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The Conversation

  • Written by The Conversation
imageThere is an abundance of baby fur seals on the west coast of South Africa, but true seals, which inhabited the area five million years ago, no longer do so.Supplied

Five million years ago, the southwestern tip of Africa was teaming with animals such as the African bear (Agriotherium), the giant wolverine (Plesiogulo), the short-neck long-horned giraffe (Sivatherium), ancestral forms of the elephant and mammoth as well as modern elephants and various raptors, seabirds and ground birds.

Today, many of the species that roamed inland from Saldanha Bay on the southwest coast of South Africa are extinct. There are some exceptions, including the earless, also known as true seal (phocid) seals. They flourish in great numbers in other parts of the world, but they no longer inhabit the west coast of South Africa – though they do appear from time to time as vagrants visiting from the Antarctic.

One of the major influences on survival was climate and sea level changes over time. The disappearance of the phocid seal, also known as the true seal, from the area can, most likely, be linked to decreases in sea levels that resulted in the loss of islands along the coast during the Pleistocene, about 2.6 million to less than 100,000 years ago. This reduced haul-out sites and removed breeding areas.

Studying the conditions under which phocid seals lived and why they disappeared off the South African coast is of academic interest, but it also offers an opportunity to better understand the effect of modern-day climate change on various species.

There are 19 species and 16 subspecies of phocid seals in the northern and southern hemispheres. These include the monk seal, which lives in Aegean and Ionian Seas (Greece and Turkey), northwest Africa, the Canary Islands and northwestern Hawaiian Islands, and the Baikal seal – the only fresh water seal which lives in Russia’s Baikal lake. This type of seal lived off South Africa’s coast five million years ago.

Today, the southern African coast is dominated by the South African fur seal, an otariid or eared seal. Fur seals have a visible ear flap, are mobile on land because they can turn their hindlimbs/flippers forward, and use their front flippers for swimming.

The phocid seal has no visible ear flaps, cannot move around on land because they can’t turn their hindlimbs/flippers forward, and use their back flippers to swim.

New light has been shed on the relationship between these seals through shape and phylogenetic analyses of the six fairly complete fossil seal skulls from Langebaanweg, and why phocid seals vanished from South African shores.

Rich fossil territory

South Africa’s west coast is renowned for its cold ocean, as the home of great white sharks, Seal Island and migrating humpback and right whales. It is also home to the world renowned Mio-Pliocene palaeontological site, Langebaanweg, which is now part of the West Coast Fossil Park.

Langebaanweg, just over 100km north of Cape Town, is a prolific source of well-preserved Mio-Pliocene fossils which were exposed during phosphate mining. It was considered a prime locality for palaeoecological studies because it has yielded a rich early Pliocene animal and bird assemblage as well as fossil pollen information during a period of change from moderate to more cooler climates.

Studies of the fossils have provided a detailed understanding of the terrestrial fauna and flora along the west coast of South Africa.

Shedding new light

Previous studies identified a single phocid seal, Homiphoca capensis, which lived on South Africa’s coast during the early Pliocene some five million years ago. It was considered to be intermediate between the monk and Antarctic phocid seals (excluding elephant seal) but a later study suggested that it was more closely related to the crabeater seal.

This latest study was conducted using 3D landmark morphometrics. This process identifies landmarks on the fossil and the distances between them are measured. Once these are calculated it is possible to see the shape of the skull in relation to other seal skulls.

Our landmark morphometric analysis showed that the shape variation in the fossil seal skulls is greater than any single extant seal species. It also suggested that three skulls formed an ontogenetic, or growth, series. The anatomical and quantitative analyses suggested that there were two or three seal species present at Langebaanweg.

Analysis of the seal relationships showed that the Langebaanweg seals are grouped together with the Antarctic phocid seals. They are closely related to the Antartic’s Ross seal.

A second weighted analysis produced a grouping which included the Langebaanweg seals, Antarctic seals and extinct South American phocid seals – but these relationships are not well supported in the analysis. The Langebaanweg skulls that grouped together in the morphometrics may represent different species and not a growth series.

In the current analysis, the exact nature of the Langebaanweg seals relationships was not resolved though the phylogenetic results support presence of at least two seal species at Langebaanweg.

When Table Mountain was an island

How did two, maybe three, seal species come to live off South Africa’s west coast? Five million years ago an archipelago existed along the coast at Saldanha while high points of the peninsula including Table Mountain formed islands off the coast as high sea levels flooded the Cape Flats and connected the bays of St Helena and Saldanha Bays.

Phocid seals lifestyle is independent of land suggesting that island hopping was a possible way that South Africa’s west coast was colonised. The nutrient rich Benguela Upwelling System was established off the coast by this time resulting in an increased food supply. Seals would have used the Antarctic circumpolar current and the South Atlantic current to colonise islands in the Southern Ocean, then colonise islands off South Africa’s coast.

These islands were surrounded by shallow water making them ideal haul out and breeding sites. This would have allowed speciation of the seals through segregation and different feeding strategies allowed them to co-exist.

During the Pleistocene the sea levels decreased, resulting in the loss of islands along the coast. This potentially reduced haul-out sites as well as breeding areas. It would have resulted in the extinction of these seals.

Today we are faced with climate changes that not only affect the land but will greatly influence the oceans. An aim of these studies is to not only understand the conditions under which seals lived five million years ago, but how this affects our understanding of modern-day climate change.

Romala Govender Curator of Cenozoic Palaeontology in the National History Department Iziko South African Museum. My postdoc where this work began was funded by the Claude Leon Foundation.

Authors: The Conversation

Read more http://theconversation.com/unlocking-the-mystery-of-how-true-seals-disappeared-from-the-cape-44344

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