An introduction to the Mesozoic biotas of Scandinavia and its Arctic territories

Abstract The Mesozoic biotas of Scandinavia have been studied for nearly two centuries. However, the last 15 years have witnessed an explosive advance in research, most notably on the richly fossiliferous Triassic (Olenekian–Carnian) and Jurassic (Tithonian) Lagerstätten of the Norwegian Arctic Svalbard archipelago, Late Cretaceous (Campanian) Kristianstad Basin and Vomb Trough of Skåne in southern Sweden, and the UNESCO heritage site at Stevns Klint in Denmark – the latter constituting one of the most complete Cretaceous–Palaeogene (Maastrichtian–Danian) boundary sections known globally. Other internationally significant deposits include earliest (Induan) and latest Triassic (Norian–Rhaetian) strata from the Danish autonomous territory of Greenland, and the Early Jurassic (Sinemurian–Pliensbachian) to Early Cretaceous (Berriasian) rocks of southern Sweden and the Danish Baltic island of Bornholm, respectively. Marine palaeocommunities are especially well documented, and comprise prolific benthic macroinvertebrates, together with pelagic cephalopods, chondrichthyans, actinopterygians and aquatic amniotes (ichthyopterygians, sauropterygians and mosasauroids). Terrestrial plant remains (lycophytes, sphenophytes, ferns, pteridosperms, cycadophytes, bennettitaleans and ginkgoes), including exceptionally well-preserved carbonized flowers, are also world famous, and are occasionally associated with faunal traces such as temnospondyl amphibian bones and dinosaurian footprints. While this collective documented record is substantial, much still awaits discovery. Thus, Scandinavia and its Arctic territories represent some of the most exciting prospects for future insights into the spectacular history of Mesozoic life and environments.

The Mesozoic fossil record of Scandinavia and its Arctic territories of Greenland and Svalbard span the dawn of the Triassic some 252 myr ago (Wordie Creek Formation, East Greenland: Nielsen 1935;Bendix-Almgreen 1976;Looy et al. 2001;Stemmerik et al. 2001;Bjerager et al. 2006) through to the terminal Cretaceous -Palaeogene boundary 66 myr ago (Møns Klint Formation, Denmark: Damholt & Surlyk 2012;Surlyk et al. 2013;Adolfssen & Ward 2014;Hansen & Surlyk 2014). This interval is marked by the nascence of modern faunal and floral biodiversity, and culminated in one of the most cataclysmic extinction events in Earth history. Much of our knowledge about the Mesozoic world has derived from the long tradition of palaeontological research in Europe (Rudwick 2008;Evans 2010), and yet many key biotas and bioevents from this continent remain comparatively underexplored. Scandinavia and its Arctic territories are therefore extremely important because they encompass not only a Boreal mid -high palaeolatitude setting (Surlyk 1990;Ditchfield 1997), but have also witnessed a burgeoning of novel discoveries that reveal significant insights into the global spectrum of Mesozoic organisms, ecosystems and environments.
This Special Publication aims to encapsulate these latest palaeontological advances, and augments them with topical synopses from leading specialists in the field. Our introduction is intended to provide additional contextual background, and, in particular, emphasizes the broad trends in floral successions and the distribution of faunal finds. Together, these highlight Scandinavia and its Arctic territories as a regional centre for Mesozoic biotic radiations, and a spectacular area for future field exploration with landmark research potential.

A synthesis of Scandinavian Mesozoic biotas
The Triassic The long history of Scandinavia's terrestrial biotas is charted through the palynological record, which manifests liverworts as the seminal colonizers of continental ecosystems in the early Palaeozoic (Late Ordovician) of southern Sweden (Badawy et al. 2014). Increasing abundance and diversity of bryophytes and vascular plants occurred throughout the Silurian and Devonian in Skåne (Mehlqvist et al. 2015) and Gotland (Hagström 1997), with the genesis of characteristic Mesozoic floras around the Permian-Triassic boundary in Greenland, Svalbard and the Oslo Rift: these collectively indicate turnover of regional biomes coincident with increasing aridity (Bercovici et al. 2015). The Permian-Triassic extinction event is otherwise expressed by the disappearance of dominant hygrophilous Cordaites (which equate to gigantopterids in Cathaysia and glossopterids in Gondwana) and their replacement by emergent seed plants (Anderson et al. 1999;McLoughlin 2011).
The coeval chronicle of Triassic terrestrial faunas is not well represented until the Norian-Rhaetian of the Fleming Fjord Formation in Jameson Land, East Greenland (Klein et al. 2015;. Here, body fossils and footprints evidence various dinosaurian taxa, especially sauropodomorphs, together with plagiosaurid and capitosaurian temnospondyl amphibians, rare rhamphorhynchoid pterosaurians, and early mammaliforms (e.g. Bendix-Almgreen 1976;Jenkins et al. 1994;Milàn et al. 2012a;Sulej et al. 2014;Clemmensen et al. 2015;Hansen et al. 2015;Klein et al. 2015). Fragmentary Late Triassic (Carnian -Rhaetian) temnospondyls are likewise known from both Svalbard and southern Sweden  and references therein), and coincide with lush vegetation comprising ginkgoes, cycads and bennettites, lycophytes, sphenophytes, and ferns (Vajda et al. 2013). Fossilized fungi and bacterial traces have also been reported from Hopen Island in the Svalbard archipelago (McLoughlin & Strullu-Derrien 2015). A bone fragment of a Late Triassic sauropodomorph was also recovered from a drill core in the North Sea 2256 m below the seabed (Hurum et al. 2006a).
Globally renowned Triassic marine amniote fossils were recovered from Spitsbergen during the Nordenskiöld expeditions of 1864 and 1868 (Hulke 1873). More complete material was subsequently collected by Swedish scientists in 1908 and 1909 (Wiman 1910(Wiman , 1916a(Wiman , b, 1928(Wiman , 1933, and constitutes a diverse assemblage of ichthyopterygians ( Fig. 1g), including the phylogenetically important basal taxon Grippia longirostris . Isolated pistosaurid sauropterygian remains have also been discovered , and Hurum et al. (2014) documented Triassic ichthyosaurian material from Edgeøya (Vigran et al. 2014). The classic vertebrate successions of Wiman (1910) are, however, still used to subdivide the horizons on Spitsbergen (see : the lithostratigraphical work of Mørk et al. (1999), equating the actinopterygian-and temnospondyl-dominated 'Fish Niveau' to the lower Olenekian Lusitaniadalen Member of the Vikinghøgda Formation; the 'Grippia Niveau' and 'Lower Saurian Niveau' -both representing sequential components of the Late Olenekian-Anisian Vendomdalen Member of the Vikinghøgda Formation; and derived mixosaurid and shastasaurid ichthyosaurians from the 'Upper Saurian Niveau' characterizing the Landinian Blanknuten Member of the upper Botneheia Formation and the Carnian Tschermakfjellet Formation.

The Jurassic
The Triassic -Jurassic transition is marked by extinctions coincident with emissions from the Central Atlantic Magmatic Province (Sha et al. 2015). In the Scandinavian territories, this is evidenced by successions from East Greenland (Klein et al. 2015). These reveal an abrupt replacement of the Rhaetian 'Lepidopteris flora' (typified by seed ferns, Taxodiaceae and the enigmatic Ricciisporites-producing plants) by the Hettangian 'Thaumatopteris flora' (Harris 1931), which was dominated by ferns, Cheirolepidaceae, Pinaceae and new groups of cycadophytes (Vajda et al. 2013). Compatible earliest Jurassic strata are exposed in southern Sweden and on the Danish Baltic island of Bornholm (Vajda & Wigforss-Lange 2009). Ornithopod and potential thyreophoran footprints (Gierliński & Ahlberg 1994;Milàn & Gierliński 2004), together with isolated dinosaurian vertebrae (Bölau 1954), have been described from the Rhaetian-Hettangian Höganäs Formation of the Höganäs Basin in southern Sweden.
Intense Jurassic volcanism, today revealed by volcanic necks in southern Sweden (Bergelin 2009), created lahar deposits that preserve plant remains in exceptional detail, even including visible cell nuclei (Bomfleur et al. 2014). More recent excavations in similar sediments overlying the Sinemurian-Pliensbachian Höör Sandstone have produced conifer wood with growth increments, permitting reconstruction of palaeoclimate, and pollen assemblages that evince the vegetative community .
The Early-Middle Jurassic outcrops on Bornholm are situated within a complex fault block of the NW -SE-trending Sorgenfrei-Tornquist Zone (Gravesen 2009). The stratigraphically oldest finds occur in the Hettangian Sose Bugt Member of the Rønne Formation, and comprise deformation structures interpreted as dinosaurian tracks (Clemmensen et al. 2014). Associated organic-rich beds and abundant plant material otherwise infer a warm and humid palaeoenvironment (Petersen et al. 2003).
Late Jurassic (Kimmeridgian) plesiosaurians have been found on Milne Land in Greenland (Bendix-Almgreen 1976;Smith 2007), as well as on Spitsbergen, where both plesiosaurian vertebrae (Wiman 1914) and articulated skeletons  were recovered with ichthyosaurian remains that have not yet been formally described. Subsequent systematic exploration of the Spitsbergen Jurassic outcrops by field teams from the University of Oslo  has correlated this material with the late Tithonian Slottsmøya Member of the uppermost Agardhfjellet Formation   (Fig. 2c). Since then, numerous plesiosauroid and large pliosaurid taxa, as well as ophthalmosaurid ichthyosaurians (Fig. 2d), have been identified (Knutsen et al. 2012a, b, c, d;Druckenmiller et al. 2012;Roberts et al. 2014). Rich ammonite assemblages (Wierzbowski et al. 2011) (Fig. 2e) and methane seep horizons have further revealed a diverse ecosystem of bivalves and echinoderms (Hryniewicz et al. 2014 and references therein). Delsett et al. (2015) reviewed this current record in the context of its preservation and geological setting.

The Cretaceous
The terrestrial Jurassic -Cretaceous transition is distinguished at Eriksdal in Skåne, southern Sweden (Vajda & Wigforss-Lange 2006). This time frame marks the nascence of angiosperms, the oldest Scandinavian pollen records of which occur in the Hauterivian Nytorp Sand (Vajda 2001;Vajda & Wigforss-Lange 2006). Latest Jurassic -earliest Cretaceous plant fossils, bivalves, ammonites and an ophthalmosaurid ichthyosaurian skeleton are known from Andøya island in northern Norway (Norborg & Wulff-Pedersen 1997;. Early Cretaceous strata are also exposed on Bornholm, where the Berriasian Rabekke, Robbedale and Jydegaard formations represent an interlinked barrier spit and lagoonal complex (Noe-Nygaard & Surlyk 1988). These rocks crop out along the coastal cliffs east of Arnager (Gravesen 2009), with the Rabekke Formation having produced a prolific bone-bed assemblage of atoposaurid, bernissartiid and goniopholidid crocodyliforms (Schwarz-Wings et al. 2009), actinopterygian fishes, urodelan and anuran amphibians, indeterminate turtles and lepidosaurians, dromaeosaurid and possible avian theropods, and a single tooth of the multituberculate mammal Sunnyodon (Lindgren et al. , 2008. A trample ground with abundant large dinosaurian tracks (up to 700 mm in length) and possible lungfish aestivation burrows is also evident in overlying beds (Surlyk et al. 2008).
The uppermost horizons of the Jydegaard Formation likewise hosts a diverse range of hybodontiform sharks and bony fish, including the lepisosteiform Lepidotes, amioids, pycnodonts and stem teleosts: these occur in conjunction with unidentified turtles, the neosuchian crocodylomorph Pholidosaurus and a scincomorph lizard (Bonde 2004(Bonde , 2012. Finally, isolated teeth of a dromaeosaurid and possible juvenile sauropod , vertebrate coprolites (Milàn et al. 2012a, b), and mass accumulations of non-marine bivalves and gastropods have been reported (Noe-Nygaard et al. 1987;Noe-Nygaard & Surlyk 1988).
Barremian-Aptian ornithopod tracks are known from the Festningen Sandstone Member of the Helvetiafjellet Formation on Spitsbergen (Hurum et al. 2006b). These were first published in the 1960s (Lapparent , 1962, and have been used to elucidate Boreal high-latitude dinosaurian assemblage composition in Fennoscandia during the Early Cretaceous (Gangloff 2012;Hurum et al. 2016a).

Future directions for research
Mesozoic research has a long history in Scandinavia that has contributed to the development of palaeontology as a modern science (Ebbestad 2016). This proud tradition continues to this day, with dynamic international collaborations and cutting-edge infrastructure facilitating innovative approaches and intensive exploration of its unique fossil resources. In particular, work undertaken in the remote Arctic regions of Svalbard and Greenland has garnered popular appeal, yet continued investigations into the well-documented localities of southern Sweden and Denmark have, over the last 15 years, generated more novel data than ever before. Aspects of this rapidly expanding work are highlighted in this Special Publications volume, which we hope will inspire new lines of inquiry. Indeed, a number of key areas are already attracting attention, such as the Triassic of Greenland, Svalbard and southern Sweden, and the Cretaceous -Palaeogene transition in Denmark. The rapid progress of these studies bodes exciting potential for the future, with Scandinavia and its Arctic territories likely to reveal further significant discoveries that will have a major impact on the global perspective of Mesozoic biotas and bioevents.
Many have contributed to the successful completion of this work. However, foremost are the authors of the constituent papers, all of whom generously gave of their knowledge, time and support. The Geological Society of London Publishing House also skilfully handled production of the volume and ensured its timely completion. We extend our deepest thanks to all.