Browsing by Person "Cooper, Samuel Lewis Alan"

Now showing 1 - 1 of 1

Pachycormid fishes from the early jurassic (Toarcian) Posidonienschiefer formation of southern Germany
(2023) Cooper, Samuel Lewis Alan; Schoch, Rainer
Pachycormiformes were a successful clade of Mesozoic actinopterygian fishes, with a patchy fossil record spanning the Lower Jurassic to Upper Cretaceous. Current phylogeny resolves pachycormids as a monophyletic grouping of stem-teleosts (Teleosteomorpha) and therefore they hold an important evolutionary position in the debated Neopterygii to Teleostei transition. The Toarcian Posidonienschiefer Formation – a marine Konservat Lagerstätte deposited predominately in Baden-Württemberg (SW Germany) – records the oldest occurrences of Pachycormiformes in the fossil record, along with the first ~3 million years of their evolutionary history. The formation is therefore ideal for testing major evolutionary concepts for this clade; including their origins, early diversity, palaeoecology, early adaptive radiation, and survivorship across the Early Toarcian Oceanic Anoxic Event (ETOAE). Pachycormids are geographically confined the north-western Tethys during the Toarcian, with their fossils found in Liassic black shales and marls of the UK, France, Belgium, Luxembourg, Germany and northern Italy. Of the supposed 17 pachycormid species previously cited as being present in Posidonienschiefer Formation by various authors, I consider only 9 species to be valid; although just 7 of these are actually present in the German Lias: Euthynotus incognitus, Euthynotus intermedius, Sauropsis veruinalis, Pachycormus macropterus, Saurostomus esocinus, Germanostomus pectopteri, and Ohmdenia multidentata. Unfortunately, most of these taxa are poorly described and inadequately diagnosed, thus creating a caveat of problems surrounding their taxonomic identities. Many specimens are commonly misidentified, both in museum collections and published datasets, which is a major hindrance preventing reliable anatomical and evolutionary studies on Pachycormiformes. The primary objective of this thesis is to resolve the taxonomic identifies of certain Toarcian pachycormids in the Posidonienschiefer Formation, and provide a constrain on their stratigraphic and palaeobiogeographic ranges (Chapters 2, 3, Appendix 1). Only after this is resolved can the bigger questions surrounding pachycormid evolution and diversity through time be more accurately addressed. Provided in Chapter 1 is a detailed overview of the known stratigraphic and palaeobiogeographic ranges of named pachycormid taxa across their fossil record. Herein, I highlight notable gaps in their occurrences, changes in species richness through time, and discuss the abiotic and human factors biasing the pachycormid fossil record. Peaks in taxic diversity appear to correlate with eustatic sea level rise, whilst low stand periods and an associated depletion of suitable fossiliferous deposits are responsible for gaps. In Chapter 2, I review and redescribe the problematic historic species Saurostomus esocinus, providing a revised diagnosis, phylogeny, distribution ranges, and new definitive criteria to differentiate this taxon from the commonly conflated genus, Pachycormus. S. esocinus is shown to be an early diverging member of the suspension-feeding lineage of pachycormids due the detection of important shared characters (e.g., loss of infraorbital and suborbital bones; reduced skeleton ossification). I also describe a new large pachycormid from the Posidonienschiefer Formation and discuss its evolutionary significance to the evolution of suspension-feeding capabilities in Pachycormiformes. Germanostomus pectopteri is differentiated from Saurostomus esocinus by proportionately more elongate jaws, an enlarged premaxilla, loss of the opercular process on the hyomandibula, and large inverted ‘D’-shaped pectoral fins which are suitable for upwards lift. Morphologies and trends in the pachycormiform gastrointestinal tract are also discussed in an evolutionary context. Classification of Pachycormiformes is revised with the monospecific Family Pachycormidae now divided into two Subfamilies: Asthenocorminae and Hypsocorminae. Additionally, I review the ecology of Pachycormus macropterus – the most abundant and wide spread pachycormid species in the Early Jurassic. A dietary shift in prey preference associated with ontogeny is identified in P. macropterus, with juveniles obligate piscivores and adults facultatively teuthophagous. One P. macropterus specimen preserves a large ammonite shell inside of the gut, demonstrating a previously unknown trophic relationship between pachycormids and ammonoids. Evidence for cannibalism in pachycormids, based on conspecific gut contents in Pachycormus, is presented for the first time. Body size distribution in Pachycormus specimens between different sites (Curcy, France, and Holzmaden, Germany) suggest that juveniles occupied mostly coastal environments but migrated further off-shore later in ontogeny, possibly to exploit the high abundance of pelagic coleoids in these areas. Comparisons in gut contents suggest that interspecific competition between large asthenocormine pachycormids was likely low in the Posidonienschiefer Formation with Pachycormus favouring non-belemnoid teuthids (e.g. Teudopsis sp.), Saurostomus and Germanostomus consuming mainly non-belemnite belemnoids (e.g. Clarkeiteuthis conocauda) and small fishes, whilst only Ohmdenia multidentata ate belemnites. Changes in species abundance and taxic diversity of pachycormids in the Posidonienschiefer Formation is likely associated with changing water depth. Pachycormids have a low detection rate in the lower beds (tenuicostatum Zone), but become extremely abundant following the fallout of the ETOAE in the serpentinum Zone, only to drastically decline in the upper part of the section (bifrons Zone). External to Pachycormiformes, a new genus and species of coccolepidid fish (Coccolepididae) is described from the Posidonienschiefer Formation at Holzmaden (Chapter 6); the first to be named from the Toarcian, including one specimen preserved as a regurgitalite (Speiballen).