As a result of a microfacial study in the outer and middle ramp deposits of the Vaca Muerta Formation (lower Tithonian-upper Valanginian), four ichnotaxa of crustacean microcoprolites are described: Palaxius azulensis Kietzmann isp. nov., Palaxius caracuraensis Kietzmann isp. nov., Helicerina? isp. A. aff. Helicerina siciliana and Helicerina isp. B. They represent one of the first records of crustacean microcoprolites for the Neuquén Basin and Argentina. Helicerina is reported for the first time from the Upper Jurassic and Lower Cretaceous of South America. It is inferred that Palaxius ichnospecies were produced by callianassids, while Helicerina ichnospecies could be produced by decapods of Mecochiridae, Erymidae, and/or Nephropidae affinity. Two assemblages of crustacean microcoprolites are recognised, a middle Tithonian to lower Berriasian Palaxius-dominated assemblage and an early to late Valanginian Helicerina-dominated assemblage.
Some crustaceans produce internally structured microcoprolites. The structures consist of longitudinal canals that, in cross-section, show particular characteristics which allow the discrimination of different ichnotaxa (Brönnimann 1972). The canals are formed by pyloric fingerlets inside the crustacean's gut (Powell 1974). Mass accumulation of these fecal pellets are very common in the Late Jurassic and Early Cretaceous carbonate platforms, and are mostly associated with shallow marine soft sediments (Flügel 2004). However, some studies report also the presence of microcoprolites from outer ramp environments (De Romero and Galea-Alvarez 1995; Schweigert et al. 1997) and, more recently, from deep-sea environments (Peckmann et al. 2007; Senowbari-Daryan et al. 2007; Buchs et al. 2009).
The marine deposits from the Jurassic—Cretaceous boundary interval of the Neuquén Basin contain a rich invertebrate fauna mainly represented by mollusks (Aguirre-Urreta 2003). Crustaceans and their products, however, are poorly known. The remains of decapods from the Mendoza Group (Fig. 1) include palinurids, anomurids and astacideans, which are commonly preserved as isolated chelipeds (Aguirre-Urreta 1989, 2003). On the other hand, Thalassinoides Ehrenberg, 1944 burrow systems with boxwork-like architecture, typically associated with callianassids (see Bromley 1990), are abundant in certain levels of the Vaca Muerta Formation (Kietzmann and Palma 2009). Crustacean microcoprolites in South America had been reported from Peru, Colombia, Venezuela, and Chile (Förster and Hillebrandt 1984; Senowbari-Daryan and Stanley 1986; Blau et al. 1994, 1995; De Romero and Galea-Alvarez 1995). Recently, the first record of crustacean microcoprolites in the Neuquén Basin was reported by Kietzmann and Palma (in press), who report the presence of Palaxius decaochetarius Palik, 1965.
In this paper we present two new microcoprolites ichnospecies of the ichnogenera Palaxius Brönnimann and Norton, 1960 (Palaxius azulensis Kietzmann isp. nov. and P. caracuraensis Kietzmann isp. nov.), together with two possible new ichnospecies of the ichnogenera Helicerina Brönnimann and Masse, 1968. Results could be used for comparison with other localities worldwide, and provide biostratigraphic implications to the Jurassic—Cretaceous boundary of the Neuquén Basin.
During the Late Jurassic and Early Cretaceous, the Neuquén Basin constituted the setting for the development of a wide low-gradient carbonate ramp whose distal facies are represented by lower Tithonian—lower Valanginian middle ramp to basin deposits, which have been named the Vaca Muerta Formation (e.g., Legarreta and Uliana 1991; Scasso et al. 2005; Kietzmann et al. 2008). The proximal facies are represented by lower Valanginian inner ramp deposits and are known as the Chachao Formation (e.g., Legarreta and Uliana 1991; Palma and Lanés 2001; Palma et al. 2008) (Fig. 1).
The samples containing microcoprolites come from different stratigraphic sections of the Vaca Muerta Formation outcroping in the Mendoza province. They have been obtained in levels of middle Tithonian and early Berriasian age (Aulacosphinctes proximus to Substeueroceras koeneni zones), late Berrasian (Spiticeras damesi Zone) and early Valanginian age (Lissonia riveroi and Olcostephanus atherstoni zones) (Fig. 1). Microcoprolites are found in thin sections of laminated packstones and wackestones, rich in ammonites, bivalves, and radiolarians. These facies are associated with black shales, radiolarian and bioclastic mudstones/wackestones and storm induced calcareous sandstones turbidites (Kietzmann et al. 2008). Originally, many peloids were mentioned previously as peloidal micrites by Scasso et al. (2005) and Kietzmann et al. (2008), and interpreted in some cases as the result of possible microbial activity (Kietzmann and Palma 2009). However, the reexamination of these microfacies in new localities (Fig. 1) indicates that many of these peloidal micrites are actually mass accumulations of microcoprolites.
Two types of preservation are present. The best specimens of microcoprolites are recognised in microcoprolitic grainstones that partially fill the chambers of some ammonites. Within these chambers, microcoprolite preservation improves toward the center of the chamber due to an inward gradient in the intensity of calcitic cementation. On the other hand, microcoprolite preservation is generally quite poor in wackestone or packstone microfacies. In such cases, the canals are filled with mud and the edges are diffuse, probably as a consequence of the higher availability of mud, and the effects of compaction.
(by D.A. Kietzmann)
Coprolites are considered trace fossils and their nomenclature is governed by the International Code of Zoological Nomenclature (ICZN). However, coprolites are one of those groups of trace fossils whose dependence or independence from the Linnean system is still under discussion. As Bertling et al. (2006) have stated, an amendment of the ICZN will probably be necessary in order to properly apply its nomenclatorial rules to groups such as this one. In addition to the nomenclatorial problem for fossil microcoprolites, the systematic position of fossil crustaceans in the extant groups remains uncertain (Blau and Grün 2000). At present, the ichnotaxonomy most often followed by the experts is the one proposed by Vialov (1978), who introduced the term Favreinidae for canal-bearing coprolites in order to distinguish them from their producers (crustaceans).
The features used for ichnotaxonomic subdivision of canalised microcoprolites are the number, shape and arrangement of the internal canals. Using size of canals as a diagnostic feature may be confusing because in some microcoprolites the canal-system alters during ontogeny (e.g., Schweigert et al. 1997). Presently, there are eleven ichnogenera of microcoprolites attributed to crustaceans (Senowbari-Daryan and Kube 2003). Systematic characteristics and a list of the main ichnotaxa known can be found in Blau and Grün (2000).
Ichnofamily Favreinidae Vialov, 1978
Ichnogenus Palaxius Brönnimann and Norton, 1960
Type ichnospecies: Palaxius habanensis Brönnimann and Norton, 1960, La Habana, Cuba, Miocene.
Diagnosis.—The ichnogenus Palaxius is characterised by the presence of longitudinal canals with crescent or hook-shaped outline, arranged bilaterally to a symmetry plane (Brönnimann 1972).
Remarks.—The ichnogenus Palaxius contains 27 known ichnospecies from the Late Carboniferous to the Miocene, including the 2 new ichnospecies described in this paper (cf. Senowbari-Daryan and Kube 2003; Senowbari-Daryan et al. 2007; Buchs et al. 2009). Palaxius biserialis (Kristan-Tollmann, 1989) has been reported for the Tithonian and P. decaochetarius Palik, 1965, and P. tetraochetarius Palik, 1965 from the Berriasian—Valanginian interval.
Palaxius azulensis Kietzmann isp. nov.
Etymology: Named after the Sierra Azul range, where the new ichnospecies was found.
Holotype: The specimen presented in Fig. 2A, thin section CPBA-N° 20675.
Type locality: Rahue creek, Mendoza province, Argentina (36°01 .762′S, 69°59.393′W).
Type horizon: Vaca Muerta Formation, Olcostephanus (Olcostephanus) atherstoni Zone, at approximately 340 m from the base.
Diagnosis.—Ichnospecies of the ichnogenus Palaxius with four internal canals clustered around a symmetry plane (2:2). It is differentiated from all other four-canaled Palaxius ichnospecies by the orientation of the canals (Fig. 2A).
Material.—Eight specimens: thin sections CPBA-N° 20675, CCPBA-N° 20676.
Description.°Rod-like microcoprolite with ventral groove and a cross section of 300 to 500 µm in diameter. Internally, it shows four canals. These are arranged bilaterally to the symmetry plane in two groups (2:2), each consisting of a dorsal canal (canal 1) and a ventral canal (canal 2). The canals have a crescent shaped outline, 100 µm long and 30 µm wide, with their concave side facing the symmetry plane, and with extremities characterised by rounded protuberances. Canal 1/1′ is displayed at a 60° angle from the bilateral symmetry plane, while canal 2/2′ is at a 90° angle (Fig. 2).
Comparisons.—Palaxius azulensis Kietzmann isp. nov. differs from P. caucaensis Blau, Moreno, and Senff, 1995, P. kumaensis Senowbari-Daryan and Silantiev, 1991, P. tetraochetarius Palik, 1965, and P. osaensis Buchs, Guex, Stucki, and Baumgartner, 2009 because of its cross-section morphology and the arrangement of the canals (Fig. 3). The only ichnospecies with ventral groove and crescent-shaped canals with their concave side facing the symmetry plane is P. salataensis Brönnimann, Cros, and Zaninetti, 1972, that can be distinguished by the orientation of canals 2 and 2′, which are orientated at 45° in P. salataensis and at 90° in P. azulensis in respect to the center of the microcoprolite.
Stratigraphic and geographic range.—The new ichnospecies was recognised at different levels corresponding to the Lissonia riveroi and Olcostephanus (Olcostephanus) atherstoni zones (Valanginian) in the Rahue creek and Yeso creek (Sierra Azul range) sections of the Vaca Muerta Formation (Fig. 1). Other poorly preserved microcoprolites (Favreinidae indet.) have been recognised in the Aulacosphinctes proximus (middle Tithonian) and Spiticeras damesi (late Berriasian) zones (Fig. 1), so the presence of this ichnotaxon in levels of Tithonian and Berriasian age is possible.
Palaxius caracuraensis Kietzmann isp. nov.
Etymology: After the Cara Cura range, where the new ichnospecies was found.
Holotype: The specimen illustrated in Fig. 2B, thin section CPBA-N° 20689.
Type locality: La Tosca creek, Cara Cura range, Mendoza province, Argentina (36°40′12.28″ S, 69°40′13.21″W).
Type horizon: Vaca Muerta Formation, Substeueroceras koeneni Zone, 170 m from the base.
Diagnosis.—Species of the genus Palaxius with ten canals clustered around a symmetry plane (2–3:3–2). It differs from comparable ichnospecies (P. habanensis Brönnimann and Norton, 1960 and P. decaochetarius Palik, 1965) in the orientation of canals 1 and 2 in respect to the center of the coprolite, and the different morphology of canals 4 and 5 (Fig. 2B).
Material.—Six specimens from thin section CPBA-N° 20689 and nine specimens from thin section CCPBA-N° 20690, 20691, 20692.
Description.—Microcoprolite with circular to oval cross section. The holotype has a diameter of 1,300 µm in cross section parallel to the symmetry plane and 1,800 µm perpendicularly to it. Internally, it shows ten canals arranged in two bilaterally symmetric groups of five canals (2–3:3–2). Each group consists of two “dorsal” canals (canals 1 and 2) and three “ventral” canals (3/3′, 4/4′ and 5/5′). Observed perpendicularly to the symmetry plane, canals 1/1′ and 2/2′ are crescent shaped and their concave sides face the symmetry plane, canal 3/3′ is sickle shaped with its concave side towards the symmetry plane, while canals 4/4′ and 5/5′ are sickle shaped and their convex sides face the symmetry plane. Canals are 250 µm long and 60–80 µm wide. Their extremities show rounded protuberances. Canal 1/1′ is orientated at 20° from the bilateral symmetry plane; canals 2/2′ and 4/4′ are at 45°; canal 3/3′ is at 120°, and canal 5/5′ is at 90°. In none of our specimens was a ventral groove observed.
Comparisons.—Other five ichnospecies with ten canals are known (Fig. 3). The new ichnospecies presents similarities with P. habanensis and P. decaochetarius. It is differentiated from P. habanensis by the direction of canals 1/1′ and 2/2′, as well as by the morphology of canals 4/4′ and 5/5′, which are crescent shaped in P. habanensis and sickle shaped in P. caracuraensis Kietzmann isp. nov. It differs from P. decaochetarius in the morphology of canals 4/4′ and 5/5′ which are crescent shaped in P. decaochetarius, and also in size and width of the canals.
Stratigraphic and geographic range.—The new ichnospecies was recognised in levels corresponding to the Corongoceras alternons (lower late Tithonian) and Substeueroceras koeneni (upper late Tithonian—early Berriasian) zones, in the Cara Cura range section, Vaca Muerta Formation (Fig. 1). Similar but poorly preserved microcoprolites were documented in the same levels but in the Loncoche creek, Chihuido slope, and Yeso creek sections.
Ichnogenus Helicerina Brönnimann and Masse, 1968
Type ichnospecies: Helicerina spinosa Brönnimann and Masse, 1968, southern France, late Barremian lower Aptian.
Diagnosis.—The ichnogenus Helicerina is characterised by triangular or diamond-shaped canals within the symmetry plane of cross sections. Laterally, rounded shaped canals can develop from spine-like extensions of the central canals (Brönnimann and Masse 1968 emended by Schweigert et al. 1997).
Remarks.—This ichnogenus contains 7 ichnospecies (Fig. 3) known from the Middle Triassic to the Miocene (Schweigert et al. 1997; Blau and Grün 2000), but only H. siciliana Senowari-Daryan, Schäfer, and Catalano 1979 is know from the Jurassic (Upper Triassic and Middle Jurassic; see Schweigert et al. 1997). Two possibly new ichnospecies are described herein.
Helicerina? isp. A aff. Helicerina siciliana Senowbari-Daryan, Schäfer, and Catalano 1979
Material.—Four poorly preserved specimens of thin section CPBA-N° 20675, 20676, Rahue creek (Fig. 1, 36°01.762′S, 69°59.393′W).
Description.—Microcoprolite with circular outline and a cross section of 900 µm in diameter. The specimen is characterised by one canal which lies in the symmetry plane. This canal can be compared with the canal developed in Helicerina siciliana Senowbari-Daryan, Schäfer, and Catalano, 1979. In contrast to Helicerina siciliana the canal in Helicerina? isp. A is not clearly separated in distinct canals (in Helicerina siciliana: a dorsal diamond shaped and a ventral triangular shaped canal) but seems to form one “unit”.
Contrary to all other known Helicerina ichnospecies the canal shows no connection to the “outside of the coprolite” (terminology according to Senowbari-Daryan and Bernecker 2005). However, this connection is very thin and can be clossed. If additional findings will confirm the lack of connection, our specimen belongs to a new ichnogenus and -species.
Stratigraphic and geographic range.—Helicerina isp. A aff. Helicerina siciliana was recognised in levels corresponding to the Lissonia riveroi and Olcostephanus (Olcostephanus) atherstoni zones (early Valanginian) of the Vaca Muerta Formation.
Helicerina isp. B
Material.—Four specimens in thin section CPBA-N° 20675, 20676, Rahue creek (Fig. 1, 36°01. 762′S, 69°59.393′W).
Description.—Microcoprolite with circular outline and a cross section of 500 to 800 µm in diameter. Due to the poor state of preservation it is not possible to accurately determine the exact shape and number of canals. However, it is possible to notice three canals in the symmetry plane interconnected by a pronounced dorsal groove; and some canals interconnected by thinner grooves, two of them joined to the first angular canal at each side of the symmetry plane (Fig. 2D).
Comparisons.—Helicerina isp. B shares similarities with H. kainachensis Fenninger and Hubmann, 1994 from the Santonian—Campanian and with H. ruttei Schweigert, 1997 (see Fig. 3) from the Miocene. Even when the preservation of the samples does not allow their proper classification, the ichnospecies from the Vaca Muerta Formation presents a circular canal joined to the first angular canal at each side of the symmetry plane, which permits to assure that it is a possible new ichnospecies.
Stratigraphic and geographic range.—Helicerina isp. B was recognised in levels corresponding to the Lissonia riveroi and Olcostephanus (Olcostephanus) atherstoni zones (early Valanginian) of the Vaca Muerta Formation (Fig. 1).
A generic/specific level identification of the microcoprolite producers becomes impossible when body fossils are not present. However, some crustacean microcoprolites allow identification of the producers at the family level (Schweigert et al. 1997). At the present time, the ichnogenus Palaxius is produced by the genera Callianassa and Protocallianassa (Moore 1939; Blau and Grün 2000), and this association is also known from the fossil record (Peckmann et al. 2007). According to Blau et al. (1993) the ichnogenera Payandea and Thoronetia can be associated with the family Galatheidae, while the ichnogenera Favreina, Parafavreina, and Palaxius with the family Thalassinidae. On the other hand, Helicerina has been found in association with decapods of the genus Proballaya (family Potamidae, Schweigert et al. 1997).
The distribution of crustacean fossils in the Mendoza Group has been described by Aguirre-Urreta (1989, 2003) and Aguirre-Urreta et al. (2008). Astacideans have been reported for the Berriasian and the Valanginian, while palinurids are represented in the late Valanginian. However, anomurids are the most abundant decapods in the Mendoza Group rocks (Fig. 1) and have been recognised in middle Tithonian of the Vaca Muerta Formation (Leanza and Zeiss 1990), as well as in the late Berriasian and early Valanginian deposits (Aguirre-Urreta 1989). These last records have been assigned by Aguirre-Urreta (2003) to Callianassa aff. peruviana.
Together with the four new ichnospecies described in this paper and the ichnospecies P. decaochetarius Palik, 1965 described in Kietzmann and Palma (in press), other poorly preserved Palaxius microcoprolites were observed in the Vaca Muerta Formation. They appear to have 2 or 4 canals and probably correspond to P. groesseri Blau, Grün, and Senff, 1993 (Palaxius isp. B in Fig. 1) and P. salataensis Brönnimann, Cros, and Zaninetti, 1972 (Palaxius isp. A in Fig. 1). According to the distribution of crustacean fossils in the Tithonian—Valanginian deposits (Aguirre Urreta 1989, 2003; Leanza and Zeiss 1990), it can be inferred that Palaxius ichnospecies were produced by callianassids, while Helicerina ichnospecies could be produced by Meyerella rapax (Mecochiridae), Eryma sp. (Erymidae) and/or Hoparia sp. (Nephropidae).
Although the biostratigraphic utility of crustacean microcoprolites is currently in discussion (Schweigert et al. 1997; Senowbari-Daryan and Kube 2003), this study seeks to assess a possible contribution to the biostratigraphy of the Neuquén Basin. Blau et al. (1993) proposed that crustacean coprolites could be considered as stratigraphic correlation fossils, since the larvae of the crustaceans can easily migrate through the oceans. In effect, these authors correlated deposits from the Late Triassic and Early Jurassic of the western margin of Gondwana (Colombia) with successions from the western Tethys (Austria and Italy) using Parafavreina thoronetensis, Favreina martellensis, and Thorontia quinaria. Arguments of Schweigert et al. (1997) are based on the fact that the most simple ichnospecies have a much broader temporal range than the more complex ones. In effect, these idiosyncracies of ichnospecies morphology and distribution have been recognised by Senowbari-Daryan and Kube (2003) in their revision of the ichnogenus Palaxius, and have been attributed to different ontogenetic stages (Senowari-Daryan and Kuss 1992; Schweigert et al. 1997), and to the differential evolutionary rate between the decapod morphology and their digestive system (Schweigert et al. 1997).
Taking into account these constraining characteristics of the crustaceans, the stratigraphic value of the microcoprolites may be of significance. The distribution of the microcoprolites in the Mendoza Group does not allow us to place the Jurassic-Cretaceous boundary in Mendoza Group sequence. However, there is a clear difference between the assemblage of crustacean microcoprolites from the middle Tithonian to early Berriasian (Aulacosphinctes proximus to Substeueroceras koeneni zones) and the assemblage from the Valanginian (Lissonia riveroi to Olcostephanus (Olcostephanus) atherstoni zones) (Fig. 1). Assemblage 1 is composed of Palaxius caracuraensis Kietzmann isp. nov., Palaxius decaochetarius Palik, 1965, Palaxius isp. (A and B), and other Favreinidae microcoprolites. Assemblage 2 is composed of P. azulensis Kietzmann isp. nov., Helicerina isp., Palaxius isp. B, and other Favreinidae microcoprolites. Despite poor temporal resolution, microcoprolite assemblages could be useful in subsurface studies, where other biostratigraphical information (such as ammonite biozones) is not easily established. Hopefully, future studies in other sectors of the basin will improve and/or corroborate the biostratigraphic utility of microcoprolites in these rocks.
In this paper four new ichnotaxa (two formal, two informal) of crustacean microcoprolites from the Neuquén Basin are described (Palaxius azulensis, P. caracuraensis, Helicerina? isp. A aff. H siciliana, and Helicerina isp. B). They constitute the first record of the ichnogenera Palaxius and Helicerina for the Upper Jurassic and Lower Cretaceous of South America. Considering the distribution of the crustacean microcoprolites in the Vaca Muerta Formation, there is a noticeable change in the composition of the assemblages from the Tithonian—lower Berriasian, dominated by Palaxius, and from the Valanginian, dominated by Helicerina. It is inferred that the ichnogenus Palaxius was produced by callianassids, while Helicerina may be associated with Mecochiridae, Erymidae and/or Nephropidae. The use of crustacean microcoprolites as a correlation tool is questionable and only applicable to large stratigraphic intervals. However, this work establishes the possibility of discriminating two microcoprolites assemblages, one of the Tithonian to early Valanginian and another one from the early to the late Valanginian, that could be useful in local subsurface studies.
The authors are especially grateful to Gunter Schweigert (Staatliches Museum für Naturkunde Stuttgart, Germany), Baba Senowbari-Daryan (Universität Erlangen-Nürnberg, Germany), and David Buchs (Université de Lausanne, Switzerland) for providing bibliographic material. We are indebted to Andrea Concheyro and Marina Lescano (Universidad de Buenos Aires, Argentina) for facilitating the microscopes with photographic equipment. We are grateful to the reviewers John Chamberlain (Brooklyn College, New York, USA) and Baba Senowbari-Daryan for their constructive critics on the descriptions. This research has been done in the framework of the UBACyT X-476 (Universidad de Buenos Aires) and PIP 5142 (CONICET) projects.