Scientists are finding that long before dinosaurs dominated the Earth, a small reptile was moving through what is now southern Brazil. Its unusual legs may have helped it move more efficiently than many reptile that came before it.
Now, roughly 240 million years later, scientists are using its fossilized remains to better understand one of evolution’s most important transitional periods.
The newly described species, Silescelida acristata, lived during the Middle Triassic Period. Researchers discovered its remains in Dona Francisca, a municipality in Brazil’s Rio Grande do Sul state.
The animal was roughly comparable in size to a small alligator, although it had a much more slender build. It walked on four legs and likely hunted smaller animals within the recovering ecosystems of Triassic South America.
While Silescelida acristata was neither a dinosaur nor a crocodile, its evolutionary position makes it particularly interesting. The reptile belonged to the archosauriform lineage, which eventually produced archosaurs.
Those animals would ultimately give rise to dinosaurs, birds and crocodilians.
Researchers from the Quarta Colônia Center for Paleontological Research at the Federal University of Santa Maria, alongside scientists from the Federal University of Rio Grande do Sul and Pontifical Catholic University of Rio Grande do Sul, studied the fossil.
Their findings were published in the scientific journal Scientific Reports.
Life Was Recovering From Earth’s Greatest Mass Extinction
Silescelida acristata lived in a world that looked dramatically different from the one we know today. About 12 million years before this reptile lived, Earth went through the Permian-Triassic extinction event. Often called the “Great Dying”. It was the most severe known mass extinction in the planet’s history.
The catastrophe eliminated an enormous percentage of Earth’s species and radically changed ecosystems on land and in the oceans.
However, extinction also created ecological opportunities.
During the Triassic, surviving groups began diversifying and occupying newly available ecological niches. Archosauriform reptiles were among those undergoing major evolutionary changes.
Silescelida acristata offers scientists a glimpse into this experimental period.
Researchers believe the reptile was a small predator that likely ate animals smaller than itself. More importantly, its body shows anatomical changes associated with how early archosauriforms moved.
Its femur suggests that its legs were positioned in a more semi-erect posture. Instead of extending dramatically outward from the sides of its body, its limbs were positioned farther underneath it.
That might sound like a relatively minor change, but limb posture can have enormous consequences.
Positioning the legs beneath the body can make movement more efficient and reduce the amount of side-to-side motion required while walking. Similar anatomical transformations would become increasingly important during the evolution of later archosaurs.
Dinosaurs would eventually take this concept much further, with their limbs positioned directly underneath their bodies.
That makes this little Brazilian reptile part of a much bigger evolutionary story.
Strange Legs Offer Clues to the Rise of Archosaurs
Scientists don’t need a complete skeleton to make an important paleontological discovery. Much of the known fossil material from Silescelida acristata consists of limb bones. While that may seem limited compared with the spectacular complete dinosaur skeletons displayed in museums, those bones contain important anatomical clues.
The animal’s femur is particularly significant.
Researchers found characteristics indicating its semi-erect limb posture, potentially allowing the reptile to move more efficiently than animals with sprawling limbs. Its femur also contains another unusual feature, or rather, lacks one.
The species name acristata means “without a crest.” Researchers selected it because the reptile’s femur lacks a prominent bony structure known as a trochanter.
This structure provides an attachment point for muscles associated with the tail. Its absence helps distinguish Silescelida acristata from nearly all of its close relatives.
The fossil therefore represents more than another unusual prehistoric animal.
It provides evidence of the anatomical experimentation occurring among archosauriforms before dinosaurs and crocodilian ancestors became dominant components of terrestrial ecosystems.
When we study Evolution, generally we find that it rarely progresses along a simple, straight path. Different species develop different anatomical solutions, with some lineages disappearing while others continue diversifying.
Silescelida acristata represents one branch of that complicated evolutionary process.
Researchers also discovered something surprising when attempting to determine where that branch belonged on the reptile family tree.
A Brazilian Discovery Changes What Scientists Knew About These Reptiles
Phylogenetic analysis suggests Silescelida acristata may have been closely related to Euparkeriidae, an unusual and still poorly understood group of archosauriform reptiles.
That relationship is important because of where the Brazilian fossil was discovered. Fossils associated with Euparkeriidae and closely related animals have historically been found primarily in Africa, Asia and Europe.
Because they’ve now found a potential relative in South America. it dramatically expands their known geographic range.
During the Triassic Period, Earth’s continents were connected into the supercontinent Pangaea. Animals could potentially walk across areas that are now separated by enormous oceans. This is how many fossils are found across different continents. In some maps it’s easy to see where the location of a particular group of dinosaur lived.
This discovery suggests these early archosauriform reptiles may have spread much farther across the Triassic world than the existing fossil record previously indicated. It emphasizes South America’s importance in understanding the evolution of the animals that eventually gave rise to dinosaurs and crocodiles.
Rio Grande do Sul is already particularly important to paleontologists studying this period. The state’s Triassic rocks have produced an extraordinary collection of fossils, including some of the world’s earliest dinosaurs.
The area where Silescelida acristata was discovered is now part of the UNESCO Quarta Colônia Geopark, which preserves geological formations documenting crucial moments in Earth’s history.
However, scientists almost lost part of this particular story.
The discovery of Silescelida acristata wasn’t simply the result of researchers uncovering a new fossil in the field. A portion of its story involves rediscovering something that had been sitting unnoticed for decades.
Part of the Fossil Was Lost for More Than 20 Years
Part of the Silescelida acristata specimen was accidentally separated from the rest of the fossil and remained lost for more than two decades.
Without that information, scientists couldn’t properly establish the geological context of the specimen. That made formally studying and describing the animal considerably more difficult.
Then came an unexpected breakthrough.
During a 2022 technical visit to the scientific collection at Pontifical Catholic University of Rio Grande do Sul, researchers located the missing fragment.
For a while the missing part of the animal was lost and unfortunately it held a lot of information with it. Including the geographical content that would help researchers conduct work necessary to formally describe the reptile as a new species. And now because we have a complete picture, the unusual history even inspired the name.
Silescelida combines words associated with “silence” and “leg.” The reference to silence represents the decades the fossil spent effectively forgotten, while the reference to a leg reflects the limb material preserved in the specimen.
The name turns the circumstances surrounding the fossil into a permanent part of the animal’s scientific identity.
It also demonstrates why museum and university fossil collections remain incredibly important to paleontology.
Major discoveries don’t always begin with someone dramatically pulling a complete skeleton from the ground. Sometimes, they begin with researchers opening drawers and taking another look at fossils collected decades earlier.
Old specimens can become scientifically valuable again when new technology, new discoveries or previously missing information provides researchers with another way to examine them.
Small Fossils Can Tell Enormous Stories
Silescelida acristata may never become as recognizable as Tyrannosaurus rex or Triceratops. Its remains are fragmentary, and the animal itself wasn’t especially large.
Yet its scientific importance comes from where it fits into the history of life.
This reptile lived during a critical period between the devastating Permian-Triassic extinction and the eventual rise of dinosaurs. The creature’s anatomy captures some of the evolutionary experimentation happening among reptiles during that transition.
Their limbs provide clues about changing locomotion. Its femur contains anatomical features separating it from related species. Its evolutionary relationships suggest certain archosauriforms were more geographically widespread than previously understood.
Even the rediscovery of its missing fossil material demonstrates how scientific collections can continue producing discoveries decades after specimens are originally collected.
Most importantly, the fossil adds another piece to the complicated puzzle surrounding the origins of archosaurs.
The dinosaurs that would eventually dominate terrestrial ecosystems didn’t suddenly appear fully formed. Neither did the ancestors of modern crocodilians.
Their success emerged from millions of years of evolutionary change involving countless species experimenting with different body plans, methods of locomotion and ecological roles.
Some of those experiments survived. Others disappeared entirely.
About 240 million years ago, Silescelida acristata was one of them.
Today, a handful of bones from Brazil is helping scientists reconstruct that forgotten world and better understand how the age of dinosaurs began before dinosaurs ever ruled the Earth.
