Friday mystery object #420 answer

Last week I gave you this deep sea mystery to have a go at identifying:

It was picked up on the Porcupine Bank, which is a raised area of the Irish shelf around 200km off the west coast of Ireland, just before the drop off into the abyssal depths of the Atlantic.

Although it’s a little hard to tell from the photo, the object is around 1m long. It has a conical cavity at the base, but it’s fairly shallow, extending only about 20cm before fully closing up.

The fact this isn’t a tube allows us to rule out the possibility that this is from some unholy giant scaphopod:

Or indeed a particularly well-formed giant ship-worm, like one I’ve featured before:

You can’t tell from the photo, but it’s very dense and heavy. What you can see is that it has some quite well defined longitudinal ridges:

This isn’t something you normally find in horns, but you do find in tusks. So the question has to be, what kind of tusk is this?

I’ve talked about tusks before on this blog, and I’ve spent a lot of time identifying ivories over the years, after learning key diagnostic features from the wonderfully knowledgable Dr Sonia O’Connor, both in her training courses and working alongside her when I was at the Horniman Museum and she was visiting to do some research. This tusk reminds me of one of the more tricky ones we looked at.

The marine location suggests it could be from a Walrus. The overall shape isn’t bad, but Walrus tusks tend to be no more that about 75cm long at their longest. They also have a more squared-off section at the base and often a deeper groove on the sides partway along the length from the base, so it seems unlikely.

Really that just leaves something proboscidian – but here we hit the difficult bit. Mammoth tusks have been dredged from the sea many times, from fossils in sediments that became covered by sea level rises after the melting of glaciers around 11,500 years ago. However, Elephant tusks were transported in huge nubers to Europe by ship to supply the demands of the ivory trade between the late 18th and early 20th Century, so it is entirely possible that this is a relic of that trade (as suggested by Chris Jarvis).

In my experience, submerged Mammoth tusks are seldom in such good condition as this. While there is some degration and flaking towards the tip, there is much less of the deep staining or separation of dentine fibrils that I would normally expect from Mammoth tusk submerged for several thousand years.

However, if it was more deeply buried until recently it may have avoided the worst of that degradation, so that expectation isn’t good enough.

There is a method for distinguishing between Mammoth and Elephant ivory, that relies on an artefact of the tooth development process. This involves measuring the intersection angle of Schreger lines (an optical feature resulting from light interacting with dentine tubules) in a polished section of the tusk:

In Elephants the angle of intersection tends to be obtuse (>90o), whereas in Mammoths they are more acute (<90o). Of course, to see this would require cutting a section of tusk, so it may have to remain a mystery until the desalination treatment has been completed and I can see if there is an opportunity to check any broken surfaces or prepare a small sample.

The question in this case has to be, how important is it to know the identity, compared to the importance of keeping the specimen as intact as possible? That is a bigger conversation that will need to be had with my colleagues.

Thanks for your thoughts everyone!

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