One of the most exciting things in the study of fossils is to find the so-called soft tissue preserved – basically everything that is not bone in the animal’s body. Investigating it allows more concrete discoveries about what these extinct animals were like and how they lived, and in this sense the Crato Formation, in the Araripe Basin in the states of Pernambuco, Piauí and Ceará, is prolific. A new study helps explain why.
Researchers from the Federal University of Rio Grande do Sul (UFRGS), the University of São Paulo (USP) and the Center for Paleontological Research of the Araripe Plateau found traces of fossilized bacteria in the crest of a pterosaur that lived around 115 million years ago. What happened to these microorganisms, they believe, may help us to understand of how soft tissue has been so well preserved in at least some of the fossils found in the region.
The example in question belongs to a species described in 1997 by paleontologists Alexander Kellner and Diógenes de Almeida Campos, called Tupandactylus imperator. It was found by workers in the Triunfo Mine, close to the town of Nova Olinda, in Ceará, and despite being damaged when it was being collected, this is the best example of the species found so far.
The fossil, which has been given the code CPCA 3590 in the collection at the Center of Paleontological Research of the Araripe Plateau, stands out because of its enormous crest, which has remained partially fossilized. Felipe Pinheiro, a researcher on Cesar Schultz’s team in the Paleovertebrates Sector of UFRGS, described the fossil last year in Acta Palaeontologica Polonica. “At the time, we had no idea of the presence of the bacteria,” says Pinheiro.
It was only when Paula Sucerquia, from USP, made the first micrographs of the fossil that the researchers found small structures in the form of rods on the surface of the fossilized soft tissue. The team then started investigating the hypothesis that these really were bacteria.
There are records of bacterial fossilization throughout the world, but until then none had come from the Crato Formation. Years ago there were even those who interpreted similar shapes, observed in other fossils from the region, as traces of microorganisms, but subsequent investigations indicated that there might have been a mistake – instead of bacteria, what they were seeing in other fossils might be melanosomes, cellular organelles containing the pigment melanin, which for some reason are extremely resistant to decomposition.
Illustration: Voltaire PaesThis is not the case with the small structures found in CPCA 3590. Based on the morphology, they were identified as bacteria that were decomposing the soft tissue of the pterosaur at the bottom of the then Araripe Lake, right after the death of the flying reptile more than 100 million years ago. If the analysis is correct, this is the first solid evidence of bacterial fossilization coming from that region.
Death and preservation
In fact, according to the researchers, the presence of these microorganisms may have enabled the preservation of soft tissue in the fossil. There are two ways for this to happen. In one of them, the bacteria that decompose the animals produce chemical reactions that lead to the mineralization of the tissue. “In most cases dead tissue serves as the site for the deposition of phosphate and it’s not unusual for sub-cellular structures to be preserved, such as muscle fibers and even cell nuclei, with a high degree of faithfulness,” explains Pinheiro.
The case of the pterosaur, however, is something else. “The bacteria themselves fell into a trap,” says the researcher. “The phosphate that was diluted is deposited in the cell wall of these microorganisms. This causes the death of the bacteria, but allows them to be preserved as fossils,” explains the researcher, the first author of the new article, published in Lethaia – International Journal of Paleontology and Stratigraphy. This process, called bacterial auto-lithification, is not very gentle with the animal’s soft tissue that is in the process of being fossilized. Since the bacteria form a type of fossilized mold it is impossible to study the microscopic details of what was underneath. This work, added to other recent works, helps overturn a myth of paleontology: that good fossil preservation is necessarily associated with the absence of bacterial decomposition.
An interesting characteristic of the granular structures (the fossil of ancient bacteria) found by the researchers is that they seem to be joined two by two, as if they were in the middle of a replication process when they were fossilized.
Although the researchers themselves admit that this evidence is still not very conclusive, it is important because it suggests that fossilization might occur very quickly – in hours or days after the death of the animal; it is almost as if it were a courtesy of nature that would allow beings that have been extinct for a long time to be discovered millions of years later.
PINHEIRO, F. L. et al. Fossilized bacteria in a Cretaceous pterosaur headcrest. Lethaia. 2012.