Friday, 22 July 2016

Austrian Sea Cows

As I was visiting Austria recently, I decided to do some background palaeo reading. I came across mention of a fossil group I'd never have associated with this country: the sea cow. I thought you might like to share some of what I found out about Europe, 30 million years ago. It was entirely unlike the place we know today, filled with tropical forest, flooded with warm shallow seas, and home to some unexpected animals...

Long before Austria was the mountainous land-locked country it is today, much of it was under an ancient sea called the Paratethys. This sea was the remnant of the more ancient Tethys, alongside what would become the Mediterranean Sea. During the Miocene, this shallow sea was warm and bountiful, providing the perfect habitat for Metaxytherium krahuletzi, an extinct species of dugong.

Most of Austria’s fossils were found near Eggenburg, north-west of Vienna, which gave it's name to the geological period of its rocks, the Eggenburgian (the first part of the Burdigalian, around 20 million years ago). This area represents a shallow sub-tropical sea, fringed with forest. The forest of the Miocene in Austria was rich in species we would never find there today. There were tapir, which would have been almost indistinguishable from the rainforest species still alive today. Areas of cypress swamp full of rhinos and flying squirrels led to established mixed forests on higher ground. Alongside more modest walnut, beech and elm trees, grew chocolate, ebony and tea species, as well as the occasional citrus. They would have stretched from the shores of the sea to modern day Germany and France. To support these species, the climate must have been humid and sub-tropical/tropical.

Mountain tapir (Image: World Land Trust).
Austria’s ancient sea-cows shared their habitat with dolphins such as the early species Schizodelphis sulcatus and Miocene hippos like Brachyodus onoideus*. Modern dugongs tend to be found in water up to 10 metres deep, as most grasses thrive in these shallow continental shelf waters. Like modern dugongs, Metaxytherium fed not only on sea grasses, but on their rhizomes, snuffled from the sea bed, and the occasional invertebrate such as jellyfish or sea squirts. However, modern dugings have been recorded as deep as 39 metres, pursuing the few deep-water grass species on their herbivorous menu. This is especially surprising as they are of course air-breathers, holding their breath for between two and five minutes. If Metaxytherium made similar dives it would have had the infamous Carcharocles megalodon to contend with: the 18 metre long shark beloved of school-kids and fossil shark-tooth collectors.

Image: Ruth Hartnup
There are around eight fossil species of Metaxytherium, from Europe, Africa, the Americas and Asia. They are related to the modern extant dugong (Dugong dugon), and the recently extinct Steller’s sea cow (Hydrodamalis gigas), all of them being part of the mammal order Sirenia. Like them, the skeleton is robust and thickened (a condition called pachyostosis), giving the ballast necessary for effortless underwater floating. Their hind legs were long lost, but their forelimbs retained strong paddles, and their thick tail would have gently pushed them through their watery habitat. The dugongs are related to manatees, but with some important differences. Dugongs have a forked, fluked tail, like a dolphin’s, rather than a round, manatee-style paddle. They also develop short tusks, which manatees lack, and these mark sexual maturity in male dugongs.

Both images are of Halitherium schinzi, a specimen from Hessen in Germany, approximately 30 million years old. It is held at the Pal√§ontologisches Museum M√ľnchen (Own image).
Sadly, the palaeontological evidence suggests the fossil sea-cows found in Eggenburg all died together after massive tropical storms destroyed their food source. Modern dugongs are seldom found in groups because the sea grass they feed on can’t support more than a few at a time in any one area. Assuming the ecology of Metaxytherium was similar, if a storm destroyed the sea-grass over a large area of the sea bed, it is little wonder so many perished from hunger before it could recover.

The dugong has a long and successful history, flourishing for millions of years. The combination of the cooling climate of the ice age, and more recently the impact of man – hunting and destroying habitat and food sources – has reduced this family down to a single species whose numbers are falling.The wonderful Steller's sea cow has only recently, tragically, been lost. Let's hope we can reign ourselves in soon enough to protect the beautiful dugong (and the manatee) from extinction.

Here are the modern species of Sirenia, including the extinct Steller's sea cow.

*For more on Austrain geology and palaeontology, here is a good paper that also discusses the Eggenburg sea cows.

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