La mer avant les poissons / البحر قبل الأسماك
قبل أكثر من 500 مليون سنة، خلال العصر الجيولوجي الكامبري، حدثت طفرة تطورية سريعة تُسمى الانفجار الكامبري، والتي أدت إلى ظهور أول الحيوانات المعروفة في المحيطات. كانت هذه الحيوانات أقدم بمرتين من الديناصورات المعروفة وأقدم بـ 50 مليون سنة من أول الأسماك العظمية! على الرغم من أن الأسماك العظمية الحديثة (الأوستيختيين) اليوم كثيرة ومتنوعة في المحيطات، إلا أنها لم تكن من أول الحيوانات التي ظهرت. قبلها، كانت المحيطات مأهولة فقط بالحيوانات اللافقارية التي كان بعض منها يمتلك أصدافًا أو دروعًا بدلًا من العظام، مثل المحار أو السرطانات، بينما كانت حيوانات أخرى مثل الديدان تفتقر إلى أي جزء صلب. كانت أول الحيوانات الكامبرية غريبة المظهر وصعبة الفهم، لكنها في الواقع تمثل أسلاف العديد من مجموعات الحيوانات الحديثة التي لا تزال حية اليوم.
بعد الانفجار الكامبري، حدثت دفعة تطورية كبيرة ثانية خلال العصر الجيولوجي الأردوفيشي، قبل 480 مليون سنة، والتي تُسمى التنوع البيولوجي الكبير في الأردوفيشي. في هذه الفترة، بدأت الحيوانات التي تعيش في المحيطات تأخذ أشكالًا تشبه تلك التي نراها اليوم، بينما بدأت الكائنات ذات الخصائص الغريبة بالانقراض.
تأتي الأحافير المعروضة من فترة تقع بين هذين الطورين التطوريين وتشهد على انتقال بيئي رائع. تم اختيارها من مجموعة تضم أكثر من 3000 عينة، وقد تم الحصول عليها من قبل متحف الجيولوجيا الكانتوني وجامعة لوزان في عام 2017، حيث خضعت منذ ذلك الحين لبحوث علمية هامة. ندعوكم لاستكشاف هذه الأحافير الرائعة والاكتشافات المدهشة التي تقدم لمحة مثيرة عن الحياة في المحيطات قبل ما يقرب من نصف مليار سنة.
التوضيح: تطور التنوع عبر الزمن الجيولوجي
Les fossiles des Fezouata / The Fezouata Biota
In the Anti-Atlas region of Morocco, some 300 kilometers southeast of Marrakech, a series of high cliffs and beautifully exposed stratified rocks give a striking landscape, but the magnificent fossils shown in this exhibit actually come from the low gentle hills of the desert in the foreground of these cliffs. The rocks belong to the Fezouata Shale formation, and they yield fossils of beautiful bright yellow, orange and red colours that are unique witnesses to life 480 million years ago, during the Early Ordovician geological time period. These rocks are made from fine particles of mud that settled out of ocean waters around 100 metres deep, off shore of an ancient continent located near the south pole. Trapped in these muds are animal fossils of exceptional completeness, which sometimes preserve non-shelly parts of their bodies that don’t normally make it into the fossil record – these exceptional fossils are referred to as the Fezouata Biota. The shelly fossils from these rocks have been known since the 1950’s, but the exceptional Fezouata Biota fossils were discovered nearly 50 years later, in the early 2000’s, by Mohamed ‘Ou Said’ Ben Moula, a local collector from Taichoute, near Alnif. The fossils of this exhibit were selected from a collection of over 3’000 specimens excavated by Ben Moula and his sons during 2015 to 2016, and brought to the University of Lausanne in 2017, where they have since the subject of major paleontological investigation. We invite you to explore these beautiful fossil and the remarkable insights they bring to understanding life in the oceans of the south pole half a billion years ago.
Illustrations: (Top left) Paleogeographic reconstruction with the arrangement of tectonic plates during the Ordovician, 480 million years ago. (Top right) Mohammed ‘Ou Said’ Ben Moula next to the sign on the field, announcing Fezouata as one of the top 100 sites of the World Geological Heritage. (Bottom) One of the Fezouata excavation sites.
Un trésor de fossiles aux couleurs flamboyantes / A colourful treasure trove of ancient polar fossils
Deep in Ordovician time, the continents were gathered in the southern hemisphere and the seafloor of Fezouata was located close to the south pole. But unlike today, the south pole was not a frigidly cold climate; it was much warmer, in a globally hotter context during which ocean waters could reach up to 40°C in tropical zones. At Fezouata the water was 50 to 150 metres deep, and the seafloor was periodically smothered by influxes of fine sands kicked up by storms in the shallows. Each fossil bed represents a snapshot of the animals that lived or had recently died at that place on the seafloor. As these carcasses were buried, their bodies flattened and shells, carapaces and some internal organs were replaced by iron-rich minerals during the rock-forming process. Hundreds of millions of years later, the formation of the Atlas Mountains and erosion brought the rocks close to the surface and the circulation of rainwater caused the iron-rich minerals to rust, giving the fossils their bright red, orange and yellow colours.
Illustration: The colourful Fezouata fossils.
Un étrange arthropode qui a tant à nous raconter / A strange arthropod with so much to tell us
One of the most remarkable Fezouata fossils is this marrellid arthropod, which was distantly related to trilobites and crustaceans. The marrellid’s most distinctive feature is a huge head shield relative to the size of its body, on which there were 3 pairs of long spines. What would have been the purpose of this big head shield? Protection from predators, or maybe to achieve stability when crawling on the soft sediment? What we do know is that the fossils of this head shield provide us with information about moulting – the shedding of the exoskeleton that all arthropods must do in order to grow. At Fezouata we find the discarded moults of the marrellid with distinctive breakage lines through the head shield, separating the two anterior pairs of spines from the third posterior pair of spines. Fezouata also preserves both juvenile and adult specimens, meaning we can track the growth of this arthropod from tiny baby size of just a few millimeters in length, up to the adult size ~8 cm in length. To study the smallest specimen, we employed CT-scanning, similar to a medical 3D x-ray scanner, to reveal tiny appendages and spines still embedded in the rock.
Illustration: Reconstruction of an Ordovician marrellid from Canada, which is very similar to the Fezouata marrellid. Artist: Christian McCall, from Moysiuk et al. 2022.
Les rampants des fonds marins / Crawlers of the seafloor
Currently the most numerous and diverse group of animals, arthropods—today including insects, spiders, millipedes, lobsters, and crabs—have always been important components of ecosystems in the past. The Ordovician Fezouata was no exception, with many arthropods walking or crawling on the seafloor. Here, arthropods reigned supreme over this early animal community. Trilobites are abundant and diverse, showing different life stages, both babies and adults, and many different ecologies. Some individuals preserve soft antennae or walking limbs, in additional to their thick, hard exoskeleton. Fezouata has also yielded the oldest known horseshoe crabs. They were extremely numerous, sometimes found with dozens in close proximity on the same surface of rock. Juvenile and adult life stages are recognized at Fezouata overall, with juveniles typically found in rock layers deposited at shallower water depths, while adults predominate at rock intervals deposited at deeper water depths. Individuals migrated from shallow to deep waters as they grew, similar to what modern horseshoe crabs do today – a migratory behaviour that has been conserved for 480 million years.
Illustration: Reconstruction of several Fezouata trilobites, showing a variety of sizes, life stages, and ecologies. Artist: Jiří Svoboda, from Laibl et al. 2023.
Les « géants » de l’Ordovicien / Giants of the Ordovician
In the oceans before there were fish, whales and sharks, what were the largest swimming animals? At Fezouata, a group of bizarre arthropods called radiodonts occupied a similar ecological role to whales and whale sharks – giant planktivores, meaning that they filtered plankton made up of microscopic algae, plants and animals. The broad, rounded bodies of these radiodonts were equipped with a series of lateral swimming flaps that enabled them to swim like manta rays. Their heads were covered by a carapace, and two frontal appendages possessed very fine spine-like structures to trap small plankton, like whale baleen. At Fezouata, these animals could reach an estimated size of two meters and are known from hundreds of fossils. These gentle giants evolved from Cambrian superpredators, which possessed two large compound eyes, frontal appendages adapted to grasping and slicing up large prey, and a circular jaw. These last two structures are found as rare fossils at Fezouata, showing that some predatory radiodonts were still lurking in the shadows during the Ordovician.
Illustration: Reconstruction of giant radiodont of the Ordovician. Artist: Jonathan Pople.
De l’important d’être grand / The importance of being big
As the largest animal in the Fezouata ecosystem, fossils of the giant radiodont Aegirocassis reveal important ecological interactions in unexpected ways. Their large head carapaces are sometimes found covered in small round shells of brachiopods, and a study at the University of Lausanne suggested that these were attached to the radiodont during life. These normally sedentary animals are filter feeders, just like Aegirocassis, and were hitching a ride on the giant swimmer to ensure they always have access to the richest plankton sources. After death, Aegirocassis was a hugely important food source for scavengers living on the seafloor, similar to whale falls in the modern oceans. Their carapaces are sometimes covered in tracks, trails and burrows of animals that likely ingested the organic component of their carapaces. As can be seen here, numerous large Aegirocassis carapaces are sometimes found piled up together covering a large surface area. Exactly how these accumulations of large carapaces formed is a mystery that is still being investigated by researchers.
Illustration: Reconstruction of the Fezouata sea floor, with the carcass of Aegirocassis providing an important food source for many animals in the ecosystem. Artist: Madmeg, from Lefebvre et al. 2016.
Un des premiers ancêtres des scorpions, des araignées et des limules / One of the earliest ancestors of scorpions, spiders, and horseshoe crabs
Modern scorpions, spiders, and horseshoe crabs belong to a group of arthropods known as chelicerates, equipped with pincers used for biting, grasping prey, or injecting venom—the chelicerae. The origin of this group has long puzzled paleontologists, as it has been impossible to identify with certainty any early arthropod species that shared enough similarities with modern chelicerates to be considered their ancestors. Research conducted at the University of Lausanne has identified a Fezouata animal that, for the first time, allows scientists to trace the entire lineage of chelicerates, from the appearance of the earliest arthropods to modern spiders, scorpions, and horseshoe crabs. This animal, represented by hundreds of fossils, has been named Setapedites abundantis. Measuring between 5 and 10 millimeters, it must be studied under a microscope. Like the scientists, use the microscope to observe its beautifully preserved anatomy, including remains of its head legs!
Illustration: Reconstruction of Setapedites abundantis, and photographs of two specimens. Reconstruction artist, Elissa Sorojsrisom ; photographs, Lorenzo Lustri (UNIL).
La diversité des fossiles des Fezouata / Non-arthropod Fezouata fossils
In recent years, Fezouata arthropod fossils have been the main focus of research at the University of Lausanne. Their stories are told in the four large display cases of the exhibition. These ten small display cases highlight other equally remarkable animals uncovered in the Fezouata clays. Some of the fossils presented are distant relatives of some prized seafood, such as gastropods or orthocone cephalopods, related respectively to whelks and squids. Echinoderms are also abundant and highly diverse, with close relatives of starfish and sea lilies. Among the other animals from Fezouata, there is a variety of sponges, worms, a strange pyramid-shaped fossil, and filamentous or tree-like organisms. The latter are floating colonies of tiny animals, called graptolites, which are used to date the rocks of the Fezouata precisely. A particularly important species is Sagenograptus murrayi, which is typical of the Tremadocian geological period, corresponding to the very beginning of the Ordovician, 480 million years ago.
Illustration: Reconstruction of the Fezouata animal community. Artist: Christian McCall.