Browsing by Person "Schoch, Rainer"

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Ontogeny and reproductive paleobiology in the Mesozoic marine reptile clade Ichthyosauria
(2023) Miedema, Feiko; Schoch, Rainer
Ichthyosaurs were viviparous marine reptiles that occupied many consumer niches in the marine ecosystems of the Mesozoic. Their viviparous reproductive strategy lends us the opportunity to study their prenatal development in great detail and lets us assess their birthing strategies. Moreover, some species are common in Lagerstätten deposits, which means we are likewise able to assess postnatal ontogeny. The assessment of ontogeny is important in our understanding of an extinct animal’s paleobiology and necessary for its’ correct phylogenetic placement. After studying the prenatal osteological development in the ichthyosaur Stenopterygius we now recognize 4 different stages on the basis of relative cranial ossification. The overall ossification sequence was very similar to other diapsids. The establishment of multiple prenatal stages was a first for an extinct sauropsid. We likewise erected 3 postnatal stages in Stenopterygius on the basis of cranial ossification, fusion and element morphology. Recognizing multiple postnatal ontogenetic stages in model (fossil) organism such as Stenopterygius will help recognize the ontogenetic stages of less well-known species and single specimens as well as refine phylogenetic characters which may be prone to major ontogenetic differences. Moreover, the cranial ossification and ontogeny of Mixosaurus cornalianus was studied. Due to sample size and the difficult crushed material, we were unable to establish the same stages as in Stenopterygius. However, we did clarify certain parts in Mixosaurus osteology, established some ontogenetically variable traits, most notably regarding the braincase and lower jaw, and were able to assess in detail a prenatal stage. Early cranial development in Mixosaurus differs from that of Stenopterygius regarding the precise ossification of the basioccipital and exoccipital, whereby Mixosaurus possibly shows a more ancestral developmental pathway. The notochord is a structure established early in development of vertebrates and recedes during the ossification of the vertebral centra. After studying much fetal material of several taxa of ichthyosaurs across phylogeny we established that the relative size of the notochord pit can function as a proxy for determining prenatal stage (as compared to Stenopterygius). This shows that the developmental pathway is relatively conservative across ichthyosaurs, as expected, and will help in determining prenatal stage in the absence of cranial material in subsequent finds. Lastly the birth preference of ichthyosaurs was re-evaluated. A longstanding idea within the community was that ichthyosaurs preferentially (and almost exclusively) gave birth tail-first as head-first birth would increase the risk of drowning of the fetus during birth. An early diverging ichthyosaur, Chaohusaurus, was found to have head-first birth. The authors argued that it must be the ancestral and terrestrial condition, citing the drowning hypothesis. After studying three pregnant specimens of the ichthyosaur Mixosaurus, we assessed birthing orientation in all ichthyosaurs in which pregnant specimens are known. Moreover, we looked at birthing in aquatic and terrestrial extant and all extinct aquatic amniotes. We see no link between birthing in an aquatic medium and tail-first birth and thereby deem the drowning hypothesis improbable. We propose two new hypotheses for birth orientation preference. Furthermore, based on all known ichthyosaur fetuses we propose that a preference for tail-first birth originated much later in ichthyosaur evolution than previously anticipated.
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).