Tuesday, 9 April 2002

Jurassic Sharks
by
Dr Chris Duffin

                         Notes: RC

A welcome return visit from Dr Chris Duffin who on a previous occasion, about 2 years ago, gave this Society a very entertaining talk on the historical origins of folklore surrounding fossils, minerals and rocks.

On this occasion, Chris began by defining the basic shark morphology and the evolutionary trends starting from Devonian times to the present. He paid particular attention to the development of the fins and the changes in the position of the mouth in relation to the nose, over this period of time. Chris said that the bone structure of the shark family is formed from cartilage. This made for a more flexible body form giving improved locomotive ability, which helps explain the shark's success as a predator. The down side, was that fossilization of cartilage dose not take place readily due to its high organic content.

However, the fossil record has an abundance of shark teeth. Chris described a typical shark's mouth by getting us to imagine the upper and lower jaws being divided into left and right sections. Each quarter containing 20 teeth, giving some 80 teeth in use. (One species of shark have up to 70 per jaw section making 280, what a mouthful!) These teeth would be shed and replaced every eight days. (A specimen of a sharks mouth was circulated to demonstrate the inclination and rows of immature teeth in waiting.) With the average shark's life being about 40 years Chris asked us to estimate the number of teeth per shark per life time. (LOTS!). Additionally, these teeth are formed of the mineral Apatite and are very resistant to erosion, so their preservation should come as no surprise.

Chris said that, given the wide (global) distribution of sharks, the teeth could make a good candidate for use as a 'zone marker fossil' (a species of fossil restricted to a time horizon in the geological past) and that this idea was being investigated. (The approach requires the correct identification of one species of shark based on fossil teeth of that species, and connected to a particular time in the geological past. Not easy! RC).

We were shown photographs of electron microscope studies of the teeth. These revealed a three layer structure which looked like a vertical intertwined matrix at the base, followed by a horizontal layer of fibre, toped by a final layer of a parallel vertical matrix. All this to absorb the shock of biting into prey!

Chris continued with some wonderful slides of soft body fossilization of sharks. Most were of the fossil cartilage skeleton. Interestingly, all complete specimens showed a disarticulation of the vertical column, about two thirds along its length from the head. Chris said that this was caused by the eruption of decomposition gases (from the stomach) sometime after death.

In some of the slides of complete specimens there were black areas. These were attributed to features such as fossilization of stomach contents. In one specimen this was followed be a smaller white area with what looked like a spiral winding along its length. This was said to be the small intestine. Another specimen showed dark smudges near the eye orbits in the scull, and were said to represent fossilized eyeballs. Chris delighted in explaining these smudges, as being consistent with the eyeball being released from its socket and dropping forward as the muscles relaxed, post rigor mortis, some ten hours after death.

The lecture was concluded with a description of a fossil shark that was more eel like! All the fin characteristics of the shark family were present, mostly as vestigial features, except for the anal fin, which now extended almost the full length of the underbody. Chris postulated that this specimen may have had characteristics similar to electric eels in respect of locomotion, feeding habits, and defence mechanisms.

This talk was obviously given by a man who knows and loves his subject. His expertise and enthusiasm simply oozed out. The lecture itself was highly technical at times (for me but not our shark experts) but there was something for everyone. I certainly learnt a lot and enjoyed every minute. (RC 10 April 2002)

I have been fishing with the NET for further reference: If you, like me, know little about sharks give this site a try, Buena Vista Museum of Natural History. All about sharks in a very readable form. Also via Buena Vista (but repeated here to save you experts time) is http://www.elasmo.com with plenty of teeth (but sadly not British fauna).

.

Member's Recent Finds on Display

Finds from Field Trip to Wickelsham and Baulking Pits. Many thanks to all those members who were considerate enough to bring in their finds for us to view.

Something Different!

                                                                                                                       Top View
                                                                                                            About 3 to 3.5cm in length

Phyllodus toliapicus     Agassiz

Unit C, Orwell Member,
Harwich Formation, Eocene,
Harkstead Foreshore, Suffolk TM18773353
24th June 1998

This specimen was brought in by Bob Blackburn. Having seen a few Phyllodus plates over the years, (and never finding one myself) what caught my attention was the location. Bob said that
he found this in the gravel as he was leaving the foreshore at the end of their visit. It's nice to know that some of us mere mortals have better luck than our teeth experts. However, Martin Rayner also brought in his recent Phyllodus plate from Beltinge (30 March 2002, I was there and so were a few other club members, lovely warm sunny day and a nice low tide). This was indeed a good specimen of note and also on view at the meeting.                                                                                                        

                                                                                                                  Bottom View

I find this view quite interesting as it looks like a reverse mould of the top view. Does this mean that what was underneath, was a new complete tooth plate and that the plates are shed throughout the life cycle of this fish? (But not every eight days like the shark!)

Martin & David Rayner brought in a fabulous collection of shark and fish teeth from Beltinge. Their display complemented the evening's lecture wonderfully well. (I did not take any pictures for two reasons, (a) I couldn't get near enough because of the crowds. (b) The Rayners have a second book in preparation and it will be worth the wait to see better quality pictures with their words to the "music"!

(RC 12 April 2002)