Saturday 14 January 2012

Fossil beaked whales from the seafloor of the Southern Indian Ocean.

Palaeontology is usually a dryland affair, with palaeontologists meticulously recording the horizons in which they find their specimens. The only palaeontologists who normally venture on the seas are micropalaeontologists studying tiny fossils in core samples drilled from the deeps. Sometimes though, large fossils are recovered from the sea, by dredgers or fishing trawlers (fossils recovered in this way have to be fairly robust). In the English Channel Mammoth tusks are occasionally brought up by dredgers collecting aggregates for industry, and in South Africa a number of partial whale skeletons have been recovered by fishing trawlers.

In a forthcoming paper in the journal Acta Palaeontologica Polonica, that has been available online in a draft form since 15 December 2011, Pavel Gol'din of the Taurida National University in Simferopol, Ukraine, and K Vishnyakova of the Southern Scientific Research Institute of Marine Fisheries and Oceanography, also in Simferopol, describe the partial skulls of two beaked whales trawled from the Banzare Bank in the Sub-Antarctic Indian Ocean in 1976, that have been in the Museum of the Southern Scientific Research Institute of Marine Fisheries and Oceanography ever since.

One of the whale skulls from the Banzare Bank. From Gol'din and Vishnyakova (2011).

The specimens are similar to earlier fossils recovered off the coast of South Africa, and described as Africanacetus ceratopsis, though slightly larger, and with a few minor differences. Modern beaked whales are known to be sexually diamorphic (the sexes do not look the same), and the differences between the new specimens and A. ceratopsis are within the range of sexual variation in modern beaked whales, for which reason Gol'din and Vishnyakova conclude that the new specimens are either the same species as A. ceratopsis but a different sex (probably male, with the previously known specimens being female), or a second species within the same genus, for reason which they classify the new specimens as Africanacetus sp., without making a judgement on the species of the new specimens.

It would be easier to establish the relationship between the new specimens and Africanacetus ceratopsis if it could be established to have lived at the same time, or if one was known to have lived earlier than the other. However while some of the South African specimens have approximate dates attached to them, no date can even be guessed at for the Banzare Bank specimens, given the method by which they were collected and the time since this happened.

The Banzare Bank is in the Sub-Antarctic Indian Ocean, thousands of miles to the east of South Africa, and hundreds of miles to the south. This makes the new specimens the most southerly beaked whale fossils found to date. It also has implications for our understanding of the distribution of beaked whales. Modern beaked whales have a roughly circumpolar distribution, with the greatest species diversity apparently in the South Temperate Zone, though many species are not well understood due to their deep sea habitat and low abundance. The presence of Africanacetus in the Sub-Antarctic Indian Ocean suggests that the same pattern of distribution may have been true for these ancient whales.

Map showing the known distribution of Africanacetus (1) and the Banzare Bank (2). From Gol'din and Vishnyakova (2011).