How the squid got its ink sac

Nautiloid+reconstruction_medium

An intriguing new fossil has been donated to the Museum.  It’s a large straight-shelled nautiloid with a colourful history.

Nautiloids are marine cephalopods related to modern-day octopus, cuttlefish and squid. They have an external shell and horny jaws but the soft parts of nautiloids are rarely preserved as fossils. Comparison with the living Nautilus suggests that this animal also had a ring of tentacles surrounding the mouth.

The donated nautiloid fossil in its original location in the family home

The donated nautiloid fossil in its original location in the family home

This particular specimen was handed down through five generations of one family, before finally being donated to the Museum by Mrs  Jan O’Leary via her children, Tim O’Leary and Kate Whittingham.

Accompanying the nautiloid was a lithograph of the specimen, labelled:

ACTINOCERAS.  (MOUNTAIN LIMESTONE.)  Newbury Marble Works – The Peak, Derbyshire. SECTION IN THE MUSEUM, OSMASTON MANOR

“Mountain Limestone” is an old-fashioned term for Carboniferous Limestone, which means the fossil was from the Lower Carboniferous Period, around 350 million years ago. Osmaston Manor was the historic home of the donor family until its demolition in 1965.

On the far right hand side of the donated lithograph is a reconstruction of what the nautiloid may have looked like in life (shown at the top of this post). This is very unlike modern reconstructions of nautiloids, and looks rather more like the reconstruction of a squid-like belemnoid (complete with ink sac) in William Buckland’s Geology and Mineralogy (1836). So did nautiloids have ink sacs like belemnoids and modern cephalopods? What was known at the time the reconstruction was drawn, and what do we know today?

Belemnite+reconstruction+Buckland_medium

Looking at dates of construction and demolition of Osmaston Manor, as well as the operation dates for the company credited with producing the lithograph, we can assume that it was created sometime between 1867 and 1888. So what information on fossil cephalopods would have been available at that time? Buckland provided proof that belemnoids had ink sacs, but, information on nautiloid soft parts at that time was practically non-existant. We can only suppose that the reconstruction is an extrapolation from the belemnoid evidence. Given that this specimen was living in the Carboniferous Period (359 – 299 million years ago), and the belemnoids described were from the Jurassic Period  (199 – 146 million years ago), this is quite a leap of faith.

And the state of knowledge today? Phylogenetic studies suggest that ammonoids and coleoids (belemnoids and their living relatives) split from from the nautiloids in the Silurian Period (443 – 416 million years ago), then coleoids split from ammonoids in the Devonian Period (416 – 359 million years ago). But where on this branching tree did ink sacs arise?

A paper by Doguzhaeva and colleagues (2003) described an ink sac in the Upper Carboniferous coleoid Donovaniconus, which means that ink sacs had indeed evolved as far back as the Carboniferous. But, despite intriguing historical reconstructions and beautiful fossils, we are yet to find any evidence that nautiloids really did have ink sacs at all.

Eliza Howlett, Collections Manager, Earth

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