Saturday, September 22, 2012

Giant Salamanders and peramorphosis

Giant salamanders (Cyptobranchidae) can live for a century, grow to two meters in length, and have an ancestry that extends back more than 56 million years. Fossils giant salamanders are frequently found in Eurasia and they show little variation from their modern descendants. The early giant salamanders had a body size and lifestyle similar to those alive today and they lived in East Asia and North America. But while modern species inhabit oxygen-rich, fast-flowing mountain streams in China, Japan and the US, their ancestors lived in rivers and lakes in the lowlands.
Ontogenetic development in giant salamanders.

Now, Madelaine Böhme and Davit Vasilyan at the University of Tübingen have discovered the oldest known giant salamander, Aviturus exsecratus, lived on land as well as in water. A reexamination of 56 MY fossil A. exsecratus from southern Mongolia demonstrated that the animal hunted in the water and on land. Making its life style quite different from all the later giant salamanders, which live or lived only in water.

The evolution of a species from a purely aquatic lifestyle to an amphibious-terrestrial lifestyle is linked with gigantism and sustained growth and is called peramorphosis (the opposite of paedomorphosis). It is completely unknown in modern salamanders. However, development like this is only known in palaeozoological amphibians such as Eryops, which lived 300 MYA.

The authors suspect that Aviturus exsecratus fed on fish and invertebrates in the water – as suggested by the shape of its lower jaw, and probably hunted insects on land. Terrestrial adaptation is indicated by the animal’s heavy bones, long hind legs, a well-developed sense of smell, and palatal dentition typical of a terrestrial salamander. Also, fossil remains of this huge, up to  two meters long animal were found in rock typically formed from water’s-edge sediments.

The researchers hypothesize that body size in Aviturus exsecratus was probably due to a short period of global warming 55.8 million years ago: the Paleocene-Eocene Thermal Maximum. This most sudden climate change since the death of the dinosaurs saw global temperatures rise 6 degrees Celsius within around 20,000 years.

Citation
Davit Vasilyan, MadelaineBöhme. Pronounced Peramorphosis inLissamphibians—Aviturus exsecratus (Urodela, Cryptobranchidae) from thePaleocene–Eocene Thermal Maximum of Mongolia.PLoS ONE, 2012; 7 (9):e40665 DOI:10.1371/journal.pone.0040665

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