Fenestrate bryozoan fauna from the Middle Devonian of the Eifel (western Rhenish Massif, Germany)

Seven fenestrate bryozoans are described from the Middle Devonian of the Eifel. Three species were identified as Prolixicella bifurcata Ernst and Schroeder, 2007, Rectifenestella aculeata (Sandberger and Sandberger, 1856), and Spinofenestella antiqua (Goldfuss, 1826). Four species are described in open nomenclature: Rectifenestella sp. 1 and sp. 2, Ptylopora sp., and Spinofenestella sp. Analysis of the distribution of the fenestrate bryozoans from the Middle Devonian of the Eifel shows their relatively high level of endemism revealing some few connections to the Devonian of Europe and Asia. The studied association shows similarities to the contemporary fauna from Sauerland.

The evaluation of self-collected material and museum bryozoan collections from the Middle Devonian (Eifelian-Givetian) of the Eifel revealed the presence of diverse fenestrate taxa: phylloporines Prolixicella and Bashkirella as well as fenestellid genera Fenestella, Spinofenestella, Rectifenestella, Dissotrypa, Anastomopora, Bigeyina, Loculipora, Fenestrapora, Schischcatella, Ptylopora, and Hemitrypa. The majority of these bryozoans were treated in recent studies (Ernst and Schroeder 2007;Ernst and Bohatý 2009;Ernst et al. 2012;Ernst 2016Ernst , 2020Ernst , 2022. From this fauna, the genus Hemitrypa is currently excluded for a later investigation, whereas the present paper deals with the study of the representatives of the genera Prolixicella, Rectifenestella, Ptylopora, and Spinofenestella.

Localities and stratigraphy
The material for this study comes from various localities in the Rhenish Massif (Fig. 1). Material was collected by the author in years 2008-2010. Further samples from Essingen-Hohenfels and Schwirzheim were provided by Jan Bohatý, Wiesbaden. All these specimens are deposited at the Senckenberg Research Institute and Natural History Museum, Frankfurt am Main, Germany (SMF-numbers). Material from Lüdenscheid, Sauerland, was collected by Frank Langenstrassen, Göttingen, and deposited at the Geological Centre Göttingen, Germany (GZG-numbers). Type material of Sandberger and Sandberger (1856) is deposited in Sandberger`s Devonian Collection, the Natural History State Collection of the Wiesbaden Museum, Wiesbaden, Germany.
The stratigraphy of Devonian sedimentary rocks at studied localities has been adopted from Struve and Werner (1982), with corrections by Jan Bohatý (pers. comm., 2008;Figs. 2 and 3).
This quarry contains muddy limestones of the Olifant Member of the Müllert Subformation of the Ahbach Formation (lowermost Givetian) (Fig. 3b).
The localities near Lüdenscheid in the Sauerland (51°09, 7°33) include a series of outcrops of silty and richly fossiliferous limestones of the Honsel and Werdohl formations (Givetian). Most of these outcrops are no longer accessible (Frank Langenstrassen, pers. comm., 2006). The Werdohl Formation corresponds to the lower part of the Honsel Formation (Unterhonsel Fm.) of the Sauerland and to the Loogh Formation of the Eifel (May 1992;Andreas May, pers. comm., 2008

Methods
Bryozoans were investigated in thin sections using a binocular microscope in transmitted light. Morphologic character terminology is partly adopted from Snyder (1991a, b) and Hageman (1991a, b). The following morphologic characters were measured and used for statistics in the studied material: aperture spacing along branch, aperture spacing diagonally, autozooecial aperture width, branch width, branch thickness, dissepiment width, fenestrule width, fenestrule length, distance between branch centres, distance between dissepiment centres, maximum chamber width, keel node width, and distance between node centres.
Interior description: Autozooecia triangular to pentagonal in mid-tangential section; with well-developed long vestibule; axial wall zigzag; aperture positioned at distal end of chamber. Hemisepta absent. Internal granular skeleton continuous with obverse keel, nodes, peristome and across dissepiments. External laminated skeleton well developed, traversed by abundant microstyles. Microstyles regularly arranged in longitudinal rows on colony reverse surface, 0.005-0.010 mm in diameter. Heterozooecia not observed. Rectifenestella sp. 2 (Figs. 5i-j, 7a-c and Table 3) Material: Four thin sections of a single colony SMF 62472-SMF 62475. Exterior description: Reticulate colonies with straight branches, bifurcating, joined by moderately wide dissepiments. Autozooecia arranged in two alternating rows on branches. Autozooecial apertures circular, surrounded by apertural nodes; 3-5 apertures spaced per fenestrule length. Apertural nodes 0.015-0.020 mm in diameter. Fenestrules oval to rectangular, moderately long and narrow. Median keel low. Keel nodes small, intermediately spaced, granular core stellate in shape. Nodes on reverse side present, 0.020-0.025 mm in diameter.
Interior description: Autozooecia triangular to pentagonal in mid-tangential section; with well-developed long vestibule; axial wall zigzag; aperture positioned at distal end of chamber. Hemisepta absent. Internal granular skeleton continuous with obverse keel, nodes, peristome and across dissepiments. External laminated skeleton well developed, traversed by abundant microstyles. Microstyles regularly arranged in     Interior description: Autozooecia short, triangular to trapezoid in mid-tangential section; with short to moderately long vestibule in longitudinal section. Axial wall between autozooecial rows strongly zigzag; aperture positioned at distal end of chamber. Superior hemisepta weakly developed; inferior hemisepta absent. External laminated skeleton welldeveloped on both obverse and reverse sides, traversed by small microstyles. Heterozooecia not observed. Comparison: Spinofenestella antiqua (Goldfuss, 1826) differs from S. inclara (Pŏcta, 1894) from the Lower Devonian of the Czech Republic in having wider branches (average branch width 0.27 mm vs. 0.24 mm in S. inclara) and in smaller distances between branch and dissepiment centres (average branch spacing 0.43 mm vs. 0.71 mm in S. inclara; average dissepiment spacing 0.61 mm vs. 1.23 mm in S. inclara). Remarks: Toots (1951) described and depicted the original material of Goldfuss (1826) for Retepora antiqua. He noted the presence of tubercles (nodes) on the reverse branch side which are characteristic for this species, as well as triangular shape of autozooecial chambers. The lectotype of Fenestrella subrectangularis Sandberger and Sandberger, 1856, Sandberger`s Devonian collection (Wiesbaden), shows the same type of nodes and shape of autozooecia ( Fig. 8a-b). Their dimensions are similar, so Fenestrella subrectangularis is acknowledged as a junior synonym of Retepora antiqua, which is assigned here to the genus Spinofenestella because of triangular shape of autozooecia and presence of nodes on the median keel.

Discussion
Distribution of fenestrate bryozoans in the Devonian of the Eifel As shown by recent studies, Devonian bryozoans of the Eifel are abundant and diverse (Ernst and Schroeder 2007;Ernst 2008aErnst , b, 2016Ernst , 2020Ernst and Bohatý 2009;Ernst et al. 2011Ernst et al. , 2012Ernst et al. , 2014aErnst et al. , b, 2016Ernst et al. , 2020. According to these results, the non-fenestrate bryozoans (orders Cystoporata, Trepostomata, and Cryptostomata) are represented by fortyseven species. In contrast, the fenestrate bryozoan fauna contains twenty-seven fenestrate species identified during the author's own studies. Of these, nine species occur in the Eifelian, fifteen species are restricted to the Givetian, whereas three species occur throughout the Middle Devonian of the Eifel (Table 7). A significant number of taxa established in older publications still need re-study. Type material from the collections is often unavailable for study or cannot be used for the preparation of thin sections. Moreover, the type localities do not exist anymore or are not accessible. The homepage of the International Bryozoology Association (IBA, http://bryozoa. net) lists following fenestrate species from the Devonian of the Eifel, whose assignment needs clarification: Fenestella angusta Sandberger and Sandberger, 1856 (sample 313) has been assigned to the genus Anastomopora (Ernst 2020). In the present paper, species Fenestrella aculeata Sandberger and Sandberger, 1856 (sample 311) has been placed in the genus Rectifenestella and the species Fenestrella subrectangularis Sandberger and Sandberger, 1856 (sample 312) was found being a junior synonym of the species Spinofenestella antiqua (Goldfuss, 1826). The species Polypora laxa Sandberger and Sandberger, 1856 (sample 314) belongs probably to Anastomopora; however, this specimen is badly preserved.

Palaeobiogeographic relations of fenestrate bryozoans from the Middle Devonian of the Eifel
The fenestrate bryozoan species from the Middle Devonian of the Eifel seem to be quite endemic. From the published data, few connections to the Devonian of Europe can be detected. A bryozoan very close to Bashkirella devonica (Dessily, 1967) from the Eifelian of Belgium has been identified in the Cürten Formation (Givetian) of the Eifel (Ernst and Schroeder 2007). In the same locality, the species Anastomopora inflata (Bigey, 1988b), previously known from the Frasnian of France, was found. Fenestrapora transcaucasica Morozova and Lavrentjeva, 1998 is quite common in the Eifelian to Givetian of the Eifel (Ernst 2016), whereas the original record of this species comes from the Mucrospirifer diluvianoides-Radiomena irregularis brachiopod Zone (upper Eifelian, Middle Devonian) of Azerbaijan. The species Dissotrypa sincera Ernst and Königshof, 2010 was originally described from the Givetian of Western Sahara.
The majority of fenestrate genera from the Middle Devonian of the Eifel are cosmopolitan (Fenestella, Rectifenestella, Spinofenestella, Hemitrypa, Anastomopora, and Loculipora). The others show some restrictions in their distribution. The genus Bigeyina Suárez-Andrés and McKinney, 2010 is restricted to the Lower-Middle Devonian of Europe (Czech Republic, Spain, and Germany). The genus Prolixicella was identified besides German localities in the Lower-Middle Devonian (Emsian-Eifelian) of Spain (Ernst 2012) and in the Givetian of Western Sahara (Ernst and Königshof 2010). The genus Schischcatella Waschurova, 1964 is originally known from the Lower Devonian (Emsian) of Tajikistan. Two species of the genus Dissotrypa Ernst and Königshof, 2010 are known from the Lower-Middle Devonian (Emsian-Eifelian) of Spain (Ernst 2012).