Linda and Michaela here – when we were undergraduate students, we had to do a four week internship as part of our degree. Learning a new skill beyond the university’s coursework is more fun when you get to get your hands dirty and spend time outdoors, preferably lots of it. A perfect way to do so is to do an internship at a paleontological excavation. Luckily, we both got accepted at the same place and thus, spent a month excavating fossils at the eocene Eckfelder Maar in the western German Eifel mountain range (Fig 1).
The Eocene is a geological epoch ranging from 56 to about 33 million years ago. Back then, a greenhouse effect had heated up central Europe and the world. Tapir relatives and tiny horses roamed (sub-)tropical forests, crocodiles stalked marsupials at lakeshores, small primates climbed palm trees. These are not the organisms and ecosystems associated with cold and rainy Germany today! But this is what it looked like, when the Eckfelder Maar came to life with a bang. An event that should change the rainforest and the lives of German paleontologists alike.
A maar is a volcano, although at first glance, it doesn’t look like one – there is no lava, no ash, not even a mountain. It resembles a volcano so little that early descriptions deemed maars the result of ‘cold eruptions’, which is wrong but relatable.
Maars usually present themselves as perfectly circular. The water-filled craters are the result of the sudden and violent evaporation of cold groundwater that came in contact with hot magma. The explosion tears a pointy, steep-sided hole into the landscape and is usually a singular event, at least in that exact location. At first, the crater is belted by a ring of debris that was thrown out by the force of the explosion. This wall, however, becomes eroded by wind and rain and, as the crater slowly fills up with rain and groundwater, no direct sign of the volcanic activity is left (Fig 2). An eruption like this is called a phreatomagmatic eruption.
The Eifel area, where the Eckfelder Maar is located, is the international type locality that coined the term ‘maar’. Over 75 of these round craters are speckled throughout the landscape and often referred to as the “eyes of the Eifel” because of the round shape and blue colour of the lakes. Over time a maar lake is destined to fill completely with sediment and eventually dry up. The Eckfelder Maar is 44.3 Million years old and hence much older than the others in this area, which formed between 500.000 and 11.000 years ago. Even of the younger maars only 9 still host a lake today, the Eckfelder Maar lake has long dried up. Initially, the eruption blasted a 1000 m wide and up to 210 m deep crater into the surrounding rocks. The bottom of the crater was quickly filled with a layer of debris. After the dust had settled and the lake had formed, it became quiet. For 250,000 years layer upon layer of clay, each less than a millimetre thick, accumulated at the lake floor and slowly but surely filled it up.
For the majority of its existence the lake was strictly divided into two layers: a lower, mineral-rich and oxygen-depleted part and an oxygen-rich upper part. The density difference between the two water bodies inhibited mixing and thus kept the lake floor a life-hostile environment. What is bad for sediment-dwelling organisms is good for paleontologists – the oxygen poor lake bottom served as a preservation chamber for all kinds of organisms.
Due to the steep crater being located in a (sub-)tropical, species rich forest, many organisms ended up on the bottom of the lake. In addition to (semi-)aquatic creatures such as crocodiles, turtles and fish that spent at least part of their lives in the lake, large amounts of plant fragments fell into the lake and sank to the lake floor. Leaves and leaf fragments are among the most common finds in this lagerstätte, but pollen, pieces of bark, twigs, fruits and the occasional flower have also been discovered. Especially flowers are very valuable finds, since – in cases of exceptional preservation – they can allow the extraction of pollen. In this case the scientists have proof that a certain species of plant produces a certain type of pollen and can thus confidently identify pollen found elsewhere. Plant fragments are found so commonly, that only rare and exceptionally well preserved or otherwise special finds are being collected, such as fruits, flowers, and leaves with damages suspected to be caused by insect herbivory. Other less valuable finds are given to visitors who come by to learn about the excavation.
In addition to plant material, insect fossils are recovered in large numbers. Honey bees, ants, termites, flies, wasps, grasshoppers, lice, dragonflies and others are found at the Eckfelder Maar. Among these, beetles are the most common find, as their comparatively high weight and drop-shaped bodies sink quickly. Lighter built creatures, like a dragonfly, tend to float on the surface of the lake and decompose or end up as someone’s dinner. Sometimes the attentive student spots a tiny, metallic blue or green shimmer among the sediment. This is the moment when you know you have encountered one of the most spectacular insect finds. The gemstone-like jewel beetles (family Buprestidae) are – even as fossils – colourful and shiny. The jewel beetles’ colouration is not caused by pigments, but by the microstructure of their wings, which can be preserved much easier than pigments, so they still look as fabulous as they did 44 million years ago.
Plant and invertebrate finds are usually very small, so a hand lens is a crucial tool, just as the dull knife we used to split the soft, wet sediment (Fig 4). If a slab of sediment contained a small fossil, we removed as much of the surrounding material as possible without damaging the find, then placed it in a plastic container and submerged the fossil in glycerin to keep it moist (Fig 5). Since the water content of the sediment is very high, a sudden change of conditions such as drying out of the fossil would lead to irreversible damages.
Vertebrate fossils tend to be larger, but are much rarer. Just as today there are fewer vertebrates than ants, flies or beetles around in most ecosystems. Often, you only find a single bone or a fish scale. Every once in a while, the steep crater walls caused sediment to slide into the lake in one big gush, called a turbiditic current, destroying everything in its path on the bottom of the lake. These turbidites often contain fragmented skeletons and single bones, but are also useful features as they can function as marker horizons and thus help with the stratigraphical indexing of fossils. Finds are labelled for example ‘15↓④’ meaning this fossil was collected from a layer 15 cm below marker horizon number 4. This is important because it later on allows to understand the fossil assemblages in the correct sequence. The exact location of all vertebrate finds is also documented using a theodolite, a device that measures the angles between points. If you place the theodolite on a fixed position, then measure the angles from there to reference points and then to a special marker held on top of the fossil (Fig 6), the exact location can be calculated and represented 3-dimensionally later. If you do find a complete skeleton unaffected by turbiditic currents, they are often in pristine condition, as due to the anoxic, inhospitable environment at the bottom of the paleo-lake no bioturbation or scavenging has affected them.
The Eckfelder Maar is known for its well preserved horses and horse relatives (family Equidae). Several complete skeletons of different early equid species have been discovered there. The most spectacular specimen discovered there so far was a pregnant mare and both the fetus and parts of the placenta have been preserved and studied extensively. These early horse relatives had more toes than modern horses and were only the size of a dog.
The largest complete vertebrate fossil found during our internship was a large basal ray-finned fish (family Amiidae). Since it was too large to be handled in the field, it was first coated in glue, covered in plastic foil and plaster, then lifted together with a large chunk of the surrounding rock to be carefully excavated later on in the lab (Fig 7).
One of the smaller, but more exciting finds was a complete skeleton of a young bird (Fig 8). Fossil birds are rare in these kinds of deposits, since birds don’t tend to slip and fall into a lake, like it could happen to a clumsy horse on a slippery lakeshore. The specimen appeared to be a nestling, since the preserved feathers looked very fluffy. We hypothesized that it must have fallen out of its nest directly into the lake.
It’s fossils like these, preserved under exceptional circumstances, that allow us to reconstruct and understand ecosystems that are long gone. The Eckfelder Maar is a little slice of Eocene, frozen in time, waiting to be uncovered.