Reticulosan Sponges are a group of hexactine (six-rayed spicule) bearing early Sponges, that are considered typical components of the Cambrian Evolutionary Fauna. They are abundant in Cambrian deposits, scarce in the Early Ordovician, and subsequently unknown. Isolated fragments and partial specimens of 'Reticulosans' are known from the Tremadocian (earliest Ordovician, 485.4 to 477.7 million years ago) but they are not confidently assigned to a suitable taxon. The only diverse assemblages to date are those from Little Métis, Quebec, which is of uncertain age within the late Cambrian–Early Ordovician interval, and the Tremadocian-Floian Fezouata Biota of Morocco. A new association was described more recently from the Early Ordovician of Wales.
In a paper published in the journal Ameghiniana on 2 April 2020, Josefina Aris of the Consejo de Investigaciones de la Universidad Nacional de Salta, and the Instituto para el Estudio de la Biodiversidad de Invertebrados, Marcelo Carrera of the Facultad De Exactas Físicas y Naturales at the Universidad Nacional de Córdoba and Miguel Ángel Boso, also of the Consejo de Investigaciones de la Universidad Nacional de Salta, describe a Reticulosan Sponge from the Tremadocian Floresta Formation of Salta Province, northwestern Argentina.
In northwestern Argentina the Tremadocian Hexactinellid Sponge records include isolated spicules and Protospongiod Sponges and a complete Tremadocian Hexactinellid (Pelicaspongid) Sponge from the Puna Region of northwestern Argentina. Cambrian and Ordovician sponges, including Hexactinellid spicules, have been also reported from elsewhere in Argentina, but mainly concentrated in the limestone units of the Argentine Precordillera with a remarkable different geologic history to the northwest of the country.
The Sponge described by Aris et al. was collected from an outcrop of the Floresta Formation of the Santa Victoria Group, on the western flank of the Mojotoro Mountains, in Salta Province, Argentina. This unit has been previously studied and recognized for its exceptional preserved fossils, in particular Marrellomorph Arthropods.
Map showing the fossil locality (five points star). Aris et al. (2020).
The basement of the range of the Puncoviscana Complex has an Ediacarian-Early Cambrian age. Late Cambrian sediments of the Meson Group overlie these Precambrian units. The Late Cambrian to Ordovician sediments of the Santa Victoria Group unconformably overlie the Mesón Group. The Salta Group, comprising Cretaceous to Eocene sediments, overlie the Santa Victoria Group in the southern section of the mountain range. The youngest deposits, the Neogene Oran Group, overlie the Palaeozoic units on the eastern flank of Mojotoro Mountain Range.
The Santa Victoria Group in the Mojotoro range comprises eight unitst, from bottom to the top the La Pedrera, San José, Caldera, Floresta, Áspero, San Bernardo, Mojotoro, and Santa Gertrudis formations. The Floresta Formation was deposited during the late Tremadocian, and is the most fossiliferous unit of the Santa Victoria Group.
The fossiliferous levels, from which the Sponge derives, are located in a 6.8 m thick interval at 130 meters from the base of the Floresta Formation in Tres Cerritos Creek Section. They consist in olive-green siltstones with parallel lamination intercalated with thin levels of iron hydroxide siltstones. It also contains thin levels of very fine sandstones of similar colour and parallel lamination and olive green, slightly micaceous shales.
Stratigraphic column of Floresta Formation in the Tres Cerritos Creek Section. Aris et al. (2020).
The Sponge was found parallel to the stratification plane with Tergomyan Molluscs, Orthide Brachipods, and Trilobites of Bienvillia tetragonalis biozone.Therefore, the age of this level is assessed to be early late Tremadocian. The grade of disarticulation and the fragmentary nature this association suggest some transportion of the material. In general, the environmental interpretation of these deposits at the Tres Cerrito Creek Section, suggests that the Sponge could have lived on a siliciclastic distal platform.
The recovered specimen is a single fragment (part and counterpart) that is deposited at the Department of Paleontology at the National University of Salta under the collection number CNS-I 153 (5-5´). The specimen occurs compressed as a bi-dimensional laminae and shows spicules preserved in iron replacement.
The specimen is referred to the genus 'Reticulosa', although this is now considered as a paraphyletic grouping, rather than a valid phylogenetic group. Accordingly, the term is used informally by Aris et al. (2020). The higher-level classification of complex Reticulosans and their familial levels has not yet been fully established.
The specimen is a single fragment of Reticulosid Sponge, 1.5 cm long, possibly a juvenile form, without base or osculum. It is thin-walled, with a single skeletal layer irregularly interrupted by parietal gaps. The thin skeletal layer composed of irregularly oriented stauractins of different sizes, and apparent longitudinal subparallel strands. Across some parts of the preserved fragment, larger spicules occur closely spaced, and with close overlapping of elongated spicule rays combining to form subparallel, straight or sinuous strands. There are also possible long monaxons (rhabdodiactines) intertwining with the long rayed hexactins or simply long monaxons in the strands cross-connected or overlaid by small perpendicular monaxons and hexactins.
'Reticulosa' sp. (1) Complete specimen, scale bar 1 mm. (2) Detail of the mid-central part of the specimen, scale bar 1 mm. (3) Detail of the main gaps, scale bar 1 mm. (4) Detail of the spicular structure, counterpart, scale bar 0.5 mm. (5) Detail of the same mold showing the hexactine-based structure with monaxons, scale bar 1 mm. (6) Detailed view of the spicular net and the small gaps (arrows), scale bar 0.5 mm. (7) Detail of the previous photograph showing sub-vertically oriented main axis of hexactine based spicules (stauracts) and a small square gap (lower right), scale bar 0.5 mm. (8) Detailed view of the previous photograph, showing sub-vertically oriented strands joined by small stauracts or a perpendicularly cross-connected monaxon (arrows), scale bar 0.5 mm. (9) Detail of photograph (6) showing rounded to angular small parietal gaps, scale bar 0.5 mm. Aris et al. (2020).
The hexactine spicules are of two main sizes: Large spicules with axis 1-1.5 mm long, and the smallest hexactins of 0.2-0.3 mm long. There are also intermediate sizes of 0.5-0.7 mm long. Parietal gaps are irregularly distributed and with no clear differentiated form (the largest 1-1.2 mm wide), occurring as slightly protruding mounds. Due to the fragmentary preservation of the specimen it is difficult to determine the real nature of these large apertures, whether they are gaps, an artifact of preservation or a damage of the skeleton. They occur partially outlined by small curved monaxons 0.2-0.3 mm long, united by their tips to conform and extremely irregular, sub-circular to sub-angular perimeter. There are also small and intermediate size gaps (in this case, with no doubts of their origin) ranging from 0.3 mm to 0.5 mm wide, rounded or sub-angular, commonly outlined by hexactins.
The main features of this Argentinean Sponge are the semi-regular to irregular skeletal structure in a single layer with rare irregularly distributed parietal gaps, hexactine based skeleton (stauracts of several sizes), and the presence of long apparently subparallel strands of long monaxons or hipertrofied hexactine rays. These characteristics are found in many Reticulosans, some of which possessed semi-regular skeletons with parietal gaps, bilaminar walls, or complex walls with both multiple layers and pronounced mounds. Others included features such as hypertrophied or highly modified spicules, or strengthening structures produced by modified overlapping spicule arrangements.
Members of the genus Ratcliffespongia have small parietal gaps and loosely stauracts of several sizes. In some species, such as Ratcliffespongia freuchenensis, the general structure is somewhat similar to the Argentinean Sponge, with long-rayed stauractins in irregular array and widely-separated parietal gaps. However, Aris et al.'s specimen has rare irregularly distributed parietal gaps, and it possesses monaxons in the skeletal net which are absent in Ratcliffespongia.
Stephenospongia is a conico cylindrical thin walled Reticulosid, with numerous large vertically elliptical parietal gaps outlined by tracts of irregularly oriented rhabdodiactines or fine stauractines. However, it shows a more regular structure and well defined parietal gaps, outline by tracts which is not seen in the Argentinean form.
Hintzespongia has double-layered walls, with parietal gaps of 0.5–0.7 mm in diameter, which is a clearly different structure that the one seen in Aris et al.'s material.
Valospongia has huge and regularly distributed parietal gaps compared to the Argentinean Sponge and a different structure. Lantianospongia possess basal diagonal monaxons as well as serrated oscular margin; it shares many characters with Valospongia. Both Valospongia and Lantianospongia share monaxons and the presence of several sizes of stauracts, which are also present in the Argentinean form. However, the structure of the skeletal net, more regular and defined around the parietal gaps are different from those of Aris et al.'s specimen.
The character combination in the Argentinean Sponge such as, irregular distributed stauracts of different sizes, presence of monaxons, scarce irregularly distributed small gaps, and in particular, the presence of subparallel strands joined by monaxons or small stauracts could be enough features to differentiate a new genus of Reticulosan. However, due to the particular and fragmentary preservation of the specimen, the absence of the base and osculum and the uncertainties on the real nature of the large apertures in the skeletal net, Aris et al. prefer to leave the Argentinean form in open nomenclature.
The Reticulosan genera are typical Cambrian Sponges, and this group together with the Protomonaxonids, are considered the Sponge members of the Cambrian Evolutionary Fauna. Because there are very few records of Late Cambrian and Early Ordovician non-Lithistid Sponges (part of the onset of the Ordovician Radiation), it is difficult to know when the Protomonaxonids declined, or when the Hexactinellid faunas changed from typically Cambrian groups to the more complex taxa typical of the Middle and Late Ordovician.
The occurrence of the Argentinean Reticulosan form represents a new piece of evidence that elements of the Reticulosan Sponges from the Cambrian Fauna continued into the Early Ordovician as did genera such as Valospongia and Hintzespongia.
The sponge was recovered from the same unit as other exceptionally preserved fossils (e.g. Narrellomorphs). This location in a mid- to high palaeogeographic latitude in Gondwana, implies not only that several different elements of the Burgess Shale-type assemblages survived into the Early Ordovician, but also that this Sponge group survived well into the Early Ordovician at different palaeo-latitudes and in different environments.
The occurrence of a Tremadocian Hexactinellid Pelicaspongid Sponge from the nearby Puna Region in North Western Argentina, in a shallow water environment, shows the next step in Hexactinellid Sponge evolution: Sponge taxa from the Palaeozoic Evolutionary Fauna, show the acquisition of complex three-dimensional skeletons, for the invasion of high-energy, shallow platforms settings.
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