Non-technical Summary
A newly discovered hermit crab species, Cretatrizocheles rodfeldmanni, has been identified in a limestone block from the Early Cretaceous period near Velika Strmica village in southeast Slovenia. This discovery contributes to our understanding of the Trizochelinae subfamily, which thrived in shallow-marine reef environments during the Late Jurassic and Early Cretaceous in Europe. These findings suggest a possible link between their past habitat preferences and the Mesozoic Marine Revolution, as they are now found exclusively in deep-marine settings.
Introduction
Members of the family Pylochelidae Bate, Reference Bate1888, commonly known as symmetrical hermit crabs, differ from all other marine paguroids in the presence of complete abdominal segmentation and paired appendages on each segment. Pylochelids do not usually inhabit gastropod shells but prefer decayed pieces of wood, stones, empty mollusk shells (mainly scaphopods, but also gastropods), serpulid worms, or living sponges (Forest, Reference Forest1987b). Bathymetrically, the range of pylochelids varies from 30 to 1,570 m, but most representatives have been recorded from depths between 200 and 500 m (Forest, Reference Forest1987a, Reference Forestb). Until Forest (Reference Forest1987b) published his seminal monograph, the importance of pylochelid paguroid faunas in tropical and subtropical deeper waters had been underestimated. One reason for this is that, because of their cryptic habitats, they were rarely collected (McLaughlin and Lemaitre, Reference McLaughlin and Lemaitre2009).
Currently, pylochelids are assigned to four subfamilies: Parapylochelinae Forest, 1987 (Forest, Reference Forest1987a); Pomatochelinae Stebbing, Reference Stebbing1914; Pylochelinae Bate, Reference Bate1888; and Trizochelinae Forest, 1987 (Forest, Reference Forest1987a) (see e.g., Schweitzer et al., Reference Schweitzer, Feldmann and Fraaije2023). The last-named subfamily is by far the most speciose in present-day marine environments (e.g., McLaughlin and Lemaitre, Reference McLaughlin and Lemaitre2009). To date, the genus Trizocheles accommodates 22 species (Komai, Reference Komai2013; Komai and Chan, Reference Komai and Chan2016), the majority of which are spongicoles (Forest et al., Reference Forest, de Saint Laurent, McLaughlin and Lemaitre2000).
Geographically, species of Trizocheles are generally limited to narrow areas (McLaughlin and Lemaitre, Reference McLaughlin and Lemaitre2009), which is probably because their larvae are lecithotrophic, thus limiting their dispersal ability (Komai, Reference Komai2013). As a general rule, the vertical range for each species of Trizocheles is also relatively narrow (120 m at the most); in other words, this genus appears to be the most stenobath of all pylochelid hermit crabs (Forest, Reference Forest1987a).
Fossil representatives of this paguroid group are rare and poorly documented, which may be explained, at least in part, by their small size and unremarkable appearance. However, they are much more common than previously thought, as both recent work (Fraaije et al., Reference Fraaije, Klompmaker and Artal2012, Reference Fraaije, Robins, Van Bakel, Jagt and Bachmayer2019) and the new record described herein document.
Locality and stratigraphy
Velika Strmica is situated approximately 10 km northwest of Novo mesto in southeast Slovenia (Fig. 1); structurally, the area belongs to the Dinarides (Placer, Reference Placer2008). The stratigraphical succession exposed near Velika Strmica starts with Triassic, Jurassic, and Lower Cretaceous carbonates (Buser, Reference Buser2009) deposited along the margin of the Dinaric Carbonate Platform (Vlahović et al., Reference Vlahović, Tišljar, Velić and Matičec2005). These platform carbonates are discordantly overlain either by upper Santonian to mid-Campanian marly limestones or by Campanian–Maastrichtian flysch-type deposits (Pleničar and Premru, Reference Pleničar and Premru1977; Trotošek, Reference Trotošek2002).
Simplified map of eastern Slovenia showing the locality of Velika Strmica (star) and the present-day position of Cretaceous outcrops across Slovenia (adapted after Buser, Reference Buser2009).
Lower Cretaceous (Aptian/Albian) carbonate olistolith blocks of reefal limestone are found embedded within the younger turbidite flysch series, west of the village of Velika Strmica.
The limestone olistolith measures approximately 2 m in diameter and consists of grainstone in its lower part, followed by sponge floatstone on the top (Gašparič and Gale, Reference Gašparič, Gale, Novak and Rman2018). While grainstone is less rich with fossil remains, the floatstone facies contains a rich fauna with sponges, decapod crustaceans, corals, and brachiopods. Within the grainstone, clasts ranging between 0.1 and 1.3 mm represent 70% of the sample. The sediment is moderately well sorted. Fossils are represented by rare foraminifera, bryozoans, and brachiopods. Bryozoan colonies are fragmented and later abraded. Zooecia are filled with micrite. Most of the benthic foraminifera are fragmentary, whereas planktic forms are better preserved.
In the sponge floatstone, clasts larger than 2 mm represent 20–40% of the area. Sorting is very poor. Most of these are stromatoporoid and chaetetid sponges; corals and brachiopods are subordinate. Calcified sponges and corals are commonly encrusted by Lithocodium/Pseudolithocodium-like crusts, sessile foraminifera, serpulids, and red algae. Serpulids are also found within internal canals of sponges. Microborings are also very common on the outer surface of sponges and corals. Brachiopod shells are preserved with closed valves. Complete bivalve and gastropod shells are rarely preserved. Other grains include foraminifera, echinoderm ossicles, echinoid spines, fragmented bryozoans, and ostracods.
The present pylochelid paguroid originates from this olistolith, from which a diverse decapod crustacean fauna has previously been recorded (Gašparič and Gale, Reference Gašparič, Gale, Novak and Rman2018; Gašparič et al., Reference Gašparič, Robins and Gale2020, Reference Gašparič, Van Bakel, Fraaije and Jagt2022). The olistolith has been dated as late Aptian to middle Albian on the basis of the presence of orbitolinid foraminifera, mainly the species Mesorbitolina texana (Roemer, Reference Roemer1849) (Gašparič et al., Reference Gašparič, Van Bakel, Fraaije and Jagt2022).
Material and methods
This study is based on a single anterior dorsal carapace (shield), registered as RGA/SMNH 4543 (part) and RGA/SMNH 4586 (counterpart) in the R. Gašparič Collection at the Institute of Palaeobiology and Evolution (Ljubljana, Slovenia). The specimen was collected by one of the authors (R.G.) in 2020 and prepared using fine needles under a Leica EZ 4D stereomicroscope. Photographs were taken with a digital camera Nikon Z 7II with a Laowa 25 mm/f2,8 5x Ultra Macro lens. The photographs were subsequently stacked using the Helicon Focus 8 software to improve depth of field, and levels were adjusted in Adobe Photoshop CS6. For some photographs, the specimen was whitened with ammonium chloride sublimate before photography to enhance details of ornamentation.
Repository and institutional abbreviation
RGA/SMNH—R. Gašparič Collection, part of the paleontological collections of the Natural History Museum of Ljubljana (Slovenia, SMNH).
Systematic paleontology
Order Decapoda Latreille, Reference Latreille1802
Infraorder Anomura MacLeay, Reference MacLeay and Smith1838
Superfamily Paguroidea Latreille, Reference Latreille1802
Family Pylochelidae Bate, Reference Bate1888
Subfamily Trizochelinae Forest, Reference Forest1987
Genus Cretatrizocheles Fraaije, Klompmaker, and Artal, Reference Fraaije, Klompmaker and Artal2012
Type species
Cretatrizocheles olazagutiensis Fraaije, Klompmaker, and Artal, Reference Fraaije, Klompmaker and Artal2012 by original designation.
Included species
In addition to the type species, C. doerflesensis Fraaije et al., Reference Fraaije, Robins, Van Bakel, Jagt and Bachmayer2019 and C. rodfeldmanni n. sp.
Cretatrizocheles rodfeldmanni new species
Figure 2
Holotype
Holotype, and sole specimen known to date, is RGA/SMNH 4543 (part) and RGA/SMNH 4586 (counterpart). It is a well-preserved anterior dorsal carapace (shield).
Holotype (RGA/SMNH 4543) of Cretatrizocheles rodfeldmanni n. sp. from the upper Aptian–lower Albian of the Velika Strmica olistolith. (1) Dorsal shield; (2) ammonium chloride whitened dorsal shield; (3) lateral view of dorsal carapace. (1, 2) Scale bars = 2 mm; (3) scale bar = 1 mm.
Diagnosis
Shield well calcified, nearly as long as wide. Prominent triangular rostrum considerably longer than post-ocular projection. Narrow, elongated massetic region. Globose keraial region. Prominent undulose post-rostral ridge. Short yet distinct central gastric groove. Posterior carapace not preserved.
Occurrence
Known only from the type locality, late Aptian to middle Albian of Velika Strmica, Slovenia.
Description
Shield smooth, strongly calcified, about as long as wide; prominent, broad-based, triangular rostrum, considerably longer than postocular and post-antennal projections. Short central gastric groove at posteriormost part of deep, concave, and undulose post-rostral ridge. Distinct lateral gastric grooves originating posterior to keraial region and effacing into subcircular lateral edges of post-rostral ridge. U-shaped cervical groove curving anteriorly toward thin and elongated massetic region. Posterior carapace, appendages, abdomen, and cuticle not preserved.
Etymology
The specific epithet honors the paleontologist Dr. Rodney Feldmann in recognition of his considerable contributions to the crustacean paleontological record.
Remarks
Until now, the pylochelid paguroid genus Cretatrizocheles was known exclusively from Tithonian reefal limestones at Ernstbrunn (Austria; see Fraaije et al., Reference Fraaije, Robins, Van Bakel, Jagt and Bachmayer2019) and upper Albian reefal limestones in Navarra, northern Spain (Fraaije et al., Reference Fraaije, Klompmaker and Artal2012). Cretatrizocheles rodfeldmanni n. sp. is the third member of the genus to occur in reefal limestones, this time in Slovenia. The new species differs from the type species, C. olazagutiensis, in having a more pronounced and centrally concave post-rostral ridge that becomes more angular laterally and a longer central gastric groove. Cretatrizocheles rodfeldmanni differs from C. doerflesensis in having a broader, less elongate and more globose keraial region.
The only other fossil trizocheline taxon recorded to date, Cretatrizocheles doerflesensis Fraaije et al., Reference Fraaije, Robins, Van Bakel, Jagt and Bachmayer2019, from Tithonian reefal limestones at Ernstbrunn (Austria) can be differentiated from C. rodfeldmanni in having a broader rostral base, a straight to centrally slightly convex post-rostral ridge, a longer central gastric ridge, and a narrower keraial region.
Cretatrizocheles rodfeldmanni is a significant addition to the fossil record of the symmetrical pagurid subfamily Trizochelinae, whose members apparently were successful inhabitants of shallow-marine reefal settings during the Late Jurassic–Early Cretaceous in central and southwest Europe.
Discussion
Although restricted to the Indo–West Pacific at present (Forest, Reference Forest1987b), the genus Trizocheles is by far the most diverse taxon of pylochelid paguroids. There is a tendency among marine decapod crustaceans for species that have an advanced or highly abbreviated larval development to show more or less localized distributions (e.g., Komai, Reference Komai2013). This appears to be the case for species of Trizocheles because the majority have a restricted, endemic range (e.g., McLaughlin and Lemaitre, Reference McLaughlin and Lemaitre2009; Komai Reference Komai2013). In addition, they demonstrate a mutualistic relationship with thin-walled dictyonin hexactinellid sponges or with more massively built lithistid and haplosclerid demosponges (Forest, Reference Forest1987b; McLaughlin and Lemaitre, Reference McLaughlin and Lemaitre2009).
A spongicolous inquilinistic lifestyle, with lecitrotrophic larvae, is highly beneficial in a shallow-marine, reef-like environments. In deep-marine settings, a planktotrophic development is generally more advantageous, as is a less closely knit inquilinistic way of life (Wisshak et al., Reference Wisshak, Neumann, Jakobsen and Freiwald2009). It is quite possible that the possession of lecitrotrophic larvae and a spongicolous inquilinistic lifestyle are relicts from the Mesozoic. The lack of appropriate abodes may have induced some species of Trizocheles to settle for various shells, mainly dentaliid scaphopods, but gastropod shells or serpulid tubes are occasionally chosen (Forest, Reference Forest1987b).
Having been recovered exclusively from shallow-marine, reef-related facies of Middle and Late Mesozoic age and occurring nowadays only in deep-marine settings, suggests that the Trizochelinae present yet another example of the effect of the Mesozoic Marine Revolution (e.g., Vermeij, Reference Vermeij1977; Tackett, Reference Tackett2016), similar to stalked crinoids (Isocrinida), which, with a few exceptions, all migrated to deeper waters by the Late Cretaceous (Jagt, Reference Jagt1999; Hess, Reference Hess, Hess and Messing2011; Gorzelak et al., Reference Gorzelak, Salamon, Trzęsiok, Lach and Baumiller2012). The shift in the bathymetric range among sessile stalked crinoids during the Late Mesozoic, from shallow shelves to habitats further offshore, is likely a result of increased predation pressure in shallow waters (Oji, Reference Oji1996).
Of note is that the cyrtocrinid crinoid Proholopus holopiformis (Remeš, Reference Remeš1902) has recently been recorded by Zamora and López Horgue (Reference Zamora and López-Horgue2022) from upper Albian strata (Albeniz unit), close to the locality that yielded Cretatrizocheles olazagutiensis in Navarra (western Pyrenees, Spain). This record ranks among the youngest occurrences of cyrtocrinids in shallow-marine environments before their migration into the deep sea as a result of the Mesozoic Marine Revolution.
Additional fieldwork and screening of collections for extinct pylochelids should clarify when these symmetrical hermit crabs also migrated from shallow- to deep-marine settings. We suggest this must have occurred in post-Albian times. To date, intensive fieldwork over years in the Maastrichtian type area (southeast Netherlands and northeast Belgium) by ourselves and in Eocene shallow-marine facies of northern Italy by our Italian colleagues has not yielded any trizocheline paguroid remains.
Acknowledgments
We thank the journal reviewers, G. Schweigert (Staatliches Museum für Naturkunde, Stuttgart, Germany) and F.A. Ferratges (Instituto Geológico y Minero de España, Zaragoza, Spain), for constructive comments on an earlier version of the manuscript.
Declaration of competing interests
None of the authors received any specific grants from funding agencies in the public, commercial, or nonprofit sectors for the present study.
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