Traces of fins reveal the presence of ancient seals on the South African coast
The largest and heaviest living carnivore in the world is not a big cat, a bear or a wolf: it is southern elephant seal (Mirounga leonina). Bulls of this species can be 5 meters long and weigh up to 3,500 kg. The Cape fur seal, on the other hand, can reach 3.9 meters and weigh up to 360 kg. A large African lion, by comparison, weighs 250 kg.
So you might think that the ancient ancestors of these seals and other large seals would leave plenty of evidence of their presence in the fossil record. This is partly true; there is a extensive worldwide record of body fossils for the seals. But, so far, there has been no documented evidence of fossilized seal tracks – the telltale signs of these large animals dragging their flippers while hauling themselves along ancient beaches.
Our new search, from South Africa’s Goukamma Nature Reserve on the southern coast of Cape Town, changes that. The traces we have discovered and described come from two sites dating back approximately 75,000 years.
This adds to our existing knowledge of the types of animals that roamed this landscape long ago, as well as what the landscape might have looked like – an important part of understanding what has changed over time and how it might change as the climate changes.
Many different creatures
The Cape South Coast Ichnology Project began in 2008. Ichnology is the study of tracks and traces. Since then, our team has identified over 300 vertebrate track sites along a 350 km stretch of South Africa. littoral. Traces and traces date back to the Pleistocene epochwhich began about 2.6 million years ago and ended 11,700 years ago.
Traces and traces are found in eolianites (cemented remains of dune surfaces) and cemented deposits of beaches. Fossil sites have provided a census of the creatures that have left their mark on these ancient dunes and beaches: the elephants, crocodiles, big birds – and even hominins, human ancestors. There were also tiny members of this varied community; we reported gerbil and spider tracks and traces.
The absence of seals from the world’s ichnological records can probably be explained by the fact that these mammals spend much of their time in the ocean and by a preference for rocky islands, cliff-side ledges and beaches. pebbles when they land. None of these environments would preserve fossil traces of seals.
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However, large colonies can be found on remote sandy beaches, and individual seals often haul themselves onto beaches, both of which can leave fossil traces.
Two construction sites
One of the sites we studied is located on the northern slope of a sandbar and has a furrow created by sliding down the slope, complete with trail marks from the hind flippers. There are parallel grooves and adjacent interlocking grooves, consistent with the front fin tracks. Taken together, these features more closely match the tracks of a fur seal than those of an elephant seal. Both species are present on the coastline today, but Cape Fur Seals are much more common.
Cape fur seals often congregate in dense colonies, some of which contain hundreds of thousands of seals; such a colony can be found at Cape Cross in Namibia. Seals in rookeries on beaches can create depressions in the sand that are molds of their outlines, and this is what evidence at the second site suggests. Four large prints match mold from juvenile seals, and one of them contains what appears to be a fin print.
Microscopy of thin sections taken from the rock surfaces allowed us to determine that the two sites belong to beach rather than dune environments. This helped support our interpretation of the tracks as seal tracks. Meanwhile, optically stimulated luminescence dating studies carried out at the University of Leicester indicated that seals inhabited these beaches around 75,000 years ago.
More to find?
The notion of seal tracks, made so long ago on these beaches, being evident today on the rocky surfaces can be considered remarkable. Our findings help fill what was a significant gap in the global ichnology record. Our findings will hopefully prompt researchers elsewhere to search for similar traces to better understand what parts of the ancient world seals occupied and when.