Does anyone know this strange fossil?

A friend of mine asked me if I knew this fossil. She got it from her uncle who keeps saying it is very interesting, although he does not have the slightest idea what it might be. Neither do I. The piece is about 35 cm long and very heavy. It looks somewhat like a horn to me, but that is all I could come up with to date. Does anyone know what this is?

Side view of the piece.

Top view.

Detail.

Photographs: Jan Bosselaers

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