Homeomorphy in Lunostoma, a new Middle Devonian cryptostome bryozoan
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- Ernst, A., Taylor, P.D., Bohatý, J. et al. Paläontol Z (2012) 86: 135. doi:10.1007/s12542-011-0127-8
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A new genus and species of rhabdomesine cryptostome bryozoan, Lunostoma pulchra n. gen. n. sp., is described from the Lower Givetian (Middle Devonian) of the Eifel, Germany. It differs from all previously known rhabdomesines in having crescent-shaped structures (“scuta”) on the proximal sides of the apertures. These scuta resemble the lunaria that characterise cystoporate bryozoans, providing yet another example of homeomorphy in the Bryozoa. The function of scuta is unclear as, in contrast to lunaria, they do not project sufficiently from the apertures to constrain the everting lophophores.
KeywordsDevonianBryozoaTaxonomyHomeomorphyEvolutionEifel (Rhenish Massif, Germany)
Eine neue Gattung und Art einer rhabdomesinen cryptostomen Bryozoe wird als Lunostoma pulchra n. gen. n. sp. aus dem unteren Givetium (Mittel-Devon) der Eifel (Rheinisches Schiefergebirge, Deutschland) beschrieben. Morphologisch unterscheidet sie sich von allen bekannten Rhabdomesina durch halbmondförmige Strukturen („Scuta”) an den proximalen Seiten der Aperturen. Diese Scuta ähneln frappant den für cystoporate Bryozoen charakteristischen Lunarien und liefern damit Beispiel für Homeomorphie bei paläozoischen Bryozoen. Die Funktion der Scuta ist nicht klar, anders als bei Lunaria, da sie nicht weit genug aus den Aperturen hinausragen um die Ausstülpung der Lophophoren einzuschränken.
SchlüsselwörterDevonBryozoaTaxonomieHomeomorphieEvolutionEifel (Rheinisches Schiefergebirge, Deutschland)
Cryptostome bryozoans are an important group of Palaeozoic Stenolaemata, displaying a long record from the Early Ordovician to the Late Permian (McKinney and Jackson 1989; Gorjunova 1996), although a recent report of a dubious rhabdomesine cryptostome bryozoan from the Late Cambrian of Mexico (Landing et al. 2010) may extend the range of the group. Cryptostomes are characterised by erect colonies in which usually short autozooecia bud in a more or less regular pattern so that their apertures are arranged in regular longitudinal or spiral rows, being shaped elliptical, subcircular or rectangular (Blake 1983). A characteristic feature of many cryptostome bryozoans is the development of different kinds of hemisepta, shelf-like skeletal projections into the autozooecial chambers. However, hemisepta can be found also in all other groups of Stenolaemata (notably in bifoliate cystoporates).
Cystoporate bryozoans are another large group of Palaeozoic Stenolaemata which are characterised by encrusting, massive and diverse erect colonies. Cystoporates appeared in the Early Ordovician, and persisted apparently until the Late Triassic (Boardman 1984; Schäfer and Fois 1987). The most important cystoporate characters are long conical or tubular autozooecia, vesicular skeletons and lunaria (Utgaard 1983). The two latter characters do not necessarily occur in all genera. Whereas a vesicular skeleton is also known in some genera of Trepostomata and Cryptostomata (Ptilodictyina), lunaria seem to be unique to cystoporates, although a few examples of lunaria or lunarium-like structures are known in post-Palaeozoic cyclostomes (Borg 1965; Utgaard 1968). Lunaria represent hood-like projections over the autozooecial apertures and are absent in heterozooecia. In most genera, lunaria are radially arranged around maculae, located on the sides of zooecial apertures nearest to the macular centres.
Lunarium-like structures were also mentioned in fenestrates (e.g. Schulga-Nesterenko 1941; Morozova 1970). However, these structures were subsequently revealed to be parts of sharply inclined vestibules (Morozova 2001). Therefore, fenestrates seem to lack any structures closely comparable to lunaria. Among Cryptostomata, lunaria have been described in two genera: Tavayzopora Kiseleva 1969 (Suborder Timanodictyina) and Junggarotrypa Lu 1999 (Suborder Ptilodictyina). Lunaria in Tavayzopora are small, semicircular projections in walls of autozooecial chambers which are moderately calcified. This genus was described only from thin sections, so that it is impossible to determine whether these structures represent hood-like projections over the autozooecial apertures as in true lunaria. Junggarotrypa contains two species, but lunaria were described only in J. gratiosa, the type species. Lu (1999, p. 178) noted “lunaria distinct and semicircular in outline (…) and deeply indenting their strongly projected ends.” However, examination of the figures in his original publication fails to show any structural features that resemble lunaria in this genus. As a result this record of lunaria in ptilodictyinids cannot be confirmed.
Geological and palaeontological setting
The studied bryozoans come from the Lower Givetian Loogh Formation (hemiansatus Conodont Biozone) of the south-western part of the Gerolstein Syncline. Regionally, the base of the Loogh Formation (Fig. 2) consists of the Dachsberg Member comprising homogeneous limestones containing sparsely preserved macrofossils (Winter 1965). According to Winter (1965, p. 289), the Dachsberg Member is restricted to the southwest of the Gerolstein Syncline. These limestones were deposited under quiet conditions (Winter 1965, p. 307). Progressive shallowing led to the inception of biostromal growth and facies differentiation characterising the Baarley Member of Winter (1965, pp. 289–290). Massive trochite-dominated limestones and “matrix limestones” characterise this member. Local facies differences within the Baarley Member often modify this basic lithology. The bryozoan locality “Mühlenwäldchen” (Fig. 1) is characterised by both fine-grained homogeneous limestone and marl banks. More regular limestone–marl interbeds increase towards the boundary of the overlying Hustley Member (Winter 1965, pp. 290–292), which represents a temporary decrease in sedimentation marked by the appearance of stromatoporoid/coral biostromes. These biostromes, with partly limy, partly marly deposits, mark the maximum level of facies differentiation.
The “Mühlenwäldchen” site in the SW-Gerolstein (Gerolstein Syncline, Eifel, Rhenish Massif, north-western Rhineland-Palatinate, Germany) is located at UTM 50°13′16.14”N, 6°39′01.00”E (Fig. 1). Stratigraphically, the bryozoan outcrop is presumably positioned within the lower Baarley Member [regional valid denomination of the uppermost Wotan Member (Hotz et al. 1955) of the Gerolstein Syncline (sensu Winter 1965)] of the middle Loogh Formation, Lower Givetian (Middle Devonian; hemiansatus Conodont Biozone). This historic fossil locality is most famous for well-preserved crinoid cups (comprising melocrinititids, rhodocrinitids, megaradialocrinitids and hexacrinitids, as reported by Bohatý 2006, p. 472), echinoids, brachiopods, rugose and tabulate corals (e.g. Bohatý et al. in press), platyceratid gastropods, phacopid trilobites, cyclostome, trepostome, as well as a diverse spectrum of fenestrate bryozoans. The latter include the aberrant semicosciniid Schischcatella heinorum Ernst and Bohatý 2009 typically found attached to mimatrypid brachiopods or, rarely, to crinoid pluricolumnals.
Material and methods
Lunostoma pulchra n. gen. n. sp. (Middle Devonian, Rhenish Massif)
Studied material is housed at the Senckenberg Museum (Frankfurt am Main, Germany). Five colonies, numbered SMF 20.927–SMF 20.929, were studied using SEM at the Institute of Geosciences, University of Kiel (CAMSCAN-Serie-2-CS-44). Thin sections were prepared from 12 colonies (SMF 20.915–SMF 20.926).
Lunostoma pulchra n. gen. n. sp., by monotypy.
Derivation of name
The genus name refers to the presence of crescent-shaped partitions in the apertures (from Latin Luna = moon, and Greek stoma = mouth, opening).
Colonies branched, arising from basal discs. New branches arising at angle of 90° from the older branches. Axial region formed by well-defined median axis. Autozooecia moderately long, initially polygonal, budding in spiral order at angles of 19–23° from the median axis in endozones, then bending gently and intersecting the colony surface at angles of 60–63°. Autozooecial apertures arranged rhombically. Distally curved multiple hemisepta occurring on the distal autozooecial wall. Diaphragms rare to absent. Aktinotostyles abundant, spaced regularly between autozooecial apertures, locally forming small clusters. Autozooecial walls finally laminated, with median dividing layer in endozone; coarsely laminated, thickened in exozones. Scuta present, positioned on proximal ends of autozooecial apertures, directed toward colony base.
Lunostoma n. gen. differs from all other rhabdomesine cryptostomes in having apertural partitions (herein called scuta) and in the distally curved multiple hemisepta. The new genus is similar to Saffordotaxis Bassler 1952 in both the presence and distribution of aktinotostyles and the shape of the autozooecia.
Occurrence and geological age
Derivation of name
The species name derives from Latin pulchra = beautiful.
Fourteen colonies SMF 20.915, SMF 20.917–SMF 20.926 (thin sections), SMF 20.927–SMF 20.929 (SEM samples).
“Mühlenwäldchen”, SW-Gerolstein, Gerolstein Syncline, Eifel, Rhenish Massif, Germany; UTM: 50°13′16.28”N, 6°39′02.28”E.
Lower Baarley Member of the middle Loogh Formation, Lower Givetian, Middle Devonian; hemiansatus Conodont Biozone.
As for genus.
Colonies branched, growing from basal discs, branches 0.96–1.82 mm in diameter, with 0.60–1.20 mm wide endozones and 0.18–0.31 mm wide exozones. Axial region formed by well-defined median axis. Autozooecia moderately long, initially polygonal, budding in spiral order at angles of 19–23° from the median axis in endozones, then bending gently and intersecting the colony surface at angles of 60–63°. Autozooecial apertures oval, arranged rhombically in longitudinal rows on colony surface. Long multiple hemisepta occurring on the distal autozooecial wall throughout autozooecial chamber, distally curved, closely spaced. Diaphragms rare to absent. Aktinotostyles moderately large, abundant, spaced regularly between autozooecial apertures in 1–2 rows, locally forming small clusters, originating at the base of exozone. Autozooecial walls finally laminated, with median dividing layer, 0.015–0.035 mm thick in endozone; coarsely laminated, thickened in exozones. Scuta present, horseshoe shaped, positioned on the proximal edge of autozooecial apertures.
As for genus.
Scutum: a new zooecial structure and its function
In Lunostoma, horseshoe-shaped skeletal shields—here termed “scuta”—are situated on the proximal edge of autozooecial apertures and positioned suborally, i.e. below the level of the exterior surface of the aperture. In plan view, scuta are similar in shape to the lunaria present in most cystoporate bryozoans (Utgaard 1983). However, cystoporate lunaria differ from scuta in that they usually extend into a hood-like structure that projects over the autozooecial aperture above the colony surface. The scuta of Lunostoma do not appear to be the abraded bases of true lunaria, but rather are crescent-shaped suboral shields, hence the name “scutum” (plural “scuta”) from the Latin for the semi-cylindrical shields carried by Roman legionaries. Whereas projecting, hood-like lunaria may have functioned in directing the lophophore distally, the function of scuta is unclear. They resemble the poster of ascophoran cheilostome bryozoans, i.e. the part of the orifice proximal of the opercular condyles and leading to the ascus or compensation sac involved in lophophore eversion (Hayward and Ryland 1979). However, stenolaemates including Lunostoma lack an ascus. Furthermore, it is improbable that Lunostoma possessed an operculum: opercula are absent in all but a few stenolaemates (cf. eleid cyclostomes, see Taylor 1985) and apparently require exterior wall on which to hinge whereas the skeleton of Lunostoma comprises interior wall only.
Homeomorphy in stenolaemate bryozoans
The development of scuta in these Devonian cryptostomes is another example of homeomorphy in bryozoans. This term was first coined by Buckman (1895) (Afanas’yeva 1977), who realised that similar or identical morphological characters could be recognised in different taxa that may or may not be separated by time. Homeomorphy is a result of convergent evolution of traits (homoplasy), sometimes through heterochrony (Anstey 1987). Homeomorphy has been described in brachiopods (Cloud 1941), cephalopods (Reyment 1955; Evans 2007) and echinoderms (Blake and Kues 2002) amongst other groups, and is frequent in bryozoans (e.g. Voigt and Flor 1970; Hinds 1975; Blake 1980; McKinney et al. 1993; Taylor and Badve 1995).
Both colonial- and zooidal-level homeomorphy can be found in bryozoans; for example, colonies with distinctive lyre-shaped colonies occur in the fenestrate genera Lyropora and Lyroporella from the Mississippian of North America and are also found in a cyclostome species from the Eocene Castle Hayne Limestone Formation of North Carolina (McKinney et al. 1993). Similarly, the development of a spiral axis in the Mississippian fenestrate Archimedes and the Eocene cyclostome Crisidmonea (Taylor and McKinney 1996) represents homeomorphy at colonial level. McKinney et al. (1993) cited additional examples of homeomorphy in bryozoan morphology that involve branch development (Blake 1980) and surface topography (Winston 1979). At zooidal level, Hageman (1991) noted homeomorphy in the Carboniferous cryptostome Worthenopora with zooids superficially resembling those of some post-Palaeozoic cheilostomes. More remarkable is the homeomorphy between Cretaceous–Paleocene melicerititid cyclostomes and cheilostomes, the former being highly unusual among cyclostomes in possessing cheilostome-like opercula to close the aperture and developing polymorphic zooids with enlarge opercula that resemble cheilostome avicularia (Taylor 1985). The long, horn-shaped zooids of the Cretaceous cheilostome Chiplonkarina are strongly homeomorphic with stenolaemates, such as the cyclostome Ceriocava (Taylor and Badve 1995). Homeomorphy of internal characters in stenolaemate bryozoans includes the development of four-sided autozooecial chambers in trepostome bryozoans, which arose independently at least four times between the Ordovician and the Permian (Boardman and McKinney 1976).
The extent to which homeomorphy compromises stenolaemate bryozoan taxonomy is contestable. Boardman (1984), in a discussion of the origins of the post-Triassic stenolaemates, believed that some post-Palaeozoic cyclostomes sharing morphological traits with Palaeozoic stenolaemates were direct descendants of these supposedly extinct taxa rather than their homeomorphs. In contrast, molecular sequence data obtained from Recent cyclostomes has shown numerous skeletal morphological characters to represent homoplasy (Waeschenbach et al. 2009), implying high levels of homeomorphy among stenolaemates.
Phylogenetic position of Lunostoma
Lunostoma in its general morphology is close to the rhabdomesine Family Rhomboporidae Simpson, 1895. Shared characters are: rhombic arrangement of apertures, budding of autozooecia from a distinct median axis, autozooecial shape and orientation, and presence of aktinotostyles (Blake 1983: 576). However, Lunostoma possesses two cardinal characters which do not occur in the Suborder Rhabdomesina: scuta and distally curved multiple hemisepta. Both these morphologies are apparently unique among Bryozoa (see also previous discussion). Rhomboporids usually lack hemisepta, which are normally superimposed superior and inferior hemisepta, or have only one type. Rarely, multiple hemisepta occur in rhabdomesines, but these are proximally curved structures on the proximal wall like those in some species of the rhabdomesid genus Ascopora (Blake 1983: 571). The most similar genus to Lunostoma appears to be Saffordotaxis Bassler 1952 of the Family Rhomboporidae.
We thank Wolfgang Reimers (Kiel) for his assistance in preparing the thin sections, and Ute Schuldt (Kiel) for help with SEM processing. Robert Anstey, East Lansing and Kamil Zágoršek, Prague, are thanked for their helpful and constructive reviews of the manuscript. Most of the studied bryozoan material was collected during the crinoid research project of the Deutsche Forschungsgemeinschaft (DFG project HE1610/16-1); J.B. gratefully acknowledged the financial support. Also A.E. thanks the Deutsche Forschungsgemeinschaft for financial support (DFG project ER 278/4.1 and 2). This paper is a contribution to IGCP 499.