Stratigraphy and geological setting

The sponge specimens come from the Lower Devonian Talacasto Formation (Padula et al., Reference Padula, Rolleri, Mingramm, Criado Roque, Flores and Baldis1967), which is mainly exposed in the central Argentine Precordillera of San Juan Province (Fig. 1), even though isolated outcrops are known in the northernmost extent of the Precordillera, in La Rioja Province (Rustán et al., Reference Rustán, Vaccari and Astini2011).

The Talacasto Formation is composed of a marine succession of intensely bioturbated, greenish gray mudstones with intercalated beds of sandstone; it starts with dark, fine-grained, muddy levels basally, passing upward into sandy levels with fossiliferous nodules (Fig. 1). In its type locality at Talacasto Creek, Talacasto Range, in San Juan Province (Fig. 1), the formation reaches ~300 m in thickness, but a maximum of >1000 m is recorded in the north at the Loma de los Piojos section, near Jáchal.

The Talacasto Formation represents a muddy shelf depositional system developed during a highstand, and overlies the mainly Silurian shelf succession of the Los Espejos Formation (Astini, Reference Astini1991). The turbiditic system of the (mainly) Middle Devonian Punta Negra Formation (Bracaccini, Reference Bracaccini1949) overlies the Talacasto Formation (Bustos and Astini, Reference Bustos and Astini1997).

The Talacasto Formation has yielded the majority of the Devonian macrofossils described from Argentina. The main fossil groups include brachiopods, trilobites, bivalves, gastropods, cephalopods, echinoderms, corals, tentaculitids, ostracods, hyoliths, and conularids. There are abundant ichnofossils, and plant remains are scarce.

According to brachiopod and palynological information (Benedetto et al., Reference Benedetto, Racheboeuf, Herrera and Brussa1992; Racheboeuf and Herrera, Reference Racheboeuf and Herrera1994; Lé Herissé et al., Reference Lé Herissé, Rubinstein and Steemans1997; Herrera and Bustos, Reference Herrera and Bustos2001; García Muro et al., Reference García-Muro, Rubinstein and Steemans2014) the Talacasto Formation spans the Lochkovian to the Emsian. The base of the Devonian is virtually missing due to the presence of a discontinuity between the Los Espejos and Talacasto formations. The stratigraphic record of the Silurian-Devonian boundary can only be recognized in the uppermost interval of the Los Espejos Formation in a few localities toward the north of the Talacasto area, at Cerro del Fuerte, La Chilca (Benedetto et al., Reference Benedetto, Racheboeuf, Herrera and Brussa1992; Carrera et al., Reference Carrera, Montoya, Rustán and Halpern2013), and Jáchal River sections (García-Muro et al., Reference García-Muro, Rubinstein and Steemans2014).

The fossil sponges reported herein come from the Lochkovian lower stratigraphic interval of the Talacasto Formation, in the type area of Talacasto Creek (Fig. 1). Aside from brachiopod biostratigraphic precisions (Racheboeuf and Herrera, Reference Racheboeuf and Herrera1994; Herrera, Reference Herrera1995b; Herrera and Bustos, Reference Herrera and Bustos2001), the Lochkovian age of the fossiliferous beds is based on palynological information and stratigraphic correlation from equivalent levels of the Cerro del Fuerte and Jáchal River sections (Lé Herissé et al., Reference Lé Herissé, Rubinstein and Steemans1997; García-Muro et al., Reference García-Muro, Rubinstein and Steemans2014).

Further aspects of the geologic setting, relevant here, have been treated in several previous works such as those by Baldis (Reference Baldis1975), Herrera (Reference Herrera1995a, Reference Herrerab), Herrera and Bustos (Reference Herrera and Bustos2001), Rustán and Vaccari (Reference Rustán and Vaccari2010), Carrera et al. (Reference Carrera, Montoya, Rustán and Halpern2013), and Salas et al. (Reference Salas, Rustán and Sterren2013).

Material and methods

A total of 14 specimens has been collected in the lower (Lochkovian) part of the Talacasto Formation. The bulk of the material comes from the type section at Talacasto Creek (Fig. 1), but two additional specimens were collected at Los Algarrobos Creek section, nearly 22 km toward the south (see map by Carrera et al., Reference Carrera, Montoya, Rustán and Halpern2013) and from Las Aguaditas Creek section, nearly 12 km toward the southwest of Jáchal (see map by Edgecombe et al., Reference Edgecombe, Vaccari and Waisfeld1994).

The fossils are preserved mainly as reddish-yellowish (oxidized) molds of usually complete individuals contained in greenish black, fine, clayey mudstone matrix, yet in some cases the wall is preserved, so that spicules have been identified. Because of the size and fragility of the fossil material, it was carefully prepared using pneumatic vibrotools and needles. As usual, polished thin sections of specimens, ~30 µ in thickness, were prepared.

The material is housed under the prefix CEGH-UNC in the paleontological collection of CIPAL (Centro de Investigaciones Paleobiológicas, Córdoba, Argentina), at the building of CICTERRA (Centro de Investigaciones en Ciencias de la Tierra), Universidad Nacional de Córdoba, Córdoba, Argentina.

Systematic paleontology

Class Hexactinellida Schmidt, Reference Schmidt1870

Order Hexactinosa Schrammen, Reference Schrammen1903

Superfamily Pileolitoidea Finks, Reference Finks1960

?Family Pileolitidae Finks, Reference Finks1960

Genus Talacastospongia new genus

Remarks

The new genus is included in the Superfamily Pileolitoidea (Finks and Rigby, Reference Finks and Rigby2004) and tentatively in the Family Pileolitidae because of the structure of the spicular net of dictyonal strands and subparallel rhabdodiactines locally united by synapticulae, and irregularly oriented hexactines. However, in Talacastospongia n. gen., the spicular net is dominated by the dictyonal strands or rhabdodiactines with very few hexactines. Furthermore, Talacastospongia possesses a well-developed spongocoel, which is absent or only slightly developed in the genera included in the Pileolitidae (Finks, Reference Finks1960).

The erection of a new family was considered among possibilities at the time of preliminary taxonomic examination. However, the obscure preservation of spicule details prevents a complete view of their arrangement and characteristics for a clear familial differentiation. This led us to a more conservative decision, and we therefore leave the genus, with doubts, in the Family Pileolitidae. The type genus of the family, Pileolites Finks, Reference Finks1960, is a short cylindrical to conical sponge with a shallow depression at the top. It is composed of dermal and inner layers of dictyonal strands sometimes united by synapticulae and with distal rays replaced by knobs. The other genus included in the family is Hexactinoderma Pisera and Bodzioch, Reference Pisera and Bodzioch1991, which is a thick-walled, cylindrical to cup-shaped sponge with layers mainly composed of fused hexactins with highly ornamented globular distal rays. The structure differs from the rhabdodiactine-dominated, spicular net observed in our material.

The Family Wareembaiidae Finks and Rigby, Reference Finks and Rigby2004 shows layers with different structure: an external subeuretoid with dictyonal strands with rhabdodiactins and another internal layer, the aulocalycoid. The genera of this family show hexactins typically ornamented by globular distal rays forming a regular quadrate structure. This is different from the single-layered structure of the Euretidae Zittel, Reference Zittel1877 and clearly different from the more irregular structure observed in Talacastospongia n. gen.

The Family Pileospongiidae Rigby, Keyes, and Horowitz, Reference Rigby, Keyes and Horowitz1979 includes sponges built of several layers, parallel to the surface, composed of irregularly oriented rhabdodiactins and few hexactins. The genera in the family are massive encrusting sponges with inhalant and exhalant canals opening on the same surface, which is clearly different from the morphology seen in Talacastospongia n. gen. In addition, the structure of the skeletal net differs substantially.

The Superfamily Pileolitoidea has earlier records in the Ordovician, but the Family Pileolitidae, in which the new form is tentatively included, has so far been restricted to the Permian. Thus, the Lower Devonian Talacastospongia could represent the oldest record of the family.

Talacastospongia minima new species

Figures 2.1–2.6, 3.1–3.8

Figure 2

Talacastospongia minima n. gen. n. sp. (1, 2) External views (both sides) of the complete sponge, holotype CEGH-UNC 26054. (3) Complete specimen with a budding process, CEGH-UNC 26056. (4) Detail of the pedunculate base and origin of vertical canals, CEGH-UNC 26057. (5) Close-up of (2.3) showing columnar tracts and the porous external surface. (6) Underside view of a complete specimen, showing rounded pedunculate attachment and divergent subparallel external tracts, CEGH-UNC 26055. Scale bars=5 mm (2.1–2.4, 2.6), 1 mm (2.5).

Figure 3

Talacastospongia minima n. gen. n. sp. (1) Thin section of a general view of the endosomal layer marked with pores (dark gray rounded areas), and at the top, the summit of the spongocoel marked with small vertical canals, CEGH-UNC 26059. (2) Thin section of the external layer showing external tracts and the porous external wall, CEGH-UNC 26058. (3) Close-up of (3.2) showing detail of the granulose appearance of tracts. (4) Thin section showing the endosomal layer with the irregular distribution of dictyonal strands; note the continuous nodes along the length of some dictyonal strands, CEGH-UNC 26060. (5) Thin section of a regularly distributed endosomal layer showing two series of parallel strands crossing each other; on the upper part some slightly preserved rhabdodiactines, locally united by synapticulae, CEGH-UNC 26059. (6) Irregularly distributed rhabdodiactins and small hexactins, CEGH-UNC 26061. (7) Irregularly distributed rhabdodiactins, some united by synapticulae, CEGH-UNC 26061. (8) General view of some knobby rhabdodiactins and some small hexactins (base of figure), CEGH-UNC 26062. Scale bars=5 mm (3.1), 1 mm (3.2–3.8).

The Argentinean sponge in the Devonian paleogeographical context

Spiculate sponges are common elements in Devonian faunas only locally. However, studies on Devonian sponges have significantly increased in recent years; the records are very limited in the Lower and Middle Devonian with an increase in the Upper Devonian (Rigby, Reference Rigby1979; Rigby et al., Reference Rigby, Pisera, Wrzolek and Racki2001; Garcia-Bellido and Rigby, Reference García-Bellido and Rigby2004). As a consequence, a detailed regional synthesis on their Lower and Middle Devonian distribution would be premature.

Excluding hypercalcified sponges (stromatoporoids, sphinctozoans), the Lower Devonian records occur mainly in New York (Hall and Clarke, Reference Hall and Clarke1898), Spain (Garcia-Bellido and Rigby, Reference García-Bellido and Rigby2004), Tennessee, (Rigby and Clement, Reference Rigby and Clement1995), and Australia (Pickett, Reference Pickett1969; Pickett and Rigby, Reference Pickett and Rigby1983). Other scattered occurrences, mainly belonging to calcareous spicular sponges (mostly heteractinids), were recorded in different sporadic records from North America of single specimens or isolated spicules (Reimann, Reference Reimann1945; Richardson, Reference Richardson1950; Rigby, Reference Rigby1976, Reference Rigby1991).

Lower and Middle Devonian demosponges are mainly recorded in Spain (Garcia-Bellido and Rigby, Reference García-Bellido and Rigby2004), Tennessee, (Rigby and Clement, Reference Rigby and Clement1995) and Australia (Pickett, Reference Pickett1969; Pickett and Rigby, Reference Pickett and Rigby1983). Other demosponge occurrences are known from single specimens or very limited suites (Rietschel, 1968; Rigby and Murphy, 1976, 1983; Rigby, 1977).

The record of hexactinellids is even more limited. Lower and Middle Devonian hexactinellids occur in New York (Hall and Clarke, Reference Hall and Clarke1898), but are relatively scarce when compared with the abundant assemblages found in younger rocks of the same region. Some other minor records of Lower and Middle Devonian hexactinellids occur in Tennessee (Rigby and Clement, Reference Rigby and Clement1995) and Canada (Rigby and Topor, Reference Rigby and Topor2003).

The Upper Devonian has far higher sponge diversity, and extensive faunas are known from several areas. The best known and first described are the Upper Devonian sponge localities in New York and Pennsylvania (Hall and Clarke, Reference Hall and Clarke1898; Caster, Reference Caster1939). Other important occurrences are in Alberta (Rigby, Reference Rigby1970, Reference Rigby1977; Birkhead and Murray, Reference Birkhead and Murray1982), Poland (Rigby et al., Reference Rigby, Pisera, Wrzolek and Racki2001), France (Pickett and Plusquellec, Reference Pickett and Plusquellec1998), Australia (Rigby, Reference Rigby1986), and China (Pickett, Reference Pickett2007).

In a general paleobiogeographical context, the Devonian faunas from Argentina have been classically considered to be of Malvinokaffric affinities. The Malvinokaffric Realm is a major paleobiogeographical unit, which mainly embraces marine Early–Middle Devonian records belonging to a paleocircumpolar austral region. Because Devonian faunal records from India and Australia do not support a paleobiogeographical link to this great region, the Malvinokaffric Realm involves only some Gondwanic marine basins, including only those from southern South America, South Africa, Ghana, and Antarctica.

Following pioneering proposals from Clarke (Reference Clarke1890) on the ‘austral’ signature of its faunas, this paleobiogeographical unit was formally proposed as a Devonian province by Richter and Richter (Reference Richter and Richter1942) based on trilobite distribution, and later recognized as a realm using brachiopod data (Boucot, Reference Boucot1974). In turn, in addition to high endemicity of taxa at the suprageneric level, this realm is also characterized by scarcity (or absence) of some typical Paleozoic groups such as stromatoporoids, graptolites, and conodonts, and in contrast, relative abundance of others, such as hyoliths and conularids (Boucot and Racheboeuf, Reference Boucot and Racheboeuf1993).

The Porifera are one of major taxonomic groups virtually unknown in Malvinokaffric basins. Until now, a single certain occurrence has been reported, in outcrops of the Lower–Middle Devonian Accraian Series from Ghana, Africa (Saul et al., Reference Saul, Boucot and Finks1963), which was referred to the hexactinellid dyctiospongid genus Prismodyctia Hall and Clarke, Reference Hall and Clarke1898, typical of the Naples Group of New York. Although fairly complete, these specimens are mostly represented by impressions of a highly regular net in clayey sediments, which preserve spicules just as traces of light limonitic stains. Despite the fact that Saul in his M. Sc. thesis (1960, p. 53) suggested the putative presence of Prismodictya in the Middle Devonian Maecurú or Ereré formations in the Amazonas Basin, State of Pará, Brazil, based on the report of Clarke (Reference Clarke1890), these records are uncertain, because no mention of sponges was provided in this last work. The reference by Saul (Reference Saul1960) to this sponge in the Early Devonian of the Ponta Grossa Formation in the State of Paraná, Brazil, based on material mentioned by Kozłowski (Reference Kozłowski1913), is also doubtful. In a recent paper on a Permian hexactinellid sponge from Brazil (Mouro et al., Reference Mouro, Fernandes, Rogerio and Fonseca2014), no records of sponges older than the Carboniferous were mentioned for this country.

An additional record in a clear Malvinokaffric context is that of Receptaculites bolivianus Braniša, Reference Braniša1965 from Pragian levels of the lower part of the Belén Formation, in the area of Belén and Pujravi, Bolivia (Braniša, Reference Braniša1965). However, the status of Receptaculites has been controversial, and the latest consensus is that receptaculitids are not sponges but calcareous algae (Nitecki et al., Reference Nitecki, Mutvei and Nitecki1999, Reference Nitecki, Webby, Spjeldnaes and Yong-Yi2004).

Apart from these mentions, no sponges are known from other classic areas with Malvinokaffric records, such as South Africa, Uruguay, the Malvinas Islands, and Antarctica. In this context Talacastospongia minima n. gen. n. sp. represents the best Malvinokaffric sponge record to date. However, although clearly endemic in comparison with Prismodyctia, it does not supply further insights on several issues related to contrasting paleobiogeographical patterns in Malvinokaffric basins.

Information from other fossil groups shows a particular paleobiogeographic scenario. Trilobites, for example (Eldredge and Ormiston, Reference Eldredge and Ormiston1979), exhibit endemic signatures (despite a number of suggestions to the contrary by Rustán and Vaccari, Reference Rustán and Vaccari2010, Reference Rustán and Vaccari2012; Holloway and Rustán, Reference Holloway and Rustán2012). The same situation occurs with upper Lower–Middle Devonian ostracodes (Salas et al., Reference Salas, Rustán and Sterren2013) and hyoliths (Malinky and Racheboeuf, Reference Malinky and Racheboeuf2011). In contrast, a composite paleobiogeographic signal has been recognized in brachiopods (Herrera, Reference Herrera1995b; Herrera et al., Reference Herrera, Salas and Giolitti1998; Isaacson, Reference Isaacson2007) and bivalves from the Argentine Precordillera (Sterren et al., Reference Sterren, Rustán and Salas2015).

Postulated extrinsic controls driving evolutionary and biogeographical processes in this realm include complex paleogeographical settings (Abe and Lieberman, Reference Abe and Lieberman2009), latitudinal position (Boucot et al., Reference Boucot, Johnson and Talent1969), oceanographic factors such as paleocurrents (Eldredge and Ormiston, Reference Eldredge and Ormiston1979; Isaacson, Reference Isaacson2007), and variations in sea level (Rustán and Vaccari, Reference Rustán and Vaccari2010). In particular, the hypothesis of a sharp paleoecological barrier due to a high paleolatitudinal position has been regularly suggested for supporting Malvinokaffric endemism (Eldredge and Ormiston, Reference Eldredge and Ormiston1979; Boucot et al., Reference Boucot, Rowell, Racheboeuf, Pereira, De Melo and De Siqueira2001; Morzadec et al., Reference Morzadec, Mergl, Villarroel, Janvier and Racheboeuf2015).

Despite the well-known fact that several taphonomic issues affect the apparent paleogeographic distribution of sponges (see Muir et al., Reference Muir, Botting, Carrera and Beresi2013 for a review), it is important to note the significant sampling effort in the Devonian of the Argentine Precordillera for more than 50 years. We can therefore state, at least in a general view, that the absence of the typical, carbonate-related hypercalcified sponges (stromatoporoids, sphinctozoans), as well as the extremely depauperate Malvinokaffric sponge records can be visualized as a real pattern. The scarcity of sponges in general, and particularly of hexactinellids, contrasts with their conspicuous records in equivalent siliciclastic localities situated in lower latitudinal regions. This would be in agreement with the general idea of a discrete major biogeographic unit (the Malvinokaffric Realm) developed in high-latitudinal settings, in accordance with classical interpretations based on sedimentological and paleogeographical data, with the South Pole being located between South America and Africa during the Early Devonian (Boucot and Racheboeuf, Reference Boucot and Racheboeuf1993). Alternatively, all these lines of evidence might also suggest the effect of environmental gradients on biogeographical patterns, rather than distinct paleoecological barriers, a largely unexplored aspect pending the application of analytical approaches.