Wednesday 28 January 2015

A fossil Porpoise from the Early Pliocene of Antwerp Harbour.

Porpoises, Phocoenidae, are small Dolphins, Delphinida, found today across most of the world’s oceans, but with a fossil record restricted almost entirely to the North Pacific. Only a single fossil species from outside the Pacific Basin has been described, Septemtriocetus bosselaersi from Pliocene sediments at Verrebroek Dock in Antwerp Harbour, Belgium, with a few isolated bones tentatively assigned to the group from the Miocene of Malta and the Pliocene of Italy.

In a forthcoming paper in the journal Acta Palaeontologica Polonica available online from 14 December 2014, Wouter Colpaert of the Research UnitPalaeontology at the Universiteit Gent and Mark Bosselaers and Olivier Lambert of the Department of Terre et Histoire de la Vie at the Institut royal desSciences naturelles de Belgique, describe a second species of fossil Porpoise from Antwerp Harbour, collected from the slightly earlier Kattendijk Formation on the left bank of the River Scheldt at the entrance of the Beveren Tunnel.

The specimen was collected in several fragments by Robert Marquet, a volunteer palaeontologist at the Institut Royal des Sciences Naturelles de Belgique. It is named Brabocetus gigasei, where ‘Brabocetus’ means ‘Brabo’s Whale’, in reference to SilviusBrabo, the mythical hero who liberated the city of Antwerp from the giant Antigoon, and ‘gigasei’ honours Paul and Pierre Gigase, a father and son fossil collecting team who have for many years collected marine Mammal remains from the Antwerp area, and who donated the specimen to the Institut royal des Sciences naturelles de Belgique.

The specimen comprises a partial skull in three parts, lacking the tip of the snout as well as the the nasals, the pterygoids, and some elements of the cerebral cavity and basicranium as well as part of the left supraorbital process. The snout appears to have been more elongate than any extant Porpoise but similar to that seen in Septemtriocetus bosselaersi as well as in some other extinct Porpoises (Lomacetus, Piscolithax and probably Salumiphocaena), however unlike in these species (which are not all closely related) the snout in the two Belgian species is shorter than the cranium. However Colpaert et al. note that this analysis is based upon a reconstruction of the missing parts of the skull, and cannot be considered reliable without additional material being found.

Skull of Brabocetus gigasei,from the Pliocene of northern Belgium, in dorsal view.Photo (A) and corresponding line drawing (B). Dotted lines indicate reconstructed parts and poorly visible structures. Dark grey surfaces indicate fracture surfaces. Light grey surface indicates the original position of the lost right nasal. Scale bar is 50 mm. Colpaert et al. (2014).

A phylogenetic reconstruction of known members of the Phocoenidae suggests that while Brabocetus gigaseiis closely related to Septemtriocetus bosselaersi, both being members of an extinct group of Porpoises which also includes several Pacific species, it is not the closest relative of that species, which is more closely related to five North Pacific species than it is to the new member of the group. It is not closely related to the modern Harbour Porpoise, Phocoena phocoena, the only modern Porpoise found in the North Sea.

Consensus tree of the 27 most parsimonious trees, showing the phylogenetic relationships of Brabocetus gigasei with other extinct and extant Phocoenids. † indicates extinct taxon. IO, Indian Ocean; NA, North Atlantic; NP, North Pacific; NS, North Sea; SA, South Atlantic; SO, southern oceans; SP, South Pacific; 1, 1', and 1'' correspond to a first trans-Arctic migration scenario with three successive dispersals to the North Sea. 2, 2', and 2'' correspond to a second scenario with two dispersals to the North Sea (2, 2'') and a dispersal back to the North Pacific (2'). Colpaert et al. (2014).

This leads to two possible evolutionary scenarios, each of which are considered equally likely. Either Brabocetus gigasei and Septemtriocetus bosselaersi (or close ancestors of these species) migrated from the North Pacific to the North Sea via the Arctic Ocean separately, or the common ancestor of these two species made a similar migration, with the common ancestor of the more derived members of the group subsequently migrating back to the North Pacific.

The modern Phocoena phocoena is thought to have reached the North Sea much later, part of a recent global expansion which saw members of the group migrate from the North Pacific across the world’s oceans, and which coincides with a similar global expansion of other Dolphin groups.

See also… fossil Dolphin from the Late Miocene of Hokkaido, Japan, which may be the oldest True Dolphin.                                                              Dolphins, Delphinidae, are the most diverse group of Whales (or indeed any form of Marine Mammals) alive today, with 36 described species including charismatic animals such as Bottlenose Dolphins and Killer Whales. Historically a large number of fossils have also been...

The Squalodelphinidae are a small group of small to medium-sized Toothed Whales known from the Miocene of Europe and North and South America. They are thought to be related to the modern Asian River Dolphin, Platanista gangetica, which lacks any close living relatives. The group is not well understood, with most described specimens being fragmentary in nature.
Porpoises (Phocoenidae) are small Whales, related to Dolphins (Delphinidae). They tend to have shorter snouts than Dolphins, with flattened, spade-shaped teeth, as opposed to the conical, pointed teeth of Dolphins. They are among the smallest and shortest lived Whales, ranging from 1.2 to 2.3 m in length and typically reaching sexual maturity at about eight...

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