In 2021, mysterious black structures appeared during excavations at the Polzberg fossil deposit in Lower Austria. Researchers have now examined more than 80 of these finds and clarified their identity with the help of digital 3D models: Apparently they are fossil remains of the cartilage of squids that lived around 233 million years ago. The cartilage was replaced by carbon through geochemical processes, so that the structure of the actually perishable material has been preserved to this day. According to the researchers, these are the oldest known cartilage fossils in the world.
One of the most important fossil sites in Austria is at the Polzberg north of Lunz am See in Lower Austria. In the area originally developed for coal mining, exceptionally well-preserved late Triassic fossils were discovered as early as the late 19th century. The remains of marine animals and plants found date from the time of the Carnic Crisis around 233 million years ago, a major mass extinction event in which volcanism and rising CO2 levels in the atmosphere caused entire ecosystems to collapse.
Findings by researchers and citizen scientists
During excavations in 2021, researchers and citizen scientists discovered a previously unknown type of fossil in the Polzberg deposit: black structures up to three centimeters in size, which were often symmetrical. “We examined the microstructure of these mysterious fossils using scanning electron microscopy and determined the geochemical composition using X-ray spectroscopy,” explain paleontologists Petra and Alexander Lukeneder from the University of Vienna and the Natural History Museum Vienna.
The research team analyzed more than 80 finds in this way. Using micro-CT images, they created 3D models of the fossils and discovered that the structures were criss-crossed by countless branched veins. On the basis of comparisons with extinct and modern-day animal groups, Petra and Alexander Lukeneder determined that it was apparently the head of squid, originally made of cartilage. “So far there have only been a few fossil finds of cartilaginous structures,” the researchers explain. “Occurrences of mineralized fossil cartilage are therefore of great importance for evolutionary biology and paleontology.”
Obtained thanks to carbonization
But how could the structure of this actually very ephemeral material be preserved to this day? The material analysis showed that the objects consist largely of pure carbon. The fossils were created by what is known as carbonization. In the process, elementary carbon gradually accumulates as a result of geochemical processes under pressure and the absence of oxygen. While the original material disappears, the structure remains.
In addition to the carbonized bits, the researchers found the squid’s chalky shells in close proximity, as well as hundreds of squid arm hooks. According to the researchers, they come from the now extinct squid species Phragmoteuthis bisinuata, which lived around 233 million years ago in what was then the Reiflinger Meer in what is now Lower Austria. The cartilaginous part protected the brain and the area around the eyes of the animals.
Understanding the evolutionary development of cartilage
“The evolution of cartilaginous structures in marine invertebrates is still a matter of intense debate,” the researchers write. In her view, the new find can help to better understand the evolutionary history of the origins and importance of cartilage in invertebrates. In future work, they want to take a closer look at the channel system inside the fossils in order to better understand the connections between the biology of the squid, the environmental conditions and the processes of cartilage mineralization.
Source: Petra Lukeneder (University of Vienna) et al., Plos One, doi: 10.1371/journal.pone.0264595