellent clues to the early adaptation of lobe-finned fishes to life on land, but the discovery presented some puzzles as well.

While some Carboniferous tetrapods were clearly optimized for living on land, those of the Late Devonian appeared to consist of a hodgepodge of fish and tetrapod features. What had previously been viewed as the relatively straight-line evolution of legs so that animals could walk on land has now been clouded by the realization that the first animals with legs probably rarely left the water.

Ichthyostega (Late Devonian). Ichthyostega is known from several excellent specimens and was the first taxon to push back the origin of tetrapods to the Late Devonian. As such, Ichthyostega is often thought of as the archetype of all early tetrapods, even though other specimens such as the much less complete Tulerpeton feature somewhat more derived traits found in later tetrapods. In truth, Ichthyostega was very much at home in the water, although it did not have gills. Its limbs were more robust than those of Acanthostega, and its abdomen was fortified with long, thick, overlapping ribs. Its spine was fishlike but differed from that of Acanthostega by possessing bony projections called zygapophyses; these projections served as points for muscle attachment to strengthen the body. The hefty limbs, robust ribs, and sturdy spine enabled Ichthyostega to support its body while on land and to walk stiffly, if not nimbly.

Known specimens of Ichthyostega do not yet show the presence of wrists or ankles, but the animal had seven digits on each of its feet. The hind limbs were considerably shorter than the front limbs; this suggests that the back legs were probably used for paddling in the water and the front legs for propping the animal up and walking on land using the tetrapod equivalent of front-wheel drive. Ichthyostega was moderately large; it measured about four feet (1.2 m) long. Its skull was broad and flattened, with orbits for the eyes positioned on the top of the skull. Ichthyostega's teeth were remarkably different from the short teeth of Acanthostega. The upper jaw of Ichthyostega had an outer row of long, conical teeth and an inner row of smaller teeth. The bottom jaw had only a row of smaller teeth. These teeth might have given Ichthyostega many types of prey to choose from, including fish, aquatic and terrestrial arthropods, and perhaps other tetrapods. Like Acanthostega, Ichthyostega was probably at home in the water; however, its sturdier, less fishlike skeleton and its lack of gills suggest that it was also at home on land.

Tulerpeton (Late Devonian): Recovered from a fossil site near Tula, Russia, Tulerpeton is known from less-complete skeletal elements than either Acanthostega or Ichthyostega but is nonetheless significant because of what those few elements reveal. The remains include a forelimb, a hind limb, part of the shoulder girdle, a piece of jaw, and bony belly scales, called scutes, that protected Tulerpeton from scraping against the ground. As with Ichthyostega, this tetrapod had a peculiar number of digits: six. Clearly, by the Late Devonian, tetrapods had not yet settled on five as the optimum number of digits, as seen in the Carboniferous. The shoulder bones and longer forelimbs of Tulerpeton are more suited for life on land than those in either Acanthostega or Ichthyostega, and it is assumed that Tulerpeton was capable of terrestrial locomotion. Tulerpeton s hind limbs, however, lacked a true ankle and probably were also good at paddling in the water; this shows once again that tetrapods from the Late Devonian were still undergoing evolutionary changes that would free them from the water.

The above specimens, although fragmentary, represent the most complete fossil material of Devonian tetrapods currently known. The paleontology of Devonian tetrapods is remarkably limited when it comes to the abundance of specimens. For those who study the very earliest forms of tetrapods, a highly valued fossil specimen often amounts to no more than a single jaw or leg bone. A postscript should therefore be made regarding several even less informative partial specimens that have affinities with tetrapods but about which little more can accurately be surmised. If they say nothing else, these partial specimens speak to the worldwide distribution and diversity of early tetrapods and proclaim the hope that more discoveries are imminent—discoveries that will help complete the story of the first Devonian limbed vertebrates. Most of these specimens were described during the past 12 years and include the pectoral girdle and jaw of Hynerpeton (Red Hill, Pennsylvania), the first tetrapod found in the United States; the jaw of Densignathus (Red Hill, Pennsylvania); Jakubsonia (Russia), known from a partial jaw bone and upper shoulder; and Metaxygnathus (New South Wales, Australia) and Sinostega (northwest China), both known only from lower jaw fragments and the first such early tetrapod specimens from their respective parts of the world.

By the end of the Devonian Period, the first limbed vertebrates had begun to find their niche in the river floodplains of a greening world. Their existence was not entirely divorced from the aquatic habitat of their ancestors, and they certainly spent much of their life in the water, particularly for feeding and reproduction. This was a workable scheme in the humid, rain-drenched world in which these creatures lived, and it gave rise to an increasingly diverse variety

LATE DEVONIAN LOBE-FINNED FISHES, BASAL TETRAPODS AND THEIR LOCATIONS

Locality

Age*

Taxon/Taxa

Classification

Eastern Greenland

Famennian

Acanthostega Ichthyostega

Early tetrapod

Red Hill, Pennsylvania, United States

Famennian

Densignathus Hynerpeton

Early tetrapod

Ellesmere Island, Canada

Early to Middle Frasnian

Tiktaalik

Lobe--finned fish

Scat Crag, Scotland

Late Frasnian

Elginerpeton

Early tetrapod

Escuminac Bay, Quebec, Canada

Elpistostege

Lobe--finned fish

Oryl Region of Russia

Lower Famennian

Jakubsonia

Early tetrapod

Guaha Formation of Latvia and Estonia border area

Late Givetian, Late--Middle Devonian or Early Frasnian, Early--Late Devonian

Livoniana Panderichthys

Lobe--finned fishes

New South Wales, Australia

Frasnian--Famennian boundary to the Late Famennian

Metaxygnathus

Early tetrapod

Eastern Latvia

Late Frasnian

Obruchevichthys

Early tetrapod

Ningxia Hui Autonomous Region, northwest China

Late Famennian

Sinostega

Early tetrapod

Central Belgium

Late Famennian

Strud jaw**

Early tetrapod

Tula region of Russia

Famennian

Tulerpeton

Early tetrapod

Western Latvia

Late Famennian

Ventastega

Early tetrapod

^Devonian Period

The Middle Devonian Epoch is divided into two stages: Eifelian (397.5 million to 391.8 million years ago) Givetian 391.8 million to 385.3 million years ago) The Late Devonian Epoch is divided into two stages: Frasnian (385 million to 374 million years ago) Famennian (374 million to 359 million years ago) **The "Strud jaw" is a lower-jaw fragment discovered at Strud in 1888 that measures about 7 cm in length and that represents the earliest known Late Devonian tetrapod from western continental Europe.

of amphibians. The story of land vertebrates was barely underway, however; soon, these animals would experience even greater diversification. This diversification eventually would lead to the evolution

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