A collection of fossils from the Socorro and Urumaco Formations in Venezuela has helped understand the evolution of gigantism in anacondas (genus Eunktis) in South America, according to a study published in the specialized journal Journal of Vertebrate Paleontology this Tuesday (2).
According to the results, giant tropical snakes have maintained these dimensions for at least 12 million years, since the middle Miocene, in swampy (hot and humid) environments, similar to the habitat of the green anaconda (the common name for the anaconda in Brazil).
A team composed of researchers from the Universities of Cambridge (UK) and Zurich (Switzerland), the Paleontological Museum of Uromaco, and the National Experimental University Francisco Miranda, both in Venezuela, was responsible for finding the fossil remains of anacondas in rocks ranging in age from the Middle Miocene (14.5 to 12.4 million years ago) to the Upper Miocene (12.4 to 6.8 million years ago).
The collection consists of 183 snake vertebrae attributed to 32 different individuals, according to location, color and position in the spine. Paleontologists use a database that includes characteristics of current organisms. For each vertebra, two measurements were taken that are considered standard in the fossil study of snakes, says Andrés Alfonso Rojas, a doctoral candidate at the University of Cambridge and first author of the study.
They also compared it with measurements from the collection’s specimens and other fossil species known from the Miocene of South America. Columbovis. Linear regression analysis was then performed to arrive at estimated body size.
After analysis, the researchers estimated the minimum size at 3.95 meters (for upper myosin vertebrae) and the maximum at 5.23 meters (middle myosin material), with an average size of 4.14 meters. The average size of anacondas living today is five metres, with some reports of snakes reaching six metres. According to the study, the difference Eunktis With its closest relative, the genus Hippocrateswhich has an average size of 2.5 metres, occurred 38 million years ago.
“What makes this study different is that we have a robust amount of fossil material from a snake still extant today, with specimens dating from 12 to 6.8 million years ago – approximately 6 million years of evolutionary history – in the Urumaco. Using reconstructions based on phylogenetic analysis, we confirmed that anacondas were already large in size 12 million years ago,” says the paleontologist.
The rocks in the Urumaco and Socorro formations represent a well-known ancient ecosystem in northern South America in the Miocene called the Pibas System, according to the author. “Pipas were also home to large crocodiles, tortoises and giant ground sloths, as well as abundant fish, medium-sized crocodiles, and rodents such as the capybara, which are commonly hunted by anacondas today. Although today’s climate is cooler and the pipas system no longer exists, the ecological role of anacondas has remained the same over the past 12 million years, which explains why they remain as large as they once were. In the Miocene.”
Modern snakes include more than 4,000 species worldwide ranging in size from ten centimeters (the smallest known snake) to more than ten meters (such as the reticulated python, native to Asia). In the Americas, large species are part of the clade Booidea, which includes constrictor snakes, such as anacondas and boas.
In Brazil there are at least three species of anaconda or anaconda: the green anaconda (Unectes morenus) Northern green anaconda (E. Akiyama), which was recently discovered, and the yellow anaconda or Pantanal anaconda (E. Notaeus). There is also a record of a species restricted to Marajó Island in Pará (E. DeShaughnessy), but recent studies indicate that it is synonymous (same species) as the green anaconda.
Lazar Elbaz, a doctoral candidate at the University of the South Pacific Museum of Zoology who specializes in studying the evolution of extinct and present-day large snakes based on the fossil record, said that the authors of the new article combined two methods used to calculate the approximate size of anacondas already presented in other studies conducted by the group and that the results, in general, are based on the current population of snakes used as a reference.
“The same author, Jason Head, also made a rough estimate based on the Booidea vertebrae of the giant snake Titanoboa, excavated in Miocene rocks from Colombia, that it reached 13 meters in length, which is a bit too much,” says Elbaz, who was not involved in the research. “In the case of the new study, where it was compared to cattle, which is an existing group, there is already very advanced knowledge of the group, which makes it possible to verify the estimate more easily.”
El-Baz also points out that evolutionary analyzes of snakes based exclusively on vertebrae are often difficult to draw conclusions, because this is an anatomical region subject to modifications according to lifestyle habits, and that analyzes using cranial material provide additional data on the origin and evolution of large groups of snakes.
“They attribute the gigantic size of the fossils to the ecosystem, but that’s pretty coherent, actually, because it’s the same environment that today’s anacondas live in,” he says.
Alfonso Rojas and his colleagues plan to carry out further expeditions in this area in search of additional fossil material, ideally including skulls.