Success and extinction of pycnodont fishes (Neopterygii, Pycnodontomorpha)

Pycnodonts are an ancient and iconic group of extinct fishes. Although extinct today, they were extremely successful for almost 175 million years originating ca. 227 million years ago and surviving the extinction events at the Triassic/Jurassic (ca. 200 million years) and the Cretaceous/Palaeogene (ca. 65 million years) boundaries. In the early Eocene (ca. 52 million years) they finally disappeared without any living relatives. During the Mesozoic, they rapidly diversified in both species numbers and morphological bauplans and are generally considered the coral fishes of this period occupying fundamental and important ecological niches in many shallow marine ecosystems. The reasons for their seemingly rapid diversification early in their evolutionary history, subsequent success and thus their possible superiority over other fish groups inhabiting same environments, habitat use through specific adaptations, possible size aggregations due to ontogenetic shifts, reasons for diversity fluctuations, and events triggering their final disappearance more or less simultaneously with the appearance and evolution of teleostean fishes, which are major elements of modern coral-fish assemblages still are ambiguous or have not yet been studied with rigorous analytical methods. We intend to determine possible correlations between diversification patterns and biotic factors, particularly competition, that enabled pycnodonts to occupy their niches despite supposed competition by ecological similar fishes during their evolution, and whether competition from teleosts in the Eocene triggered their extinction. To answer these questions, a multidisciplinary palaeobiological approach is necessary that will allow us to explore morphological changes, adaptations, and ecological functions of pycnodonts, and to subsequently identify possible competition patterns within pycnodonts and between pycnodonts and other fishes with similar adaptations. For this, we will analyse their phylogenetic relationships, the quality of their fossil record for ascertaining the reliability of data for studying their evolutionary history, and reconstruct their ecological role based on morphospace analyses within the habitats they occupied through time. By doing this, we also will identify possible body size segregations, which might be caused by intra- and inter competition patterns and to understand the relationships between morphological changes and increases in species numbers through time (chronophylomorphospace). This project thus will significantly augment our understanding of macro- and co-evolutionary patterns of fishes through time by integrating different aspects. We expect that the results will significantly advance and inform how certain groups prevail over others and give insights into the evolutionary success and ecology, but also into causes triggering extinctions of marine vertebrates. Innovative aspects of this project are the integrated approaches and analytical procedures, combining up-to-date methodologies. Dr. J. Kriwet and J.J. Cawley, MSc (PhD student position applied for here) will conduct this project. 1

Pycnodontiforms are an extinct group of very successful ray-finned fishes with a fossil record spanning almost 175 million years. These deep bodied, laterally compressed fishes with deep sturdy jaws displaying highly specialised dentitions for feeding on hard shelled prey were the dominant durophagous fishes in the past. They occupied reefs and subtropical to tropical shallow marine environments during the Mesozoic. A common hypothesis posits that 54 million years ago pycnodonts were outcompeted and eventually went extinct when modern acanthomorphs, which are the dominant fishes today, started to radiate into numerous lineages. The aims of this project were to analyse why pycnodonts were so successful over long geologic periods and to identify abiotic and/or biotic factors underlying their success but also their final extinction. In modern ecosystems, niche partitioning is a common response by organisms to increasing competition and it could be assumed that niche partitioning also was responsible to avoid competition in deep-time. Our results show that pycnodonts were not competing with other fishes for resources because they were adapted to tougher prey, and niche partitioning can be observed among different pycnodont families. It also is evident that acanthomorphs were not outcompeting pycnodonts. To further investigate causes of pycnodont extinction, the diversity of pycnodonts was compared to changes in sea surface temperature and sea level through time. These abiotic factors, however, also had almost no negative effect on pycnodont diversities. However, changes in reef area over the same time frame show reef decline before and after the end-Cretaceous extinction event some 65 million years ago similar to the simultaneous decline of pycnodont diversity. This could indicate that habitat loss might have contributed to pycnodont extinction. But it is most likely that the very long and successful evolutionary history of pycnodonts ultimately resulted in genetic depletion making these fishes a "dead clade walking". New and hitherto unknown taxa also were discovered that enabled to identify sexual morphological differences and ontogenetic shifts in habitat occupation for the first time. Importantly, they also evidence a close relationship between distribution patterns and climate. This project thus provided deep insights into the mechanisms underlying the success, but also decline of an important marine fish group over long geologic periods. The results highlight the importance of the methods employed here to tackle important palaeobiological aspects. The results underline the importance to take the evolutionary history but also ecological interactions between groups through time into account for understanding the extinction risk and the underlying abiotic and biotic causes for diversity changes of marine fishes today. In this, the results contribute largely to various research agendas and the gained knowledge provides promising future research directions.