Middle Triassic ichthyosaurs are rare and mostly small. New Besanosaurus specimens described in the peer-reviewed journal PeerJ – the Journal of Life and Environmental Sciences – by Italian, Swiss, Dutch and Polish paleontologists provide new information on the anatomy of this ancient fish-like reptile, revealing its diet and exceptionally large adult size: up to 8 meters, a real record among all marine predators of this geological period. In fact, Besanosaurus is the oldest large marine diapside – the group to which lizards, snakes, crocodiles and their extinct cousins belong – with a long, narrow muzzle.
Besanosaurus leptorhynchus was originally discovered near Besano (Italy) three decades ago, during systematic excavations carried out by the Museum of Natural History in Milan. the PeerJ The article re-examines his skull bones in detail and attributes five additional fossils to this species: two previously undescribed fossil specimens, and two previously referred to a different species (Mikadocephalus gracilirostris), which is found to be invalid due to the absence of significant anatomical differences with Besanosaurus.
The six specimens vary mainly in size and probably represent different stages of growth. According to this re-analysis, Besanosaurus is the oldest and most basal representative of a group of ichthyosaurs known as shastasaurids.
All the specimens, preserved in the museums of Milan, Zurich and Tübingen were collected in the last century in the black bituminous shales of the region of Monte San Giorgio (Italy / Switzerland, UNESCO World Heritage), which were deposited there about 240 million years ago at the oxygen-depleted bottom of a particular sea basin. The locality is famous all over the world for its rich fossil fauna which, besides ichthyosaurs, includes many other marine and semi-aquatic reptiles, a variety of fish and hard-shelled invertebrates.
“The extremely long and slender platform suggests that Besanosaurus feeding mainly on small and elusive prey, feeding lower in the food chain than an apex predator: a new ecological specialization never reported before this Triassic period in a large diapid reptile. This could have triggered an increase in body size and reduced competition between the various ichthyosaurs that coexisted in this part of the Tethys Ocean “, explains Gabriele Bindellini of the Department of Earth Sciences at the University of Milan, first author of this study.
“Studying these fossils was a real challenge. Besanosaurus the specimens were extremely compressed by deep time and rock pressure, so we used advanced medical computed tomography, photogrammetry, and comparisons with other ichthyosaurs to reveal their hidden anatomy and reconstruct their 3D skulls, bones by bone ”, remarks Cristiano Dal Sasso of the Natural History Museum of Milan, main author of the PeerJ article which, in 1996, originally described and named Besanosaurus.
Interestingly, Italian researchers started to re-study Milan Besanosaurus around the same time, an international team including Andrzej Wolniewicz (IP PAS, Warsaw), Feiko Miedema (SMNS, Stuttgart) and Torsten Scheyer (UZH, Zurich) started working on the Swiss specimens. “Rather than doing parallel studies, we pooled our data and efforts and pulled together to improve our understanding of these fascinating extinct animals,” adds Torsten Scheyer.
Image 1: The first and most complete fossil of Besanosaurus leptorhynchus is a pregnant female (containing an embryo) on display at the Natural History Museum in Milan, with a fiberglass reconstruction of her living appearance. 16,500 hours of manual preparation were required to expose the entire skeleton, embedded in a 330×270 cm layer of black schist. Credit: Gabriele Bindellini, © Museo di Storia Naturale di Milano.
Image 2: The skull of the type specimen of Besanosaurus leptorhynchus is characterized by extreme longevity (i.e., a slender, elongated muzzle) and equipped with tiny sharp teeth, perfect for catching small fish and extinct cousins of squid with quick movements of the head and jaws. Credits: Gabriele Bindellini and Marco Auditore, © Museo di Storia Naturale di Milano.
Image 3: View of Lake Lugano from Monte San Giorgio, between Lombardy (Italy, left) and the canton of Ticino (Switzerland, right). Ichthyosaurs are among the most abundant fossils of this UNESCO World Heritage Site, which protects a unique paleo-biodiversity dating from the Middle Triassic (240 million years ago). Credit: Gabriele Bindellini.
Image 4: At the Institute and Museum of Palaeontology in Zurich, Gabriele Bindellini measures the orbital diameter of a subadult Besanosaurus leptorhynchus. This fossil preserves a certain three-dimensional anatomy, which has helped to redefine the “identity card” of the species. Credit: Cristiano Dal Sasso.
Image 5: At the Institute and Museum of Palaeontology in Zurich, Cristiano Dal Sasso opens the display case of a remarkably large ichthyosaur specimen which, if complete, would have been 8 meters long. Although disarticulated, the skull indicates that it is another Besanosaurus… Credit: Gabriele Bindellini.
Image 6: Reconstruction of Besanosaurus. Despite their fishy appearance, ichthyosaurs were reptiles. They were perfectly adapted to marine life thanks to large modifications of the limbs of their terrestrial ancestors in paddles. Dorsal fins and crescent tails also developed into later and more advanced forms. Pencil by Fabio Fogliazza digitally modified by Gabriele Bindellini, © Museo di Storia Naturale di Milano.
Image 7. The “Sasso Caldo” site, near Besano (Varese, Italy) in 1995. This outcrop is characterized by a regular alternation of thin black oil shales and thicker gray-whitish dolomitized layers. Fossils are found in both, although in shales the specimens are very distorted. Photo by Giorgio Teruzzi, © Museo di Storia Naturale di Milano.
Image 8. Spring 1993, Besano (Varese, Italy): a snapshot of the recovery of the most complete specimen of Besanosaurus leptorhynchus. To avoid damaging the fossil, the bones of the large ichthyosaur were detected in cross-section, along the cuts of the slabs, without splitting the layer that firmly embedded them. The scissors in the photo were only used to detach the fossil layer from the underlying layer. Photo by Cristiano Dal Sasso, © Museo di Storia Naturale di Milano.
Image 9. At the Fondazione Ospedale Maggiore di Milano, on the CT screen appears the Besanosaurus skull, with the orbital cavity (bottom center) surrounded by the bones of the temporal region. Photo by Cristiano Dal Sasso.
Image 10. At the Museum of Natural History in Milan, the preparers of the Laboratory of Paleontology remove the layer of silicon which, poured over the reconstructed skeleton of Besanosaurus leptorhynchus, makes it possible to produce identical replicas of the original fossil. © Photo by Guido Alberto Rossi and Museo di Storia Naturale di Milano.
Image 11. In the thoracic region of Milan Besanosaurus, where the stomach used to be, leftover food is preserved, including that tiny hook from the arm of a squid parent. Cephalopods were the preferred prey of several species of ichthyosaurs. Photo by Gabriele Bindellini, © Museo di Storia Naturale di Milano.
Image 12. The site “Sasso Caldo” of Besano (Varese, Italy) as it appears today. The outcrop is extremely rich in fossils in almost all rock layers. In this photo, the paleontologist Cristiano Dal Sasso indicates with his right hand the bituminous layer n. 65, where the old Besanosaurus was integrated, and with his left hand the layer n. 63, which provided two small ichthyosaurs of the genus Mixosaurus. Photo by Gabriele Bindellini.
Image 13. At the Fondazione Ospedale Maggiore di Milano, paleontologist Gabriele Bindellini (left) and graduate student in medical radiology, Alessandro Crasti (right), scan the skull of Besanosaurus leptorhynchus before moving it into the CT ring. Photo by Cristiano Dal Sasso.
Image 14. At the Institute and Museum of Palaeontology of the University of Zurich, palaeontologists Cristiano Dal Sasso (left) and Torsten Scheyer (right) discuss in front of the largest Besanosaurus – recently identified as such – unearthed in the first half of the last century in the Swiss mines of Monte San Giorgio. If it were complete, this specimen would have approached 8 meters in length. Photo by Gabriele Bindellini.
Dr Cristiano Dal Sasso
Sezione di Paleontologia dei Vertebrati
Museo di Storia Naturale di Milano
Corso Venezia 55 – Milan 20121
Phone. 02 88463301
Email: [email protected]
Dr Gabriele Bindellini
Dipartimento di Scienze della Terra “Ardito Desio”
Università degli Studi di Milano
Phone. 349 6220170
Email: [email protected]
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