Monticuliporidae, Nicholson, 1881

Jiménez-Sánchez, Andrea, 2010, New Monticuliporidae (Bryozoa, Trepostomata) from the Cystoid Limestone Formation (Upper Ordovician) of the Iberian Chains (NE Spain), Geodiversitas 32 (2), pp. 177-199 : 196

Monticuliporidae sp. indet. ( Figs 8 ; 9; 10 ; Table 5)

Orbignyella sp. Jiménez-Sánchez et al. 2007: 687, fig. 4 (1-2).

MATERIAL EXAMINED. — Zoarium MPZ 2006/110.

DESCRIPTION

General characters

Zoarium with incrusting growth habit, composed of two layers with variable thickness in different parts of colony. Zoarium with maximum height of 4.3 mm, minimum height of 1.5 mm and maximum aparent diameter of 32.0 mm. It is incrusting the external and internal surface of a brachiopod shell and the zoarium of Prasopora carnica described above.

Tangential section

Autozooecial apertures with very regular hexagonal form, giving a high degree of packing, with average diameter of 0.39 mm and average of 7.2 autozooecia per mm2 and 2.6 autozooecia per linear mm. Mesozooecial apertures polygonal (rectangular to pentagonal), with average diameter of 0.20 mm and average spacing of 2.2 mesozooecia per mm2 and 0.6 mesozooecia per linear mm. Mesozooecia located in the space left by less regular hexagonal autozooecia. Acanthostyles rather scarce, averaging one per mm 2, large (mean diameter of 0.065 mm), mainly located in junction of three autozooecia. Acanthostyles with light-coloured cores and darkcoloured laminated sheaths. Zooecial walls are thick (0.032 mm) with beaded outline, ( Fig. 9). Maculae not observed.

Longitudinal section

Zoarium composed of two layers of irregular thickness, with overgrowth surface marked by development of new basal diaphragms. Autozooecia grow at an angle of 26° from the substrate, bending sharply and intersecting zoarial surface at an angle of 90°. Autozooecia with tubular shape, containing diaphragms and cystiphragms. Diaphragms mainly straight and perpendicular to autozooecial walls, but occasionally also oblique, concave or convex; regularly spaced inside autozooecia with an average spacing of 3.9 diaphragms per mm. Cystiphragms developed in upper three quarters of autozooecia with a spacing of 2.5 per mm. Th ey are attached to one side of the wall curving and leaning on a lower diaphragm or on a cystiphragm developed in the opposite wall. Mesozooecia scarce, consisting of narrow tubes originating at the base of the exozone. They have more diaphragms than autozooecia, an average of 4.6 per mm. Acanthostyles long, up to 1.14 mm in length, developed at different levels, but only up to the point where autozooecia curve ( Fig. 10 ). Zooecial walls with irregularities in form of small beads visible at low magnification; their thickness increases upwards in the colony. At higher magnification an exterior dark line that continues in the diaphragms can be observed. Between the two dark lines there is a layer with an obvious granular microstructure.Endozone-exozone boundary marked by change of inclination of autozooecia, development of mesozooecia and acanthostyles, and higher density of cystiphragms in autozooecia.

REMARKS

This specimen exhibits the main diagnostic characters of the family Monticuliporidae : encrusting growth habit, polygonal autozooecial apertures, presence of diaphragms and cystiphragms, densely tabulate mesozooecia and presence of acanthostyles.The only diagnostic character not observable are maculae, but it could be the result of the small number of sections studied.Th is specimen has a combination of features that are not present in any other Monticuliporidae genus: very regular hexagonal apertures, walls with a distinctive beaded outline in cross section, and very large and scarce acanthostyles with a core.These three features have been used in traditional trepostomate systematic to distinguish and classify genera.The occurrence of these characters would merit the erection of a new genus in the family Monticuliporidae . The formal definition should await until more colonies are found.

This Iberian zoarium is similar in the shape of its cystiphragms to Orbignyella Ulrich & Bassler, 1904 , originally ascribed to the monticuliporids but presently considered to be best placed within the family Atactotoechidae Duncan, 1939 . Nevertheless, the Iberian zoarium cannot be included in Orbignyella which, like the other atactotoechids, has exilazooecia instead of mesozooecia and much smaller acanthostyles than the studied sample. Th is zoarium is also similar to Prasopora spjeldnaesi n. sp., but it can be clearly distinguished by the shape of its autozooecial apertures (polygonal vs oval), the number of mesozooecia (much less abundant than in the Prasopora species ) and the presence of acanthostyles.

Figures

FIG. 8. — Monticuliporidae sp. indet., MPZ 2006/110: A, longitudinal thin section showing the large acanthostyles (ac), the peculiar cystiphragms (cy), and the narrow mesozooecia (mz); B, tangential thin section showing the regular hexagonal autozooecia (az) with wall sections having gently beaded (bd) outline,the mesozooecia (mz) in an area where hexagonal autozooecia are not so regular,and large acanthostyles (ac); C, a high-magnification detail of the longitudinal section of an acanthostyle (ac) showing the central lumen on the lower part of the figure and lamination around the lumen; D, a high-magnification detail of the cross section of an acanthostyle (ac) showing the light coloured central lumen (lu) and surrounding dark lamina. Scale bars: A, B, 1 mm; C, D, 0.1 mm.

FIG. 10.— Monticuliporidae sp.indet.,schematic longitudinal section showing a prominent acanthostyle (ac), the shape of diaphragms (dp), cystiphragms (cy) and autozooecial tubes (az), as well as the narrow mesozooecia (mz). Scale bar: 0.1 mm.