Ukrainian fossil giant salamander provides new insights into group’s origins

Amphibians are perhaps one of the less popular tetrapod groups. Most people would rather look at a furry mammal or feathered bird than a slimy newt or toad. This doesn’t mean that the group, consisting of frogs, toads, salamanders and the limbless caecilians, aren’t interesting. In fact, the first vertebrates that made the transition to life on land some 370 million years ago were amphibians so we all owe a debt of gratitude to our moist-skinned cousins! Since then, they have continued to successfully adapt and survive, with around 7,000 species alive today. The largest amphibians in the world today are the Chinese giant salamander (Andrias davidianus) and the Japanese giant salamander (Andrias japonicas), which can reach lengths of over 1.5 m. In the video below (narrated by the one and only, legendary, awesome and über-cool David Attenborough) you can see how amazing these animals really are and why they should maybe get more attention than they do.

But what is even cooler than giant salamanders? Fossil giant salamanders of course! And a new paper by Vasilyan et al. published in the most recent issue of the Journal of Vertebrate Paleontology has just described a new species of giant salamander from Ukraine and revealed new insights into the origins of the group (known scientifically as Cryptobranchids). The new species, Ukrainurus hypsognathus is middle-late Miocene in age (around 11.5 million years) and was found in an abandoned quarry just southeast of the village of Grytsiv, western Ukraine. The holotype is a left dentary (shown below) and other cranial elements and several postcranial elements have also been assigned to the same individual.

The holotype dentary of Ukrainurus hypsognathus shown in A, lingual; B, dorsal; C, labial; D, ventral views. Scale bar = 1 cm. Image from Vasilyan et al. 2013.
The holotype dentary of Ukrainurus hypsognathus shown in A, lingual; B, dorsal; C, labial; D, ventral views. Scale bar = 1 cm. Image from Vasilyan et al. 2013.

By studying the dentary and other cranial elements of U. hypsognathus, Vasilyan et al. were able to establish that it would have had a considerably strong bite force, with which it would have used to feed on prey such as fish, frogs and even other salamander species that are also known from the locality. They also found that the jaw of U. hypsognathus would have been less flexible than that of modern giant salamander species, meaning its method of prey capture may have differed slightly from that of modern taxa.

The phylogeny of Vasilyan et al. 2013. You can see that Ukrainurus hypsognathus is sister to all crown Cryptobranchids. Image from Vasilyan et al. 2013.
The phylogeny of Vasilyan et al. 2013. You can see that Ukrainurus hypsognathus is sister to all crown Cryptobranchids. Image from Vasilyan et al. 2013.

This paper is also the first study in which the relationships between fossil and recent species of giant salamanders have been analysed in a phylogenetic context. The results of the phylogenetic analysis show that U. hypsognathus is the sister taxon to crown group Cryptobranchidae. The single living American species Cryptobranchus alleganiensis was also found to be the sister taxon of all Eurasian cryptobranchids. A fossil cryptobranchid from Saskatchewan, Canada (excluded from the final phylogenetic analysis) was found not to be closely related to C. alleganiensis, but would have placed more basally in the phylogeny, closer to U. hypsognathus.

What this all implies is that Cryptobranchidae originated in Asia and dispersed to North America on two separate occasions. In terms of the timing of these events, the oldest crown Cryptobranchid is Aviturus exsecratus from the Palaeocene of Mongolia. This means that the split between crown and stem Cryptobranchidae would have had to occur in the Palaeocene at the latest and possibly even in the Late Cretaceous. When exactly the C. saskatchewanensis and C. alleganiensis lineages dispersed to America remains uncertain however.

With the majority of fossil cryptobranchids being known from Central rather than Eastern Europe, it would appear that there could well be more new insights to come on the evolution of the giant salamanders, surely an interesting group of amphibians if there ever was one.

Reference

Davit Vasilyan , Madelaine Böhme , Viacheslav M. Chkhikvadze , Yuriy A. Semenov & Walter G. Joyce (2013): A new giant salamander (Urodela, Pancryptobrancha) from the Miocene of Eastern Europe (Grytsiv, Ukraine), Journal of Vertebrate Paleontology, 33:2, 301-318.