Rectifenestella exiliformis, Ernst & Fernández & Fernández-Martínez & Vera, 2012

Ernst, Andrej, Fernández, Luis Pedro, Fernández-Martínez, Esperanza & Vera, Carmen, 2012, Description of a bryozoan fauna from mud mounds of the Lebanza Formation (Lower Devonian) in the Arauz area (Pisuerga-Carrión Province, Cantabrian Zone, NW Spain), Geodiversitas 34 (4), pp. 693-738 : 720-724

Rectifenestella exiliformis n. sp. ( Figs 17 A-I; 18A-C; Appendix)

HOLOTYPE. — SMF 21.314 .

PARATYPES. — SMF 21.315- SMF 21.332.

ETYMOLOGY. — The species name refers to the similarity with the species R. exilis (Počta, 1894) from the Lower Devonian (Pragian) of Czech Republic.

TYPE LOCALITY. — Arauz Sur (Arroyo section), Province of Palencia, NW-Spain (Cantabrian Mountains).

TYPE HORIZON. — Lebanza Formation, Lower Devonian (Pragian).

DIAGNOSIS. — Reticulate colonies with straight branches joined by moderately wide dissepiments; autozooecia pentagonal in mid tangential section, arranged in 2 alternating rows on branches, with an additional row shortly before bifurcations, 4-7 spaced per length of a fenestrule; axial wall zigzag; peristomes containing 10-15 variously sized nodes, with 3-6 larger than the others; fenestrules oval to rectangular; keels wide, low, with widely spaced, stellate nodes; microacanthostyles abundant; large styles present on the reverse colony surface.

DESCRIPTION

Reticulate colonies with straight branches, bifurcating, joined by dissepiments. Autozooecia arranged in 2 alternating rows on branches, with additional row shortly before bifurcation, having circular apertures with moderately high peristomes, 4-7 spaced per length of a fenestrule. Peristomes containing 10-15 variously sized nodes, of which 3-6 are significantly larger than the others. Smaller nodes 0.010 -0.015 mm in diameter, larger nodes 0.020 -0.030 mm in diameter. Fenestrules oval to rectangular. Median keel wide, low. Keel nodes widely spaced, granular core stellate in shape. Microacanthostyles on the colony surface abundant, regularly spaced in longitudinal rows, 0.01-0.02mm in diameter. Large styles irregularly spaced on the reverse colony surface, 0.03-0.06 mm in diameter.

Interior description

Autozooecia pentagonal in mid tangential section; with well-developed long vestibule; axial wall zigzag; aperture positioned at distal end of chamber. Autozooecial chambers diverging laterally at angles of 42-70°. Superior hemisepta indistinct; inferior hemisepta absent. Internal granular skeleton continuous with obverse keel, nodes, peristome and across dissepiments, 0.020 -0.045 mm thick on the branch reverse wall. Outer lamellar skeleton well developed, 0.015 -0.060 mm thick on the branch reverse wall. Terminal diaphragms common.

COMPARISON

Rectifenestella exiliformis n. sp. is similar to R. exilis (Počta, 1894) from the Lower Devonian (Pragian) of Czech Republic, but differs in having thicker branches (average branch width 0.31 mm vs 0.21 mm in R.exilis ), more closely spaced autozooecial apertures (average distance between aperture centres 0.18 mm vs 0.23 mm in R. exilis ), and more widely spaced branches and dissepiments. Furthermore, R. exilis (Počta, 1894) has a row of small nodes on the keel instead of the large stellate nodes seen in R. exiliformis . Fenestella aff. parallela Hall, 1881 described by Ernst et al. (2011: 318-320, figs 8e-g, 9a-d) has a similar peristome morphology with variously sized nodes and stellate nodes on the median keel. However, this species has rectangular shaped autozooecia in mid tangential section and a smaller number of autozooecia per fenestrule (2-4 in Fenestella aff. parallela vs 4-7 in R. exiliformis ). Fenestella (Rectifenestella) asterogrumosa Waschurova, 1964 from the Lower Devonian (?Emsian) of Tajikistan has a similar peristome morphology, stellate nodes on the keel and pentagonal shaped autozooecia in mid tangential section. However, this species lacks styles on the reverse surface and has 2-3 autozooecia per fenestrule length instead of 4-7 in R.exiliformis . Waschurova (1964: 80) mentioned 7-20 apertural nodes 0.005 -0.015 mm in diameter,without stressing variation in size. Figure 1 on Waschurova’s plate 27 shows some significantly larger nodes in the aperture, however.

The present material is placed in Rectifenestella Morozova, 1974 because of the autozooecial chamber shape and character of the meshwork. However, the morphology of the peristome with variously sized nodes is quite unusual for Rectifenestella , which usually has stellate structures consisting of 8 rays in the apertures.

Figures

FIG. 17. — Rectifenestella exiliformis n. sp.: A, B, tangential section showing obverse branch surface with autozooecial apertures and nodes, holotype SMF 21.314; C, mid tangential section showing autozooecial chambers,paratype SMF 21.323; D, tangential section of reverse side showing styles,holotype SMF 21.314;E, mid tangential section showing autozooecial chambers,paratype SMF 21.323; F, tangential section of obverse side showing stellate nodes, holotype SMF 21.314; G, H, tangential section showing autozooecial apertures, paratype SMF 21.326;I, branch transverse section,paratype SMF 21.327. Scale bars:A, 2 mm;B-D, 1 mm; E-H,I, 0.2 mm.

FIG. 1. — A, schematic map showing the location of the studied outcrops in the Arauz area (north of Palencia province, Spain); B, schematic geological map of the Cantabrian Zone (modified from Pérez-Estaún et al. 1988) showing the location of the studied outcrop in the Alto Carrión Unit.