New Fossils Provide A Link Between Seahorses And Their Ancient Ancestors

The 12.5 million year old fossil and a reconstruction of Hippotropiscis frenki

Sea horses are bizarre little creatures. Small and rather delicate, they have found many ways of surviving in the dangerous seas. Some look like scraps of seaweed, others like pieces of coral. Some hide, while others have a certain degree of mobility which allows them to escape from small fish. Unlike, as in most species, it is the male who gives birth to the young.

Their body structure alone is unusual. They do indeed have a very horse-like head, a small fin on their back and a rather thin and wispy tail. Yet while strange, scientists have been able to place them with relative ease on the tree of life.

A close up of the head

Their closest relatives and therefore evolutionary ancestors are the pipe fish. We have fossils of seahorses, we have fossils of pipe fish and now a group of palaeontologists excavating in Slovenia have fossils of the evolutionary links in between.

The Tunjice Hills are composed of limestones laid down during the Miocene epoch. They are highly fossiliferous and have revealed a large number of fantastically well preserved specimens. A team led by Jure Zalohar, a geologist at the University of Ljubljana, found remains of a creature known as a pygmy pipehorse. Just 2.5 centimeters in length, it was named Hippotropiscis frenki after Frenk Stare, a famous Slovenian geologist. A long dorsal fin on its back suggests that it was a good swimmer.

Further analysis of its morphology shows that, like its modern counterparts, Hippotropiscis swam upright. Found in what was a formerly a semi-enclosed lagoon, similar to the Jurassic Solnhofen limestones, known as the Coprolitic Horizon, it is likely that this creature lived close to the bottom in dense vegetation, filtering food from the sediment, the classic lifestyle of pygmy pipehorses. In 2009, Zalohar also found, in the Tunjice Hills, the remains of the oldest known seahorse, Hippocampus slovenicus.

Dated to 13 million years old, the new fossils are just 500,000 years younger. While genetic evidence suggests that pygmy pipehorses adopted an upright stance in the water and diverging from their ancestors some 20 million years ago, the new fossils do still allow for such a date gap. Such fossils are incredibly rare and as a result valuable. Sea horses are primarily soft-bodied, and it is likely that the Tunjice lagoon was highly anoxic which prevents against decomposition, preserving the ancient fossils in great detail.