A specimen of Psammolepis in the Gauja Formation (Estonia)

The Devonian Period is sometimes called „The Age of the Fishes“. It is the period when fish and agnathan diversity exploded and many modern fish families developed. Studies on Devonian agnathans and fishes represent an important part of evolutionary research telling us how physiology and morphology of aquatic animals changed in changing environment. The evolution of Devonian fishes and agnathans was related to the evolution of first tetrapods in mid-Devonian when vertebrates moved to the terrestrial environment. In Estonia, the Devonian System has long time been a subject of studies. One of the first to describe fossil fishes in Estonia was Hermann Martin Asmuss (1812-1859). Thanks to his excavations of fishes in the Aruküla Caves, the Estonian fish fossils became known around the world. Thanks to extensive studies by Elga Mark-Kurik, the collection of Devonian agnathans and fishes in Estonia has grown considerably and is today one of the best in the world. The Devonian of Estonia is rich in psammosteids. According to E. Mark-Kurik, they were likely the dominant species during the Middle Devonian (Mark-Kurik, 2007). Thanks to this, psammosteids can be used in stratigraphy as they are quite common in the Devonian outcrops of the Baltic area. Fish fossils are one of the few sources of biostratigraphic information that can be used in the Devonian stratigraphy of Estonia. Fish species are also known to have large distribution areas. They can be used for correlation of the Estonian sections to the global stratigraphic standard and support comparisons with other regions, e. g. Scotland. Fish fossils can also be used in palaeoenvironmental and palaeoecological studies. All the same is valid for agnathans. Material for the present study was collected in the Kalmetumägi locality. The recovered specimen was particularly hard to prepare because of extensive fragmentation. Fragmented material, however, contained pieces of a shield with fragmentarily preserved sensory system, thus making the material potentially very rare and important. Our sense of hearing is the most familiar example of a sensory system that is sensitive to changes in pressure when sound is transmitted through air. The lateral line system in Devonian agnathans and fishes could provide similar type of information. It was sensitive to changes of pressure transmitted through water. The lateral line system allows the animal to determine direction and rate of water movement. The animal could then gain a sense of its own movement, on the activities of nearby predators or prey and even the displacement of non-living objects in water. More complex canal systems are indicative of more turbulent and dynamic living environment which required better movement control for maintaining a stable position (Elliott & Mark-Kurik, 2005). Furthermore, the electroreceptors of sharks are also derived from the lateral line system. The lateral line system is of vital importance within the sensory system of aquatic vertebrates but is different in different taxa. It may slightly vary also between the individuals of same species, but also due to the age of the animal, but can still be used as a species character in taxonomy. This study is positioned on the border between geology and biology. It implies basic understanding of fish anatomy, physiology, palaeoecology and sedimentology, in order to describe a vertebrate fossils and differentiate between the anatomical and post-mortem features. Uncovering and identifying a new fossil is a challenge and opportunity to acquire various experiences and valuable information. Various publications in English, Russian and German were used to get more information about the subject. The database of the Estonian geological collections was used to learn about other fossils from the same area. The Latvian Museum of Natural History and the Geological Institute of Tallinn University of Technology were visited to study fossils from the same area. The geological maps of the Land Board of the Republic of Estonia were used to study and discuss stratigraphy of the area. The primary aim of this thesis was to identify the collected material. This was a stepwise procedure starting with preparation and assembling the fragments, figuring and describing the fossil in order to find characters of identification. The second of this study was to analyse the geological (biostratigraphic, taphonomic, etc.) value of the material. This required access to geological databases and literature on the fossils from the same area.