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Precambrian Fauna

Life protected by armor

The Northern Region of Paraguay might have the highest diversity of fossils of the earliest animals with skeletons

056-059_esqueleto_199-1ASandro CastelliThe only cement factory in Paraguay is in the area that surrounds Puerto Vallemí, a village of 9 thousand inhabitants in the north of Paraguay. A few kilometers away from the village, the Indústria Nacional del Cemento company has been digging into a 640-meter high rock wall, which provides most of the limestone used by the Paraguay civil construction industry. A blanket of white dust covers the village on windy days. Brazilian geologist Lucas Warren has spent time searching the mining company’s excavations and digging through ravines lining the region’s highways. He recently found what he refers to as “the gold mine of paleontology.”

The rocks he brought from Paraguay are currently lying on a big table in his office at the Geosciences Institute of the University of São Paulo (USP). Small, elongated structures – on average 1 centimeter long – are embedded in the rocks. The structures resemble worms imprisoned in a block of mud that had hardened under the sun. These actually very rare structures are found in only a few areas of the world and are fossils of what were probably the first living creatures with skeletons to appear on our planet.

Lucas, an expert on sedimentology and paleontology, estimates the age of the fossils at 550 million years, the same as the rocks in Puerto Vallemí. Geologist Eric Tohver, a researcher at the University of Western Australia, which has a partnership with USP, is currently trying to date fossil-containing rocks by using more accurate techniques. If their age is confirmed, they will be among the oldest fossils ever found of animals with biomineralized skeletons. Similar fossils were found in Namibia, southwest Africa. The African fossils lived 549 million years ago. More recently, similar finds occurred in China, suggesting that this kind of animal may have existed even earlier. However, the Chinese fossils have not been clearly identified yet.

There are only a few species – no more than five or six – of the first beings with a skeleton visible to the naked eye. According to fossil records, the species existed for only a short time – from 550 million to 542 million years ago. In Puerto Vallemí, Lucas and Paraguayan geologist Alberto Cáceres found examples of two species that had already been identified and at least one example of a species that has not yet been identified by science. The two men also identified signs of soft-bodied live beings that lived during the same period and left similar vestiges to those embedded in the rocks.

This is no trivial matter. It is very uncommon to find vestiges of two or more species of this kind from the same period and in the same region. Before the findings at Vallemí, similar fossils had been found in Namibia, Canada, Brazil, China, Oman and Russia. “The quality of the fossils found in Paraguay and the variety of the species make this one of the most comprehensive and representative collections of the fauna from that period,” says paleontologist Thomas Fairchild, from USP’s Institute of Geosciences (IGc). Fairchild, together with Lucas, Mírian Pacheco, Claudio Riccomini, Marcelo Simões and other collaborators described the fossils found in Puerto Vallemí.

Lucas found those fossils in a region bound by the Paraguay River to the west and by the Apa River to the north, on the border with the State of Mato Grosso do Sul, where geologists Paulo Boggiani and Claudio Gaucher had already found a similar fossil. Many of the samples collected by Lucas – some of them fit into two hands – have hundreds of fossilized skeletons, imprisoned in a layer nearly one centimeter thick.

056-059_esqueleto_199-1BSandro CastelliLucas was not looking for fossils when he came to the region. During the first expeditions in 2006, when he had begun his doctorate under Boggiani, Lucas had planned to map the evolution of the sedimentation basin in the region that extends through the State of Mato Grosso do Sul, Bolivia, the north of Argentina and part of Chile. The rocks found in that region indicated that it had been under the sea; 550 million years ago, the continents were shaped very differently from how they are today. The huge continental block on which the Amazon Region and Paraguay were located was more to the south and was isolated from the rest of South America (see map on the side). This part of the South American continent was actually a shallow sea, with clear and hyper saline waters.

This is the setting in which the Puerto Vallemí skeletal beings probably lived. The way they are preserved in the rocks indicates that they were anchored to the seafloor sediments, a greenish mat of cyanobacteria that, upon going into photosynthesis, removed carbon gas from the water and transformed it into calcium carbonate.

Most of the fossils in that region belong to two kinds of animals: Corumbella and Cloudina. The Corumbella were first described in 1982 by a team headed by German geologist Detlef Walde, from the University of Brasília. Rocks collected in the region of Corumbá, State of Mato Grosso do Sul, contained fossils of skeletons in the form of an inverted pyramid. The biggest examples of this species, named Corumbella werneri, were 10 centimeters long. The species found in Paraguay were 5 centimeters long. Even though this species was identified three decades ago, the composition of its skeleton was not well known. By analyzing examples of the Corumbella, paleobiologists Mírian Pacheco and Juliana Basso, from IGc, found that the skeletons of these fossils contain a significant concentration of organic matter– possibly chitin, the polysaccharide that is part of insect skeletons.

Lucas, Mírian and Fairchild also found microscopic pores and papilla in the said skeletons. Described in an article published this year in Geology, these characteristics indicate that the skeleton belongs to the Cnidaria, a phylum that includes medusae, anemones and jellyfish. These animals have a simple, jelly-like body, with a digestive orifice and an oral cavity, sometimes surrounded by tentacles with cnidocytes.

056-059_esqueleto_199-2Sandro CastelliIt is widely believed that the distribution of Corumbella is restricted. In addition to being found in Corumbá and Puerto Vallemí, examples of these animals have only been found in California. The Cloudina, however, were more cosmopolitan. The earliest examples, believed to have lived 549 million years ago, were identified in Namibia in 1972. Subsequently, their existence was confirmed in a dozen other countries and now in Paraguay.

The smaller Cloudina fossils measure no more than 3 centimeters. Their skeleton resembles an ice cream cone or a pile of coffee cups and is composed of layers of calcium carbonate, deposited as the animal inside it grew. This skeleton, which is stiffer and exclusively made of minerals, which facilitates fossilizing, seems to have ensured the animal’s mobility, enabling it to move with the waves. This animal had a more complex body, and it was probably an annelid, the group that includes worms and polychaeta, or bristle worms.

Nobody knows why the ability to produce a skeleton first appeared in the animal kingdom, which probably happened more than once. There are three theories that try to explain this. One of the hypotheses suggests that the ability to produce a mineral skeleton could be a way of eliminating high levels of calcium carbonate – extracted from seawater – from the organism. In other words, the skeleton acted as a detoxification mechanism. Another hypothesis is that the skeleton appeared by chance and became an adaptive advantage, providing the necessary support for these animals to reach the food available above the layer of sediments. “Staying 1 centimeter above the bottom of the sea may have enabled the animals to explore a region where they had no competitors,” says Lucas.

Thrombolite collected in Vallemí

LUCAS WARREN/IGC-USPThrombolite collected in VallemíLUCAS WARREN/IGC-USP

However, Lucas, Fairchild, and other researchers from the IGc believe in the third hypothesis: the skeleton, having appeared by chance, acted as an armor that increased the animals’ possibility of survival from attacks by predators. They believe that this is the strongest hypothesis because of the co-existence of beings with different skeleton-produced strategies – namely, the strategy of the Cloudina to extract raw material from water, and the strategy of the Corumbella to synthesize material based on organic compounds.

Indeed, predation was an entirely new way of interacting. Life appeared on Earth 3.5 billion years ago. Bacteria, the earliest living beings, had only one cell, which resembled a tiny pouch containing genetic material and proteins. Very few changes occurred over the next 3 billion years. Some single-cell beings began to live in colonies, where each group of cells carried out different functions, but they did not form an organism. The first multi-cell organisms began to appear between 580 million and 560 million years ago. These organisms, known as the Ediacara biota, had a jelly-like body, arranged into tissues and unusual shapes (tubular or frond disks or feathers).

Fairchild says that “this is when the first living beings appeared. They were able to move on sediment at the bottom of the sea.” Before that, the beings had not been able to move and made their own food using sunlight and the nutrients available in the environment. “Life was peaceful and quiet before skeletons appeared,” he jokes.

Whatever the origin of the skeleton may be, the fact is that this structure seems to have had a radical influence on life on our planet. Soon after the first skeletal had disappeared 542 million years ago, an enormous variety of live beings with increasingly complex bodies flourished; they were the precursors of all the organisms that exist nowadays. This change is referred to as the explosion of life during the Cambrian period. “Anybody who wants to have a better understanding of what happened during this phase of transformation of the life on our planet cannot ignore the Vallemi fossils,” says Lucas.

Projects
1. Stable isotopes (C, O and Sr) of the Itapucumi Group and correlations with the Corumbá Group (Ediacara biota) (nº 2010/02677-0); Modality Regular Research Funding; Coordinator Paulo Cesar Boggiani – IGc/USP; Investment R$ 88,107.25 (FAPESP).
2. Tectonics and sedimentation of the Itapucumi Group in the context of the carbonate ediacarian platforms; geochemical, geochronological, paleomagnetic, and biostratigraphic approach (nº 2010/19584-4); Modality Regular Research Funding; Post-doctorate in the country; Coordinator Lucas Verissimo Warren – IGc/USP;  Investment R$ 150,870.57 (FAPESP).

Scientific articles
WARREN, L.V. et al. The dawn of animal skeletogenesis: Ultrastructural analysis of the Ediacaran metazoan Corumbella werneri. Geology. v. 40. p. 691-94. Aug. 2012.
WARREN, L.V. et al. Corumbella and in situ Cloudina in association with thrombolites in the Ediacaran Itapucumi Group, Paraguay. Terra Nova. v. 23 (6), p. 382-89. Dec. 2011.

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