ALBERT

Description: The Saxonian Cretaceous Basin constitutes an important source of rare Late Cretaceous marine amniote fossils from Germany. It is also historically famous, having been documented in a series of monographic works published by the distinguished German palaeontologist Hanns Bruno Geinitz in the nineteenth century. The most productive rock units include the upper Cenomanian Dölzschen Formation and upper Turonian Strehlen and Weinböhla limestones (lower Strehlen Formation). A survey of curated specimens recovered from these deposits has now identified isolated teeth of probable polycotylid and elasmosaurid plesiosaurians, as well as several humeri that are referred to protostegid marine turtles. The Saxonian Cretaceous Basin formed a continuous epeiric seaway with the Bohemian Cretaceous Basin during late Cenomanian – Turonian time. A western connection to the North Sea Basin also existed via the North German and Münsterland Cretaceous basins. The Mesozoic marine amniote remains from these regions therefore record a coeval northern European fauna that was probably homogeneous across the northern peri-Tethyan margin during Late Cretaceous time.

Description: Reconstructing the swimming capabilities of extinct marine tetrapods is critical for unravelling broader questions about their palaeobiology, palaeoecology and palaeobiogeography. Ichthyosaurs have long been the subject of such investigations because, alongside cetaceans, they are one of the few tetrapod lineages to achieve a highly specialized fish-like body plan. The dominant locomotory mode for the majority of derived, post-Triassic ichthyosaurs is hypothesized to have been caudal fin-driven propulsion. Limb-based swimming has however been suggested for some highly autapomorphic forms, such as the Cretaceous genus Platypterygius, which has a remarkably robust humeral morphology and exceptionally broad paddle-like limbs. To evaluate this atypical lifestyle model, we conducted a comprehensive comparative osteological assessment of Platypterygius in relation to extant marine mammals, whose analogous skeletal frameworks provide a structurally compatible selection of alternate propulsive strategies. Based on a proxy exemplar of the most completely known species, P. australis from the Early Cretaceous of Australia, the propodial shape, absence of functional elbow/knee joints, tightly interlocking carpals, hyperphalangy and extreme reduction of the pelvic girdle are most similar to cetaceans as opposed to pinnipeds or dugongs. There is no obvious structural consistency with aquatic mammals that use sustained forelimb-driven swimming. The exceptionally broad fore-paddle (a product of hyperdactyly) and extensive humeral muscle insertions might therefore have had a cetacean-like role in enhancing manoeuvrability and acceleration performance. We conclude that, despite its atypical features, P. australis was most likely similar to other ichthyosaurs in using lateral sweeps of the tailfin to generate primary propulsive thrust.

Description: Amniotes first invaded saline lagoons and coastal seaways towards the end of the Palaeozoic (Early Permian, ~ 280 Ma: Piñeiro et al. 2012), but by the dawn of the Mesozoic (Early–Middle Triassic, ~ 250–235 Ma: Rieppel, 2002; McGowan & Motani, 2003) they had achieved a diversity of specialized body-forms requisite for an obligate oceanic lifestyle. Such an explosive ecomorphological radiation paved the way for amniote dominance of large-bodied aquatic carnivore/omnivore niches over the next 185 Ma, with some lineages (e.g. dyrosaurid crocodylomorphs and bothremydid turtles: Gaffney, Tong & Meylan, 2006; Barbosa, Kellner & Sales Viana, 2008) even persisting on into the Palaeogene (until ~ 50 Ma), and diversifying (i.e. chelonioid sea turtles: Hirayama, 1997) alongside emergent marine mammals through the Neogene (from ~ 23 Ma) and up until today.

Description: Despite being known for over 155 years, the Late Cretaceous marine amniotes of the Bohemian Cretaceous Basin in the Czech Republic have received little recent attention. These fossils are however significant because they record a diverse range of taxa from an incompletely known geological interval: the Turonian. The presently identifiable remains include isolated bones and teeth, together with a few disarticulated skeletons. The most productive stratigraphical unit is the Lower–Middle Turonian Bílá Hora Formation, which has yielded small dermochelyoid sea turtles, a possible polycotylid plesiosaur and elements compatible with the giant predatory pliosauromorphPolyptychodon. A huge protostegid, together with an enigmatic cheloniid-like turtle,Polyptychodon-like dentigerous components, an elasmosaurid and a tethysaurine mosasauroid have also been found in strata corresponding to the Middle–Upper Turonian Jizera Formation and Upper Turonian – Coniacian Teplice Formation. The compositional character of the Bohemian Cretaceous Basin fauna is compatible with coeval assemblages from elsewhere along the peri-Tethyan shelf of Europe, and incorporates the globally terminal Middle–Upper Turonian occurrence of pliosauromorph megacarnivores, which were seemingly replaced by mosasauroids later in the Cretaceous.

Description: Ichthyosaurians provide some of the most famous examples of ‘live birth’ in the amniote fossil record based on spectacular complete skeletons of gravid females. Such remains facilitate direct comparisons between adult and foetal life stages, and thus have significant impact on phylogenetic hypotheses that require discrete character states to be ontogenetically uncoupled. This is especially true for Cretaceous ichthyosaurian taxa, the majority of which have been established using single specimens of assumed osteological maturity. Our assessment of in utero ichthyosaurian remains from the late Albian of Australia was therefore aimed at testing ontogenetic stability amongst key traits defining the most ubiquitous Cretaceous taxon: Platypterygius. Surprisingly, almost all of the salient features were identifiable in our sample of undoubtedly immature individuals. Indeed, only the proportions of the sclerotic ring, relative ossification and fusion of various basicranial elements, development of the axial skeleton, prominence of the deltopectoral crest and dorsal trochanter, and formation (but seemingly not number) of distal articular facets on the humerus were found to vary from larger-bodied members of the same species (P. australis). Ontogenetic continuity amongst the majority of other phylogenetically pertinent skeletal structures advocates their application for cladistic analyses, and suggests that many classic characters used to differentiate Platypterygius remain diagnostic irrespective of growth stage.

Description: Elasmosauridae constitutes one of the most immediately recognizable plesiosaurian radiations. Their distinctive body plan represents the popular model for Plesiosauria, and is typified by an osteological morphology especially adapted for hyper-elongation of the neck. Nevertheless, many archetypal elasmosaurids are known only from incomplete and/or inadequately documented material, a problem that has contributed to their uncertain intra-clade relationships. A prime example of this isLibonectes morganifrom the Upper Cretaceous of Texas, USA, which is frequently presented as an elasmosaurid structural proxy because of its three-dimensionally preserved holotype skull. Perplexingly though, both the taxonomic diagnosis and phylogenetic placement ofL. morganirely primarily upon the cervical vertebrae, together with the pectoral girdle and forelimb, yet most of these elements are now lost and figured only as line drawings. We therefore reviewed the remnant postcranial skeleton ofL. morganifirst-hand with the objective of clarifying its defining character states. Our observations showed that the existing diagnosis ofL. morganiis indeed inadequate. Moreover, the only identifiable autapomorphies occurred within the axial skeleton. This concurred with an examination of character scores used in published plesiosaurian phylogenies, and highlights the persistent significance of postcranial elements for discriminating elasmosaurid taxa.