With a population of fewer than 70,000, the municipality of Januária, in northern Minas Gerais State, is known today for its waterfalls, limestone caves and artisanal cachaça, the Brazilian rum that, according to producers, owes its virtues to the climate and the natural moisture in the local soil, which are favorable for growing the sugarcane that is used to make it. The area’s strategic geographic location on the left bank of what becomes the huge São Francisco River, called opará (sea river) in ancient times by the local Indians, made Januária a major port and commercial hub during the colonial era. Vestiges of a much more remote past, nearly forgotten but marked by an intimate connection to the water, have just come to light in still-active stone quarries on the outskirts of the city.
A team of geologists and paleontologists from the University of São Paulo (USP) and the São Paulo State University (Unesp) has found a special type of fossil there: tiny fragments of marine animals of the genus Cloudina, tubular creatures consisting of a series of calcareous cones nested within one another. The remains of these animals, which lived on Earth around 550 million years ago, were embedded in a steep wall and in other outcrops of rocks of the Sete Lagoas Formation, part of the Bambuí Group. A sedimentary unit of the São Francisco watershed, the Bambuí stretches over approximately 300,000 square kilometers and includes vast portions of the states of Minas Gerais and Bahia, extending as well into the states of Goiás, Tocantins and the Federal District.
The fossils are virtually irrefutable proof that, a little over half a billion years ago, a shallow seaway no more than 10 meters deep covered this part of Brazil. “That must have been the last beach Minas Gerais ever had,” was the good-humored comment from geologist Lucas Warren, now a professor at the Rio Claro Institute of Geosciences and Exact Sciences (IGCE) at Unesp. Warren was doing post-doctoral research at USP under a FAPESP grant at the time of the discovery last year. He is the principal author of an article on the fossil discovery in Januária, which appeared in the May 2014 issue of the scientific journal Geology. “Until now, there had been no definitive animal fossil findings in the Bambuí Group,” says Warren, who collaborated with Fernanda Quaglio, an expert in paleobiogeography, to identify the fossils. “In addition to Cloudinas, we also found at least three fragments attributed to the genus Corumbella, and traces in rock that were probably left by a soft-bodied animal.” The Corumbellas, which may have also had a skeleton, shared their marine environment with the Cloudinas. The team that collected the fossils in Januária also included geologist Nicolás Strikis, a PhD student at USP and co-author of the article, and biologist Hamilton dos Reis Salles, who is from Januária. In 2012, Warren and colleagues from South America had found Cloudinas and Corumbellas in Puerto Vallemí, a village in northern Paraguay (see Pesquisa FAPESP Issue No. 199).
ILLUSTRATION SANDRO CASTELLIIn the new study, the researchers support the hypothesis that this shallow seaway covered not just the part of Brazilian territory having rocks of the Bambuí Group, but vast portions of eastern South America, western Africa and southern Antarctica (see map). “This sea linked the three continents and connected to the ocean,” says biologist Pedro Strikis of the Geosciences Institute at USP (IGc-USP), another co-author of the paper. A little over half a billion years ago, the shape of the reasonably stable rocky blocks that form the continental crust—which geologists call cratons—differed from their present-day configuration. South America, Africa and Antarctica were interconnected and formed part of Gondwana, the southern supercontinent that held most of the landmasses now situated in the Southern Hemisphere. Although there is still intense debate among Brazilian researchers as to precisely how and when all the pieces of Gondwana came together—whether it occurred 520 million or 620 million years ago—there is consensus on the view that most of South America was already connected to Africa and Antarctica around 550 million years ago.
The idea that there was a shallow sea that flooded large sections of Gondwana is principally based on the geographic distribution of the Cloudinas found in various parts of the world. Fossil specimens have been obtained in places such as Namibia, Oman, Argentina, Paraguay, Spain and China. In Brazil, prior to the discovery of the specimens in northern Minas Gerais, traces of these marine creatures had been recovered in Corumbá, in the state of Mato Grosso do Sul. Measuring up to three centimeters in length, Cloudinas are one of the first macroscopic marine animals to have a calcium carbonate-based exoskeleton, shell or carapace. Because they are hard to classify, they were initially thought to be annelids, which include earthworms; but now they, like Corumbellas, are usually classified as members of the Cnidaria, a group that includes corals. Their habitat was the carbon dioxide-rich floor of shallow seas, at a depth where light can pass through the water. The Cloudinas lived on the seabed, attached to microbial mats, which are fine layers of cyanobacteria that derive their energy from photosynthesis. In some cases these mats are associated with the formation of calcareous rocks that, when fossilized, can produce formations called stromatolites (if their layers are visible) or thrombolites (when the layers have a clotted appearance).
The fragments of Cloudina specimens are considered guide fossils. In the jargon of paleontologists, this means that they are a type of record found in several places on Earth, but whose occurrence is limited to a well-defined period of time. Because of these characteristics, guide fossils are used throughout the world to correlate and date geological layers and, by extension, the depositional environment associated with them. Cloudinas occur only in sedimentary rocks of marine origin that were deposited onto the Earth’s crust between 550 million and 542 million years ago, at the end of the geological period known as the Ediacaran, immediately prior to the beginning of the Cambrian, when marine invertebrates with biomineralized carapaces diversified over a short period of time.
Cloudinas have a fragile carapace that contains a small amount of calcium carbonate. “The shells were not mechanically strong, and wouldn’t be able to “survive” vigorous transport or the continuous action of running water,” says paleontologist Marcello Guimarães Simões of the Botucatu Biosciences Institute (IB) at Unesp, another co-author of the paper in Geology. “In other words, they were autochthonous or parautocthonous.” For this reason, the fossils of these animals are thought to have originated at the sites where they were found, or very close by. That particular feature reinforces the idea that a shallow sea did indeed once cover the places where these fossils were found. Since the Cloudina sites were part of cratons roughly contiguous with what is believed to be Gondwana some 550 million years ago, it is reasonable to assume that this ancient shallow sea connected South America and Africa.
The age of the Bambuí
In addition to providing evidence that oceanic waters inundated parts of the southern supercontinent in its early days, the Cloudina specimens help Brazilian geologists establish a more accurate chronology for the sediments that lie at the base of the Bambuí Group. The age of this geological unit has been the subject of controversy in recent decades. Estimates for the period when its rocks were formed vary widely, from 740 million to 550 million years ago. In 2012, geologist Márcio Pimentel, then at the Federal University of Rio Grande do Sul (UFRGS) and now at the University of Brasília (UnB), determined the age of 25 samples of detrital zircon collected in areas of the Bambuí Group in northern Minas Gerais and central Bahia State. Zircons are minerals crystallized in granites or in volcanic rocks that are subsequently eroded, transported with sediments and deposited in basins. They contain significant amounts of uranium, and their age can be calculated through radioactive decay. The age that Pimentel obtained for the crystals found in the Bambuí was between 600 million and 550 million years, younger than the age normally associated with the group (see Pesquisa FAPESP Issue No. 195). “Finding animal fossils in Januária was a pleasant surprise, and it virtually ends the debate about the age of the Bambuí Group,” Pimentel says.
Geologist Claudio Riccomini of IGc-USP thinks along the same lines, though with one reservation. “The discovery of Cloudinas as well as fragments of Corumbellas definitively answers the question of the age of the Bambuí Group, at least with the current state of knowledge,” says Riccomini, another co-author of the article on the new marine fossils. “But the debate is not completely over. Among other matters, it is important to confirm whether the Bambuí Group shows the same age in different parts of its watershed and to ascertain the relationships between the rocks of the Sete Lagoas Formation and the glacial deposits that lie beneath them.”
Experts generally agree on the importance of the Januária fossils for establishing a more precise chronology of the Bambuí Group and developing the hypothesis that significant portions of South America, Africa and Antarctica were covered by a shallow sea about 550 million years ago. But discovery of the Cloudinas in northern Minas Gerais intensifies the debate around a fundamental question: a little over half a billion years ago, had the southern supercontinent Gondwana already been completely formed, or not? Researchers are divided on this issue and, in recent years, have aligned into two groups with different views. Each current of thought is based on different types of data, such as rock dating and information on paleomagnetism, which help determine where the cratons of Gondwana must have been during a given period and how they moved and interacted on the earthly globe over time.
The authors of the paper on the Januária fossils support the hypothesis that Gondwana, particularly its western section (which today includes South America), was not yet fully formed at the time when Cloudinas and Corumbellas lived. According to this theory, most of the large continental blocks, or cratons, that formed the supercontinent were already joined, but one of them—the large Amazonia craton—was already separate from the others around 550 million years ago. An ancient ocean, given the name Clymene in 2006 by geologist Ricardo Trindade of the Institute of Astronomy, Geophysics and Atmospheric Sciences of USP (IAG-USP), is thought to have separated most of Gondwana from the Amazonia craton. Under this scenario, the Clymene was likely the source of the saltwater that created the shallow sea over a significant portion of the land masses of the nascent Gondwana in the time of the Cloudinas. Only around 520 million years ago did the ocean close, completing the assembly of the jigsaw puzzle of the southern supercontinent. “The formation of western Gondwana is more complex, and it happened later than was thought,” Trindade says.
In the opinion of geologist Umberto Cordani of IGc-USP, the waters of the shallow sea that probably covered part of South America and Africa in the late Ediacaran cannot have come from the Clymene. It is impossible for one simple reason, he says: that ocean never existed. Cordani, Márcio Pimentel of UnB and other researchers hold the more classic view on the establishment of Gondwana. According to this hypothesis, the western portion of the supercontinent, consisting of Africa and South America, came together around 620 million years ago through the closing of a large ocean, the Goiás-Pharusian, that separated the Congo and Saara cratons from the continental blocks of Amazonia and western Africa. At the time of the Cloudinas, therefore, South America and Africa had no internal oceans, according to this view. The small marine animals that have now been found in Minas Gerais and in other Gondwana sites are thought to have populated a vast, shallow interior sea that stretched over a continental lithosphere (crust). “There is no geological evidence of an oceanic lithosphere in central Brazil during the Ediacaran or Cambrian period that could be associated with the possible existence of the Clymene,” Cordani says.
The two groups with differing views on the formation of Gondwana have amicably published articles and comments questioning data and interpretations of colleagues who do not share their position. The discovery of the marine fossils in northern Minas Gerais—which for some serves as evidence that the Clymene Ocean covered South America and Africa—is one more ingredient to fuel the debate.
Tectonics and sedimentation of the Itapucumi Group in the context of Ediacaran carbonatic platforms: a geochemical, geochronological, paleomagnetic and biostratigraphioc approach (No. 2010/19584-4); Grant mechanism Post-doctoral research grant in Brazil – Regular; Principal investigator Claudio Riccomini (IGc-USP); Grant recipient Lucas Verissimo Warren – IGc/USP; Investment R$150,870.57 (FAPESP).
WARREN, L.V et al. The puzzle assembled: Ediacaran guide fossil Cloudina reveals an old proto-Gondwana seaway. Geology. v. 42, n. 5, p. 391-94. mai. 2014.