Cantabria labyrinthica, Clausen & Álvaro, 2006

Clausen, Sébastien & Álvaro, J. Javier, 2006, Skeletonized microfossils from the Lower-Middle Cambrian transition of the Cantabrian Mountains, northern Spain, Acta Palaeontologica Polonica 51 (2), pp. 223-238 : 233-236

Cantabria labyrinthica sp. nov.

Figs. 6–8.

Etymology: After “labyrinthine”, the diagnostic character of the meshwork microstructure.

Type locality: Eastern Esla nappe, in the vicinity of the locality of Crémenes, Cantabrian Mountains, Spain.

Type horizon: Up to 10 m above the base of the “Beleño” facies, upper member of the Láncara Formation.

Holotype: Plate DGO 21149 ( Fig. 6B).

Material.—More than 100 specimens preserved as phosphatic sclerites in the “Beleño” facies, sections Cr 2 and Cr 4.

Diagnosis.—Phosphatic sclerites, subcircular in outline and trapezoidal in cross−section, composed of a labyrinthine meshwork of tubes that centripetally increase in diameter and length; tubes interconnected by transverse smaller tubes; upper surface perforated by meshwork of holes, round to oval in shape.

Description.—Phosphatic sclerites, subcircular in outline and trapezoidal in cross−section, with flat bottom, up to 0.8 mm in diameter and 0.2 mm thick, constructed of two parts: a framework forming a labyrinthine meshwork of tubes, and an upper surface making up the uppermost wall junctions of tubes; in some cases the latter is absent because of erosion or incomplete preservation. Edge of sclerites formed by peripheral girdle, completely perforated: holes 5–15 µm in diameter on bottom and lateral sides, and 50–200 µm on upper cap. Basal and lateral girdle rough containing grooves joining small perforations. Upper surface usually flat to slightly convex, perforated by meshwork of holes, round to oval in shape, highly variable in size (from 50 to 200 µm in diameter) but distinctly decreasing in size close to periphery; wall junctions between holes thin (ca. 30–35 µm) and smooth.

Thin sections through sclerites ( Fig. 7 ) reveal a complex internal structure, composed of a labyrinthine meshwork of tubes centripetally increasing in diameter and length by anastomosing their walls ( Fig. 8 ); lowermost and lateral tubes randomly oriented, closed basally by thin hemispherical but perforated part of framework, passing upward into vertical, parallel and larger tubes, with annulated walls ( Fig. 7A, B , see also Fig. 6D 1 –D 2 ). Tubes laterally perforated by transverse small synapticula−like tubes, up to 10 µm in diameter, connecting neighboring tubes ( Fig. 7E 3, see also Fig. 6D 3). As a result, whole tube meshwork (and, originally, soft parts filling it) intercommunicating. Tube walls composed of two distinct apatitic layers, possibly diagenetic in origin (see discussion below), 0.25–0.3 µm thick ( Fig. 7E 2 , E 3).

Remarks.— Cantabria labyrinthica may be related to the eoconchariids based on the characters observed on the upper surface: the lack of protruding nodes or spines on the wall junctions separating the chaotic hole meshwork is a character shared by Fusuconcharium Hao and Shu, 1987 . However, the holes of Microdictyon Bengtson, Matthews, and Missarzhevsky (in Missarzhevsky and Mambetov 1981), Fusuconcharium and Quadratapora Hao and Shu, 1987 (even if the two latter are considered synonyms of the former) can be both open and closed basally, are laterally unperforated, and continue as a single assemblage of parallel tubes through the plate, which has a bidimensional shape lacking a cup−shaped form.

The labyrinthine microstructure of Cantabria can also be related to stereom microstructures (see e.g., Bengtson et al. 1990: fig. 174A–C; Clausen and Smith 2005). Nevertheless, a labyrinthic stereom ( Smith 1980: 12), which would be the most comparable known echinoderm−microstructure, consists of an unorganized mesh of trabeculae (which are not hollow) that behaves as a single crystal, whereas the meshwork of Cantabria is composed of (hollow) tubes with walls containing two distinct apatitic layers ( Fig. 7E 2 , E 3), although this last character may be diagenetic. These skeletons were either originally phosphatic in composition or secondarily replaced by phosphate. However, the apatitic meshwork is not likely to represent phosphatic crusts lining trabeperforated parallel tubes labyrinthine meshwork culae and pores (external molds) as they have a constant wall thickness and both their inner and outer sides are ornamented with transverse crests ( Fig. 6C). In addition, although the “Beleño” facies is dominated by “encrinitic” (echinodermrich) packstone (see Álvaro et al. 2000b), echinoderm ossicles are absent after etching as they were not secondarily phosphatized: phosphatization was selective and did not apparently affect stereoms. In summary, even if we interpret the described structure as a coated stereom, the preservation of the upper surface of the plates ( Fig. 6B, D) indicates that their tubes would from a stereom porosity and, therefore, the density, coarseness (sensu Smith, 1980) and structure would be quite different from other known stereom types.

The taxonomic affinity of Cantabria is uncertain. Nevertheless, the similarities with eoconchariids (discussed above) suggest it might be considered as Middle Cambrian lobopodian external sclerites phylogenetically related to the Early Cambrian eoconchariids. However, new sclerites found in other outcrops, and eventually with other mineralogical composition, would help to improve this uncertain assignment.

Stratigraphic and geographic range.—Lowermost Middle Cambrian “Beleño” facies, upper member of the Láncara Formation, eastern Esla nappe, in the vicinity of the locality of Crémenes, Cantabrian Mountains, Spain.

Figures

Fig. 7. Thin sections on backscattered SEM of Cantabria labyrinthica gen. et sp. nov. (apatite is white and secondary silica is grey), enigmatic fossil, possible external sclerite of lobopodian affinity from the “Beleño” facies (section Cr4), upper member of the Láncara Formation, early Leonian.A, B, D. Cross−sections with chaotic meshwork microstructure of the sclerites (DGO 21152, DGO 21153, and DGO 21156 respectively) with tubes centripetally increasing in size; note their whole perforated pattern of outer wall. C, E. Transverse sections close to the basal (C, DGO 21154) and the upper side (E, DGO 21155) with subcircular to irregular sections of individual tubes. Overall view (E1) and enlargements (E2, E3) showing the tube walls built of two distinct apatitic layers; see a synapticule−like tube (arrowed) connecting two transverse tubes (E3). Scale bars 250 µm. +

Fig. 8. Sketch of Cantabria labyrinthica gen. et sp. nov.“ (enigmatic fossil, possible external sclerite of lobopodian affinity) illustrating the cross sections of the previous figure.

Fig. 1. A. Pre−Hercynian outcrops of the Iberian Peninsula (stippled) and setting of the Esla Nappe in the Cantabrian Zone (grey area). B. Geologic map of the Esla nappe and stratigraphic sections sampled in this work (Cr1–4 and V1; after Truyols et al. 1990; Aramburu et al. 1992; Álvaro et al. 2000b).

Fig. 2. Stratigraphic units of the Lower–Middle Cambrian boundary interval in the Esla nappe, and setting of the fossils described in the text.