Embryos in Deep Time: The Rock Record of Biological Development

4. Bones and Teeth under the Microscope

66 As unlikely as it may seem, the most important piece of equipment for most paleontologists, besides the hammer, is the microscope . A large proportion of people studying extinct biodiversity work for the oil industry, examining the very small pollen of fossil plants or extinct foraminifera, the latter members of a group of single-celled organisms important for stratigraphic correlation between geologic sections. For paleontologists, the microscope enables also the study of the tissue microstructure of fossils, in particular the bone, which has become an important matter of investigation concerning development in extinct taxa. The study of living tissues, or histology, is a vast field, and much of it is concerned with the identification of pathologies, an important diagnostic procedure in studies of cancer for example . Comparative studies of tissues of different organisms have been a subject of investigation for centuries, made possible by technological advances in performing thin sections of delicate and often small complex structures of different consistencies and shapes. Pioneering work on tissues at a microscopic level foUr Bones and Teeth under the Microscope Bones and Teeth under the Microscope / 67 goes back to the Swiss Wilhelm His (1831–1904), a native of Basel. His invented the microtome, a mechanical device consisting of a cleverly disposed blade used to slice thin tissue sections for microscopic examination. With this new technological development , he was able to trace the embryonic origins of different types of animal tissue. As far as I know, His did not study the microstructure of fossil bones; rather, he was concerned with embryos and soft tissues, for instance, making discoveries that led him to coin the term dendrites, the conducting projections of nerve cells. Histological sections using basically the same method that His used some 150 years ago are currently used in the study of embryos of living species that, by virtue of their size, are difficult to study by dissection.1 From quite early on, paleontologists realized that thin sections could reveal important anatomical details of fossils. The British specialist in fossil fish, William Johnson Sollas (1849– 1936), a professor of geology at Oxford, and his daughter, Igerna, pioneered this study when in 1903 they published an account on a Devonian fish of uncertain affinity. Their technical breakthrough consisted of sequentially grinding through a specimen, making drawings of the revealed sections, and translating these into a wax model, larger than the original. Sollas had initially made models of fossil brittlestars (ophiuroids) and graptolites, a group of extinct invertebrates related to acorn worms, which were exhibited at the British Association in 1901.2 Sollas made models of many fossils, but few appear to have survived Sollas’s method was later masterfully used by a school of Swedish comparative anatomists. But wax models are extremely time-consuming to produce. For example, a famous wax model of the head of an important animal for the fish/land vertebrate transition Eusthenopteron took fifteen years of two technicians’ time to 68 / Bones and Teeth under the Microscope produce. A digital camera and the right hardware and software can now do the same job in days. This is technical progress on a grand scale. The next major technological advance was presented in a 1970 paper by the Frenchmen Cécile Poplin and Armand J. de Ricqlès, who invented microtome slicing for fossils. They were searching for a technique to slice fossils into sections that could be mounted onto glass, as done with standard histological methods. This was no trivial challenge, as in fossils the hard tissue is porous, breakable, brittle, more heterogeneous in composition, and less resilient to strain than extant bone. With the impregnation of resin under vacuum and pressure into the fossil bone, each slice becomes more stable. The chemistry of fossil bone is also different from that of extant taxa. The resulting sections preserve the outlines of the different structures in the fossil. Hence, even if the chemistry has changed or the original components have been replaced, the boundaries between them are preserved. This microtome technique was the first step toward replacing the serial grinding method, which sacrifices the original fossil. The newest technique for visualizing the internal structure of a fossil does not require sectioning. With powerful highresolution computer tomography, it is possible to study details of microanatomy without even cutting a fossil. informative fossil Bone seCtions The reason the histology of fossil bone is rich in information is that bone is a...