MUSEUM OF NATURE

Press, Volume CVI, Issue 31256, 31 December 1966, Page 16

MUSEUM OF NATURE

Crabs In Stones

TJAVE you ever thought as you break open a stone that you are the first to have ever seen inside it? If stone breaking is your hobby or your trade this thought may make you wield your hammer with some reverence. Should the stone be of sedimentary’ rock, originally silt or sand laid down on the sea floor, there may be a prize waiting for you within.

Perhaps lying in the stone, about to be released, are the remains of an animal which lived on that sea floor some millions of years ago.

Some of the most interesting fossils that have been found in Canterbury over the last few years are fossil crabs. The North Canterbury coast north of the Waipara river is formed of soft bluegrey siltstone. Some layers within this siltstone are much harder having been cemented by calcium carbonate. These lime-cemented layers are called concretions, and as the siltstone cliffs are eroded by the sea, the hard concretionary layers break up and form boulders on the beach.

A very few of these concretionary boulders contain beautifully preserved fossil crabs. Sometimes parts of these can be seen on the outer surface but often they are revealed only by splitting the boulder. Crabs are comparatively rare as fossils and some specimens from North Canterbury, which are outstanding by world standards,

are on display in the Von Haast Hall of Geology at the museum.

A crab grows many shells during its life; each shell is shed before a period of growth and replaced by a larger shell. Thus a fossil crab may be only moulted shell and an individual crab may have produced a number of fossil crabs.

A large crab some 15 million years old is found in miocene siltstone. Its smooth rounded body may be six inches across and the male may have an enormous right nipper. A specimen from Taranaki, at the museum, has a nipper eight inches across In 1960, Professor Martin Glaessner, of the University of Adelaide, a world authority on fossil crabs, named this species Tumidocarcinus giganteus.

The most exciting recent discovery is a five million year old crab found in the younger Pliocene rocks. This has a spiny body up to five inches across and had not previously been found living or fossil. It is, however, very similar to a living crab

Trichopeltearion fantasiticum, first found in the stomach of a dog fish caught near Wellington in 1949.

Dr. R. K. Dell, newly-ap-pointed director of the Domin ion Museum, Wellington, is the expert on these crabs and is at present studying the Canterbury ' Museum’s collection of fossil Trichopeltarions. He has decided that they belong to a new species and is writing a paper describing them that will be of great interest to those studying the ancestry of our modern crabs. Thus are advances made in scientific knowledge. Members of the public are collecting increasing numbers of these fossils and some of these may be of great scientific value. The museum would gladly give them a home to ensure their safe-keeping, to make them available for study by scientists, and to display them to the public.—D.R.G.

The picture shows a fossil crab, Tumidocarcinus giganteus, 15 million years old, collected by Mr C. B. Ricketts from the North Canterbury coast.