Unraveling the hidden paleobiodiversity of the Middle Devonian (Emsian) crinoids (Crinoidea, Echinodermata) from Poland

Most previous publications on Devonian crinoids from the Holy Cross Mountains in Poland have concentrated on crinoid columns, and until now, little has been published about crinoid cups and calyxes. Herein, five crinoid taxa are described from an abundant occurrence of aboral cups and partial crowns from the Bukowa Góra Member (Emsian) in the Holy Cross Mountains of southern Poland. The following taxa are described: Bactrocrinites sp., Codiacrinus sevastopuloi sp. nov., Halocrinites geminatus (Bohatý, 2005), Halocrinites schlotheimii Steininger, 1831, and a single brachial plate from a flexible crinoid placed in Flexibilia incertae sedis. Simple discoid holdfasts are also present encrusted to cylindrical stromatoporoids. These taxa are the first crinoids described from the remains of partial crowns from Emsian strata of Poland.

Full-size DOI: 10.7717/peerj.12842/ fig-1 mainly columnals; and they were described using the principles of artificial classification of crinoid remains proposed by Moore & Jeffords (1968). Głuchowski (2003) added that the applicability of crinoid stems may be useful for stratigraphic and correlation purposes.
Here we report complete or almost complete Emsian cups associated with numerous isolated calyx and column remains from the Bukowa Góra Member in the Holy Cross Mountains of southern Poland. These include Bactrocrinites sp., Codiacrinus sevastopuloi sp. nov., Halocrinites geminatus (Bohatý, 2005), Halocrinites schlotheimii (Steininger, 1831), and Flexibilia incertae sedis. Simple discoid holdfasts are also present. Remains of unidentifiable specimens indicate that several other crinoids also existed in the Bukowa Góra Member fauna.

GEOLOGIC FRAMEWORK
The Holy Cross Mountains are located in the southern part of Poland. Their main element is the Paleozoic core, divided into two parts: the Łysogóry region (northern, connected with the Łysogóry Block) and the Kielce region (southern, connected with the Małopolska Block; see Fig. 2A). These regions differ from each other by facies development of contemporaneous deposits. Devonian sediments of the Łysogóry region were formed in the deeper basin in contrast to the shallower facies exposed in the Kielce region (Szulczewski, 1995).
Outcrops of Lower Devonian rocks in the Łysogóry Region are connected with the southern limb of the Bodzentyn Syncline. The sedimentary rocks of the upper Emsian are best exposed in the active quarry ''Bukowa Góra'', located about 16 km northeast of Kielce (see Fig. 2A). The section includes sediments belonging to patulus Conodont Zone and douglastownense-eurypterota Miospore Zone Malec, 2005;Filipiak, 2011;Fijałkowska-Mader & Malec, 2011).
As a result of the progressive deepening of the marine basin, deposits of the Grzegorzowice Formation were formed, which is also present in the Kielce region (Malec, 2005;Wójcik, 2015). In the Bukowa Góra quarry section the two members are present: Bukowa Góra Member and the Kapkazy Member. The Bukowa Góra Member occurs only within the patulus Conodont Zone in the western part of Bodzentyn Syncline (see Fig. 2B). In the eastern part of the Bodzentyn Syncline, the Bukowa Góra Member appears earlier, i.e., in the serotinus Conodont Zone (Malec, 2005). Malec (2005) marked the lower boundary of the Bukowa Góra Member in the bottom of the complex of dark claystones, whereas Szulczewski & Porębski (2008) put this boundary on the pebble conglomerate that begins in the lower shoreface to offshore transitional series.
In the lower part of interbedded sandstones, the Bukowa Góra Member is comprised of sandstones, siltstones, and claystones, which are about 7 m thick (see Szulczewski & Porębski, 2008). Above them appears the offshore facies represented by black to dark-gray claystones and silty claystones with a thickness of ∼13 m and containing discontinuous beds of dolomitic limestones up to 10 cm thick (see Fig. 2B). Both claystones and limestones contain a rich faunal assemblage related to the colonization of the soft sea bottom. There are massive colonies of both stromatoporoids and tabulate corals accompanied by solitary rugoses, brachiopods, crinoids, ostracods, gastropods, and trilobites (Malec, 2005; see also Głuchowski, 1993b;Fijałkowska-Mader et al., 1997).
Claystones of the Bukowa Góra Member are overlain by sandstones of the Kapkazy Member, which is ∼34 m thick. The lower part of the Kapkazy Member is comprised of coarse-grained and conglomeratic sandstones, containing rare crinoids, brachiopods, and gastropods. Above this is fine-grained sandstone, which is indicative of a clear shallowing of the sea basin (Malec, 2005).

MATERIALS AND METHODS
The studied material from Bukowa Góra Quarry was collected in 2019 and 2021. The first step consisted of examination of slab surfaces in the field. At this stage, numerous crinoid remains (isolated columnals and complete or nearly complete crowns) were collected. The next step consisted of soaking the respective samples (11 shales samples weighing each ca. 10 kg) only with hot water. Limy samples (4 samples weighing each ca. 5 kg) were soaked with Glauber's salt. These samples were then boiled and frozen (2-3 times). The residues were finally washed with running tap water and sieved on a sieve column (Ø1.0, 0.315 and 0.1 mm mesh). The final step consisted of drying the shaly and limy residues at 160 • C. Residue was hand-picked from each macerated sample for microscopic study.
All crinoids were photographed by a SONY DSC-RX10M3 digital camera. Specimens discussed here are deposited in the University of Silesia in Katowice, Faculty of Natural Sciences, Institute of Earth Sciences, Poland (GIUS 4-3696) and in the Senckenberg Forschungsinstitute und Naturmuseum, Frankfurt am Main, Germany (SMF).
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Diagnosis
Aboral cup medium bowl shape; three or more radiating ridges from center of basal plates that project onto radial and infrabasal plates, also very fine nodose sculpturing across calyx plates; basal plates largest plates of aboral cup; radial facets ∼50% of distal width of radial plates (arms and column characters not known).

Description
Aboral cup medium globe shaped (Figs. 3A4, 3B1), height to width ratio ∼1.0, maximum width at middle aboral cup height; three or more radiating ridges from center of basal plates that project onto radial and infrabasal plates (Fig. 3A1), also very fine nodose sculpturing across calyx plates. Infrabasal circlet ∼9% of aboral cup height, extends proximally in a short neck that is truncate proximally with a shallow, circular basal concavity that occupies ∼75% of proximal aboral cup width (Figs. 4A2, 4B2). Five pentagonal infrabasal plates, ∼3.8 times wider than high, outer surface concave, sculpturing irregular nodose. Basal circlet ∼55% of aboral cup height; basal plates largest plates in aboral cup, hexagonal, ∼1.2 times higher than wide; sculpturing with radiating ridges and nodes, ridges from near the center of the plates to ridges on adjoining proximal and distal plates. Radial circlet ∼36% of aboral cup height; radial plates ∼1.2 times wider than high, pentagonal; plate sculpturing with ridges and nodes, ridges diagonal from base of radial facet to like ridges on adjoining basal plates. Radial facets angustary (∼52% of radial plate distal width), horseshoe shaped (Figs. 4A3, 4B3). Radial facets, arms, and column unknown.

Etymology
The species name is in recognition of the substantial contributions that George D. Sevastopulo made to crinoid paleobiology, as well as paleontology and stratigraphy in general.

Remarks
One well-preserved and two poorly preserved aboral cups are assigned to Codiacrinus sevastopuloi sp. nov. Both poorly preserved specimens have their shapes distorted through compaction. Ten species of Codiacrinus, including C. sevastopuloi, are recognized herein. C.? weyeri is excluded, and it is regarded either as an aberrant individual or a member of another genus. Of these ten species, only three have arms and proximal columnals preserved (C. robustus, C. schultzei, and C. secundus). Thus, species diagnoses are largely based on characters of the aboral cup, which vary widely.
The most noticeable character used to differentiate species of Codiacrinus is the aboral cup shape, which may be low bowl, high cone, medium globe, high globe, or medium vase in shape. Codiacrinus granulatus, C. nicolli, C. robustus, and Codiacrinus sevastopuloi sp. nov. all have a medium globe-shaped aboral cup. Codiacrinus granulatus has a medium globe-shaped aboral cup, two poorly defined radiating ridges from the base of the radial facet onto each subjacent basal plate and perhaps some poorly developed concentric ridges, radial plates are the largest plates in the aboral cup, and the radial facets occupy ∼50% of the distal radial plate width. Codiacrinus nicolli has a medium globe-shaped aboral cup, very fine nodose sculpturing, radial plates are the largest plates in the aboral cup, and the radial facets occupy ∼60% of the distal radial plate width. Codiacrinus robustus has a medium globe-shaped aboral cup, three radiating ridges from the basal plate center and otherwise smooth sculpturing, basal plates are the largest plates in the aboral cup, and the radial facets occupy ∼50% of the distal radial plate width. Alternatively, Codiacrinus sevastopuloi sp. nov. has a medium globe-shaped aboral cup, three or more radiating ridges from center of basal plates that project onto radial and infrabasal plates, also very fine nodose sculpturing across calyx plates, basal plates are the largest plates in the aboral cup, and the radial facets occupy ∼50% of the distal radial plate width.

Description
Relatively large aboral cup, aboral cup plates with pustulose plate sculpturing (Fig. 3E). Infrabasal plates not known. Basal plates partially preserved, inferred to be the dominant plate circlet in aboral cup (Fig. 3E). C radial plate supported beneath by radianal plate and BC basal plate; D radial plate larger that C radial plate, supported beneath by CD and DE basal plates. Radial facets large, semicircular, angustary, declivate. Two anal plates in aboral cup. Radianal presumably tetragonal, below and to the left of the C radial plate and supports the anal X plate on the upper left (Fig. 4). Anal X plate hexagonal, supported beneath by the CD basal plate and the radianal, separates and articulates with lateral sides of the C and D radial plates.
Other aspects of the aboral cup, anal sac, arms, and column are not known.

Remarks
Species diagnostic characters for Devonian Bactrocrinites include shape of the aboral cup, plate sculpturing, relative heights of aboral cup plates, and the dimensions of the infrabasal and basal plates. Unfortunately, the single specimen of Bactrocrinites from the Emsian of Poland is not complete (GIUS4-3693/Bactrocrinites), so aboral cup shape, relative proportions of aboral cup plates and the dimensions of the basal plates cannot be determined. The pustulose aboral cup plate sculpturing and what are inferred to be prominent (high) basal plates most closely ally this specimen with B. fusiformis. However, a more complete accounting of the morphology of this Emsian specimen is required before a confident species assignment can be made.

Remarks
As discussed in Bohatý & Ausich (2021), generic and specific assignments of the Cupressocrinitidae have been varied, commonly changed, and confused until recently (e.g., Bohatý, 2005;Bohatý, 2006;Bohatý, 2009b;Bohatý & Herbig, 2010;Bohatý & Ausich, 2021). Two species of Halocrinites are recognized from the Bukowa Góra Member in Poland, including H. geminatus (Bohatý, 2005) and H. schlotheimii (Steininger, 1831). Most Halocrinites specimens from Poland are lacking the exoplacoid layer or are sufficiently worn that the character of the exoplacoid layer cannot be determined. One exception is specimen GIUS 4-3696Hscholth6 (Fig. 5E), although even this specimen is worn. They are differentiated on the basis of aboral cup shape, basal plate morphology, size of the infrabasal circlet relative to the size of the proximalmost columnal, and the size of the basal concavity, as described below. Halocrinites schlotheimii has a bowl-shaped aboral cup with a ratio of aboral cup diameter versus crown height ∼1:1.15-2.0; aboral cup typically ∼2.0 times wider than high; infrabasal plates fused into a single pentagonal plate that is confined to the basal concavity; brachials wider than high (height to width ratio ∼1:2.0-2.5); proximal columnal circular not filling entire basal concavity. In contrast, H. geminatus aboral cup bowl to moderately conical in shape; ratio of aboral cup diameter versus crown height ∼1:1.15-2.0; typically 2.0 times wider than high; infrabasal plates fused into a single pentalobate plate that is confined to the basal concavity; brachials wider than high (height to width ratio ∼1:2.0-2.5); proximal columnal circular not filling entire basal concavity. Similar to the Cupressocrinitidae described by Bohatý (2009b) and Bohatý & Ausich (2021), Halocrinites from Poland have a variety of epizoans encrusting the outer surface of crown plates. These include trepostome bryozoans encrusted on aboral cup and brachial plates. A presumable microconchid that is attached to a radial plate, and a juvenile pelmatozoan holdfast is attached to a different radial plate. These encrustations did not induce a recognizable response from the crinoid host, so it is probable that these encrustations occurred after the death of the crinoid and, thus, are episkeletozoans (see Taylor & Wilson, 2002).

Type
The type specimens for this taxon are not known.

Diagnosis
Halocrinites with bowl-shaped aboral cup with a ratio of aboral cup diameter versus aboral cup height ∼1:1.15-2.0; aboral cup typically ∼2.0 times wider than high; infrabasal plates fused into a single pentagonal plate that is confined to the basal concavity; brachials wider than high (height to width ratio ∼1:2.0-2.5); proximal columnal circular not filling entire basal concavity.

Occurrence
In Poland, H. schlotheimii is from the Bukowa Góra Member (Emsian), Bukowa Góra quarry, Holy Cross Mountains, Poland. Previously, this species has been described from the Eifelian and Givetian of Australia, China, Germany, Poland, and Spain (Webster & Webster, 2019).

Remarks
Six specimens of H. schlotheimii are known from the Emsian of Poland (GIUS4-3693/Hschloth1 to GIUS4-3693/Hschloth6). In the collection of Polish specimens, small inidividuals tend to have more pronounced convex basal plates that nearly produce a central node.

Diagnosis
Halocrinites with aboral cup bowl to moderately conical in shape; ratio of aboral cup diameter versus aboral cup height ∼1:1.15-2.0; typically 2.0 times wider than high; infrabasal plates fused into a single pentalobate plate that is confined to the basal concavity; brachials wider than high (height to width ratio ∼1:2.0-2.5; proximal columnal circular not filling entire basal concavity.

Remarks
A single brachial plate is identified as a flexible crinoid (GIUS4-3693/flexible; Fig. 7I). The brachial plate is ∼4.0 times deeper than high with, only the distal facet is visible and part of the sides of the plate are visible that includes the aboral indentation where a patelloid process from the distal adjoining brachial would reside. The brachial plate is as wide as deep. A crenulated articular ridge is present along the abaxial portion of the facet, and the lateral sides of the facet are crenulated. A narrow, shallow aboral groove is present along the adaxial margin of the facet.

Remarks
Solitary rugose coral and a presumable stromatoporoid specimens associated with the described crinoids have small, discoid holdfasts cemented to their outer surface (Figs. 8A, 8B). These holdfasts are subcircular in outline and some have a slightly digitate outer (G) columnal facet with a narrow peripheral lumen and a narrow, raised perilumen; (H) columnal with a wide lumen and a narrow, raised perilumen: (I) Flexible crinoid brachial. GIUS 4-3696/flexible; note crenulate sides and on the upper margin a notch to hold a patelloid process from the next highest columnal. All specimens are from Bukowa Góra Member (Emsian), Bukowa Góra quarry, Holy Cross Mountains, southern Poland. All scale bar equals 10 mm but in case of flexible crinoid brachial it is 1 mm. margin. In one example, the holdfast articulation to the column was canted toward the long axis of a rugose coral, suggesting the crinoid was encrusted to the coral when both were alive. Therefore, these holdfasts should be considered epizoozoans (Taylor & Wilson, 2002). It is not possible to speculate on the identity of the crown that was attached to these holdfasts, and the smaller specimens may have been juveniles or from multiple radices of a single adult.

CONCLUDING REMARKS
The first Emsian crinoids described on the basis of aboral cups and crowns are reported here from the Bukowa Góra Member in the Holy Cross Mountains of southern Poland. Named taxa include Bactrocrinites sp., Codiacrinus sevastopuloi sp. nov., Halocrinites geminatus (Bohatý, 2005), Halocrinites schlotheimii (Steininger, 1831). Taxa that can only be recognized as incertae sedis include one flexible crinoid, as many as three camerate crinoids, as many as four additional cladid crinoids, and a number of distinctive holdfasts, columnals, and pluricolumnals that cannot be matched with the crown to which they were attached. Additional collecting in the Bukowa Góra Member should yield remains of many crinoids. Previously, Halocrinites (including H. schlotheimii) have been described from younger Devonian strata in Poland (see Fig. 1). Further, species of Bactrocrinites, Codiacrinus, and Halocrinites occur in other Devonian crinoid faunas from Germany and Spain (Webster & Webster, 2019).
The new crinoids reported here are from the Bukowa Góra Member of Poland (Emsian) and are an extension of the Lower to Middle Devonian crinoid faunas from across Europe, which are best represented by Emsian to Givetian crinoids from Germany and Spain (e.g., Bohatý, 2005;Bohatý, 2006;Bohatý, 2009b;Bohatý & Herbig, 2010;Hauser, 2001;Hauser, 2002;Hauser, 2007). Halocrinites has been reported from Germany, Spain, Belgium, and Russia (Eifelian-Frasnian); wheras both Codiacrinus and Bactrocrinites have longer ranges and are cosmopolitan in distribution. In addition, to Western Europe, Codiacrinus is known from Gondwana terrane (northern Africa and Australia). The oldest recognized species of Bactrocrinites is from the middle Silurian of North America, and this genus is only known from North America and Europe. As known, Bactrocrinites became extinct at the Givetian-Frasnian extinction and Bactrocrinites and Halocrinites became extinct at the Frasnian-Famennian extinction.