Clues found in ancient remains and messages left by extinct ecosystems, waiting millions of years to be read, attract researchers to ichnology. It is an area of paleontology that, instead of focusing directly on the fossilized organism, such as skeletons, pollen, or insect wings, investigates elements that were the consequence of their activity while alive, such as footprints, nests, feces, and the traces of bites on bones.
It was through this specialty that geographer Lucca Cunha, of the Federal University of Rio Grande do Sul (UFRGS), in partnership with colleagues, discovered that insects fed from an already buried carcass, behavior that is not found among current invertebrates involved in the decomposition of organisms. The article was published in January in the scientific journal Acta Palaentologica Polonica.
The researchers analyzed bone fragments from a rhynchosaur, an herbivorous reptile that lived in what is today inland Rio Grande do Sul during the Triassic period (between 250 million and 199 million years ago). The fossils were found on the Buriol paleontology site in São João do Polêsine, near Santa Maria. The area is known for containing the oldest dinosaur bones ever found. What caught Cunha’s attention, however, were the signs left by another group of living beings — insects.
The recurrent flooding in the period covered the rhynchosaur bones in mud and minerals, which fossilized them. Among the 520 skull fragments examined, 29 presented fossilized trails and tunnels dug by different species of insects. Using photographs and computerized tomography, the team from UFRGS analyzed the shape of the trails within the bone and concluded that, at least in one of the types, the body was attacked by insects when it was already buried. “The pattern of deposition indicates that the sediment was remobilized by the action of the insect as it perforated the buried bone, filling the spaces left behind,” explains the researcher.
Biologist Voltaire Paes Neto, who did not take part in the study and is affiliated with the National Museum at the Federal University of Rio de Janeiro (MN-UFRJ) and with the Federal University of Pampa (UNIPAMPA), states that until recently it was assumed that the action of prehistoric invertebrates always occurred on the surface. “None of the existing present-day species that paleontologists use for reference act like that,” he explains. “This means that they were very distant relations of the current species or they were completely different from those known today.”
Luís Flávio Lopes / UFRGS
The biologist was a pioneer in the country for research into corrosion made by living organisms on a hard substrate, called bioerosion. In 2016, he found the oldest bite mark made by insects on bones. The estimate he made, of 240 million years ago, is slightly earlier than the dates of the fossils studied by the UFRGS team, of around 233 million years ago — when small dinosaurs had already started roaming there. The Triassic was a period of explosive biodiversity production, in which the ancestors of crocodiles and mammals also appeared.
Currently, termites and the larvae of one genus of dermestid beetles are the insects which leave marks most similar to those found by the UFRGS group. They reached the traces of meat that remained on the surface or inside the bone tissue, inaccessible by other animals. The current species, however, do not have the habit of infiltrating and operating below ground.
“It would be extremely difficult to unravel who exactly these insects of over 200 million years old were, but we now know that this underground behavior occurred, which says something about ecological interactions in the environment during the Triassic,” explains Cunha. A variety of species of vertebrates, invertebrates, bacteria, and fungi act in the decomposition of vertebrates, an essential action for the nutrient cycle in a natural ecosystem. Detecting the process from an ancient moment in evolutionary history reveals part of the role arthropods play in the ecology of the environment.
This attempt at the environmental reconstruction of the past, using clues left by the action of organisms, makes Paes Neto compare ichnology to “a type of CSI,” referring to the US police investigation series in which criminal experts rely on evidence left by larvae on the corpse to decipher what happened. Cunha’s work drew from forensic science, as well as records of fossilized insects.
Paleontologist Marina Bento Soares, a specialist in vertebrate fossils at the MN-UFRJ, highlights the boom of the appearance and diversification of life forms from the Triassic and recalls that the period culminated in a series of extinctions in the fauna, concurrent with drastic climate change. “We are now experiencing an intense period of climatic events and, despite the current ones being aggravated by human action, knowledge about what happened with organisms during accelerated climate change, like during the Triassic, could help to infer future trends,” she states.
Long before Rio Grande do Sul was devastated by water this year, the region was already a floodplain. During the Triassic (up to 250 million years ago), the entire area that today makes up the state was joined with the other territories of the single continent of Pangea. The environment was arid and dry at the beginning of the period, but changed drastically until it became a plain full of rivers and lakes that received constant rain for thousands of years. During the climate transition, many species went extinct and others, better adapted to the moisture, prevailed.
With the abundant rain, the rivers overflowed from time to time and flooded the plain. Mud and minerals covered what was in the way, including the carcasses of dead animals. The sequence of flooding of the Late Triassic helped with the episodic sedimentation of exposed remains of many species and contributed to the current southern state being one of the world’s richest regions for fossil records.
In the millions of years that separate the moment in which the rhynchosaur was buried by the constant floods and the present day, continents formed, tectonic plates shifted, plains disappeared and reappeared, and entire groups of fauna went extinct and were succeeded by other species.
Scientific article
CUNHA, L. S. et al. New bioerosion traces in rhynchosaur bones from the Upper Triassic of Brazil and the oldest occurrence of the ichnogenera Osteocallis and Amphifaoichnus. Acta Palaeontologica Polonica. vol. 69, no. 1, pp. 1–21. jan. 30, 2024.
