Bivalve molluscs are one of the most present animals in the world paleontological register, although they do not unleash the same passions as dinosaurs among amateurs. However, they have always played a key role in the balance of ecosystems and when the great Mesozoic vertebrates inhabited the province, they were already essential for cleaning the waters of the rivers of Teruel. Scientific research has just added to the world record a new genus of bivalves that lived 129 million years ago in the current area between Galve and Miravete de la Sierra.
The most spectacular paleontological discoveries that always resonate in the media and circulate on social networks are those of the dinosaurs, but on the other hand when it comes to other tiny beings such as bivalve molluscs, so important because they are essential for ecosystems , often go unnoticed.
One of the few specialists of these animals in the Mesozoic is Graciela Delvene, from the Spanish Geological and Mining Institute (IGME). Her passion for these beings, which led her to have an aquarium as a student to see how they lived, she transmits it to those who approach her to find out how they were in the past; and manages not only to do this but to arouse interest in certain animals that we really like to eat a paella without being aware of their importance afterwards.
Bivalves are that, mussels, clams, clams and others that live in rivers and seas; certain molluscs with shells which, in addition to serving as food for others in the food chain, use their filtering to purify the waters. If now shellfish enthusiasts devour them, 129 million years ago it was the dinosaurs who sank into them with full teeth and not always successfully because the specimens described in Teruel had a shell 4 mm thick. .
The work that Delvene has just published in the magazine Cretaceous Research, with paleontologists from the Dinópolis Foundation and the Isle of Wight Dinosaur Museum in England, describes a new family of bivalves and names a new genus.
These invertebrates were already known and had been described by a French paleontologist as a new species, although belonging to an already existing genus. Until now the material was classified as Elliptio galvensis and what the new research has done is reclassify it as describing a new genus, Monginaia, although the species name has been retained in honor of Galve, where these fossils were found. .
The scientific work, authored by Martin Munt with Delvene for the Isle of Wight Dinosaur Museum, and Rafael Royo-Torres and Alberto Cobos for the Teruel-Dinópolis Paleontological Joint Foundation, studies in depth the fossils of bivalves excavated in the sixties of the last century by the French paleontologist Albert-Félix de Lapparent in Galve, as well as other extracts during the last decade of this century by the authors of the publication themselves, in addition to a collection collected by a doctoral student from the University of Zaragoza in Miravete de la Sierra.
Delvene studied them all, including the Lapparent collections which are deposited in France and which he reviewed, and the result was the description of the new genus and the new family of bivalves, which he called Monginaiidae in honor of the French expert who studied fossils in their origins, Denis Mongin.
“We reviewed this genus, typical of North America, compared it to known genera and observed differences that allowed us to create a new genus, Monginaia, to include galvensis. Its characteristics have also made it possible to define a new family in which to include this genus”, explains Delvene, who assures that one of the important advances that has been made is that “this study highlights the differences between the European and American faunas”.
Bivalves like those of the Monginaiidae family are also known as naiads, which is how the protective river nymphs were called in Greek mythology. The scientist explains that the genus Ask the “it filtered water and lived in large numbers in colonies, so it can be said that it oxygenated and cleaned the waters it inhabited”.
“Bivalves are in general great water filters, they take water and filter it through their gills, taking advantage of a large amount of dissolved substances in the water for their food. In this way they oxygenate and clean the environments they inhabit,” explains the paleontologist.
In addition to being natural water purifiers, they were part of the food chain and could serve as food for vertebrates such as crocodiles or dinosaurs. In fact, these bite marks were found on the shells, which could be associated with certain spinosaur-like carnivorous dinosaurs, which are those whose heads are elongated and resemble that of crocodiles.
“We will continue to work in this direction because we have specimens from other sites where we have seen similar marks”, explains the specialist, who recalls that a few years ago they published some examples of bivalves from fresh water with bite marks “that we have attributed to lower Cretaceous crocodiles from the Cameros Basin”.
Paleontologists arrive at this conclusion by looking at the interior and exterior surfaces of the shells to see “if there are any marks, their shape, their size, their depth, their symmetry, if they pierce the shells”.
To determine whether any of these signs could correspond to the teeth of a dinosaur or a crocodile, they look at morphological data, which “allows the markings to be assigned to certain types of teeth”, which they are tapered, beveled or of another style.
“From there we can propose a hypothesis on the possible authors who nibbled and possibly fed bivalves”, indicates the scientist, without being able to specify for the moment if it is a carnivorous dinosaur which left traces. tracks. “Today, freshwater bivalves are eaten by birds, crocodiles and fish among other predators,” he says.
At school, Graciela Delvene had a teacher who was a geologist and she admits that this marked her so much that it was with him that her vocation was born in two aspects, geology and biology. “The bivalves seemed to me an interesting group because I was able to learn how they lived in a simple and direct way; when I was a student, I had an aquarium of marine bivalves where I could see how they burrowed or ate,” he says. He found that it was a group in which there were few specialists and he says that he “launched” for that.
Today she is one of the great specialists that exist in Mesozoic bivalves and that is how she became aware of those of Miravete de la Sierra that have been attributed to the same genus, and of the new discoveries that have been made on the site in the past. La Maca de Galve, where Lapparent had already extracted similar fossils more than half a century ago. In the first case, it was a student from the University of Zaragoza who contacted her, and in the second the paleontologist Rafael Royo-Torres, a classmate of Delvene.
Recently, the collaboration with the team of paleontologists from Teruel was formalized through a research project of the Ministry of Science and Innovation directed by the Dinópolis Foundation.
The importance of studying these fossils is great even if they are not the gigantic bones of the dinosaurs, since they are one more piece of the puzzle for discovering the history of life on the planet. Thanks to bivalves like those of Galve and Miravete, it is possible to know what the paleoenvironments looked like millions of years ago.
“Benthic invertebrates, that is, those that live at the bottom of aquatic ecosystems, provide us with a lot of information about the environment in which they live. Thus, invertebrate fossils tell us about the salinity, the temperature, the oxygenation of the water, the nature of the substrate in which they lived”, comments Delvene, who considers that the ecosystems of the past are “like puzzles, and just study every piece of it,” from invertebrates, vertebrates, plants, and other types of fossils, “you can achieve a complete paleoenvironmental reconstruction that’s as close to reality as possible.”
In Spain, Delvene worked on all possible aspects of these freshwater invertebrates, from their taxonomy to paleecology or issues as specific as the microstructure of their shells.
This is how he manages to establish new theories about these fascinating animals of the past, which are also very present today both in rivers and in the seas. Those described at Galve from the Lower Cretaceous had an average size of 5 centimeters at most, lived buried and were filter feeders.
“They fed by filtering water through their gills, taking advantage of food substances and expelling waste,” explains the paleontologist. And about their thick calcium carbonate shells of up to 4 mm, he explains that this is from what is inferred that they lived in river systems, in addition to revealing the sedimentological characteristics of the Camarillas Formation where they are found.
A very important feature that helped define the new genus, and that makes them “unique”, is that with the valves closed, they have two openings, anterior and posterior. “These characteristics have certain paleoecological implications related to their feet and their ability to burrow into the depths,” explains the specialist.
So they spread
The expert says that freshwater bivalves have a highly specialized reproductive cycle. “They need a vertebrate host, usually a fish,” says Delvene, because the bivalve larvae, called glochids, complete their metamorphosis in their gills or fins, giving rise to “free-living juveniles that burrow to the bottom. lakes or rivers”. .
For this reason, the dispersal of bivalves in ecosystems depends on the distance traveled by the hosts. So far, the oldest larvae have been recorded in the Pleistocene and Holocene, since
2.58 million years to the present, specifies the expert, even if she recognizes that the specialists of these invertebrates “assume that the reproduction of freshwater bivalves in the past was similar and that they needed their host in order to be able to disperse”. In any case, he declares that he does not lose hope of being able to find some in the Cretaceous as well.
Currently, this researcher has several works in progress, both on the Spanish and English continental Cretaceous, as well as on the Asturian Jurassic, as well as research on the fossils of Teruel, in particular the bivalves of the mine of Santa María de Ariño . “It’s a spectacular material that will give rise to new taxa,” he says.