|
Tuesday 12 June 2001 Lecture |
|
Firstly, an apology from the writer of this article. He says he knows nothing about fossil vertebrates and does not feel worthy to compile an account of June's guest speaker's talk. He apologizes to her for any inaccuracies or misrepresentation of her talk and to members of a more knowledgeable understanding of the subject. He pleads, "that any member, who feels up to the task, to send copy to Roger for inclusion on our website as quickly as possible." (Days, a week but not months please). A
Devonian Fish with Fingers? The Early Evolution of the Tetrapods. |
|
Doctor Jenny Clack started by defining the term tetrapod as: Anything with, or that shows evidence of having had, 4 limbs. Several slides of various examples were shown to fix in our minds the diversity of the definition. Humans to snakes! Dinosaurs to Birds! One common key feature, and clue to their past origin, was that all tetrapods have their limbs connected to the body by a single bone. For example the humorous (upper arm bone) in humans. When did all this start? Jenny took us back in time to the Late Devonian period and outlined the proposed evolutionary routes for tetrapods, starting with fish with early tetrapod features. Two examples of the single bone to body in the lobe-finned fish have survived the evolutionary route to the present unchanged. These are the coelacanths and the lungfish. However, Jenny was concentrating her talk on two other candidates for the transitional phase from fish to tetrapod. These were the Acanthostega and Ichthyostega but she said that within the last ten years a couple of other new intermediate fossil specimens had been discovered. Tantalizingly, the fossil record seems almost devoid of specimens for a 30Ma period in the early Carboniferous, until about 320Ma, when terrestrial tetrapods (did she say all with the now common five finger pattern?) seemed to have appeared and flourished. Jenny reviewed her expeditionary trips to Devonian outcrops, rich with fossil remains of Acanthostega and Ichthyostega, in Greenland. It looked remote and was! Access to camp was by light aircraft. Contact was maintained daily by Radio and supplies where flown in when needed. (It is incredible for a place so inaccessible and vast, that someone has walked over it and noticed fossil evidence. I think she said in the 1930's? This should clearly illustrate to all budding geologists the importance of keeping notes and records. Not only for yourself but for future researchers.) There where rich pickings of Acanthostega. Eight individuals came from a site half way up a mountain. She described the climb to the site as taking 4 hours from camp but as you got fitter the journey reduced to 2hrs 30mins (Daily! Plus you have to come back!). Jenny ran through some slides of the fossil preparation work on the specimens. Stunning! is the only word that springs to mind. From rocks, the untrained eye would have ignored, to detailed fossil skeletons and skulls. Truly an awesome, painstaking work of many years. The limbs of Ichthyostega where equipped with 7 digits, 4 being equal spaced and a tighter group of 3. Acanthostega had 8 digits, all seemingly of equal spacing and probably of more use in swimming. Jenny said, that nature was possibly still experimenting and had not settled on its evolutionary trail at this time. She also remarked on a 6 digit specimen tetrapod from Russia. (Is there no transitional specimens with five fingers from the Devonian?). Other features of Acanthostega included gill bars and it was concluded that it was more fish like at this stage. (I think it was described as, 1/3 fish, 2/3 tetrapod, although this comment may have been attributed to Ichthyostega?) Jenny concluded her talk by asking the question (unanswered): Where do you draw the line between fish and Tetrapods? (I left the talk thinking that we too must be an intermediate or transitional type of something to come. How will life in 300 to 400Ma time view us? An evolutionary dead end that failed! My bet is the coelacanth and lungfish will still be around, we wont. Sadly, modern man seems to have only use for one finger, typing! Our thanks to Dr Jenny Clack for a BRILLIANT TALK on our origins. To absent members; you missed another good one.)
|