New early Paleocene (Danian) paguroids from deep-water coral/bryozoan mounds at Faxe, eastern Denmark

During recent decades, decapod crustacean faunas from middle Danian (lower Paleocene) strata at Faxe (Sjælland, Denmark) have been studied in detail. However, paguroid anomurans have not yet been described formally. Two new species of hermit crab have lately been recognised in the collections of the Geomuseum Faxe. Percentages of total paguroid assemblages and feeding behaviour derived from the morphology of its chelae indicate that one of these, Dardanus faxensis sp. nov., as a generalist, was better adapted to inhabit the deep-water reefal environment of the Faxe carbonates than the more specialised, suspension-feeding Paguristes frigoscopulus sp. nov.


Introduction
In general, records of paguroid remains from Danian-aged rocks are scarce and hermit crab assemblages of this age remain largely understudied. Vega et al. (2007) recorded a right and an incomplete left chela of a diogenid from the Rancho Nuevo Formation at Coahuila (Mexico), while Armstrong et al. (2009) described a right paguroid palm, without systematic placement, from the Midway Group of Texas, USA. In addition, Cope et al. (2005) noted Paguristes johnsoni Rathbun, 1935, on the basis of a single, well-preserved propodus and an additional fragment of a carpus that were closely comparable to the type material illustrated and described by Rathbun (1935) from the Sucarnoochee Formation of Alabama (USA). Robin et al. (2017) mentioned an indeterminate species of Paguristes Dana, 1851 from a privately held collection of Danian decapod crustaceans from Vigny, France. It would appear that this locality was very rich in paguroids of earliest Paleocene age (Wallaard et al., 2020), but unfortunately this material is not currently accessible for scientific assessment and publication.
A brief visit to the collections of the Geomuseum Faxe (September 2019) has now resulted in the recognition of fourteen hermit crab speci-mens along with the material from the collections at the Natural History Museum (Copenhagen), all partially complete chelae. The two new paguroid taxa erected to accommodate these forms from the middle Danian of Faxe present a valuable addition to the hermit crab faunas from this time interval.

Geological setting
A large, cool-water, subphotic coral/bryozoan mound complex at Faxe developed in the Danish Basin during the early Danian, shortly after the Cretaceous/Paleogene boundary event (e.g., Bjerager et al., 2018;Schrøder & Surlyk, 2019). During the middle Danian, a low-diversity scleractinian coral fauna initiated the formation of extensive cold-water coral mound complexes that intercalated with bryozoan mounds (e.g., Lauridsen et al., 2012). The coral mounds started to grow below the photic zone over the easternmost part of the Ringkøbing-Fyn High, only 2 million years after the mass extinction at the K/Pg boundary (Lauridsen et al., 2012).
The middle Danian limestones exposed at Faxe quarry ( Fig. 1) document the extraordinary preservation of a 63-myr-old, cold-water coral mound ecosystem. Bryozoan mounds dominate the stratigraphically lower parts and are overlain by interfingering coral and bryozoan mounds. The Faxe quarry exposes a large mound complex that includes individual coral mound bodies. The mound complex is dominated by the frame-building scleractinian coral species, Dendrophyllia candelabrum (Hennig, 1899), which is associated with common Faksephyllia faxoensis (Lyell, 1837) and minor numbers of Oculina becki (Brünnich Nielsen, 1922). A highly diverse fauna is found in middle Danian strata at Faxe quarry, including serpulid annelids, arthropods, brachiopods, bryozoans, coelenterates, echinoderms, sponges and molluscs (Lauridsen et al., 2012, table 1).

Material and methods
The taxonomic descriptions are based on material stored at the Geological Museum (now Natural History Museum of Denmark) and and Geomuseum Faxe. Our material comprises fourteen specimens belonging to two genera, represented by two new species. The material included in the present study was collected mainly by Curator Søren Bo Andersen and the late amateur palaeontologist, Alice Rasmussen, of Faxe. In 2012, the latter was awarded the prestigious Mary Anning Prize of the Palaeontological Association (London) in recognition of her outstanding work on fossils from the type locality of the Danian Stage at Faxe. The present material was collected in various parts of the quarry over many years.
The chelae are preserved mainly in the form of internal and external moulds in the coral limestone and therefore our descriptions are in part based on casts. However, that of the holotype of Paguristes frigoscopulus relied on the internal mould ('steinkern'). Preparation and photography of the material followed the procedures outlined in Jakobsen & Feldmann (2004 2010 Eremitkrebs 20 mm**; Damholt et al., p. 23, unnumbered figures. 2010 Klo fra eremitkrebs 20 mm***; Damholt et al.,p. 42,unnumbered figure. Diagnosis: Palm transversely oval; inner and outer sides longitudinally and transversely convex; outer side of palm covered with forwardly directed, largely spinose tubercles; proximal part of outer side of palm and distal part of inner side irregularly covered with capsulated setae arranged in a curved row; fixed finger elongated, with a horseshoe-shaped, spoon-like tip. Etymology: In reference to the type locality. Type material: The holotype, OESM 6811, is a near-complete right palm with fixed finger, measuring 10 mm in length and 6 mm in greatest width; both external and internal moulds are present ( Fig. 2A-G Locality and stratigraphy: The coral limestone assigned to the Faxe Formation has been known for centuries due to the extensive quarrying. The middle Danian levels exposed at Faxe quarry represents an extraordinary preservation of a 63-myr-old, cold-water coral mound complex. This coral limestone complex was formally described as a new formation by Lauridsen et al. (2012). The type locality of the Faxe Formation is a Danish GeoSite and is accessible to all members of the public. The formation was dated as middle Danian (calcareous nannoplankton zones D5-6 of Perch-Nielsen, 1979 and NNTp3-NNT-p4a of Varol, 1998).
Description: Cross section of palm transversely oval; inner and outer sides longitudinally and transversely convex, curving slightly inwards; dorsal edge straight, ventral edge concave at central part; outer side of palm covered with forwardly directed, largely spinose tubercles, largest tubercles irregularly arranged along dorsal edge and outer distal half and proximal half of fixed finger, proximal part of outer side of palm irregularly covered with multiple capsulated setae arranged in circles or rows; inner side of palm smooth with on distalmost part few curved rows of multiple setae (Fig. 3B); fixed finger elongated, curved inwardly, with horseshoe-shaped, spoon-like tip, which is covered on outer side with a row of very large pits; the cutting edge is covered with about three variably sized, molar-like calcareous teeth.
Remarks: Normally, setal pits are situated around the bases of tubercles and/or on the adjacent integument, but usually not on the tubercles themselves. Multiple capsulated pits have previously been recorded for both fossil and extant species of paguroids (see Fraaije et al., 2011Fraaije et al., , 2015aHyžný et al., 2016). During the Paleogene, the genus Dardanus appears to have been highly successful and widely distributed (Fraaije & Polkowsky, 2016). In the case of Dardanus faxensis sp. nov. all tubercles on the outer side of the palm have large setal pits ( Fig. 2A); this morphological trait differentiates it immediately from all congeners. For a list of all known fossil species referred to the genus, reference is made to Fraaije et al. (2011) and to Fraaije & Polkowsky (2016). The shape of the chelae is indicative of a deposit feeder that was also capable of browsing, scavenging and suspension feeding. This type of paguroid fed by scraping material from corals or other solid surfaces using the tips of the spoon-like chelae (compare Schembri, 1982). One specimen, MGUH 33406, is totally covered with epibionts, mainly subcircular colonies of bryozoans (Fig. 4).  Three specimens of D. faxensis sp. nov. bear remains of serpulid/spirorbid annelids on the outer palm surface at the base of the fixed finger (Fig. 5). This appears to have been a symbiotic relationship, rather than post-mortem serpulid attachment to moulted anomurans, as described by Jakobsen & Feldmann (2004).
( Fig. 6A-E) Diagnosis: Right cheliped transversely oval; outer, dorsal and ventral sides convex, inner side almost straight; outer side covered with regular, dense cover of fine, forwardly directed tubercles, fixed finger short, stout and triangular.
Type material: The holotype, OESM 6705, is a near-complete right palm with fixed finger, measuring 8.5 mm in length and 5.5 mm in greatest width. The paratype, MGUH 33407, is an incomplete right palm, still embedded in coral-limestone.  Description: Cross section of right palm transversely oval; outer, dorsal and ventral sides convex, inner side almost straight; outer side with regular, dense cover of fine, forwardly directed tubercles; fixed finger short, stout and triangular; occlusal margin straight without teeth, surrounded by few large pores; interdigital margin oblique; length of fixed finger about one quarter of total length.
Remarks: Fraaije et al. (2015b) listed all extinct species of Paguristes known at that time. With a convex dorsal and ventral edge and a short, convex triangular fixed finger, P. frigoscopulus sp. nov. is morphologically closest to the late Albian P. liwinskii Fraaije, van Bakel, Jagt & Machalski, 2015 from east-central Poland and P. santamartaensis Feldmann, Tshudy & Thomson, 1993, from the Campanian of Antarctica. It differs in having a less dense, more forwardly directed, tuberculose ornamentation.

Conclusions
In total, fourteen specimens were available for the present study. Of these, about 86 % (12 specimens) are attributed to D. faxensis sp. nov. and only c. 14 % (2 specimens) to P. frigoscopulus sp. nov. Unfortunately, there are no detailed studies yet that relate cheliped morphology to feeding strategy. Schembri (1982) is one of the few studies that documented a close relationship between cheliped morphology and types of feeding. If we extrapolate his data to Dardanus faxensis sp. nov., that species may be considered to have been a generalist and better adapted to the Danian deep-water reefal environment of Faxe than P. frigoscopulus sp. nov., which probably was mainly a filter feeder.