ENHS Nature Trails October 2005 Article 3

October's Amazing Grace
By Reida Kimmel

I've had a lot to say about our weekend adventures in England last winter and spring, but I have never said anything about the real reasons for Chuck's half-year stay in Cambridge. He wanted to learn more about the ancestors of modern vertebrates and to become familiar with new methods of measuring and analyzing changes in morphology. He studies development, specifically the development of the cranial and facial bones of zebrafish. Many of the genes that control normal or abnormal development in these tiny fish are present in other vertebrates, even humans. The fact that they are so highly conserved means that similar genes should have even been present in early vertebrates, controlling the patterning of skulls and teeth, and even involved in the transition from the jawless state of the earliest vertebrates to the primitive-jawed fish that swam the seas over a third of a billion years ago. Of course it is impossible to hunt for genes in lumps of mineralized bone. But one can learn a lot about the evolution of bones in the face from studying a succession of fossils of different species representing changes in morphology over a long period of time. In the end, Chuck decided to study the skulls, specifically the bones of the palate, not of fish, but of very early tetrapods. He was looking at the ancestors of modern amphibians and the first amniotes. There are many questions concerning which fossil species can be declared to be the likely ancestors of frogs and salamanders. As for amniotes, only by studying anatomical features other than their eggs, which have never been found, can a scientist claim that another species is probably capable of living and reproducing exclusively on dry land. The Zoology Department at Cambridge University has a huge collection of fossils, a wonderful library, and a talented and very helpful faculty. Chuck's mentor, Dr. Jenny Clack, immediately presented him with 'Grace', a three hundred and seventy million year old Acanthostega. This fiercely toothed, grapefruit sized skull belongs to a species which is the earliest tetrapod animal known to science. However, the fact that Grace had four limbs does not mean that 'she' walked on land. The earliest tetrapods were water dwellers, using their limbs to move about and hunt or forage along the bottom of their shallow seas. It makes sense; there really was not much to eat on land yet. Another interesting property of Acanthostegans is that these primitive creatures had eight, not five digits on each limb. Scientists had always thought that five digits was the ancestral condition for tetrapods, but no, that genetic change came later. Chuck learned a lot about fossil cranial anatomy from Grace, but he was not allowed to meddle or chip away at this beautiful ancient specimen. When a fossil is prepared by traditional methods, there is always some loss of material. For instance, a fish fossil must have the operculum or gill covering removed if one is to see the bones of the jaws and inside the skull. To have an even clearer view of the interior of the brain case, preparators must remove other layers of structure or section the fossil with diamond tipped grinding machines for microscopic study. This has always been the way to study fossils. To really learn certain things, the student must destroy the fossil.

When it was still very cold and wintery, we went to Uppsala, Sweden, so Chuck could talk to a young paleontologist about his ideas for bone studies. There to our amazement, playing on a computer screen, was a three dimensional model of the skull of one of Grace's primitive tetrapod relatives, showing all the internal anatomy, the bones surrounding the brain case and the roof of the mouth. This was no imaginative creation of a computer graphics artist, but a demonstration of a new and wonderful technology that is revolutionizing the study of fossils. For about eight years, paleontologist Tim Rowe at the University of Texas has been using a very powerful CT scanner, shielded by a fifteen ton lead box to bombard fossils with lethal amounts of radiation for about six hours per specimen. The digital result can be measured, rotated, assembled in various three dimensional views, and studied in incredible clarity. Among the specimens that Rowe has scanned are the skull of an Archeopterix and the skull and braincase of the oldest and most primitive dinosaur, Herrerasaurus ischigualastensis. Scientists from all over the world, like our Swedish friend, Per Ahlberg, send their fossils to Rowe's lab, where they are CT scanned and their mysteries revealed without any harm to the fossil. Now, perhaps, someday someone will find the egg of the first amniote and scan it to reveal the secrets of its embryology. As for Chuck, he contented himself with having amazing Grace smiling at him on his desk, and he did his research using new computer programs and old published drawings of forty species of early tetrapods. He discovered some interesting things. Ask him about it. Meanwhile, see the wonderful new CT scan technology for yourselves. Play the movies.

Reida Kimmel

http://www.ctlab.geo.utexas.edu/bio/bio/herrbrain.html

http://digimorph.org/publications.phtml

Editor's Note: Just Google "CT lab UT" and the first link takes you right in to the lab. This website too might be saved for that rainy day--there's so much to see and learn. How exciting for our scientists. Thanks for the insight Reida.

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