A fossilized partial rostrum of a shark with a large node was found from the Pleistocene Ichijyuku formation, Chiba prefecture, Japan. Two modern species, Lamna ditropis Hubbs & Follett, 1947and L. nasus (Bonnaterre, 1788) are known to have such a rostrum. I examined a rostrum of a modern Lamna ditropis and found that the fossilized partial rostrum is that of Lamna ditropis. Lamna nasus is known to bear the rostrum with wider angle formed by lateral rostral cartilages¹⁾ than those of L. ditropis. The shark which yielded the Ichijyuku specimen is estimated to be about 200 cm total length (sex unknown) because the specimen is slightly smaller than that of the modern Lamna ditropis of 230 cm total length.

left: a partial rostrum from the Ichijyuku Formation, right: rostrum of Recent Lamna ditropis of 230 cm TL.
Terminology: RN - rostral node, MR - medial rostral cartilage, LR - lateral rostral cartilages.

Neurocranium of 22.5x12.5x9.0 cm in size and Jaw of Recent Lamna ditropis, a female of 230 cm TL.

1: Frederik H. MOLLEN, PARTIAL ROSTRUM OF THE PORBEAGLE SHARK LAMNA NASUS(LAMNIFORMES, LAMNIDAE) FROM THE MIOCENE OF THE NORTHSEA BASIN AND THE TAXONOMIC IMPORTANCE OF ROSTRAL MORPHOLOGY IN EXTINCT SHARKS. GEOLOGICA BELGICA (2010) 13/1-2: 61-76 A

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