Minitaspongia parvis, Carrera & Rustán & Vaccari & Ezpeleta, 2018

Carrera, Marcelo G., Rustán, Juan Jose, Vaccari, N. Emilio & Ezpeleta, Miguel, 2018, A new Mississippian hexactinellid sponge from the western Gondwana: Taxonomic and paleobiogeographic implications, Acta Palaeontologica Polonica 63 (1), pp. 63-70 : 65-68

Minitaspongia parvis sp. nov.

Figs. 2 , 3 .

Etymology: From Latin parvis , small, young.

Type material: Holotype: PULR-I 4 nearly complete sponge specimen . Paratype: PULR-I 5 , fragmentary specimen .

Type locality: Sierra de las Minitas section, La Rioja Province, Argentina. Holotype comes from tectonic block 1 at GPS: 28° 47 43 S / 68° 45 22 W . Paratype comes from tectonic block 2 at GPS 28 ° 46 28 S / 68 ° 44 40 W ( Fig. 1 ) .

Type horizon: Agua de Lucho Formation, Tournaisian. The sponge comes from nodules within a thick black fossiliferous mudstone interval, approximately 100–150 m underlying a diamictitic, glacigenic bed, and approximately 200–250 m below the record of the Tournaisian guide spore Waltzispora lanzonii Daemon, 1974 (see Prestianni et al. 2015).

Material.— Type material only.

Diagnosis.— Obconical to palmate thysanodictyine sponge with subcylindrical base expanding upward to define a broad spongocoel, with a wall consisting of a few layers of spicules. Skeleton clathrate, three-dimensional with at least two ranks of rectangular openings. First order quadrules 2× 1.5–1.8 mm and the second order approximately 0.5× 0.5 mm across. Vertical tracts supported by two or three aligned long-rayed hexactins, 3–4 mm in maximum length and a shorter ray 1.0– 1.2 mm.

Description.— The specimens are small, obconical to palmate sponges. The holotype, a nearly complete specimen, shows an upward-expanding obconical shape, approximately 3–4 cm high. The specimen begins with a small cylindrical base 3.5 mm in diameter, apparently broken, from which the body expands abruptly in a fan or obconical shape reaching 20 mm wide. The walls are 1–2 mm thick and outline a wide and deep spongocoel that reaches almost to base.

The specimens lack nodes, wrinkles or other sculpture that typically characterizes other genera of the family. Externally, the mesh is marked by rectangular skeletal openings (defined mainly by impressions and limonite stains in silty matrix). The skeleton shows a tridimensional clathrate wall of regular vertical and horizontal blade-like skeletal straps.

The wall consists of box-like meshes outlined by spicular bundles of hexactins and hexactine-based spicules. The skeletal grid has typical rectangular reticulation of dictyosponges;

A with primary quadrules nearly 2 mm high and 1.5–1.8 mm wide, and spicule tracts of 0.1–0.3 mm wide ( Figs. 2 , 3 ).

Vertical tracts are coarse (tridimensional) and mainly supported by two or three aligned long-rayed hexactins (probably stauracts). These primary hexactines have a long vertical tangential ray, 3–4 mm in maximum length and a short tangential horizontal ray 1.0– 1.2 mm, disposed perpendicularly to the vertical tracts ( Fig. 2B 4 –B 7). The third axis, perpendicular to the sponge surface, appeared as a knob-like distal ray, the other end is uncertain, it probably continues to the interior of the wall, in which case the spicule is a pentactin not a stauract ( Fig. 2B 7).

Second order quadrules, are approximately 0.5 mm high and 0.5 mm wide. In well preserved patches small hexactins, with equal ray length (0.8–1.0 mm), have been observed as main part of the quadrules. In the external surface this second order quadrules occur as a cross-like center dividing the first order quadrules ( Fig. 2A ). In the periphery of the sponge wall a marginal concentration of second order quadrules, implies a more complex structure including fine autodermal and hypodermal quadrate meshes ( Fig. 2B 1 –B 3 ).

Remarks. — The specimens are included in the subfamily Thysanodictyinae Hall, 1882 (family Dictyospongiidae Hall, 1882 ) and are related to Clathrospongia Hall, 1882 or Thysanodictya Hall and Clarke, 1899 . Both genera exhibit a typical coarse ridge-like, quadrate mesh of spicule bundles based on pentactins and hexactins. This skeletal structure is uncommon among dictyosponges, which in general have very thin skeletons made of thin strap-like tracts.

Thysanodictya and Clathrospongia both have a clathrate or grid-like skeleton, which is three-dimensional with box-like mesh openings of at least two ranks in a very symmetrical pattern.

Thysanodictya has a discoidal base (wide basal disc), and a cylindrical morphology with big and rectangular, vertically oriented quadrules.

Clathrospongia is a conico-cylindrical sponge with coarse ridges and more quadrate openings. Hall and Clarke (1899) referred four species of Devonian and Carboniferous dictyosponges to the genus Clathrospongia , in addition other four species were referred to the genus with question. Variation in grid sizes is significant and it was used for species differentiation: the type species Clathrospongia abacus Hall, 1882 , has a primary grid of 9 × 10 mm. C. fenestrata ( Hall 1863) , C. caprodonta Hall and Clarke, 1899 , and C. vascellum ( Hall, 1890) , all have a coarser grid, ranging from 0.9 × 10 mm, to a maximum of 26 × 30 mm in C. vascellum (the original spelling is C. vascellum and not C. bascellum , as cited by Rigby and Keyes 1990).

Rigby and Keyes (1990) described Clathrospongia bangorensis from the upper Mississippian of Alabama which is similar in its external morphology to the Argentinean species. However, the grid is bigger, 4 × 6 mm for the primary grid with second order openings 3 × 3 mm across.

These grid sizes are considerably larger than the primary grid of our specimens (2.0 × 1.5 mm). However, the size of the grid alone should not be taken as a reliable character. The reticulosans grew by expansion ( Botting 2003) and skeletal dimensions will be dependent on growth stage. Only when the proportion of the quadrule sides is maintained in relation with other taxa known from larger specimens, a more confident comparison could be considered.

Other related genus is Thamnodictya Hall, 1884 which has a very acute base that expands very rapidly given the sponge a fan-like appearance. This morphology is clearly different from the features observed in the Argentinean material.

The Argentinean sponge is a small and probably young specimen. The clathrate skeleton, three-dimensional with at least two ranks of rectangular openings is shared by Thysanodictya and Clathrospongia . However, the overall structure and arrangement of the quadrules in the wall, and their relative side dimensions are different from those seen in these genera. The vertical tracts supported by two or three aligned long-rayed hexactins could be considered as a distinct characteristic of the new genus. Unfortunately, there are no descriptions of the spicule details and size in the related genera of the subfamily.

There are very few records of preserved spicule net among the family. The Devonian Mattaspongia apache Rigby, 1970 shows spicule tracts composed of subparallel hexactines and smaller rhabdodiactines, that also occur as prostalia. This spicule net composition clearly differs from the structure observed in the Argentinean sponge.

The genus Corticospongia Caster, 1939 , belonging to the subfamily Calathospongiinae Hall and Clarke, 1899 ( Finks and Rigby 2004), shows preserved spicules forming the reticulate skeleton with main hexactine spicules similar to those of Minitaspongia parvis gen. et sp. nov., a knoblike distal ray and a long main tangential ray but disposed horizontally. The general structure and the overall morphology of this form are clearly different from the Argentinean species.

The marginal distribution of the second order quadrules in the sponge wall of Minitaspongia gen. nov. ( Fig. 2B 2 ), implies a more complex structure, fine autodermal and hypodermal meshes, which is comparable, to the restoration of Hall and Clarke (1899) or Finks and Rigby (2004) for Clathrospongia or Thysanodictya .

Hall and Clarke (1899) interpreted the quadrate mesh as representing radially, solid erected lamellae, as a common pattern seen in the subfamily. The coarse ridge mesh in our material is three-dimensional forming an apparent vertical wall expanded towards the interior of the sponge body. The vertical ridges are internally irregular composed of vertically and horizontally arranged bundles of subparallel hexactine-based spicules. This structure is different to the interpretation of quadrules with laminar solid walls envisaged by Hall and Clarke (1899), a view also questioned by Rigby and Keyes (1990) and Finks and Rigby (2004).

A remaining unclear feature is the whole composition of the vertical and horizontal parietal tracts. It is clear that the main scaffold structure of the spicular skeleton is supported by the long vertical rayed hexactines, but unknown smaller elongated spicules (monaxon types?) may also participate in the spicular bundle construction.

Stratigraphic and geographic range.— Tournaisian, Carboniferous; Agua de Lucho Formation, Las Minitas section, La Rioja Province, northern Argentine Precordillera.

Figures

Fig. 2. Hexactinellid sponge Minitaspongia parvis gen. et sp. nov. from the Tournaisian Carboniferous, Sierra de las Minitas section, La Rioja Province, Argentina. A. Holotype PULR-I 4, complete obconical specimen, showing the cylindrical acute base. External surface of the holotype showing deep impressions of vertical and horizontal tracts and cross-shaped spicule molds as second order elements in the middle of first order quadrules (A2). B. Paratype PULR-I 5, external impression with original spicules forming the reticulate skeleton (B1); detail showing a well preserved external area of the second order quadrules (B2); detail of the first order quadrules showing preserved hexactine-based spicules aligned (B3); detail of the tract with vertically oriented, main long axis hexactine- based spicules, bundles of two or three long spicule axis occur in the tract (B4); enlarged section of B4 showing nodes of the perpendicular axis of the hexactins (B5); details of the spicules in the tracts, long rayed hexactins, and small second order hexactins with equal-ray dimensions (B6, B7).

Fig. 3. A. Schematic reconstruction of the complete sponge Minitaspongia parvis gen et. sp. nov. B. The spicule framework of the wall as main parts of the vertical and horizontal bands.

Fig. 1. A. Geographic location of the studied area. B. Geological map and location of the studied section. The west flank of the Agua Quemada Syncline is divided in some tectonic blocks (1 and 2) by inverse and dextral strip faults. Sponges come from nodules of equivalent fine-grained dark stratigraphical intervals of the Agua de Lucho Formation (Tournaisian), cropping out under glacigenic diamictitic in both tectonic blocks. C. Stratigraphic column of the Agua de Lucho Formation and the location of the sponge-bearing levels.