ICHTHYOLITES OF THE OLD RED SANDSTONE

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TETRAPODA

 Arguably the greatest event in the history of vertebrate evolution was the emergence from water onto land and the subsequent diversification into amphibian and reptiles. This transition from fish to tetrapod is poorly represented in the fossils record and hence poorly understood. Carboniferous amphibians had been known since the middle of last century but it is only relatively recently that well preserved Devonian examples were found. In reality the transition from osteolepidiform to tetrapod was not such a great leap. The former had developed a bone pattern in their strong fins that would be retained as arms and legs in the tetrapods, swim bladders would become lungs and the skull structure could remain unchanged.

Because of the rarity of fossil remains, we have three successive snapshots of the structure and ecology of the early land vertebrates. The fossil record starts in the Frasnian/Famennian (374-360myn) with the Ichthyostegalids from Greenland . These early tetrapods were little more the highly modified panderichthyid fishes. Their skulls  were very similar to the panderichthyids (although lacking the intercranial joint) and study of the gill arches indicate they spent much of their time in water. They had a long tail with a fin and limbs with seven or eight digits.

 Until the mid 1980’s just a few fragmentary Tournaisian/Visean tetrapod fossils were known from the Midland Valley of Scotland . More specimens were discovered in Nova Scotica and West Virginia , with a single Namurian Anthracosaur from the Ruhr, Germany . All these remains pointed to specialised aquatic forms with few relatives in the upper Carboniferous. In the 1970/80’s Mr Stanley Wood discovered two assemblages of early Carboniferous amphibians. At  the Dora open cast coal mine he discovered a bone bed containing six types of tetrapods including Crassigyrinus (aquatic) and Eoherpeton (tererrestrial). Ten years later at East Kirkton , he discovered a second earlier assemblage from the Brigantian stage of the Visean, including the earliest well preserved amphibian fossil ever discovered. What was even more exciting about this site was the complete absence of fish and presence of terrestrial scorpions and spiders- the first truly terrestrial amphibians had been found and much earlier than the previous fossil record of the upper Carboniferous.

 The Temnospondyl amphibian Balanerpeton woodei is the commonest form (around 30 skeletons)  at this site. Such was the importance of this discover it was described in Nature (Vo314) in 1985 and later by Milner AR and Sequeira SEK (Trans of the Royal Soc of Edinb 1994) . The latter publication describes Balanerpeton in great detail including the following morphological features suggestive of a terrestrial habitat for the adult:

 1) Absence of lateral line sulci, 2) Absence of  ossified branchial system , 3) Possible eyelids, 4)Presence of a large tympanic ear with rod-like stapes (suggestive of ability to hear high freq airbourn sound), 5) Ossified carpals and tarsals

            Balanerpeton woodi  from  THE SEARCH FOR EARLY TETRAPODS,  Milner et al.

East Kirkton has also yielded a single skull from the earliest known loxommatid and the remains of the earliest reptiliomorph, Anthracosauroids:  Silvanerpeton miripedes and Eldeceeon rolfei.

Tetrapoda in my collection (click on thumbnails to see larger images)

Balanerpeton woodi   Temnospondyl Amphibian,  Visean,     East Kirkton, Scotland

   Skull      Lower Jaw

    1985 nature article on Balanerpeton

              Balanerpeton images from Milner & Sequeira

   Anthracosaur jaw  East Kirkton, Scotland

    Silvanerpeton skull image from Clack

Other Vertebrates in my collection (click on thumbnails to see larger images)

    Claudiosaurus, ( basal diapsid reptile) Upper Permian of Madagascar   

    Letoverpeton austriacus  Labyrinthodont, Branchosaurid   Bachov, East Czech