The globally unique fossil site Bromacker in the Thuringian Forest provides insights into a geological era when early tetrapods lived on the supercontinent Pangaea – millions of years before the dinosaurs. Together with research partners in Thuringia, the Museum für Naturkunde Berlin is assembling the fossil puzzle pieces in order to reconstruct a comprehensive picture of a prehistoric ecosystem.
Around 290 million years ago, a wide riverbed most likely cut through the landscape of what is now the Thuringian Forest. The river extended across a broad plain, surrounded by high mountains. This fertile river valley lay on the supercontinent Pangaea and was much closer to the equator than present-day Thuringia. Heavy rainfall in the hinterland caused the river to swell, regularly flooding the floodplains with sediments. During dry phases, the soil cracked open in honeycomb-like patterns.
In this valley, the inhabitants were primarily herbivores. Not tall dinosaurs – they would not appear until 50 to 60 million years later – but rather early tetrapods, including amphibians, proto-reptiles, and pelycosaurs, the reptile-like ancestors of mammals. Most of these terrestrial vertebrates were less than one metre in body length, with only a few reaching up to two metres. The most striking among them was likely the carnivorous Dimetrodon teutonis, distinguished by its prominent dorsal sail. In the floodplains, it probably hunted for prey.
Only shortly before – at least in evolutionary terms – the first vertebrates had emerged from the water and conquered new habitats on land. Their eggs developed embryonic membranes and a robust shell that protected the offspring from desiccation outside of water. Over time, herbivorous species gradually evolved from carnivorous ones. By this point, during the Permian period, there was already a great diversity of terrestrial early tetrapods – a profusion of life forms, of which nearly three-quarters perished about 40 million years later, during the greatest mass extinction in Earth’s history at the end of the Permian.
Traces of this past biodiversity have been preserved within the rock layers. Where the ancient river overflowed its banks millions of years ago, sediments washed over animals and plants. Over millions of years, these became fossils that remain to this day. At Bromacker, an old quarry between the Thuringian towns of Tambach-Dietharz and Georgenthal, a four- to five-metre-thick rock layer – lying just below the Earth’s surface – contains extensive evidence of the fossilized diversity of the Permian. Here, paleontologists coax secrets from the mudstone with hammer and chisel – and continually make astonishing discoveries.
“The state of preservation of the fossils at Bromacker is excellent; in some cases, we even find completely preserved skeletons,” says Prof. Jörg Fröbisch, Museum für Naturkunde Berlin and Humboldt Universität zu Berlin, who leads the research project. “It is an ideal site for investigating the origin of modern terrestrial ecosystems and the diversity of early tetrapods in the Permian.”
Fossilized skin impressions and traces
For the past five years, a team of around 40 paleontologists, geologists, preparators, and other researchers has been working to recover parts of the immense fossil treasure at Bromacker and to assemble the finds into a comprehensive picture of a Permian ecosystem. In addition to the Museum für Naturkunde Berlin, three other institutions are currently involved in this endeavor: the Friedenstein Stiftung Gotha, the Friedrich Schiller University Jena, and the UNESCO Global Geopark Thuringia Inselsberg – Drei Gleichen, where the site is located.
Bones, entire skulls, and traces of around 14 vertebrate species have already been discovered at Bromacker. These are unique records of long-extinct species, including numerous footprints as well as the world’s oldest evidence of reptile-like skin impressions from early mammalian ancestors. They suggest, for example, that the predator Dimetrodon teutonis exhibited social behavior; several individuals were found lying together around a watering hole or fleeing from it, just before a mudflow covered their resting marks – preserving them to this day.
It seems almost unbelievable: even the impressions of insect wings, millipedes, Conchostraca – clam shripms –, plant cones, water-level marks, and raindrops can still be clearly identified in the rock layers after 290 million years.
Since 2020, researchers at Bromacker have recovered more than 1,500 fossils; two newly discovered vertebrate species have already been described, and additional finds are currently under study. “The density of discoveries is enormous, we keep finding new species,” says Fröbisch. “Bromacker still holds research potential for many decades, as surprises continue to surface.”
As early as 1887, quarry workers at Bromacker discovered the first fossilized footprints in the reddish Tambach Sandstone. They looked as if they had just been made. Many more perfectly preserved track finds followed. Wilhelm Pabst, curator of the natural history collection at the Ducal Museum in Gotha then, studied these discoveries, became one of the co-founders of so-called ichnology, the study of prehistoric trace fossils.
Finally, in 1974 a geology student from the Freiberg University of Mining and Technology (Bergakademie Freiburg) made a discovery that brought Bromacker worldwide recognition. At the age of 22, Thomas Martens had come to document fossil clam shrimps (Conchostraca), but incidentally uncovered a fossilized bone of an early tetrapod that should not have been there at all. It lay in the sediment layers above the tracks. The find was a sensation that continues to resonate to this day. Nowhere else in the world is there a fossil deposit where bones and footprints occur together in such unique quality and abundance. “This gives research two reference systems with which to characterize early tetrapods: their bones and their tracks,” Fröbisch points out. “In some cases, we can even trace both back to the same species, or at least to the same group of animals.”
Thomas Martens made Bromacker his life’s work. For decades he carried out his own excavations, purchased a parcel of land on site with private funds after German reunification, and assembled an international excavation team that brought to light unique discoveries there, including the “Tambach Lovers” (Tambacher Liebespaar): two stem-amphibians (Ur-Amphibien) of the species Seymouria sanjuanensis, nestled head to head, a form previously known only from the United States. Martens and his team also recovered the complete skeleton of an early tetrapod – the first to be perfectly preserved from snout to tail tip: Orobates pabsti. And what is likely the first lizard capable of bipedal locomotion: Eudibamus cursoris. Excavations ceased in 2010 and were only resumed in 2020 with the new project. Now 73 years old, Martens remains actively involved in today’s digs.
Detective work with a miniature jackhammer
“When the finds first arrive, they don’t look like much,” says Pia Kain, one of the two geoscientific taxidermists of the BROMACKER project. In her workshop at the Perthes Forum of the Friedenstein Foundation Gotha, red plastic crates are stacked high, filled with reddish siltstone from which Kain teases out its secrets. Significant finds from the excavation site 20 kilometres away are catalogued at the Perthes Forum and stored in archive cabinets – up to 300 crates full of new fossil material per season. The most promising finds Kain begins to prepare immediately. Using a kind of miniature jackhammer, she works under the microscope, advancing millimetre by millimetre through the rock, exposing bone fragments and tracing the parting surfaces where sediments were deposited 290 million years ago. She can complete around 40 objects per year, while larger ones can take months. At the Museum für Naturkunde, her colleague Moritz Maier is carries out the preparation work.
“For large finds, we perform a block excavation (Blockbergung); for this we encase the rock in plaster so that it does not break apart during lifting,” Kain explains. For example, when fossilized bones of a hand protrude from the rock – possibly attached to an entire skeleton. “That’s like a surprise package that I can only open once I’m back in the workshop.”
Once, this “surprise package” weighed 500 kilograms. A stonemason delivered the block to the Perthes Forum by crane. Kain worked her way through the layers and uncovered the scattered bones of a disintegrated skeleton. “It was complete, but jumbled as if it had been through a washing machine,” she jokes. She found one vertebra, then another, and another – until an entirely preserved spinal column lay before her. It belonged to a plant-eater of the genus Diadectes, about 1.20 metres in size. “It is incredibly exciting to work one’s way through the rock and keep uncovering more bone material that no one has ever seen before,” Kain says in describing her work.
Preparation work forms an essential foundation for paleontological research. Only a largely exposed fossil can be comprehensively studied and analyzed. With a computed tomography (CT) scanner, hidden areas within the rock can be further examined to capture all dimensions and details and to generate 3D models of the bone. Using these models, researchers can describe even more features of the new species and investigate them in depth with state-of-the-art methods.
A pivotal moment in evolution
The excellent preservation of the Bromacker fossils and the unique parallel occurrence of bones and tracks enables researchers to ask very profound questions: What did the animals eat? How widespread were particular species? How fast did they grow, and how old did they become? How did they move? Which species burrowed into the ground and built tunnel systems? In what climate did they live?
The BROMACKER research project brings together several disciplines to assemble the many pieces of the puzzle. Researchers from the Geosciences Institute at the Friedrich Schiller University Jena carried out drillings 250 meters deep into the subsurface sediments. Based on the deposits identified in the drill core, they were able to demonstrate that the site must once have been a vast river valley with a floodplain.
They also determined that, despite the Tambach Formation’s location near the equator at the time, conditions there were not excessively warm – likely due to its elevated mountain setting. “Average annual temperatures were about 15 degrees Celsius, and we estimate an altitude of around 1,000 meters,” explains Fröbisch. This conclusion is consistent with bone structure: histological examinations of thin sections and bone imaging at the European Synchrotron Radiation Facility in Grenoble revealed cell sizes and growth rings. The analyses indicated that some early tetrapods from Bromacker were relatively small in stature. For comparison, fossils of the genus Dimetrodon in the United States reach four meters in length at the same age – roughly twice the size. The harsher environmental conditions of the Tambach Formation, such as pronounced seasonal fluctuations and limited food resources, were most likely responsible for the smaller body size of the Bromacker animals.
Another discoery was that most terrestrial vertebrates in the Bromacker ecosystem were herbivores. To investigate this, the team examined jawbones, their morphology and biomechanics, bone structures, and isotopes in the tooth enamel. “The composition of isotopes, which remain exceptionally well preserved in fossils due to the stability of enamel, allows us to determine whether the animals fed primarily on plants or on meat,” notes Fröbisch. For this analysis, the BROMACKER team collaborates with experts at Johannes Gutenberg University Mainz. Roughly one-third of the identified species preferred a plant-based diet. However, herbivorous species were especially common – together they accounted for about half of all individuals. “For the first time, we can see the trophic pyramid, the modern food pyramid with numerous herbivores at its base and only a few predators at the top.” One objective is to draw more precise conclusions about the food web: who actually ate whom – or what? Fossilized stomach contents, known as regurgitaliths, and other digestive remains help address this question.
The finds from the Bromacker also shed light on a decisive moment of evolution, since among the early tetrapods of the time there were already early ancestors of mammals: alongside Dimetrodon teutonis also Tambacarnifex unguifalcatus and Martensius bromackerensis. “They did not yet have hair or mammary glands and looked more like reptiles,” says Fröbisch. Contrary to what is often claimed in textbooks, however, mammals did not originate directly from reptiles. Rather, they are their sister group and together form the amniotes adapted to terrestrial life, which jointly arose from amphibians still dependent on water – and developed further in parallel to them.
Excavation techniques against extinction
The team made a truly sensational discovery in the summer of 2021. It uncovered conspicuous bone remains. As a precaution, the excavation team recovered a large block of rock, which soon afterwards landed under the pneumatic chisels of Pia Kain: the above-mentioned 500-kilogram “surprise package.” Kain gradually exposed the fossilized living chamber of a burrow, so carefully that the architecture of the structure remained clearly recognizable. Inside she found bones – a unique discovery for that geological period and at such a size. These probably come from a plant-eating diadectid, most likely from a Diadectes absitus. Possibly an animal of the same species also left the scratch marks that Kain uncovered in the living chamber, as well as the footprints preserved outside.
“The vertebrate fossil inside is among the largest ever found within a burrow,” states Lorenzo Marchetti, a specialist in fossil tracks at the Museum für Naturkunde Berlin, who created a 3D reconstruction of the burrow. As an ichnologist, Marchetti primarily searches for footprints as well as body and skin impressions. Yet at the Bromacker, so many burrows and living chambers were found that they became a research subject of their own. “The animals probably protected themselves from drought in these underground structures, reproduced, or sought safety from predators,” he says. Fossilized water-level rings provide evidence that there must have been dry periods lasting at least two months.
So far, the researchers have discovered more than 120 burrows and living chambers at the Bromacker, including complex burrows that rank among the oldest yet found. Using state-of-the-art techniques, they attempt to determine which animals may have constructed the structures. They produce CT scans of the skeletons of potential burrow producers, 3D models, and animated simulations in order to infer burrowing movements or scratch marks from limbs.
“In the Permian, more and more species appear that actively constructed burrows and applied new excavation techniques,” notes Marchetti. Presumably, through their ability to dig, the animals escaped the increasingly hostile aridity of the Permian. “The burrows may have had an influence on who survived and who did not.”
The Bromacker is for everyone!
From the beginning, the BROMACKER project was also a project for the public. In the summer months, one can visit the excavation and engage in conversation with the team. At the same time, the entire research process is communicated through innovative knowledge-transfer formats. “We want to show how exciting our finds and findings are and which rigorous scientific methods underlie our work,” explains Fröbisch. For him, the exchange with the public has become a highlight. “We ourselves are so enthusiastic that this also transfers to the visitors.” Countless further fascinating questions can be asked of the site. The state of Thuringia has just pledged additional funding until 2026, and at the federal level there are also plans to make the project permanent.
In order to disseminate knowledge about the Bromacker, the team is also working on an AI assistant, which can answer all questions at the most up-to-date state of knowledge. At Schloss Friedenstein in Gotha, the exhibition “Bromacker Lab” meanwhile invites visitors to dig for fossils themselves using the tools of paleontologists. Here visitors can also marvel at many of the fascinating original Bromacker finds, such as the “Tambach Lovers”.
And whoever looks to the left-hand wall in the foyer of the Museum für Naturkunde Berlin sees three slabs of rock. They show the fossilized diversity of the Bromacker through the traces of early tetrapods that lived 290 million years ago. “The view into these early ecosystems up to the greatest extinction event in Earth’s history at the end of the Permian is so exciting because we today again find ourselves in a phase of mass extinction,” observes Fröbisch. “It is of inestimable value to understand how primeval ecosystems developed, also in order to understand and protect nature in the present.”
Text: Mirco Lomoth
Photos: Alice End und Pablo Castagnola