Impact of the Late Devonian and Earliest Carboniferous environmental disturbances on faunal dynamics and evolution of conodonts

Conodonts are an extinct group of chordates characterized by the presence of a complex oral apparatus composed of a set of phosphatic elements. Isolated conodont elements are among the most ubiquitous fossils in the Palaeozoic and Triassic rocks. The ease of their extraction from the rock enables quantitative studies based on thousands of specimens collected thanks to dense sampling of geological sections in various regions of the world (Chapter 1). The unique structure of elements, owing to which rhythmic increments are easily recognizable within the element basal cavity, enables tracing their ontogeny in days. I performed such a research in respect to conodonts of the genus Alternognathus obtained from the latest Famennian part of the Kowala section in the Holy Cross Mountains, Poland (Chapter 2). The distribution of their mortality (population dynamics) shows a cyclicity probably corresponding to the lunar month cycle. A similar increase of mortality occurs in present-day populations of planktonic animals, when in low geographic latitudes the Moon quickly emerges above the horizon and illuminates the water, helping the vision-guided predators to hunt. Finds of the conodont apparatus elements in life arrangement are rarely available, but they enable testing hypotheses on the composition and three-dimensional organization of the apparatuses. Chapter 3 presents the results of studies on such findings from the mid Famennian of Kowala, with application of, among others, computed microtomography. It revealed that the composition of the natural assemblage is identical with its restoration proposed for the species Ctenopolygnathus brevilaminus on the basis of isolated elements. This increases the believability of the widely used (although not necessarily for the Late Devonian) apparatus approach to the conodont taxonomy based on large samples of conodont elements extracted chemically from carbonate rocks. The extraordinary completeness of the fossil record and abundance of conodont elements in the Kowala locality enables testing of the reliability of the classic method of biostratigraphy referring to the first (FAD) and last (LAD) appearances of particular species in the rock successions. The comparison of species ranges recorded in the Late Devonian of Europe (Kowala) and Vietnam (Si Phai) near the one of the most dramatic environmental events in the history of the world (the Kellwasser Event) showed that this is not a reliable method of the age correlation of rocks, because species ranges depend mostly on diachronous environmental factors, not their evolution (Chapter 4). Recognition of the course of evolution of extinct organisms remains the best way of reliable dating of the rock formations, although the immanently low rate of evolution limits time resolution of this method. The fossil record of global environmental events may allow to overcome this limitation, as long as it is used with appropriate care. The conodonts from Kowala enabled application of a quantitative research approach analogous to that used in the pollen analysis to recognize the effects of the dramatic environmental changes near the end of the Devonian. Sedimentation of a black shale both in Kowala and Cát Bà in northern Vietnam was an expression of the global Hangenberg Event. A dense sampling applied to the Kowala section showed the faunal dynamics of conodonts and ammonoids between the Hangenberg black shale and the preceding it analogous Kowala black clay bed. A surprising result of this study is that these evidently dramatic changes of the global environment did not result in sudden transformations of the conodont and ammonoid faunas (Chapter 5). Traditionally, the age correlation of the latest Famennian is based on ranges of the conodonts ‘Siphonodella’ praesulcata and Tripodellus expansus, both with unknown origin and limited geographic distribution, probably being subject of diachronous migrations. A much more reliable alternative for this time span seems to be the evolutionary transformation within the lineage Dasbergina marburgensis → D. trigonica. Populations representative of this lineage are known not only from the Devonian continent of Laurentia, on the margin of which Kowala was located, but also on the other shore of the Rheic Ocean (Dzikowiec in the Sudetes), and on the region of South China (Cát Bà in northern Vietnam). The rich fossil record of Dasbergina in Kowala allows to trace the evolution of its ontogeny calibrated in Devonian days (Chapter 6). This also clearly shows that the real course of evolution cannot be documented with application of the typological (vertical) concept of palaeontological species. The tremendous biodiversity of the Famennian pelagic ammonoid and conodont faunas terminated in central Europe with the deposition of the Wocklumeria Limestone, to be restored, after a rather long gap, with the sedimentation of the Tournaisian Gattendorfia Limestone. It is usually difficult to trace connections between these two episodes in the evolution of the living world at the conodont species level. However, this has become possible in respect to typically Carboniferous clades of conodonts, with their roots identified in the Devonian of Kowala (Chapter 7). The finding of a new species of the aberrant genus Dollymae demonstrated that this is a member of the Icriodontidae, already relic in the Famennian. Another finding, of the non-platform elements of the apparatus of the Devonian ‘Siphonodella ’praesulcata, supported its affiliation to the clade Elictognathidae and its genus Dinodus, highly diverse in the Tournaisian. A rich in specimens sample from the Carboniferous of Kowala offered a solution of the controversy about the composition of the apparatus in these conodonts but their evolutionary origin remains a mystery.