The earliest deep-sea vertebrates

revealed by unusual fossils

Scientists discover missing evolution puzzle piece in 130-million-year-old rocks

Genova, September 5, 2023 – Extremely rare fossils reveal the earliest evidence of deep-sea fishes, pushing back the invasion of the abyssal plain by 80 million years. This revolutionary conclusion has been presented in a new study conducted by an international team of scientists led by palaeontologist Andrea Baucon. The study has been published in the September issue of the Proceedings of the National Academy of Sciences (https://doi.org/10.1073/pnas.2306164120), one of the world’s most-cited peer-reviewed multidisciplinary scientific journals.

“When I first found the fossils, I can’t believe what I was seeing,” says Baucon, who discovered the fish fossils in the NW Apennines, close to Piacenza, Reggio Emilia, and Livorno (Italy). The reason for the astonishment is the remote ageof the fossils, which predate any other evidence of deep-sea fish by million years. The newly discovered fossils date back to the Early Cretaceous (130 million years ago). “The new fossils show the activity of fishes on a dinosaur-age seafloor that was thousands of meters deep,” Baucon says.

The newly discovered fossils are rare and unusual. They comprise bowl-shaped excavations produced by ancient feeding fishes, as well as the sinuous trail formed by the tail of a swimming fish, incising the muddy seafloor. These trace fossils do not comprise fish bones, but they record ancient behaviour. As such, the Apennine fossils mark a critical point in space and time. It is the point at which fishes moved out of the continental shelf and colonized a new harsh environment, located far away from their original habitat. “The studied trace fossils are akin to the astronauts’ footprints on the Moon,” says Baucon.

Thousands of meters below the surface of the Tethys Ocean, the earliest deep-sea fishes faced extreme environmental conditions relative to their shallow water origins: Total darkness, near-freezing temperatures, and colossal pressures challenged the survival of these pioneers of the abyss. “As if that wasn’t enough, turbid currents swept the vast muddy plains patrolled by ancient fishes,” says Luca Pandolfi. Such extreme conditions required adaptations for deep-sea life that are evolutionary innovations as significant as those that allowed the colonization of the land and the air (e.g., wings and limbs).

The newly discovered fossils represent not just the earliest deep-sea fishes but the earliest deep-sea vertebrates. The evolution of vertebrates – backboned animals – has been punctuated by habitat transitions from shallow marine origins to terrestrial, aerial, and deep-sea environments. Invasion of the deep sea is the least-understood habitat transition because of the low fossilization potential associated with the deep sea. “The new fossils shed light on an otherwise obscure chapter of the history of life on Earth,” comments Carlos Neto de Carvalho.

The Apennine fossils force scientists to reconsider which factors might have triggered the vertebrate colonization of the deep sea. Baucon and colleagues propose that the trigger was the unprecedented input of organic matter that occurred between the Late Jurassic and the Early Cretaceous. “The availability of food in the deep seas favoured bottom-dwelling worms, which, in turn, attracted fishes that used specific behaviours to expose them,” explains Annalisa Ferretti. “Behaviour: that’s what the new fossils are all about,” says Girolamo Lo Russo.

In the new study, researchers used a peculiar approach to understand fossil behaviour. “We turned to present-day seas for understanding the past,” says Fernando Muñiz. Baucon and colleagues studied the behaviour of modern fishes in their habitats. “The coasts of Spain and Italy have provided the key to interpreting the fossil structures,” reveals Zain Belaústegui, supported by the words of Chiara Fioroni: “Observing modern fishes has been illuminating”. Scientists explored the depths of the Pacific Ocean to study chimaeras, also known as ghost sharks, in their living environment. “At 1500 m of depth, we observed a chimaera plunging its mouth into the sediment. We now know it was a glimpse into the past!” says Thomas Linley.

The new fossils are identical to structures produced by modern fishes that feed by either scratching the seafloor or exposing their bottom-dwelling prey by suction. This reminds of Neoteleostei, the group of vertebrates that includes modern jellynose fishes and lizardfishes. “A key feature of Neoteleostei is the highly developed suction feeding apparatus, therefore, the Apennine fossils may represent a very early stage of diversification of Neoteleostei into the deep sea,” explains Imants Priede. “The present is key to the past… and vice-versa!” says Mário Cachão.

The newly discovered fossils may represent the first major step in the origins of modern deep-sea vertebrate biodiversity. “Fishes such as the bathysaur and the tripod spiderfish are an important component of modern deep-sea ecosystems,” reveals Armando Piccinini. The roots of modern deep-sea ecosystems are in the Apennine fossils, witnessing a key habitat transition in the history of the oceans. “Our fossil discoveries reassess the mode and tempo of the vertebrate colonization of the deep sea. The newly discovered fossils contain fundamental clues about the very beginnings of vertebrate evolution in the deep sea, having profound implications for both Earth and Life Sciences”, summarizes Andrea Baucon.

Research paper

The earliest evidence of deep-sea vertebrates. Andrea Baucon, Annalisa Ferretti, Chiara Fioroni, Luca Pandolfi, Enrico Serpagli, Armando Piccinini, Carlos Neto de Carvalho, Mário Cachão, Thomas Linley, Fernando Muñiz, Zain Belaústegui, Alan Jamieson, Girolamo Lo Russo, Filippo Guerrini, Sara Ferrando, Imants Priede, Proceedings of the National Academy of Sciences, Volume: 120, Issue: 37, DOI: 10.1073/pnas.2306164120

Images and videos

Journalists may use the images and videos below with appropriate credit. The full media package can be downloaded here (download size: 170 Mb). Scroll down the page for media captions.

Talks covering the new fossils

Two science talks covering the newly discovered fossils will be offered at the Natural History Museum of Piacenza (via Scalabrini 107, Piacenza, Italy). These talks will be presented by Andrea Baucon, lead author of the study:

• Press conference: Friday 22 September 2022, 11.00. The talk is aimed at journalists.
• Public talk: Friday 22 September 2022, 18.00. The talk is aimed at the general public and journalists

Images

Journalists may use the following image content with appropriate credit. Each image can be downloaded by right-clicking on it. Alternatively, you can download the full media package here.

◀ Pictured is a rock slab with the earliest evidence for deep-sea vertebrates. The fossil evidence consists of pits and trails produced by feeding fishes during Early Cretaceous times. The dish-like structures are approximately 4 cm wide.

Image credit: Andrea Baucon.

▶ Height map of the rock slab above, preserving the earliest evidence for deep-sea vertebrates. Color coding is related to height, with the warmer colours at the higher elevations.

Image credit: Girolamo Lo Russo.

◀ Chimaera swimming over the sediment at the Kermadec Trench in the Pacific Ocean (depth: 1544 m). In the new study, scientists studied the behaviour of modern deep-sea fishes to understand the formation mechanism of Cretaceous trace fossils. Results reveal the earliest evidence for deep-sea vertebrates.

Image credit: Thomas Linley, Alan Jamieson.

▶ Rock slab with two feeding pits produced by Cretaceous fishes. They represent the fossil evidence for the earliest deep-sea vertebrates. The dish-like trace fossils are approximately 3 cm wide.

Image credit: Girolamo Lo Russo.

◀ Height map of the rock slab above, showing two fossil feeding pits. Using photogrammetry, the scientists delivered photo-textured height maps for the specimen.

Image credit: Girolamo Lo Russo.

▶ Red mullet producing a feeding pit in the shallow seafloor of the Ligurian Sea. The new study report identical structures from the Cretaceous deposits of the N Apennines, Italy. Results indicate that the fossil feeding pits are the earliest evidence for deep-sea vertebrates.

Image credit: Andrea Baucon.

◀ Fossil swimming trail. The trail formed by the tail of a swimming fish incising the muddy seafloor. The trail was produced by dragging the tail, which was large enough to reach the seafloor.

Image credit: Andrea Baucon.

▶ Folded layers from the Quercianella palaeontological site in Italy.
Scientists explored the Quercianella site and discovered the earliest evidence for deep-sea vertebrates. The fossil evidence consists of feeding and swimming traces produced by deep-sea fishes during Cretaceous times.

Image credit: Andrea Baucon.

◀ The Quercianella palaeontological site. In the foreground, Andrea Baucon is studying trace fossils produced by Cretaceous fishes.

Image credit: Andrea Baucon.

▶ Badlands of Vezzano sul Crostolo. Here, Lower Cretaceous rocks preserve the earliest known evidence for deep-sea vertebrates. The new study analyze this evidence, consisting of trace fossils produced by fishes.

Image credit: Armando Piccinini.

◀ The Quaraglio landslide near Piacenza. Here, scientists discovered the earliest evidence of deep-sea vertebrates in Cretaceous rocks.

Image credit: Girolamo Lo Russo.

▶ Andrea Baucon looking for fish trace fossils in the Apennine.

Image credit: Armando Piccinini.

Videos

Journalists may use the following movie content with appropriate credit. Videos can be downloaded by clicking on the three-dots icon on the right corner of each video. Alternatively, you can download the full media package here.

▲ The video shows a chimaera swimming over the sediment at the Kermadec Trench (depth: 1544 m) and then plunging its mouth into the sediment to feed. In the new study, scientists studied the behaviour of modern fishes to understand the behaviours associated with the earliest fossils of deep-sea vertebrates. Movie credit: Thomas Linley, Alan Jamieson. Download size: 7 Mb.

▲ Using photogrammetry, scientists delivered a photo-textured height map of fossil feeding pits and trails. These represent the earliest evidence for deep-sea vertebrates. Movie credit: Girolamo Lo Russo. Download size: 32 Mb.

▲ The video shows fossil feeding pits produced by the earliest deep-sea fishes. Using photogrammetry, scientists delivered a photo-textured height map for the specimen. Movie credit: Girolamo Lo Russo. Download size: 80 Mb.