Oldest articulated skeleton fossil ever identified helps shed light on animal evolution

Like certain knights of Antiquity and the Middle Ages, a mysterious organism that lived more than 540 million years ago was covered with an armor of rigid plates, fitted one inside the other, which allowed it a certain flexibility despite the protection. According to the Brazilian researchers who analyzed these fossils, it is the oldest articulated skeleton identified so far, with details that can help to understand the evolution of most of the animals we know today.

The creature in question is the Corumbella, so named because many of its fossils were found in Corumbá, Mato Grosso do Sul, although there are also records of the invertebrate in rocks from Paraguay and the USA.

Based on exceptionally well-preserved animal remains and advanced microscopy and tomography techniques, scientists have just published the most accurate description of the animal’s anatomical structure so far, in an article in the specialized journal iScience.

In addition to researchers from Brazil, the group includes collaborators from Scotland and Germany.

Understanding how Corumbella lived and where it fits in the animal family tree is particularly important because it roamed the oceans at one of the most enigmatic times in the history of life on Earth. This is the Ediacaran, a geological period in which multi-celled organisms visible to the naked eye first became common.

The problem, however, is that the Ediacaran fossils have characteristics that are difficult to connect with later animal groups, which are much more clearly related to the species we know today. Many have the structure of “fronds” (resembling large leaves), others are disc-shaped, and there is also the presence of what appear to be worm tracks on the sea floor.

“In this biota, many organisms are ‘halfway’ between animals and something else that we don’t understand”, explains the study coordinator, Mírian Forancelli Pacheco, from UFSCar (Federal University of São Carlos). In any case, there are those who associate some of these fossils with cnidarians (a group of today’s jellyfish).

Incidentally, a connection with cnidarians was the predominant hypothesis until now for anyone trying to classify Corumbella. However, “the new data to which we are having access may reveal evidence in favor of a new phylogenetic hypothesis”, says Pacheco. “According to this hypothesis, this animal is halfway between animals with radial and bilateral symmetry.”

Simply put, this could mean that Corumbella would represent one of the biggest and oldest crossroads in the evolution of animals. Almost all the animals we see around us today —vertebrates, insects, arachnids, etc.— belong to the large group designated as Bilateria. That is, their body can be divided into a right side and a left side that are symmetrical to each other. A human’s right hand, for example, is a mirror image of his left hand.

Animals with radial symmetry “mirror” their body parts around a central axis. The easiest example to understand visually is that of starfish, with their repeating arms turned to various sides. This is, however, a somewhat misleading example, as starfish are actually part of the Bilateria—their larvae have bilateral symmetry, like us. The “root” radial symmetry of animals belonging to groups older than the Bilateria is present in cnidarians and sponges, for example.

In the case of Corumbella, things are more complicated. The reconstruction carried out by the Brazilian team indicates that, in the part where it was attached to the sea floor, the creature had a tubular shape; however, at its top, it acquired a symmetry of four sides crossed by a line (see artistic reconstruction).

Furthermore, it possessed what researchers have called a cataphract skeleton. “We were inspired by the way in which elements of military armor were organized”, says Gabriel Ladeira Osés, first author of the research.

The so-called cataphracts were heavily armed horse warriors, present in armies such as those of the Persian Empire and the Eastern Roman Empire from the end of Antiquity. The armor, which covered both the rider and his mount, was formed by overlapping metal scales, attached with wires (also metal) to a layer of leather underneath.

Rather than metal, Corumbella’s armor was likely formed by plates and rings of the mineral aragonite, a type of calcium carbonate. “It could have provided protection against predators and increased lift”, says Osés. According to the researchers, in fact, the possible appearance of predators must have been the main trigger for the evolution of animal skeletons in that remote period.

“Each of the parts of the skeleton was quite rigid. But the organization and arrangement of these parts allowed for a type of articulation that ensured the flexibility of the whole. It’s like a medieval knight who, wearing his armor, was protected in a rigid structure, but didn’t lose the articulation of the movements”, compares Pacheco.

One possibility is that the animal fed by filtering the seawater that passed through it. “It could also be a suspension-eater, capturing food particles with the help of cilia or tentacles, for example”, explains Osés.