The Lontras Shale Lagerstätte is a Konservat-Lagerstätte in the Late Carboniferous Campo Mourão Formation of the Paraná Basin, Southern Brazil. The deposition of this site occurred in a fjord estuary with an anoxic seafloor, during a deglaciation sequence of the Late Paleozoic Ice Age and the subsequent flooding to the surrounding paleovalleys. The shale is also known for its exceptional preservation of soft- and hard-bodied organisms such as insects, fishes, ammonoids, brachiopods, sponges and driftwood. Anoxia (which could sometimes reach the upper levels of the water column), lack of scavengers, rapid burial and the low water temperatures seem to be the main causes for such fossilisation.
The vast majority of the fossil content of this lagerstätte originates from the Campáleo outcrop, which is dated to the Permian-Carboniferous boundary.
History
The Lontras Shale was discovered by Jay Backus Woodworth and Euzébio Paulo de Oliveira in a 1908 expedition researching glacial beds in Southern Brazil, funded by the Shaler Memorial Fund.
While researching beds of glacial till between the municipalities of Rio Negro and São Bento do Sul, they discovered layers of marine fossiliferous black shale 6 km south of Rio Negro, where the municipality of Mafra would be later formed. In a 1912 letter to Woodworth, which would later be published, Oliveira describes finding brachiopods, fish scales, poriferans, as well as insect wing and bivalve molluscs in Teixeira Soares.
The fossils were taken to the Serviço Geológico e Mineralógico in Rio de Janeiro, where Orville A. Derby, director of agency, send part of the collection to John M. Clarke for study. Clarke, in turn, handed them to Rudolf Ruedemann who gave a quick analysis before handing them back. It would not be until 1929 that the fossils had a deeper examination, after Ruedemann encountered similar fossils in the Ouachita Mountains of Arkansas and Oklahoma. He identified the brachiopods Lingula and Obirculoidea, molds of hexactinellid sponges, fish scales (identified by William L. Bryant as paleoniscids) and fish coprolites. These coprolites where later re-examined and identified as caddisfly larval cases.
For the next few decades, research on the Lontras Shale site would be sparse. Oliveira (1930) described three new species of brachiopod, while Carvalho et al. (1942) collected new material. In the 1980s, Oscar Rösler started collection and research on the site. Meanwhile, two species of fish found in the site were described.
In 1997, the fossil site made controversial headlines when the multinacional company BANDAG (now owned by Bridgestone) attempted to construct a factory on the site of the Lontras Shale. While looking for business in the MERCOSUL, the company sought the municipality of Mafra for various strategic purposes. However, while during construction the engineers excavated fossiliferous rock and attempted to remove the obstacle with explosives. Despite the destruction of much of the outcrop, 60 tonnes of fossiliferous shale were left to the elements for months. News quickly spread across the scientific community about the vandalisation of the natural heritage site, and after a complaint, the responsible government agencies embargoed the construction. After a period of discussion, which involved several public agencies, universities, and scientific societies, as well as the local population, it was decided that the University of Contestado would take responsibility for displaced rock.
It was further agreed that the recently retired Professor Oscar Rösler would be made in charge of studying them at the CENPALEO Project (Paleontological Centre of the University of Contestado). Afterwards, the municipality of Mafra elected to make the surrounding lands of public utility and later donating it to the Frei Miguel Foundation, which was created to help maintain the CENPALEO. Thus, ever since the creation Paleontological Centre, the number of scientific works centred around the site has grown exponentially.
Description
Geology
The Lontras Shale is part of the Campo Mourão Formation, located on the Paraná Basin of Southern Brazil. Deposited during the Late Paleozoic Ice Age, the Campo Mourão Formation represents at least three deglaciation cycles. It is divided into two members: the lower Lapas Member, composed of sandstones, interpreted to have originated in tunnel valleys carved and deposited by the advance of glaciers; and the upper Lontras Member, which harbours the Lontras Shale Lagerstätte.
It varies between 50 metres thick at the surface to 101 metres thick at the subsurface, and consists of an interbedding of organic-rich black shale, mudstone and siltstone, with varve-like bedding and striated dropstones (stones carried by icebergs) evidencing that this member was deposited during a deglaciation and consequent rise of sea level. The Lontras Shale itself represents the maximum flood surface, a point in time at which the sea level was at its highest. Of the total 101 metre-thick member, only 1.15 metres are the Lontras Shale Lagerstätte, defined by both the high quality and diversity of its fossiliferous material. The shale itself is also subdivided, with each subdivision being based on its pyrite composition.
The majority of its fossils originate from the Campáleo outcrop, in the municipality of Mafra, Santa Catarina, while most other localities only yield trace fossils and palynomorphs. However, at least two other fossiliferous outcrops have been suggested for the Campo Mourão Formation; the Guaraúna shale, located 105 km from the Lontras Shale at the city of Teixeira Soares, which provides fossils of brachiopods, and a shale deposit less than a metre thick located 85 km from the Lontras Shale in Rio dos Cedros. The latter site has a fossil assemblage similar to the Campáleo outcrop, with larval cases, coprolites, brachiopods, sponges, and fishes. Because of a lack of studies on this locality, it is not clear whether it is part of the Lontras Shale Lagerstätte or a separate bed.
The age of the Campo Mourão Formation has been debated in literature. Although initially dated to the Early Permian, during the Sakmarian-Artinskian interval (294-275 Ma), recent biostratigraphic and chronostratigraphic analysis place the formation at the Middle Pennsylvanian, during the Bashkirian-Moscovian interval (315-307 Ma), while the CAMPÁLEO section is dated to the boundary between the Carboniferous and Permian periods.
Preservation The fossils of the Lontras Shale have been preserved in a multitude of forms, such as carbonisation, phosphatisation, silicification, pyritisation, concretions and tridimensional preservation. Some examples of these are; the preserved leg muscles of blattodea insects, completely articulated sponges, tridimensionally preserved fishes and silky larval cases of trichopteran insects.
Organisms with a siliceous skeleton, such as glass sponges, are mostly preserved by recrystalisation with their original mineralogy intact, occasionally being permineralised by phosphate. Ammonoids, malacostracans and ostracods, animals with carbonate carapaces or shells, are often fossilised as carbonaceous film or by pyritisation, without any of their original calcium carbonate. Insects, brachiopods, fish remains, conodonts and scolecodonts tend to preserve their original composition, because animal remains with chitin and phosphate in their bodies are more likely to preserve when compared to carbonate remains. However, some of these organisms can also undergo carbonisation, phosphatisation, and less often pyritisation, as seen with some fish and insect soft tissue. Plant remains normally preserve as carbonaceous compressions.
Other forms of preservation include possible polychaete annelids being preserved in concretions, with chaeta (brittles) and body outline visible alongside its jaws, fish coprolites preserved by phosphatisation or kerogenisation, and insect larval cases preserved by kerogenisation with partial pyritisation. Both processes of phosphatisation and pyritisation have been linked to sulfate-reducing bacterias, with possible microbial mats being associated to specific levels of the Lontras Shale, supporting a preservation similar to other lagerstätten such as the Soom Shale of South Africa.
Despite the Lontras Member being horizontally extensive and relatively thick, the Lontras Shale Lagerstätte and an other fossiliferous outcrops are situated in a very narrow horizon. The question of why only these layers of the member exhibit fossiliferous strata has been explained in the literature by some factors, such as; the restricted fjord-estuary environment in which the shale was deposited being a hotspot for organic diversity, whereas the other environments within the Lontra Member did not exhibit such favourable conditions for colonisation; the period in which the shale was deposited being of a maximum flood (highest sea level), with optimal climate conditions and input of continental flows carrying nutrients; the presence of anoxic bottoms and anoxic events, which would periodically kill the community and prevent microorganisms from decomposing them until burial. Once these conditions changed with the drop of sea level, it coincided with a decreasing flow of nutrients and the environment becoming increasingly inhospitable. This culminated in the following 40 metres of the Lontras Member being afossiliferous.
Another factor for the lack of fossil finds in the region can also be attributed to collection bias, which extends beyond this locality, such as geological surveys in Brazil normally not having a paleontological focus and the subtropical climate quickly weathering most outcrops before their fossils can be salvaged.
Although much of the evidence explaining this organic abundance relies on the fact that the shale was deposited on a fjord-like environment, another paleovalley has been proposed in the Alfredo Wagner municipality in which no fossils have been discovered. However, this is likely because the Lontras Shale was deposited at the base of the Lontras Member during a period of deglaciation, and specifically at a time when all the previously mentioned factors favoured the colonisation of the fjord, while the Alfredo Wagner paleovalley was deposited much later during a period of glacial advance, when conditions where not as ideal.
Paleoenvironment
The most accepted interpretation as to the depositional environment of this lagerstätte was that of a distal fjord estuary, formally known as the Mafras Paleofjord, though some authors have suggest a shallow marine environment instead. Based on the reorientation of the intact fossil material by water currents, a north/northeastern direction has been proposed for the mouth of the Mafras Paleofjord in relation to the fossil site.
Continental flows of melted water carrying nutrients would have resulted in high ecological productivity. The marine substrate was anoxic or euxinic (an environment with no oxygen and high amounts of hydrogen sulfide) and periodical events occurred where the bottom anoxia would ascend to the upper levels of the water column. Based on this, the water column was estimated to be no deeper than ~200-400 metres. The Mafra Paleofjord would thus be comparable to modern temperate fjords, such as the Framvaren Fjord in Norway or the Chilean fjords, with the rich mixing of both marine and terrestrial biotas and the high stratification of the water column.
Paleoecology