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When is a tooth not a tooth?

When is a tooth not a tooth?

Wed, 13 Jul 2016

In the recently opened Kōrero Mai, Kōrero Atu exhibition, two pieces of New Zealand-made Victorian jewellery are on display. One intriguing piece is a 65mm brooch described as being made from a tooth of the endemic Hooker’s sea lion (Phocarctos hookeri). As I am currently researching sea lion fossils my interest was immediately piqued.

Yes, it does look a bit like a large canine tooth and yes, sea lions have big canines. But some of its features were surprising. It immediately looked more geological than biological. After some internet searching, consultation with colleagues and access to sea lion material in the Land Vertebrate Collections I think we may now have a different provenance.

The "tooth"

“Hooker’s sea lion tooth” brooch.

Auckland War Memorial Museum Tāmaki Paenga Hira. 2008.4.2.

Firstly why does it look more geological than biological? Teeth are incredibly strong, canine teeth in particular. If you section a canine tooth down its length you will find it is composed of vertical conical layers of dentine. The brooch however has conspicuous horizontal banding, seen here as layering across the "tooth", but it shows no evidence of conical layering or vertical striations.

In other words, if the brooch was a tooth it should have signs of structure along the tooth, not across the tooth. Something of this shape with horizontal banding would not be very strong and would snap easily. Not what you want if your survival depends on catching your food or if you are fighting to secure your harem!

A sea lion tooth

Hooker's sea lion canine tooth.

Image: Jason Froggatt.Auckland War Memorial Museum Tāmaki Paenga Hira. LM146.

If you have ever seen sea lions close up they are magnificent animals and, in common with other members of the Carnivora (cats, bears, dogs), their biggest and most impressive teeth are canines!

The last 20mm of the canine tooth above the gum is distinctly tapered from a generally cylindrical base or root. Also the tooth is really only curved in the last 20mm, quite different to the shape of the brooch which is boomerang-shaped. You can see that the base of the tooth is hollow and has distinct lateral striations, again unlike the brooch. Yes it does have some horizontal striations near the base but these only mark the base of each of the conical layers of dentine which give the tooth its strength.

New Zealand Hooker’s sea lion skull and upper canine teeth. The Hooker's sea lion preys on squid, fish, some sharks and rays, octopus, and other invertebrates. They also eat penguins and various other sea birds.

Image: Jason Froggatt.Auckland War Memorial Museum Tāmaki Paenga Hira. LM146.

If the brooch is rock or mineral, what could it be?

Without being able to do any invasive tests until after the exhibition we have only visual evidence. If the rock is soft it could be a piece of layered limestone (possibly cave formation). If the rock is hard it could be a quartz-rich rock such as chalcedony, banded agate or a vein-fill rock type. Both can be polished, although the latter won’t scratch easily. The mottled brown colour is probably due to iron oxides and it is possible that the brooch was originally a polished river or beach-worn pebble before becoming a decorative piece.

What do you think?

You can visit Kōrero Mai, Kōrero Atu to see the brooch on display, or view the record in Collections Online. After the exhibition we will find out what it is really made of!

  • Post by: Hugh Grenfell

    Hugh is an Auckland-based geologist with a research, teaching and field geology background. His firm belief is that the best geologists are those who have seen the most rocks. Hence his current work on the Museum collections is a good fit!