Post by bluefedish on May 1, 2008 12:47:44 GMT -5
The skull roof of Acanthostega gunneri was first recovered from Famennian deposits (360 million years ago) in eastern Greenland in 1933, and was described and named in 1952 by Erik Jarvik.
Additional fossils were recovered during a 1970 geological expedition, but they languished in obscurity until rediscovered by Jennifer Clack. In a 1987 expedition led by Clack and Per Ahlberg recovered some exceptionally well preserved material from several individuals.
The abundance and quality of Acanthostega remains has made it the best known of the early tetrapods. From their investigations of these remains Clack and Michael Coates have reported a series of remarkable findings that have necessitated changes in our thinking on early tetrapod evolution.
Prior to these findings, most scientists assumed that the evolution of legs and feet was initiated and driven by the colonization of land. Here, however, was an early tetrapod that was ill-suited for life on land. It had well-defined digits (fingers and toes), but no wrists or ankles. It had relatively long limb bones, but they couldn't support much weight. Its hip also couldn't support much weight since it was weakly attached to the spine.
A firm attachment to the spine wouldn't help much anyway, since its spine was structurally based on the (ancestral) notochord rather than on a series of interlocking, yet flexible, vertebrae. The spine was well-suited for handling the mechanical stresses of swimming but was nearly useless for supporting weight.
Moreover, its short and thin ribs were incapable of protecting vital organs. Acanthostega also had a deep tail which sported a large bony fin. In short, it had a tail suited for swimming, a fish's spine and paddle-like limbs.
A primarily, if not exclusively, aquatic lifestyle for Acanthostega is further indicated by the presence of internal, fish-like gills.
(Evidence for internal gills include bony gill arches and post-branchial lamina on the leading edge of the shoulder girdle. In contrast, all Carboniferous-to-modern gill-breathing amphibians have external gills.) Other features that indicate an aquatic lifestyle include a fish-like stapes (a bone which will evolve into the middle ear of terrestrial tetrapods) and the retention of the sensory lateral line system found in fishes. Acanthostega's small, fish-like nares (nostrils) were probably used only for smelling under water; air may have been brought to the lungs by gulping.
Although Acanthostega had many fish-like characteristics it did have legs and feet rather than fins. These feet, however, also affected our thinking on the evolution of tetrapod limbs. It was practically an article of faith that the first tetrapods had five digits, but Acanthostega had eight digits on the front leg and at least eight digits on the hind. (Subsequent analyses have indicated that at least two other early tetrapods, Ichthyostega and Tulerpeton, also had more than five digits.)
With its combination of fish-like and tetrapod features Acanthostega has engendered a variety of speculation about the paleoecology and evolution of early tetrapods. Its feet may have been superior to fins in negotiating shallow waters filled with aquatic plants and woody debris. (These shallow water habitats could have been wetlands, stream margins or inundated floodplains.) Acanthostega also had a protective covering of elongated ovoid scutes on its belly, but no scales on the rest of its body. These scutes may have protected it from abrasion as it moved about.
The skull roof retains a number of primitive characters such as paired median rostrals, anterior tectals and preoperculars (bones lost in most other tetrapods), and shows unique ones including a dart-shaped supratemporal and a tabular with both a posteriorly directed horn, and an embayment which may have housed a persistent spiracle. It lacked an intertemporal. The external nostrils were small and placed close to the jaw margin (Clack 1994a,2002c, 2003).
The closed palate bore large fang-pairs on the vomer, as well as an outer row of smaller teeth and denticles, carried posteriorly along the palatines and ectopterygoids (Clack 1994a).
The braincase structure falls between that of Eusthenopteron and more derived stem tetrapods such as embolomeres. It was notochordal, with small exoccipitals. The large fenestra vestibulae accommodated the head of the stapes. The margins of the fenestra were formed by the basioccipital and otic capsule (see also Sensory Organs below). The basipterygoid processes were large and bifaceted as in other stem-tetrapods (Clack 1994b, 1998).
The hyobranchial elements were fully ossified and deeply grooved as in a lungfish such as Neoceratodus, except that they are strongly ossified whereas in Neoceratodus they remain cartilaginous. The grooves would have housed the afferent branchial artery carrying blood to the internal gills, which are thus inferred to have been functional (Coates and Clack 1991). The stapes was a short stout bone with a large proximal footplate and a flared distal end (Clack 1989, 1994b).
The slender lower jaw, in addition to the usual complement of bones, included a Meckelian bone and a parasymphysial dental plate. The prearticular was long and bore an area of shagreen dorsally. It was not sutured ventrally to the surangular, angular or postsplenial and its lower margin accommodated small Meckelian fossae. These features appear to be primitive. The coronoids bore a row of small teeth and denticles similar to that of the palate but there were no fang-pairs as there are in osteoplepiforms. The dentary housed smaller and more numerous teeth (about 75) than the maxilla plus premaxilla (about 60)(Ahlberg and Clack 1998).
Acanthostega has been recovered from river deposits, and the presence of well-articulated and minimally reworked remains indicate that it probably lived there. Fossils of lobe-fins (Holoptychius, Eusthenodon and lungfishes) and placoderms have been collected in localities yielding Acanthostega.
Acanthostega was about 60 cm (2 ft) in length.
Selected Sources:
www.devoniantimes.org/Order/re-acanthostega.html
tolweb.org/Acanthostega
Additional fossils were recovered during a 1970 geological expedition, but they languished in obscurity until rediscovered by Jennifer Clack. In a 1987 expedition led by Clack and Per Ahlberg recovered some exceptionally well preserved material from several individuals.
The abundance and quality of Acanthostega remains has made it the best known of the early tetrapods. From their investigations of these remains Clack and Michael Coates have reported a series of remarkable findings that have necessitated changes in our thinking on early tetrapod evolution.
Prior to these findings, most scientists assumed that the evolution of legs and feet was initiated and driven by the colonization of land. Here, however, was an early tetrapod that was ill-suited for life on land. It had well-defined digits (fingers and toes), but no wrists or ankles. It had relatively long limb bones, but they couldn't support much weight. Its hip also couldn't support much weight since it was weakly attached to the spine.
A firm attachment to the spine wouldn't help much anyway, since its spine was structurally based on the (ancestral) notochord rather than on a series of interlocking, yet flexible, vertebrae. The spine was well-suited for handling the mechanical stresses of swimming but was nearly useless for supporting weight.
Moreover, its short and thin ribs were incapable of protecting vital organs. Acanthostega also had a deep tail which sported a large bony fin. In short, it had a tail suited for swimming, a fish's spine and paddle-like limbs.
A primarily, if not exclusively, aquatic lifestyle for Acanthostega is further indicated by the presence of internal, fish-like gills.
(Evidence for internal gills include bony gill arches and post-branchial lamina on the leading edge of the shoulder girdle. In contrast, all Carboniferous-to-modern gill-breathing amphibians have external gills.) Other features that indicate an aquatic lifestyle include a fish-like stapes (a bone which will evolve into the middle ear of terrestrial tetrapods) and the retention of the sensory lateral line system found in fishes. Acanthostega's small, fish-like nares (nostrils) were probably used only for smelling under water; air may have been brought to the lungs by gulping.
Although Acanthostega had many fish-like characteristics it did have legs and feet rather than fins. These feet, however, also affected our thinking on the evolution of tetrapod limbs. It was practically an article of faith that the first tetrapods had five digits, but Acanthostega had eight digits on the front leg and at least eight digits on the hind. (Subsequent analyses have indicated that at least two other early tetrapods, Ichthyostega and Tulerpeton, also had more than five digits.)
With its combination of fish-like and tetrapod features Acanthostega has engendered a variety of speculation about the paleoecology and evolution of early tetrapods. Its feet may have been superior to fins in negotiating shallow waters filled with aquatic plants and woody debris. (These shallow water habitats could have been wetlands, stream margins or inundated floodplains.) Acanthostega also had a protective covering of elongated ovoid scutes on its belly, but no scales on the rest of its body. These scutes may have protected it from abrasion as it moved about.
The skull roof retains a number of primitive characters such as paired median rostrals, anterior tectals and preoperculars (bones lost in most other tetrapods), and shows unique ones including a dart-shaped supratemporal and a tabular with both a posteriorly directed horn, and an embayment which may have housed a persistent spiracle. It lacked an intertemporal. The external nostrils were small and placed close to the jaw margin (Clack 1994a,2002c, 2003).
The closed palate bore large fang-pairs on the vomer, as well as an outer row of smaller teeth and denticles, carried posteriorly along the palatines and ectopterygoids (Clack 1994a).
The braincase structure falls between that of Eusthenopteron and more derived stem tetrapods such as embolomeres. It was notochordal, with small exoccipitals. The large fenestra vestibulae accommodated the head of the stapes. The margins of the fenestra were formed by the basioccipital and otic capsule (see also Sensory Organs below). The basipterygoid processes were large and bifaceted as in other stem-tetrapods (Clack 1994b, 1998).
The hyobranchial elements were fully ossified and deeply grooved as in a lungfish such as Neoceratodus, except that they are strongly ossified whereas in Neoceratodus they remain cartilaginous. The grooves would have housed the afferent branchial artery carrying blood to the internal gills, which are thus inferred to have been functional (Coates and Clack 1991). The stapes was a short stout bone with a large proximal footplate and a flared distal end (Clack 1989, 1994b).
The slender lower jaw, in addition to the usual complement of bones, included a Meckelian bone and a parasymphysial dental plate. The prearticular was long and bore an area of shagreen dorsally. It was not sutured ventrally to the surangular, angular or postsplenial and its lower margin accommodated small Meckelian fossae. These features appear to be primitive. The coronoids bore a row of small teeth and denticles similar to that of the palate but there were no fang-pairs as there are in osteoplepiforms. The dentary housed smaller and more numerous teeth (about 75) than the maxilla plus premaxilla (about 60)(Ahlberg and Clack 1998).
Acanthostega has been recovered from river deposits, and the presence of well-articulated and minimally reworked remains indicate that it probably lived there. Fossils of lobe-fins (Holoptychius, Eusthenodon and lungfishes) and placoderms have been collected in localities yielding Acanthostega.
Acanthostega was about 60 cm (2 ft) in length.
Selected Sources:
www.devoniantimes.org/Order/re-acanthostega.html
tolweb.org/Acanthostega
