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The modern caecilians, with their limbless, superficially worm-like or snake-like bodies, are perhaps the most unusual amphibians in appearance, and their behaviour can be equally strange. The order Gymnophiona (the caecilians) is thought to have diverged from other amphibian lineages about 370 million years ago in the Devonian period, over 150 million years before the first mammal. The earliest caecilian was reportedly found in Arizona, USA, and dates back to the early Jurassic period, 190 million years ago. It was named Eocaecilia micropodia because it had very small legs and feet. It also had well-developed eyes, suggesting that it was not an entirely subterranean species and may have had more in common with salamanders than with modern caecilians. Because of this, its status as a caecilian is in some doubt.
Eocaecilia and Gymnophiona have long independent histories, and it is also thought that they may represent separate lineages that developed similar closed-roof skull morphology convergently, i.e. the same trait arose twice in different parts of the amphibian tree of life, which means that species possessing identical traits are not always closely related (e.g. both bats and birds have wings, but flight evolved independently in these two lineages). This means that the morphology of Eocaecilia may not be the best model for the ancestral caecilian.
Modern caecilians certainly evolved from limbed ancestors however, and therefore underwent major changes in their evolutionary history as they developed elongated, externally-segmented bodies and much-reduced eyes, and lost their limbs, limb girdles and, in the case of the most recently evolved species, their tails. The only currently known fossils of ‘true’ caecilians are three records of isolated vertebrae (back bones) from the Late Cretaceous period (about 100-93 million years ago) of Sudan in Africa, and the Palaeocene (65-53 million years ago) of Brazil and Bolivia. Two older fossils from the Cretaceous of Morocco and the Jurassic of North America are not ‘true’ caecilians, but have been argued to be more closely related to caecilians than any other amphibians. The oldest of these two possible members of the caecilian ‘stem’ lineage is Eocaecilia macropodia. Its small fore- and hind-limbs are thought to have been retained from the last common ancestor of the caecilians and the frogs+salamanders lineage.
Carl Linnaeus (the founder of modern taxonomy) described the first species of caecilian (Caecilia tentaculata) in 1758 and initially thought they were related to snakes. The taxonomic order name “Gymnophiona” is actually derived from the Greek words gymnos (meaning naked) and ophis (meaning snake). The caecilians where therefore originally refered to as “naked snakes” because they lack external scales covering their entire body and the first scientific efforts to classify them could not conceive that they were instead closely related to frogs and salamanders. Many caecilians do actually have scales, but these are small, more fish-like than snake-like, and hidden in folds in the skin, although the Sagalla caecilian and its close relatives (in the Boulengerula genus) have no scales.
The family Caeciliidae (the “common caecilians”) are thought to have branched off from the rest of the amphibian tree over 160 million years ago in the upper Jurrasic period when dinosaurs were still abundant in the earth’s terrestrial ecosystems. Boulengerula (the genus of the Sagalla caecilian) diverged within this family almost 100 million years ago in the late Cretaceous period and there are now just seven described species in this genus (which is also referred to as “Boulenger's Caecilians”), all present in East and Central Africa. This means they are as separated in time from their closest relative as humans are from elephants. Despite there only being seven species of Boulengerula caecilians, this genus is thought to be the most species-rich of all the African caecilian groups and it is probable that more species are yet to be found.
The Sagalla caecilian was only recently discovered as a new species in 2005 and was collected from Sagalla Hill, an isolated mountain block of the Taita Hills range in the Eastern Arc Mountains of Kenya. This species differs substantially from its closest relative Boulengerula taitanus (the Taita African caecilian – which is also found in the Taita Hills) and from all other members of Boulengerula in its colouration, phallus (a penis-like organ – all caecilians have internal fertilisation) and other morphological features. The Eastern Arc Mountains are one of the earth's major biodiversity hotspots, and many species in this region are restricted to one mountain block within the arc.
Project Manager for the Sagalla Caecilian conservation project
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