Remarks on Bačinella Radoičić, 1959 (type species B. irregularis) and its representatives

Bačinella irregularis was described by Radoičić (Bull Serv Géol Géophys Rep Serbie 17:87–92, 1959) as an alga incertae sedis. Based on material from the Lower Aptian of the western Maestrat Basin (Spain), the genus Bačinella with its various species is revised; an emended diagnosis is given as well as a detailed description. Bačinella is interpreted as an organism that excavated complex branching galleries into biogenic hard substrates, e.g., crusts of Lithocodiumaggregatum Elliott (such as in the holotype specimen of B. irregularis). Its morphology and boring pattern is comparable to that of modern filamentous-septate euendolithic green algae of the class Ulvophyceae. These forms, however, are microfilamentous (microborers) whereas Bačinella has a larger filament diameter. Nearly all of the “B. irregularis” occurrences reported in the literature do not belong to this taxon, but represent irregular vesicular crusts (“bacinellid” fabrics = B. irregularis auct. non) that display some resemblances with modern microbial mats.


Introduction
Lithocodium aggregatum ELLIOTT (1956) and Bačinella irregularis RADOIČIĆ (1959) are two enigmatic taxa of the Late Jurassic to Early Cretaceous epeiric sedimentary record, which have aroused more than 50 years of controversy concerning their taxonomic position, possible relationship and synonymies. Several authors considered Bačinella irregularis and Lithocodium aggregatum as synonyms with Lithocodium having priority as described earlier (SEGONZAC & MARIN 1972;LUPERTO SINNI 1979;BANNER et al. 1990;KOCH et al. 2002), while others treated the two microproblematica as different taxa (ELLIOTT 1963;SCHMID 1996;SCHMID & LEINFELDER 1996;CHERCHI & SCHROEDER 2006). Lithocodium aggregatum was originally described by ELLIOTT as a siphonal (= non-septate filaments) codiacean (= udoteacean) alga from the Lower Cretaceous of Iraq; other interpretations include a sponge, red algal, foraminiferal and cyanobacterial (calcimicrobial) origin. Recently, Lithocodium aggregatum was re-interpreted as a heterotrichale filamentous-septate green alga (order Ulotrichales?) by SCHLAGINTWEIT et al. (2010). The crusts as described by these authors are formed by a basal prostrate and an erect filament system, which correspond to the "medullary" and "cortical" filaments of the previous udoteacean algal terminology used by ELLIOTT (1956).
On the other hand, Bačinella irregularis was originally described by RADOIČIĆ (1959) as an alga incertae sedis. However, many authors have later assumed a cyanobacterial/microbial origin for this microproblematicum (SCHÄFER & SENOWBARI-DARYAN 1983;MAURIN et al. 1985; CAMOIN & MAURIN 1988;RIDING 1991a;SCHMID & LEINFELDER 1996;VACHARD et al. 2001;UŢA & BUCUR 2003). Alternatively, SCHLAGINTWEIT et al. (2010) re-interpreted Bačinella irregularis as a euendolithic chlorophycean alga, possibly belonging to the Ulotrichales. Given that the main goal of the last-mentioned work was the re-description of Lithocodium aggregatum, the present study deals exclusively with the genus Bačinella RADOIČIĆ. It critically discusses the species inventory, and provides a new generic diagnosis along with a detailed description of the type species. In addition, new microstructural details of "bacinellid fabrics" (= Bačinella irregularis RADOIČIĆ auctorum non) are presented.

Sample location and material
The specimens studied are from Lower Aptian sedimentary successions that crop out in the western Maestrat Basin of the eastern Iberian Chain (Fig. 1). The deposits studied lie within the marls of the Forcall Formation and constitute a sub-basin-wide horizon formed by several episodes of coral rubble encrusted by microorganisms, clearly dominated by Lithocodium aggregatum crusts , 2011. The ammonite-biostratigraphic data (MORENO-BEDMAR et al. 2009 permit to ascribe this horizon to the upper part of the Deshayesites forbesi biozone (Lower Aptian).
The material studied comprises 73 thin-sections, which were sampled from nine different outcrop sections in the vicinity of the towns of Aliaga, Montoro de Mezquita, Camarillas and Miravete de la Sierra (Teruel province, Spain; Fig. 1).  Fig. 2) displays an identical mosaic-like pattern and fine perforations (much smaller than the "dermal" cells) in the walls as in Bačinella ordinata PANTIĆ, 1972 (see below). Bearing in mind the wide moropological variability usually exhibited by thaumatoporellacean algae (DE CASTRO 1990), "Bačinella bicellularis" is here interpreted as belonging to this group. Status: not a representative of Bačinella. It has been interpreted as a thaumatoporellacean alga (SCHLAGINTWEIT 2011).
Bačinella crispa ELIÁŠOVÁ, 1981, Late Jurassic of Czech Republic. B. crispa was defined as a "Bacinella with a central tissue consisting of lobate elements". According to SCHMID (1996), the description of B. crispa was inadequate and corresponds to a consortium of the cryptobiotic foraminifer Troglotella incrustans WERNLI & FOOKES, 1992 inside "Lithocodium". This view was also accepted by SCHLAGINTWEIT (2008)  consists of cells with an elongate tubular pattern "arranged in vertical series" and with "irregularly spaced, thin micritic transverse partitions". This structure strongly recalls a solenoporacean alga. Status: this taxon must be removed from the genus Bačinella. It is unknown, and should be checked whether it corresponds to an individual taxon or belongs to an already existing solenoporacean alga. RADOIČIĆ, 1959 (type-species), Barremian-Aptian of Bosnia and Herzegovina. As can be inferred from the original diagnosis and description, the specific name refers to the irregularity of cells with respect to size and form. According to R. RADOIČIĆ (pers. comm.), the genus name refers to the region of the Bačina lakes, northwest of the delta of the river Neretva/Croatia where the taxon was observed for the first time in blocks of a megabreccia. The generic diagnosis of Bačinella was given as follows: "Nodular and incrusted algae whose interior is constructed by unregular cells of different size and form, which are filled with crystalline calcite (Remark: = sparitic), whereas the intercellular mass is cryptocrystalline (Remark: = micritic). The subdermal structure is differentiated" (RADOIČIĆ, 1959: 92). In the species description the "cells" of Bačinella irregularis were further specified by Radoičić as being "arranged into association or into some sort of unregular series which are intermingled". The thin microcrystalline cross-partitions within the sparite-filled "cells"

Bačinella irregularis
were not mentioned by Radoičić, but it can be inferred that the author was aware of this as otherwise the term "cell" would most likely have been replaced by "filament" or "siphon".
The holotype specimen shows that the "cells" are not really disorganised, but are radiating and successively branching, forming a network ( Fig. 3/1). The description of these cells as being arranged in series, the mentioning of their intermingled character (= branching-radiating pattern) and the micritic intercellular mass make it clear, that RADOIČIĆ´s description directly refers to the holotype specimen and not the sparitic vesicular meshworks (= bacinellid fabrics) that are treated later in the paper. In the original description only two figures were provided: the holotype specimen and one paratype. The paratype also clearly shows the micritic intercellular mass but the arrangement of the cells is more irregular than in the holotype. The holotype is interpreted as representing a section subparallel to the plane of filament growth direction. The paratype seems to cut the filaments obliquely in a supposedly more adult part of the thallus where successive branching and anastomization results in a more complex and irregular cell pattern. Following article 7.2 of the International Code of Botanical Nomenclature (ICBN) (MCNEILL et al. 2006), the nomenclatural type (here: holotype) "is not necessarily the most typical or representative element of a taxon". In fact, the holotype specimen of RADOIČIĆ and the specimen from the Early Aptian of Spain showing the boring character of Bačinella irregularis are untypical (as being very rare) for the thin-section material as these are cutting in the plane where the branching-radiating filamentous cell pattern of the alga is well recognizable.
The specimens from the Lower Aptian of the western Maestrat Basin correspond to the taxon described by RADOIČIĆ (1959). In the Iberian material, it is evident that Bačinella irregularis represents an organism boring into micritic crusts of Lithocodium aggregatum (= micritic "intercellular mass"). The original diagnosis of Bačinella is therefore interpreted as representing a mixture that refers to two different taxa (Bačinella and Lithocodium) and moreover does not include an adequate description of all morphological characteristics. An emended diagnosis is presented below. Status: valid. In conclusion, no other Bačinella species diagnosed after the description of the typespecies B. irregularis belongs to this genus (Tab. 1). Thus, Bačinella is interpreted as a monotypic taxon.
Emended diagnosis: Life-cycle is assumed to be heteromorphous, including a free-living (gametophytic) and a euendolithic (sporophytic) stage. The multicellular thallus of the euendolithic stage consists of irregular branching and anastomizing filaments with cross-walls forming a complex network that accommodates in a plane parallel to the substrate surface.
The upwards-directed branching with successive reduction of diameter results in a close-set layer of fine terminal branches that end shortly before the substrate surface. The filaments consist of uniseriate barrel-shaped, cylindrical to irregularly swollen cells. Rare hairs (setae) occur, one to a cell. Diagnosis: see generic diagnosis. According to our interpretation, the genus is monospecific so far. Diagnostic criteria for species differentiation might be cell dimensions and/or morphological criteria (e.g., branching pattern).

Description:
The supposed heteromorphous life-cycle includes a free gametophytic generation of unknown shape and an euendolithic sporophytic phase to which the following description refers exclusively. The multicellular thallus is composed of branching septate filaments, euendolithic typically in crusts of Lithocodium aggregatum ELLIOTT, 1956, but also in thalli of other calcareous algae or metazoan skeletons. Cells that make up the filaments are arranged in linear rows (= septate filaments). Mostly, these are barrel-shaped to cylindrical (diameter 50-150 µm; 70-220 µm measured from the holotype specimen). In the central part,  (Fig. 4/5). Starting from the substrate entrance point, the alga may bore with a single straight to slightly undulating filament before they irregularly branch in all directions ( Fig. 3/2). This network of branching and anastomizing filaments accommodates in a plane parallel manner to the substrate surface; the greatest observed thallus width is up to 5 mm (specimen shown in Fig. 3/2). Three different types of branching patterns occur: dichotomous, a Y-shaped bifurcating cell, branching by bisection of a cell during radial division resulting partitions that form a "T" (Fig. 6/1) (e.g., GRAHAM 1982 for details), and opposite, rectangular branching with the so-called "four-armed central cell" (NIELSEN, 1987) (Fig. 6/2).
Within the central part, rare hairs (or setae) one to a cell occur. They have a length of up to 0.5 mm with a basal diameter of 24-35 µm and are tapering distally (Figs. 4/2, 4/6). A basal septum towards the attaching cell is not detectable. As three-dimensional specimens are not available, it is unclear whether roundly terminating lateral appendages are rhizoidal processes or oblique sections of branches.
Within the studied material, there are all transitions from sparite-filled specimens with preserved cell cross-walls (e.g., Fig. 4/2) to specimens with microsparitic (e.g., Fig. 4/7) to micritic fillings without preserved cross-walls. In the latter case, only the empty boring trace is present.
Discussion: First of all, it must be noted that from the living alga, only the cross-walls of the cells of the filaments are preserved as a thin microcrystalline structure. An outer cell wall is not evident within the dark crusts of Lithocodium but it is detectable in the boring affecting a solenoporacean thallus (Fig. 5a). In another case, Bačinella irregularis was observed boring into a Liassic spongiomorphid skeleton (LE MAITRE 1935) (Fig. 5b-e). According to our knowledge, this finding represents the oldest record of Bačinella irregularis. With these two examples, the long-lasting discussion about a synonymy of Bačinella irregularis with Lithocodium aggregatum becomes obsolete, since in both cases the micritic "intercellular mass" sensu RADOIČIĆ (1959) (= Lithocodium crusts) is lacking.
As the description of Bačinella irregularis does not refer to a trace (here: boring gallery), it is treated taxonomically rather than ichnotaxonomically. Normally, fossil euendolithic chlorophyts are usually known from their borings. Mention should be made that the boring galleries of Bačinella resemble to some extent those produced by boring sponges: non-camerate (or open-camerate) representatives of the ichnogenus Entobia (e.g., Fig. 4/7).  , MÄGDEFRAU 1937;SCHINDEWOLF 1962;PUGACZEWSKA 1965). In any case, with filament or boring diameters > 100 µm, Bačinella must be considered a macroborer whereas modern euendolithic chlorophyts are typically microborer (e.g., SCHMIDT 1990;GLAUB 1994). Empty borings of Bačinella without preserved anatomical details such as walls or perforations must not be named by their producer. They rather constitute a discrete ichnotaxon, which remains to be named, however.
As already stated, the majority of references in the literature of Bačinella irregularis refer to "epibenthic meshwork crusts of noticeable thickness and extension" (SCHMID & LEINFELDER 1996: 31). These, however, are completely different to the taxon that RADOIČIĆ described in 1959, but identical to an illustration provided by the same author one year later that might be a reason for the following decades-lasting controversies (RADOIČIĆ 1960) (Fig.   3c). These structures have later been termed "bacinellid textures" (MAURIN et al. 1985) or "filamentous fabrics" (HILLGÄRTNER et al. 2001), and a cyanobacterian/calcimicrobial origin has been assumed by several authors (SCHÄFER & SENOWBARI-DARYAN 1983;MAURIN et al. 1985;CAMOIN & MAURIN 1988;RIDING 1991a;SCHMID & LEINFELDER 1996;VACHARD et al. 2001). Illustrated evidence for such an interpretation, however, was not provided. A more detailed interpretation/description of the micritic laminae that form these irregular vesicles was given by DUPRAZ (1999: 28) (Fig. 8/1). He remarked that they consist of many small dark (= micritic) dots that should represent calcification centers for light needles of possible primary aragonite, later transformed to a sparitic cement. The dark calcification centers could represent cyanobacteria that triggered calcification by photosynthesis upon or within a mucilageneous sheath (PENTECOST & RIDING 1986;RIDING 1991b) (Fig. 8/2).
In thin-sections of "bacinellid" vesicular fabrics from the Latest Jurassic-Earliest Cretaceous of the Getic Carbonate Platform (leg. O. Dragastan), individual small hollow tubes with a micritic envelope were detected forming the "walls" of many of the vesicles (Figs. 8/3-6). The outer diameter ranges from 9.5 to 50 µm, the width of the inner hollow tube is 5 to 25 µm. They can be compared with the so-called Girvanella group that is compared with calcified oscillatoriacean (cyanobacterian) sheats. Although being rather common in the studied material, many of the micritic "laminae" appear solid without a detectable central tubiform hollow. Referring to the morphology and the size range, it seems likely that the observed tubes represent filaments of cyanobacteria. In fact, the fossil "bacinellid" crusts show morphological features that resemble those reported from modern microbial mats such as pinnacles or reticulate surface patterns (NOFFKE et al. 2001;GERDES & KLENKE 2003;GERDES 2007;PORADA & BOUOUGRI 2007) (Fig. 9). It is worth to mention that already DRAGASTAN & RICHTER (2003: 87) mentioned a possible interpretation of "Bačinella" as an "algal mat resembling elephant skin texture". The mat-character of these vesicular fabrics is furthermore evident in cases where they form bindstones that exhibit a high lateral extension in preferentially back-reefal or peritidal depositional settings and may show features of subaerial exposure (e.g., MASSE et al. 1998;BUCUR et al. 2010). Further studies for the understanding of these three-dimensional structures (mats), however, are needed.

Acknowledgements
Ovidiu Dragastan (Bucharest), Ivo Velić (Zagreb) and Adrian Immenhauser (Bochum) are thanked for providing thin-sections for this study. We are grateful to Ioan Bucur      and HO-17V. 5: Branching filaments with terminal fine branchlets (above). Thin-section CAL-2V. 6-7: Boring galleries of Bačinella irregularis with microsparitic filling resembling to some extent those produced by non-camerate boring sponges (ichnogenus Entobia). Note the absence of preserved cell cross-walls. Note also the two thin tapering appendages (= setae) with upward growth towards the substrate surface in 6 (arrows). Thin-sections CA-3H and DE-3V.