The evolutionary origins of animal phyla are intimately linked with the Cambrian explosion, a period of radical ecological and evolutionary innovation that begins approximately 540 Mya and continues for some 20 million years, during which most major animal groups appear. Lophotrochozoa, a major group of protostome animals that includes molluscs, annelids and brachiopods, represent a significant component of the oldest known fossil records of biomineralised animals, as disclosed by the enigmatic ‘small shelly fossil’ faunas of the early Cambrian. Determining the affinities of these scleritome taxa is highly informative for examining Cambrian evolutionary patterns, since many are supposed stem-group Lophotrochozoa. The main focus of this thesis pertained to the stem-group of the Brachiopoda, a highly diverse and important clade of suspension feeding animals in the Palaeozoic era, which are still extant but with only with a fraction of past diversity. Major findings include adding support for tommotiid affinity as stem-group lophophorates. Determining morphological character homologies vital to reconstructing the brachiopod stem-group was achieved by comparing Cambrian Lagerstätten with the widespread biomineralised record of Cambrian stem-brachiopods and small shelly fossils. Polarising character changes associated with the putative transition from scleritome organisms to crown-group brachiopods was furthered by the description of an enigmatic agglutinated tubular lophophorate Yuganotheca elegans from the Chengjiang Lagerstätte, China, which possesses an unusual combination of phoronid, brachiopod and tommotiid characters. These efforts were furthered by the use of X-ray tomographic techniques that revealed novel anatomical features, including exceptionally preserved setae in the tommotiid Micrina. The evidence for a common origin of columnar brachiopod shell structures in the tommotiids is suggested and critically examined. Enigmatic and problematic early and middle Cambrian lophotrochozoans are newly described or re-described in light of new evidence, namely: the stem-brachiopod Mickwitzia occidens Walcott from the Indian Springs Lagerstätte, Nevada; a putative stem-group entoproct Cotyledion tylodes Luo and Hu from Chengjiang, China; a new enigmatic family of rhynchonelliform brachiopods exemplified by the newly described Tomteluva perturbata from the Stephen Formation, Canada; and the tommotiid Micrina etheridgei (Tate) from the Flinders Ranges, South Australia. Cladistic analyses of fossil morphological data supports a monophyletic Brachiopoda.

Die zu den Sirenia, oder Seekühen, zählende Gattung Halitherium ist mit Arten aus dem Obereozän bis Untermiozän bekannt. Obwohl Halitherium als monophyletisch angesehen wird, bestätigen alle bisherigen phylogenetischen Analysen die Paraphylie dieser Gruppe. Auch die auf Halitherium basierende nur fossil bekannte Unterfamilie Halitheriinae ist paraphyletisch und umfasst wiederum fast ausnahmslos paraphyletische Gattungen. Der Fokus liegt auf der Typusart H. schinzii. Deren Holotyp, ein Premolar, wird als undiagnostisch definiert und infolgedessen H. schinzii als nomen dubium eingestuft. Die Neubeschreibung sämtlicher dieser Art zugeordneter Skelettreste liefert neue morphologische Daten. So kann die Hypothese von zwei sympatrisch vorkommenden Morphospezies im Unteroligozän Zentraleuropas auf Basis mehrerer unterscheidender Merkmale gestützt werden. Für die Verwandtschaftsanalyse der „Halitherium“ traditionell zugeordneten Arten und die Ermittlung ihrer phylogenetischen Stellung innerhalb der Ordnung Sirenia finden strenge kladistische Prinzipien Berücksichtigung. Eine revidierte, ergänzte und erweiterte Merkmalsmatrix stellt dabei den bisher größten morphologischen Datensatz über Sirenia dar. Die phylogenetischen Analysen zeigen, dass die „Halitherium“ Arten keine monophyletische Gruppe bilden. Im Zuge dieser systematisch-taxonomischen Revision werden die „Halitheriinae“ eingezogen und vier neue Gattungen aufgestellt. Des Weiteren wird eine neue Klassifikation der Sirenia vorgeschlagen, in der eine konsequente Unterscheidung zwischen einer paraphyletischen Stammgruppe und einer monophyletischen Kronengruppe Anwendung findet. Diese Studie liefert neue Daten über die Diversität und Biogeographie von Sirenen. Die herausragendsten Ergebnisse sind zum einen die Revision einer der zweifelhaftesten Sirenia Gruppen, die „Halitheriinae“. Zum anderen wird für den Ursprung der Kronengruppensirenen ein eher unteroligozäner statt eozäner Zeitpunkt postuliert. / The genus Halitherium includes a number of fossil sirenian species, or sea cows, ranging from the late Eocene to early Miocene. Although Halitherium is assumed to be monophyletic, all previous phylogenetic analyses reveal this group to be paraphyletic. As such, the exclusively extinct subfamily Halitheriinae based on Halitherium is paraphyletic comprising mainly genera that are invariably paraphyletic as well. The focus lies on the type species H. schinzii and the morphological basis for its establishment. The holotype, a single premolar, is considered non-diagnostic, which resulted in the recognition of this taxon name as a nomen dubium. Abundant skeletal material originally assigned to “H. schinzii” is re-described providing new data on the morphology of this sirenian. In this process, the hypothesis of two sympatric morphospecies in the lower Oligocene of Central Europe is corroborated by a suite of distinguishing characters. For the analysis of the interrelationships of the species traditionally assigned to “Halitherium”, and the identification of their phylogenetic position within the order Sirenia, robust cladistic principles are applied. A revised, supplemented and extended data matrix represents the hitherto largest data set on Sirenia based on morphological characters. The phylogenetic analyses show that the “Halitherium” species do not form a monophyletic group. In the course of this systematic and taxonomic revision the “Halitheriinae” are refuted and four new genera are established. Furthermore, a new systematic framework is introduced for Sirenia primarily distinguishing between a paraphyletic stem group and a monophyletic crown group. This study provides new data on the past sirenian diversity and biogeography. The most important results are that one of the most disputed sirenian groups, the “Halitheriinae”, is revised, and that the divergence time of crown group sirenians is estimated as early Oligocene rather than Eocene.