A global perspective on Mesozoic marine amniotes

MOSASAURS are squamates that became the dominant predatory marine reptiles in the Late Cretaceous about 98–66 million years ago. Although early members of this group possessed body profiles similar to living terrestrial lizards, many of the later, more derived forms were streamlined and equipped with fin-like limbs and a bilobed (hypocercal) tail fluke that would have enabled powerful swimming (Lindgren et al. 2010). Ecomorphological comparisons indicate that mosasaurs were probably very similar to modern pelagic sharks in terms of their hydrodynamic performance, and thus are convergent in both body form and locomotory style with other highly modified secondarily aquatic amniotes, including advanced ichthyosaurians (which were extremely specialised basally-branching non-saurian diapsids) and whales (Lindgren et al. 2011). Moreover, benefits from thermoregulation and/or crypsis likely contributed further evolutionary parallels in their integumental colouration, which might have had implications for the ability of mosasaurs to exploit cold-water environments (Lindgren et al. 2014). Ultimately, these ubiquitous trends probably resulted from common constraints and the stringent selection pressures imposed by life in the sea (in addition to the need to periodically surface to breathe air).

The rapidly growing body of research on mosasaurs and their kin precipitated the 1st International Mosasaur Meeting, which was held in Maastricht, The Netherlands in 2004. Since then, the symposium has evolved into an international triennial event held in Hays (Kansas), USA in 2007, Paris, France, in 2010, and Dallas (Texas), USA, in 2013. These conferences have attracted an eclectic mix of professional scientists, students and avocational palaeontologists participating together in a forum dedicated exclusively to these magnificent fossil reptiles. However, the 5th International Mosasaur Meeting convened in Uppsala, Sweden, in 2016 considerably expanded the thematic framework to encompass a broader perspective on Mesozoic marine amniote evolution. This included oral and poster presentations on a spectrum of Mesozoic aquatic reptiles including not only mosasaurs, but also plesiosaurs (iconic long-necked marine reptiles), ichthyosaurs and sea turtles. The intention was to foster greater integration of knowledge, approaches and data between specialist researchers, and thus instigate collaborations that will advance the field over the years to come. The 6th International Mosasaur Meeting, which will be held in Drumheller (Alberta), Canada, in 2019, aims to continue this tradition of dynamic cooperative research.

A special issue of Alcheringa was commissioned to present some of the key contributions from the 5th International Mosasaur Meeting. This comprises 11 papers from 26 international authors that participated in the symposium, and covers a wide range of topics from palaeobiogeography, to phylogenetic assessments of new and existing taxa, palaeoecological, distributional and taxonomic overviews, anatomical interpretations, and stratigraphical correlations for important fossil localities. Collectively, these articles reflect both the ethos of the International Mosasaur Meeting series, and the latest developing research on the amazing, but long-extinct world of Mesozoic marine leviathans.

Content from the 5th International Mosasaur Meeting special issue of Alcheringa synthesises current studies from Australasia, Europe, the USA, and the Middle East.

Kear et al. (2018) present a palaeobiogeographical review of the Mesozoic marine tetrapod assemblages from Australasia. These include finds from Australia, New Zealand and the Chatham Islands, Timor, and New Caledonia, and represent a high diversity of Triassic–Cretaceous groups, such as possibly euryhaline temnospondyl amphibians, ichthyosaurians, plesiosaurians, chelonioid sea turtles, and various marine squamates. These disparate lineages all occupied the southern high-palaeolatitude regions during the Mesozoic, and were subject to periodically extreme climates that appear to have had major implications for clade dispersals and evolutionary radiations.

Sachs & Kear (2018) describe a new genus and species of Early Jurassic pliosaurid plesiosaurian (Fig. 1) from the upper Pliensbachian Amaltheenton Formation of Bielefeld in northwestern Germany. This new taxon is represented by an incomplete skeleton, but displays an unexpected mosaic of features shared with both later Jurassic and Early Cretaceous plesiosaurians. It also constitutes only the third formally named plesiosaurian ever identified from the Early Jurassic Pliensbachian interval worldwide, and thus adds important distributional information on this critical biostratigraphical gap.

Fig. 1 Life reconstruction of the new Early Jurassic pliosaurid plesiosaurian Arminisaurus schuberti described by Sachs & Kear (2018) from the Amaltheenton Formation of Bielefeld in northwestern Germany. Artwork courtesy Joschua Knüppe (Ibbenbüren).

The isolated mandible of a large-skulled ‘pliosauromorph’ plesiosaurian is documented from Early Cretaceous (Berriasian) non-marine deposits of the Deister Formation in northwestern Germany by Sachs et al. (2018a). This specimen is preserved as a natural mould, which has been digitally reconstructed using photogrammetry to reveal an unusually broad symphysis containing enlarged rostral-most teeth. This morphology resembles Early–Middle Jurassic rhomaleosaurids, as well as the pliosaurid Simolestes, and constitutes the first record of a larger-bodied plesiosaurian apex predator from the ‘Wealden-facies’ of Europe.

Van Vranken (2018) contributes a note summarising the mid-Cretaceous ichthyosaurian records from Texas, USA. Eleven ichthyosaur occurrences ranging from Albian to Cenomanian in age are documented from the state, with most of the material comprising isolated teeth and vertebrae attributable to indeterminate ophthalmosaurids. A partial skeleton incorporating diagnostic cranial remains is also assigned to the globally distributed ‘form genus’ Platypterygius.

Lively (2018) undertakes a taxonomic reassessment of the classic mosasaurine mosasaur genus Clidastes. He focuses on two particularly problematic nominal species, C. liodontus and C. moorevillensis. These are re-evaluated, redefined and re-classified as a nomen dubium, or doubtfully applied name, and nomen nudum, lacking an adequate description, respecively. The new designations have obvious implications for the morphological and species-level diversity within Clidastes, which accordingly warrants an extensive taxonomic revision.

Zverkov et al. (2018) report on an extremely well-preserved elasmosaurid plesiosaurian braincase from the Late Cretaceous (early Campanian) Rybushka Formation of the Saratov Province in European Russia. This fossil reveals new insights into the osteology and intracranial circulatory system (especially with regard to the carotid artery) of these quintessential Cretaceous plesiosaurians.

Hornung et al. (2018) document a Late Cretaceous (Campanian) mosasaurid assemblage from the Hannover region of northern Germany. The fossils include diagnostic shed teeth and an isolated humerus that expand the recorded distributional area of the mosasaurine Clidastes into north-central Europe. A new occurrence of Prognathodon also supports a Transatlantic dispersal of this large-bodied cosmopolitan genus early in its evolutionary history.

The historically significant elasmosaurid plesiosaurian Styxosaurus snowii is redescribed by Sachs et al. (2018b), who also re-evaluate its phylogenetic relationships based on the holotype specimen, which has not been examined in detail since its original description in the late 19^th century. Despite being one of the earliest described elasmosaurid species, Styxosaurus reveals a suite of novel character states in its skull and cervical vertebrae that necessitate a reconsideration of named higher-level groupings within the clade.

O’Gorman et al. (2018) discuss new elasmosaurid plesiosaurian material from the latest Cretaceous (Maastrichtian) López de Bertodano Formation of Antarctica. One of these specimens, characterised by diagnostically short cervical vertebrae, represents the stratigraphically youngest non-aristonectine elasmosaurid occurrence from Antarctica. Others demonstrate that at least two coeval non-aristonectine elasmosaurids utilised southern higher-latitude seaways surrounding Antarctica during the Maastrichtian.

Preliminary age correlations derived from ammonites found with latest Cretaceous mosasaurids from Jordan are compiled by Jagt et al. (2018), and show that the uppermost Maastrichtian Muwaqqar Chalk Marl Formation incorporates several marker species that are diagnostic for the middle/upper Maastricht Type Formation in the Maastrichtian type area of the The Netherlands. Stratigraphical correlations with the upper part of the Kunrade Formation also bolster observations of closely-related mosasaurid taxa in these units.

Finally, Milàn et al. (2018) describe a shed mosasaurid tooth from the uppermost Maastrichtian beds of Stevns Klint World Heritage Cretaceous-Palaeogene boundary section in Denmark. This fossil simultaneously documents the first record of the rare durophagous globidensin mosasaur Carinodens from Denmark, and also the most northerly latitudinal occurrence of the genus.

In summation these studies highlight a broad palate of perspectives on the biodiversity, ecosystems and environments of Mesozoic marine amniotes. However, each individual work also provides an important advance in its specialised field. We hope that this compilation inspires further investigations and leads to new discoveries in this dynamically evolving discipline of vertebrate palaeontology.

Acknowledgements

We are indebted to the numerous authors for their scholarly contributions to this special issue, together with the referees who generously contributed both their time and constructive comments. We also acknowledge all of the delegates, who attended the 5th International Mosasaur Meeting, and the Museum of Evolution at Uppsala University, who hosted the symposium and accompanying social events.