A remarkable trace fossil from Germany has provided the oldest known evidence of a cloaca in the mammalian lineage. The discovery helps palaeontologists better understand the evolution of early terrestrial vertebrates and fills an important gap in the fossil record. The fossil comes from the famous Bromacker fossil locality in Thuringia (Germany). Among footprints and other trace fossils the scientists identified a tail impression. The anterior portion of this tail impression preserves evidence of the earliest synapsid cloaca.

The international team of researchers, led by scientists from the Museum für Naturkunde Berlin, published their findings in the journal “iScience”. The trace fossils are approximately 294 million years old (Early Permian). This represents the first occurrence of a cloaca in the fossil record of stem mammals.

The trace fossil specimen MNG 13490 with the preserved underside of the tail impression. Note scale bar equals 10 cm. Picture credit: Marchetti et al.

Picture credit: Marchetti et al

A Rare Glimpse of Soft Tissue Preservation

Fossils usually preserve bones and teeth. Soft tissues, however, almost always decay before fossilisation can occur. Consequently, direct evidence of anatomical features such as the cloaca is exceptionally rare. The newly described specimen includes a beautifully preserved impression of the tail together with fossil footprints. Near the base of the tail, the researchers identified two rows of raised scales separated by a narrow vertical slit. They interpret these features as the lips and opening of a cloaca.

This fossil provides the first direct evidence of an earliest synapsid cloaca, offering scientists an unprecedented look at the anatomy of an animal closely related to the ancestors of mammals.

The caseid synapsid tail impression. Note scale bar equals 5 cm. Picture credit: Marchetti et al.

Picture credit: Marchetti et al

What is a Cloaca?

A cloaca is a single external opening used by the digestive, urinary and reproductive systems. Today, amphibians, reptiles and birds possess a cloaca. Among mammals, however, only monotremes such as the platypus and echidnas have retained this feature. Most extant mammals (placentals and marsupials) have separate openings for these bodily functions. Until now, scientists had little direct fossil evidence showing how this important anatomical structure appeared in the mammalian lineage.

An Early Relative of Mammals

The fossil traces have been linked to a caseid synapsid. Caseids were large-bodied, herbivorous members of the Synapsida, the evolutionary group that eventually gave rise to the Mammalia. Researchers matched the fossil impression with the distinctive Dimetropus footprints found at the Bromacker locality. Comparisons with skeletal fossils collected from the same site strengthened this interpretation.

Although caseids looked very different from mammals today, they belonged to the same broad evolutionary lineage. Therefore, this discovery provides valuable information about the early stages of mammalian evolution.

The cloacal area in the anterior part of the tail impression. Note scale bar equals 5 cm. Picture credit: Marchetti et al.

Picture credit: Marchetti et al

New study highlights the growth rates of diminutive Dimetrodon species from the Bromacker locality: Bromacker Dimetrodon Growth Study.

Why Did the Cloaca Evolve?

The study also explores why the cloaca changed during vertebrate evolution. Although the answer remains uncertain, the researchers suggest several possible explanations.

For example, changes in reproductive anatomy may have played a role. Likewise, increasingly dry environments could have influenced the evolution of different body structures. The scientists also propose that evaporative cooling through the cloacal opening may have helped animals regulate their body temperature. Furthermore, changes in body shape, including shorter tails and modified limbs, may have contributed to these evolutionary developments.

In squamates and birds, the cloaca tends to have a horizontal orientation. Cloacal evaporation for thermoregulation purposes has been observed in squamates and birds. In contrast, the Crocodylomorpha and the Caudata (salamanders and their extinct relatives), have a vertical cloaca. These animals are associated with aquatic environments. Intriguingly, the fossilised tail impression is clearly impressed in a water-saturated sediment.

The fossil specimen shows swimming traces associated with Dimetropus footprints. Therefore, it is possible that a transition from a vertical to a horizontal cloaca occurred as an adaptation to drier environments. The horizontal orientation of the cloacal orifice could favour more efficient cloacal evaporation to help maintain body temperature. This would be in agreement with the vent orientation change in stem reptiles between the Carboniferous and the Permian, in a time of global warming and increasing aridity and seasonality. Nevertheless, the causes of this correlation should be further investigated in modern taxa.

The Significance of the Bromacker Fossil Site

The Bromacker fossil locality is recognised as one of the world’s most important Late Palaeozoic fossil sites. It has produced an exceptional record of terrestrial animals and plants that lived during the Early Permian. This latest discovery also demonstrates the value of revisiting historic museum collections. Modern imaging techniques and careful re-examination can reveal important anatomical details that earlier researchers could not detect.

A false-colour depth map of the cloacal area taken from the photogrammetric 3D model. Picture credit: Marchetti et al.

Picture credit: Marchetti et al

Understanding the Evolution of Mammals

The discovery of the earliest synapsid cloaca provides much more than an anatomical curiosity. It offers scientists a rare opportunity to investigate the evolution of reproductive and excretory systems in one of the most important vertebrate lineages. As new discoveries emerge from the Bromacker site and other fossil localities, researchers will continue to refine our understanding of how the ancestors of mammals adapted to life on land hundreds of millions of years ago.

Commenting on the importance of this research, Mike from Everything Dinosaur stated:

“Soft tissue fossils are incredibly rare, so discoveries like this earliest synapsid cloaca impression are exceptionally important. The specimen shows that trace fossils can preserve anatomical details that skeletal remains alone cannot reveal. It also reminds us that remarkable scientific discoveries can still be made by re-examining specimens already held in museum collections.”

This remarkable discovery demonstrates how exceptional fossil preservation continues to reveal new chapters in the evolutionary history of the Mammalia.

Everything Dinosaur acknowledges the assistance of a media release from the Museum für Naturkunde Berlin in the compilation of this article.

The scientific paper: “The evolutionary significance of the earliest cloacal opening in Synapsida” by Lorenzo Marchetti, Antoine Logghe, Arnaud Rebillard, Mark J. MacDougall and Jörg Fröbisch published in IScience.

The multi-award-winning Everything Dinosaur website: Models of Prehistoric Life.

A team of international scientists has uncovered the remarkable mechanism behind seal underwater hearing. A new study, led by researchers from the London Natural History Museum reveals how extant seals hear equally well in air and underwater. The findings help to explain a long-standing evolutionary mystery and could support future conservation efforts.

The research, published as an open-access paper in the “Royal Society Proceedings B”, examined more than two hundred living and fossil seal specimens. By combining advanced CT scanning with evolutionary analysis, the researchers identified the specialised tissue that makes this extraordinary ability possible.

Blood-Filled Tissue Helps Seals Hear Underwater

Unlike humans and most other land mammals, seals can hear efficiently in two very different environments. Normally, mammalian hearing relies on an air-filled middle ear. However, this arrangement performs poorly underwater because the air pocket behind the eardrum reflects almost all sound. The researchers discovered that seals overcome this problem with specialised cavernous tissue inside the middle ear. During a dive, this tissue fills with blood. As blood has a density similar to seawater, sound passes more efficiently through the ear and reaches the cochlea. As a result, seals continue to hear clearly beneath the waves.

Lead and corresponding author Dr James Rule (London Natural History Museum/Monash University) explained that scientists have puzzled over this adaptation for decades. The new research finally identifies the anatomical feature responsible for amphibious hearing.

A 3D rendering of the skull of a Southern Elephant seal showing the underwater hearing adaptations. Picture credit: James Rule.

Picture credit: James Rule

Fossils Reveal the Evolution of Seal Underwater Hearing

The team’s analysis included both living species and fossil relatives of modern seals. Consequently, the researchers could trace when this remarkable adaptation first evolved. Evidence suggests that the ancestors of seals initially heard well only in air. Ancient species such as Potamotherium (Oligocene to Miocene) and the stem pinniped Puijila (late Oligocene to early Miocene) probably lacked effective underwater hearing. However, later marine relatives, including Enaliarctos, appear to have evolved the first form of amphibious hearing approximately 26.7 million years ago.

The earliest relatives of pinnipeds, such as the 23-million-year-old Puijila darwini, could only hear in-air. Like most mammals, their ears did not function properly underwater. Picture credit: Jaime Bran.

Picture credit: Jaime Bran

From this point onwards, different seal groups continued to refine their hearing. For example, true seals (phocids) developed enhanced underwater hearing. In contrast, the earliest eared seals retained adaptations that favoured hearing in air.

The study indicates that these improvements evolved independently after the earliest marine pinnipeds had already acquired the basic mechanism for hearing in both environments.

Rare Acoustic Abilities

The scientists also suggest that improved underwater hearing opened new evolutionary opportunities. Over millions of years, seals developed an impressive range of acoustic behaviours.

Today, some species produce haunting underwater vocalisations. Others can learn new sounds, keep a rhythmic beat and even imitate aspects of human speech. These unusual abilities may have become possible because their ancestors first evolved an ear capable of functioning effectively both above and below the water’s surface.

Life reconstruction of Enaliarctos mealsi, one of the earliest marine seals from around 23 million years ago, being chased by a cetacean. It was capable of hearing both in-air and underwater (amphibious hearing), thanks to a specialised tissue in their ears (called a cavernous tissue). However, their hearing ranges in both environments were quite limited. Picture credit: Jaime Bran.

Picture credit: Jaime Bran

Museum Collections Unlock New Discoveries

The research depended upon the Natural History Museum’s extensive collections. Scientists examined more than two hundred specimens representing a hundred and nineteen living and extinct species of Carnivora.

By studying fossil skulls alongside modern seals, the researchers reconstructed the evolutionary history of the middle ear. This work demonstrates the continuing scientific value of museum collections assembled over many generations.

Dr Natalie Cooper, Merit Researcher at the London Natural History Museum, London stated:

“The Natural History Museum has some of the best seal collections in the world, and this study really shows its value. This research simply wouldn’t have been possible without these specimens,
which have been collected across hundreds of years from all over the world. It’s important that we know how seals hear because noise pollution in the ocean is growing. As the seas get louder, it’s harder for seals to find mates and communicate with each other putting their survival at risk. By understanding how these animals hear, we can start to put solutions in place.”

To read an article from 2020 but the discovery of a prehistoric pinniped in New Zealand that re-writes seal evolution: Sealing the Fate of Pinniped Evolution.

Understanding Hearing Function Can Help Protect Seals

The discovery has important conservation implications. Oceans are becoming increasingly noisy because of shipping, offshore construction and other human activities. Consequently, underwater noise can interfere with how seals communicate, locate mates and navigate.

Understanding seal underwater hearing provides researchers with valuable information about how these animals perceive their environment. In turn, this knowledge could help shape future conservation measures designed to reduce the impact of underwater noise pollution.

Mike from Everything Dinosaur commented:

“This fascinating study shows how fossils continue to answer important questions about the evolution of living animals. By combining evidence from extinct and modern seals, the researchers have explained one of the most remarkable hearing adaptations found in any mammal.”

Everything Dinosaur acknowledges the assistance of a media release from the London Natural History Museum in the compilation of this article.

The scientific paper: “The origin and evolution of amphibious hearing in pinnipeds” by James P. Rule, Travis Park, Moganavalli Kattan, Camille Grohé, Roxana Taszus, Stephanie M. Palmer, David P. Hocking, Justin W. Adams, Alistair R. Evans, Ian G. Brennan, Tahlia I. Pollock, Daniela Sanfelice, Felix G. Marx, Naoki Kohno, Martin Sabol, Alexander Stoessel, John J. Flynn and Natalie Cooper published in the Proceedings of the Royal Society B.

The award-winning Everything Dinosaur website: Models of Prehistoric Mammals.