◄ Carnets Geol. 17 (13) ►
Contents
[1. Introduction]
[2. Paleontological contents]
[3. Taphonomy and paleoenvironmental implications]
[4. Conclusions] and ...
[Bibliographic references]
Universidade
de São Paulo, Instituto de Geociências, Departamento de Geologia Sedimentar e
Ambiental; Rua do Lago, 562; Cidade Universitária 05508-080 - São Paulo, SP (Brasil)
Published online in final form (pdf) on December 24,
2017
DOI 10.4267/2042/64291
[Editor: Bruno Granier; language editor:
Stephen Eager]
The silty shale facies of the Taquaral Member (Irati Formation), which is one of the Permian units in the Brazilian Paraná Basin, is discussed here based on the taphonomy relevant to the paleoenvironmental interpretation. The fossils are crustaceans (Clarkecaris and other indeterminate forms), isolated teeth, scales and bones of Actinopterygii, "Palaeonisciformes", which are the most common vertebrate remains, and also frequent scales of Coelacanthiformes. The scales, teeth and disarticulated bones are found together in accumulations, which may be interpreted as coprolites. Many of the fossil crustaceans display characteristics of ecdysis.
•
Permian;
•
Crustaceans;
•
Vertebrates;
•
Actinopterygii;
• Coelacanthimorpha;
• ecdysis.
Chahud A. (2017).- Additional contributions to the knowledge of the Taquaral Member, Irati Formation (Lower Permian, Paraná Basin): Taphonomy and paleoenvironmental implications.- Carnets Geol., Madrid, vol. 17, no. 13, p. 243-250.
Nouvelle contribution à la connaissance du Membre Taquaral, Formation Irati (Permien inférieur, Bassin du Paraná) : Taphonomie et implications paléoenvironnementales.- Le faciès des argilites de la Formation Irati (Membre Taquaral), une des couches du Permien inférieur dans le Bassin du Paraná au Brésil, est discuté dans cette étude à partir de la taphonomie des fossiles, révélant des informations paléoenvironnementales. Ces fossiles incluent des crustacés (Clarkecaris et d’autres formes indéterminées), des dents, des écailles et des os d’Actinoptérygiens, "Paléonisciformes", qui sont les restes les plus courants de vertébrés, ainsi que des écailles de Coelacanthiformes. Les écailles, les dents et les os retrouvés dans ces formations sont interprétés comme des coprolithes. De nombreux fossiles de crustacés semblent également représenter des exuvies ou mues.
• Permien ;
• Crustacés ;
• Vertébrés ;
•
Actinopterygii ;
•
Coelacanthimorpha ;
• exuvies.
The Irati Formation of the Brazilian Paraná Basin is the best known fossiliferous unit of the Lower Permian, because of its dark shales with remains of Mesosauridae reptiles, associated with crustaceans. It is divided into two members; the Taquaral Mb, above it, the Assistência Mb (Barbosa & Gomes, 1958; Chahud, 2007, 2011; Milani et al., 2007; Holz et al., 2010). The Taquaral Member has a thickness ranging from 5m, in marginal areas, to 30m, in the most central Paraná Basin (Hachiro, 1996).
The Taquaral Member exhibits diversified lithologies, mainly in its lower beds, that are distinguished by predominantly sandy facies. This facies is characterized by lenticular conglomeratic sandstones with granules and pebbles, intercalated with coarse to fine-grained sandstones and muddy sandstones. In spite of its small thickness, it is rich in fish remains, i.e., scales, teeth and bone fragments (Chahud et al., 2010; Chahud & Petri, 2008a, 2008b, 2009, 2010a, 2010b, 2014, 2015a, 2015b, 2015c, 2016).
The other facies, a thickly laminated silty-shale, is a more homogeneous and dark grey in colour (when unweathered). According to Hachiro (1996) the matrix is clayey but contains 20-40 % quartz (may reach 55 %), micaceous plates and pyrite are disseminated in local layers. At the northern border of the basin, this facies diplays an abrupt contact with sandy facies. The upper contact with the Assistência Member is gradual and marked by dark shales and carbonates, typical of this member.
All
the studied material occurs in outcrops in the State of São Paulo, southeastern
Brazil region. Most of the fossiliferous outcrops of the Taquaral Mb in this
state are located around the city of Rio Claro (Figs. 1 
- 2 ). The silty-shale
facies was described by Chahud et
al. (2012) and Chahud and Petri
(2013a, 2013b) and now, taking
advantage of the increased amount of data, its fossils were taphonomically
investigated, providing information for paleoenvironmental interpretation.
|
Figure 1:
Outcrop
of the Irati Formation (adapted from Hachiro, 1996, and Calça
& Fairchild, 2012). |
|
Figure 2: Sketched
columnar section of the Taquaral Member, Irati Formation as occurring at the
State of São Paulo. 1) Sandy facies; 2) Silty shale facies. |
Fossils in the silty-shale facies consist mainly of crustaceans and disarticulated vertebrate remains. Among the crustaceans, the most frequent is the genus Clarkecaris that is restricted to this facies. Clarkecaris is classified as a Syncarida and thus was considered a freshwater inhabitant by comparison to recent syncarids (Mussa et al., 1980) but it could also be a transition taxon between older marine taxa and freshwater genera (Schram J.M. & Schram F.R., 1974; Chahud & Petri, 2013a, 2013b). Besides Clarkecaris, a new form of crustacean, known only from its uropods and not yet classified (Chahud & Petri, 2013b), seems to be restricted to the Taquaral Member.
The vertebrates are very abundant, but little diversified, and are represented by fish bones only. The Actinopterygii, "Palaeonisciformes", is the most abundant group and is represented by teeth, scales and disarticulated bones. The Coelacanthiformes occur in great quantity, but only scales were reported (Chahud & Petri, 2012, 2013a, 2013b).
Other, very rare fossils include: small bivalves, pollens, spores, algae and indeterminate acritarchs (Lages, 2004; Chahud & Petri, 2013a).
Most of the fossils from the silty shale facies, both of invertebrates and vertebrates, are fragments of a few millimeters to several centimeters, essentially preserved in two dimensions. All the fossils show signs of flattening and compression.
The
crustacean’s carapaces display different kinds of chitin somite preservation
(Figs. 3
- 4 ). Loose articulated segments of the bodies are rather common and few
specimens of Clarkecaris are complete
or semi-complete
(Fig. 3 ).
Research on post-mortem degradation of living taxa of crustaceans similar in shape and size to Late Paleozoic Syncarida, were carried out by Harding (1973), Plotnick (1986), and Hof and Briggs (1997). They observed post-mortem changes under anaerobic and aerobic conditions. Under anaerobic condition, the cephalothoraxes were completely disarticulated and degraded four days after death, while it took twenty five weeks for the rest of body. Under oxidizing surface action of bioturbation and higher temperatures, the specimens lost their appendages in one week, but buried a few centimeters deep in the sediment, it took six weeks. These studies are useful for understanding the Late Paleozoic Syncarida fossilization as pointed out by Perrier et al. (2006) who studied the Upper Carboniferous Syncarida from the Montceau Lagerstätte.
Some Clarkecaris that are well preserved attest to little influence of bioturbation, otherwise, the depth of the burial was sufficient to prevent deterioration.
Among the best preserved Clarkecaris specimens are those that are enrolled, imbricated or stretched. Enrolled or slightly enrolled trilobites are interpreted as a protective response against predators or paleoenvironmental disturbance (Silva & Fonseca, 2005). However, there are neither indications of predators and nor biostratonomic evidence for diisturbance during the deposition of the silty shale.
The frontal part of the cephalon of Clarkecaris is rarely preserved, and Mezzalira (1952) suggested that the thorax may have consisted of material that was prone to weathering. The presence of complete specimens of Clarkecaris in the Taquaral shale of the Brazilian states of Paraná and center east of São Paulo (Brito & Quadros, 1978; Pinto, 1985) might be explained by sediment flux, with dense water clouds carrying fine detritus that smothered specimens. However, if so, that would result in mass mortality, which was not observed.
A hypothesis for the few complete or almost complete, well preserved specimens, could be a low population density of Clarkecaris thriving in this basin.
An
alternative hypothesis, not previously discussed but favoured in this paper,
would be that most of the crustacean remains were the result of ecdysis. During
ecdysis, the front part of the skeleton is completely lost (Phlippen et
al., 2000) which could explain the headless specimens. This could also
explain why enrolled uropods are the only remains of the indeterminate
crustacean reported herein (Fig. 4 ).
The ecdysis might be preserved by intense precipitation of the finest detritus and very rapid burial, "obruption deposits", resulting in a complete fossilization.
The low density (approximately 1/m²) of crustacean specimens could be the reason why they are not frequent and are restricted to certain levels.
|
Figure 3: Ecdysis of Clarkecaris.
Bar-scale 2.5mm. |
|
|
Figure 4:
Ecdysis
of uropods of indeterminate crustaceans. Bar-scale 5mm. |
It is important to emphasize that no ichnofossils (neither burrows nor tracks) were observed associated with these crustaceans. The silty shale displays widespread pyrite crystals over large areas, without interruption. The low oxygen level would represent favorable conditions for the crustaceans during ecdysis in the absence of predators.
The
vertebrates, founded in the silty shales of Taquaral Member, are rare and
dispersed,. Most of them are represented by isolated scales of Coelacanthiformes
and Actinopterygii (Fig. 5.A-B ), and bone pieces, see the
"Palaeonisciformes"
maxilla (Fig. 5.C ).
|
Figure 5: Fragmented
parts of Osteichthyes. A) Palaeonisciformes scale part (GP/2E-6222a); B)
counterpart (GP/2E-6222b); Bar-scale: 2 mm. C) Palaeonisciformes maxilla
(GP/2E-6231). Bar-scale: 4 mm. |
Accumulation
of either crustacean fragments or bone elements and scales of Coelacanthiformes
and "Palaeonisciformes" are rare (Fig. 6 ). Many of these are probably parts
of worn out ecdyses (Fig. 6.A ). Others may be loose fragments of the same
specimen, not yet completely disarticulated (Fig. 6A). Most of the vertebrate
fragments accumulation do not belong to the same specimen (different kinds and
sizes of the elements, jointed together) and possibly represent weathered
coprolites of large animals (Fig. 6.B-C ).
|
Figure 6:
Punctual fossils accumulations. A) Crustaceans fragments; B-C) Scales and bone
parts. Bar-scale: 2mm. |
Paleoenvironmental interpretation of these fossils wasnot conclusive for determining whether the environment was marine, freshwater or brackish. Some fossils reported from the same levels in the literature, were not observed, such as the gastropods of the genera Loxonema and Bellerophon, initially mentioned by Clarke (1920), but not confirmed by Beurlen (1931). Other fossils are restricted to particular parts of the basin, such as small bivalves in thin calcareous layers (Kazubek & Simoes, 2003a, 2003b; Rohn et al., 2003; Lages, 2004).
Besides the fossils referred to here, other researchers reported spores, pollens, acritarchs and the alga Bothryococcus (Cazzulo-Klepzig et al., 1989; Souza et al., 1992; Holz & Dias, 1998; Lages, 2004; Cardoso, 2010). These fossils can be found in waters of different salinities, and thus do indicate a unique specific paleoenvironment.
Clarkecaris are preserved in different ways: rarely as complete articulated and semi-articulated specimens, and more often fragmented. The presence of complete specimens of Clarkecaris from head to tail and some of larger fragments, should be the result of extremely rapid depositional episodes, caused by major disturbances, like storms, burying the specimens.
Likewise, the presence of coprolites in low energy deposits without significant bioturbation, very rapid burial in deeply located and undisturbed deposits.
No complete fish skeletons were found in the Taquaral Member, only bone pieces, well preserved loose scale and some punctual accumulations. Some scales and bone pieces accumulations look like coprolites, other accumulations might be due to short flooding episodes possibly responsible for their burial.
The Clarkecariidae is endemic in the silty-shale facies and unknown in other formations and basins. The presence of this family is indicative of the long isolation of the Taquaral Member paleoenvironment. Other fossils from the silty-shale facies are not ideal as paleoenvironmental indicators.
The author thanks reviewers, Philippe Janvier and Vincent Perrier, for pertinent questions, the Prof. Dr. Setembrino Petri and Dr. Giselle Utida for support and jointing field researches. The author thanks also the Departamento de Geologia Sedimentar e Ambiental of the Institute of Geoscience of the University of São Paulo which allowed the preparation of fossils in its laboratories.
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