For those wishing to leave a lasting mark of their existence on Earth, here’s a tip: walk along the edge of a lake where there is mud or fine, moist sand covered with algae. Hundreds of dinosaurs did just that, and their footprints have remained intact, sculpted into rocks found in the municipality of Sousa in the backlands of the northeastern Brazilian state of Paraíba, through the action of green and blue algae in the mud where they walked more than 100 million years ago.
This was the conclusion of paleontologists Ismar Carvalho of the Federal University of Rio de Janeiro (UFRJ), and Giuseppe Leonardi of the Cavanis Institute, in Kinshasa-Ngaliema, Democratic Republic of the Congo. Along with geologist Leonardo Borghi of UFRJ, in May 2013 they published a paper in the journal Cretaceous Research, the first physical evidence of the significance of algae in the preservation of fossil footprints. The gelatinous film created by the microorganisms growing on the trodden mud apparently prevented the footprints from being erased by wind and rain before it hardened and was covered over by a new layer of sediment that protected it from erosion.
“It’s incredible how microorganisms help record the lives of some of the largest animals that ever lived,” says Leonardi, who is regarded as a leading specialist in ichnology, the study of traces left by extinct animals, or ichnofossils, in order to determine their carriage and behavior. It was through footprints, for example, that paleontologists were able to get away from improperly assembling fossil skeletons in museums. We used to think that dinosaurs walked around like crocodiles, dragging their belly and tail on the ground. Footprints, however, show that the creatures walked with their tail and body suspended, with weight distributed equally on their feet.
The Sousa footprints were described for the first time in 1924 by Luciano Jacques de Moraes, a mining engineer. The study of these traces, however, only began in 1975, when Leonardi spent a year exploring the region. Born in Italy to a family of geologists and paleontologists, Leonardi, 74, has always divided his time between a research career and his calling as a Catholic priest. He is preparing to publish a book on Sousa, written in collaboration with Carvalho, while he spends time teaching children in the Congo.
ARIEL MILANI MARTINEThe rocks in Sousa were formed from sediments that accumulated in an open valley early during the separation of South America and Africa, in the beginning of the Cretaceous period. Between 142 million and 130 million years ago, the valley contained rivers and lakes that attracted the fauna of the region. Its mud-turned-rock recorded the passage of nearly 400 individuals—dinosaurs, crocodiles, frogs and tortoises. There are also undulation marks produced by flowing water and even tiny holes created by raindrops.
Scenes from the past
There are no fossil remains in Sousa, however, unlike the nearby sedimentary Araripe Basin in the state of Ceará, where many dinosaurs from the Cretaceous have been found. Leonardi explains that the two basins had different sediments and environments. Sousa’s more acid environment eroded the bones, while in Araripe, rushing water swiftly dragged and buried the animal carcasses, maintaining the bones under conditions favorable to petrification.
“Generally speaking, fossils are records of death, while footprints are records of life,” Carvalho says. It is difficult to identify the species of the animal by its footprints. Nevertheless, researchers are able to classify them into certain dinosaur groups and, at sites where many prints have been found, they can reconstruct scenes from the past.
The daily life of the Sousa dinosaurs is suggestive of that of large mammals in the present-day savannahs of Africa. There are tracks made by numerous bands of sauropods—huge quadrupedal herbivores similar to the brontosaurus. At one spot we can see that an adult sauropod slowed its gait to match the pace of a young one. In other places, the bands are pursued by small groups of theropods—carnivorous bipeds similar to tyrannosaurids or velociraptors. Theropods were more active than herbivores and left more recorded footprints, though their numbers were probably smaller.
“These marks are such delicate structures, so easily erased by bad weather,” Carvalho notes. “We wanted to understand how they were preserved.” He says that researchers generally used to agree that in order for footprints to be preserved, the sediment where they were imprinted just needed to have certain special characteristics. It had to be fine, moist and moldable to just the right degree like clay. All of the experimental studies to date, however, show that this is often insufficient.
Over the past decade, evidence has begun to show that the footprints exhibiting less erosion are ones that were covered with algae. In 2009, for example, a group of Swiss archeologists observed precisely that, while studying the hardening of human footprints imprinted just a few years ago on the shores of lakes in the Caribbean and the Middle East. Carvalho noted something similar in the Lake Region in the state of Rio de Janeiro. Other paleontologists began to suspect that what are known as microbial mats composed of algae served as an adhesive between the grains of sediment, thereby preserving the traces of footprints and protecting them from wind and rain. These microorganisms are thought to have also aided petrification by accumulating calcium, which hardens sediment.
Carvalho and his colleagues discovered the first physical evidence of the phenomenon when they used a microscope to analyze sheets of rock taken from a well at Fazenda Cedro, in Sousa. They found several layers of microbialites, a type of rock formed from the remains of microbial mats from the Cretaceous period.
Additional indirect evidence can be seen in Sousa, in the form of fossilized conchostracans, crustaceans protected by two shells and similar in appearance to crabs and shrimp. Conchostracans still exist today and almost never exceed half a centimeter in length. One of the species from Sousa, however, measures 4.5 centimeters. Carvalho believes that the conchostracans in Sousa grew so large because of the environment of warm, calm, nutrient-rich water that supported the proliferation of microbial mats on the banks of the lakes where the dinosaurs walked.
More algae, more details
The footprints with the richest detail, that when seen close up reveal everything from claw marks to grooves in the soles of the feet and toes, are thought to have been formed where there was more mat growth. The algae may also have helped preserve the rims that can be seen around some of the footprints. The rims were formed when mud squirted out as the animal stepped, and can give an indication of its weight.
In addition to the clayey sediment and microbial mats, the cycles of sediment deposition following the dry and rainy seasons also helped preserve the footprints in Sousa. Footprints were imprinted and hardened during the dry season, and then buried under a new layer of sediment washed in by rain. The new layer would then serve as a substrate for new footprints to be imprinted during the next dry season. At a site in Sousa known as Passagem das Pedras, Leonardi excavated 25 of these layers containing footprints produced by cyclical changes along the shores of a lake.
Carvalho, whose research is supported by the Rio de Janeiro Research Foundation (Faperj) and the National Council for Scientific and Technological Development (CNPq), now hopes to examine rock sheets from sites in other parts of the world that have fossil footprints. The largest of these is in Sucre, Bolivia. “I’m almost certain that there are microbialites there,” he says.
“Microbial mats are in vogue,” observes Marcelo Adorna Fernandes, a paleontologist at the Federal University of São Carlos, whose laboratory has Brazil’s largest collection of ichnofossils. Many of these were collected in inland São Paulo State, particularly in Araraquara, where footprints were even discovered in the stones used in city sidewalks. Fernandes says he soon hopes to examine what he believes to be tracks made by invertebrates as they tore microbial mats growing at the bottom of glacial lakes, which gave rise to the sedimentary rocks known as Itu varvite.
CARVALHO, I. et al. Preservation of dinosaur tracks induced by microbial mats in the Sousa Basin (Lower Cretaceous), Brazil. Cretaceous Research. Published on-line. May 10, 2013.