First evidence of the Cretaceous decapod crustacean Protocallianassa from Sweden

Abstract An assemblage of the burrowing ghost shrimp, Protocallianassa faujasi, is described, providing the first evidence of this decapod species from Sweden. The fossils occur in successions of the informal earliest late Campanian Belemnellocamax balsvikensis zone at Åsen and the latest early Campanian B. mammillatus zone at Ivö Klack, both in the Kristianstad Basin of NE Skåne. Numerous, heavily calcified chelipeds were found within a restricted bed at Åsen that was rich in carbonate-cemented nodules. Based on the burrowing lifestyle of modern mud shrimps, we interpret these nodules as infilled burrow chambers. The low abundance of molluscs within the Protocallianassa beds is also consistent with analogous extant communities, indicating that a similar ecologically exclusive relationship ruled within the Late Cretaceous shallow-marine ecosystems.

The Kristianstad Basin in the southernmost Swedish province of Skåne hosts a richly fossiliferous succession of Upper Cretaceous marine deposits. The Kristianstad Basin incorporates two half-graben that are bordered by horsts to the SW; the entire faulted system occurs within a zone of tectonic deformation known as the Tornqvist Zone (Erlström & Gabrielson 1992). During the mid-Late Cretaceous, repeated transgressive episodes occurred in what today represents southern Scandinavia (Vajda & Solakius 1999;Larsson et al. 2000). These created islands in the Kristianstad Basin that were surrounded by shallow seas forming paralic environments (Kominz et al. 2008;Surlyk & Sørensen 2010) adjacent to the deeper Tethyan shelf to the SW. Today, these palaeoenvironments are represented by several well-known fossil deposits, such as those exposed at Ignaberga, Ullstorp, Ivö Klack and Å sen ( Fig. 1) (Bergström & Sundquist 1978;Erlström & Gabrielson 1992). The successions at Å sen and Ivö Klack are especially rich in vertebrate remains, including sharks, rays, actinopterygian fishes, mosasaurs, plesiosaurs, marine and freshwater turtles, aquatic birds, and rare non-avian dinosaur bones (Einarsson et al. 2010;Sørensen et al. 2013;Bazzi et al. 2015;Poropat et al. 2015;Siversson et al. 2015). Coeval invertebrate assemblages comprise bivalves, cephalopods, gastropods, brachiopods, bryozoans and echinoderms, mainly represented by sea-urchin spines (Erlström & Gabrielson 1992;Surlyk & Sørensen 2010;Sørensen & Surlyk 2011;Sørensen et al. 2013). The material for this study was recovered from the earliest late Campanian succession at Å sen and the latest early Campanian succession at the Ivö Klack localities, and represents the first evidence of Protocallianassa, a burrowing shrimp, from Sweden. The morphology, stratigraphical provenance and environmental context of the studied specimens are discussed.

Methods
Eighteen calcified chelipeds of Protocallianassa faujasi were measured based on their characters, illustrated in Figure 3, following the methods of Swen et al. (2001). The material is hosted at the Department of Palaeobiology, Swedish Museum of Natural History, Stockholm, Sweden. Institutional abbreviations: NRM-PZ Ar 63926-NRM-PZ Ar 63944 (Table 1).

Systematic palaeontology
Order Decapoda Latreille, 1802 Infraorder Axiidea de Saint Laurent, 1979 Family Callianassidae Dana, 1852 Subfamily Protocallianassinae Beurlen, 1930 Genus Protocallianassa Beurlen, 1930 Protocallianassa faujasi Desmarest, 1822 Description. The decapod remains, all chelipeds of moderate quality preservation, occur within carbonate-cemented nodules that were collected from the earliest late Campanian, informal B. balsvikensis zone at Å sen between 2010 and 2012 (NRM-PZ Ar 63926-NRM-PZ Ar 63933). Isolated fossils were also found in slightly older strata (B. mammillatus zone) at Ivö Klack (NRM-PZ Ar 63934 -NRM-PZ Ar 63944) by Richard Hägg in 1903. This assemblage also shows moderate preservation. The recovered propodi all have intact fixed fingers, but the accompanying movable finger is preserved in only a few specimens. The propodi size is typical for Protocallianassa faujasi (Swen et al. 2001;Mourik et al. 2005). Other notable features include a rectangular (longer than wide) manus, setal pits on the manus and fingers, in some  specimens (probably abraded in others), and a carpus-propodus angle exceeding 1008.

Discussion
The present study adds to the distribution of Late Cretaceous Protocallianassa, extending its northern range into Scandinavia. At the close of the Cretaceous, Protocallianassa was replaced by representatives of other subfamilies such as Corallianassa. This might be linked to the Cretaceous -Palaeogene extinction event, the consequence of an asteroid impact in Yucatan, Mexico around 66 Ma (Schulte et al. 2010). Several families of decapod crustaceans went extinct at that time, including the Mecochiridae, Dakoticancridae and Carcineretidae (Schweitzer 2001;Feldmann 2003). The turnover amongst Protocallianassinae appears to have been more severe in Europe, where the youngest record  is from the Maastrichtian in The Netherlands and Belgium (Swen et al. 2001). The replacement by Corallianassa and other taxa appears to have been delayed in high-latitude southern hemisphere settings (Feldmann & Wilson 1988;Aguirre-Urreta 1989). This is consistent with the extinction patterns seen in the post-impact vegetation, with more severe extinctions seen in northern hemisphere floras (Vajda & Bercovici 2014). Feldmann (2003) suggested that decapods are under-represented in the fossil record because their exoskeleton is weakly calcified, leaving a brittle and disarticulated exterior carapace after rapid decomposition of the soft tissues. However, thick calcification of chelipeds increases their preservational potential (Glaessner 1960), and the latter is enhanced by the specific lifestyle of thalassinoidean shrimps, which inhabit burrows that infill with sediments after the animal's death (Griffis & Suchanek 1991). In recent studies from the Upper Miocene deposits of Slovakia, Hungary and Austria, Hyžný (2011) and Hyžný et al. (2015) showed that Callianassidae also produced burrows in brackish lake environments, causing significant bioturbation. The extant mud shrimp Neotrypaea californiensis excavates burrows up to 24 cm deep, and can incorporate as many as seven chambers (Volkenborn et al. 2012). This is consistent with the Protocallianassa fossils from Å sen and Ivö Klack, which comprise isolated chelipeds (often represented by internal casts) but no recognizable cephalothoracic or abdominal components. Moreover, they occur preserved within carbonate-cemented nodules that resemble chambered hollows of thalassinoidean burrows, presumably infilled and cemented by CaCO 2 after decomposition of the remainder of the carapace and soft tissues. A comparable process has been observed in the Maastrichtian of Sweden and Denmark, where the Thalassinoides burrows in chalk are filled with precipitated silica-forming chert (Surlyk et al. 2006).
In modern ecosystems, Thalassinoides species inhabit the littoral zone down to a shelf depth of 100 m (Müller 1984). This is especially consistent with the shallow-marine depositional setting for Å sen. The abundance of decapod remains within the informal B. balsvikensis zone (particularly Balsvikensis yellow) might explain the concomitant dearth of bivalves and other molluscs. It might further explain the slightly yellowish colour for this part of the succession compared to the underlying Balsvikensis green, as the bioturbated interval might have been more prone to weathering. This is based on comparisons with the extant ghost shrimp Neotrypaea californiensis, which displays a similar inverse relationship driven by the consumption  of mollusc larvae during sediment disturbance by the shrimps during burrowing (Peterson 1977;Posey et al. 1991). Such intense bioturbation has also been shown to impact on decompositional processes via aeration of subsurface sediment layers (Tudhope & Scoffin 1984). Similar interactions probably occurred with Protocallianassa in the Campanian of Sweden, where the 'shrimp bed' strata contain a larger fraction of coarser-grained material than the clay -silt-dominated underlying beds.
Conclusions † This study constitutes the first record of decapod crustaceans from the Mesozoic of Sweden and adds to the global record of Protocallianassa. The 18 measured specimens were recovered from the Kristianstad Basin, southern Sweden and are represented by chelipeds, chiefly propodi, identified as Protocallianassa faujasi (burrowing shrimps). † The beds hosting the fossil assemblage have been assigned to the informal Belemnellocamax balsvikensis zone and dated to the earliest late Campanian by belemnites. The decapods where encountered in calcified concretions, most probably representing burrow chambers. † The bed hosting the fossil decapods is poor in other fossils (e.g. molluscs) that are otherwise abundant in the succession. This is best explained by the ecological competition between these groups, which has been noted in comparable modern shallow-marine ecosystems.