Besanosaurus
Besanosaurus is an extinct genus of Middle Triassic ichthyosaur from Monte San Giorgio of Italy and Switzerland, containing the single species B. leptorhynchus. Besanosaurus was named by Cristiano Dal Sasso and Giovanni Pinna in 1996, based on the nearly complete flattened skeleton BES SC 999, the holotype specimen. This skeleton is preserved across multiple thin rock slabs spanning when assembled and took thousands of hours to prepare. Additional specimens from Monte San Giorgio that have previously been considered separate genera, including a partial skull named Mikadocephalus and a well-preserved, largely complete skeleton, have been reinterpreted as additional specimens of Besanosaurus. Putative specimens of Besanosaurus have been discovered in the Norwegian archipelago of Svalbard and Germany, although their attribution to this genus remains disputed.
As an ichthyosaur, Besanosaurus had flippers for limbs and a fin on the tail. Besanosaurus is a large ichthyosaur, with the largest known specimen estimated to measure about long. It has a long, slender body with a small head and long tail. The snout of Besanosaurus is long and thin, and contains numerous small pointed teeth. In the upper jaw, the teeth are mostly set into sockets but the rearmost teeth are implanted in a groove. The lower jaw bears enlarged coronoid processes for the anchorage of jaw muscles. There are 61 vertebrae in front of the hips, two in the hip region, and at least 138 in the tail. The tail made up more than half the animal's length and displays a downward bend. The forelimbs are longer than the hindlimbs, and the humeri are round and squat. The phalanges are elliptical in the forelimbs but constricted in the hindlimbs.
While it is understood to be a shastasaurid-type ichthyosaur, how exactly Besanosaurus and other members of this group are related to each other is unclear. The skull bones of Besanosaurus indicate that it would have possessed strong jaw muscles, but its delicate snout suggests it would have fed on small fish and coleoid cephalopods, which it could have caught with rapid, snapping bites. Ichthyosaurs gave birth to live young, and the holotype may contain the remains of an embryo in its chest cavity. All definite specimens of Besanosaurus come from the Besano Formation. During the Anisian, this region was a lagoon populated by a wide variety of marine life, including a variety of other ichthyosaurs. These different ichthyosaurs are thought to have used different feeding strategies to avoid competition.
History of study
Holotype and naming
In 1985, paleontological work began at a site known as Sasso Caldo on the Italian side of Monte San Giorgio. This site is located about above sea level, and was fairly easy to reach. The rocks of this quarry were unmodified by faulting and consisted of layers of oil shale and dolomite pertaining to the geological unit known as the Besano Formation. The excavation at Sasso Caldo is the longest-lasting excavation that was conducted at Monte San Giorgio, and was part of an operation organized by the Milan Natural History Museum, with the excavation performed by the Gruppo Paleontologico di Besano, a volunteer group. In the spring of 1993, while extracting a block of dolomite containing the skeletons of a pair of mixosaurid ichthyosaurs, a piece of shale from the layer below broke loose. Preserved within this piece of shale from bituminous level n. 65 were the parts of the jaws of a large ichthyosaur. The rest of the skeleton of this ichthyosaur was extracted through the following summer by collecting the entire rock layer it was preserved in. The shale was exposed by using sledgehammers, wedges, and a jackhammer to clear out the dolomite above it, then divided into slabs and extracted with less forceful tools. Being rather fragile, the shale often broke during the excavation.The large ichthyosaur skeleton was deposited in the Milan Natural History Museum, where it was given the specimen number BES SC 999. To determine the extent of the skeleton, the museum collaborated with a hospital in the city to scan the slabs using X-ray analysis. Each radiogram covered an area of, and 145 were needed to show the entire specimen, which was ultimately revealed to be spread across a total of 33 slabs. While the slabs are only about thick, when put together, they cover an area of about. The skeleton is flattened from top to bottom, with the exception of the skull, which was instead crushed flat from side to side to a thickness of under. Most disarticulation of the skeleton is fairly minimal, though the bones of the fingers are scattered. The slabs contain an additional small mixosaurid as well as the large ichthyosaur.
By February 1996, 2500 hours of preparation had been performed by three or four preparators on the specimen, exposing the skull and appendicular skeleton. While the preparation was not yet complete, a preliminary report on this large ichthyosaur by Cristiano Dal Sasso and Giovanni Pinna was published later that year, the authors considering it warranted by the skeleton's distinctiveness and completeness, with the unprepared portions of the skeleton studied through radiography. Dal Sasso and Pinna found the specimen to differ from other shastasaurs, and made it the holotype of a new genus and species, Besanosaurus leptorhynchus. The genus name means, referencing a village in Varese Province, Lombardy, while the Greek words leptòs for or and rhynchos for make up the species name. The authors stated that a more detailed study could be produced once preparation was finished, a task they predicted could take as long as 8000 more hours. It ultimately took 16500 hours spread over 5 years in total to fully reveal the skeleton, which was concealed under of rock. The holotype subsequently entered storage as a total of 25 slabs, while a cast of the entire specimen put on display in 1999. BES SC 999 remains the most complete specimen of Besanosaurus. While P. Martin Sander and Christiane Faber in 1998 considered it possible that Besanosaurus represented another specimen of the related Shastasaurus, Besanosaurus has otherwise been accepted as a valid taxon.
Further specimens and ''Mikadocephalus''
The holotype of Besanosaurus was not the first shastasaur known from Monte San Giorgio. Two shastasaurid specimens from Switzerland were deposited in the collections of Paläontologisches Institut und Museum der Universität Zürich in the 1920s, both of them being mentioned in passing in the literature of the century. The smaller of the two, numbered PIMUZ T 4376, is a skeleton with a somewhat articulated skull and trunk but disarticulated limbs and tail. While the tail is missing its end the specimen is otherwise nearly complete, and the preservation of the bones is good. This specimen was recovered from layer 71 in the Valle Stelle mine. The other specimen, hailing from layer 116 of the Cava Tre Fontane mine, is cataloged as PIMUZ T 4847 and is very large, and poorly preserved and disarticulated, missing the tail and limbs. Sander and Jean Michel Mazin considered both of these specimens to represent a distinct genera in 1993. The medium-sized specimen was studied by David Cook, who had an abstract published in 1994, in which this specimen was interpreted as pertaining to a new genus. Dal Sasso and Pinna considered this specimen similar to Californosaurus and proportionally distinct from Besanosaurus in their description of the latter genus, following discussion with Robert Appleby. However, Cook's study of PIMUZ T 4376 did not lead to a paper, and the specimen remained incompletely known.In 1997, Michael Maisch and Andreas Matzke described an ichthyosaur skull housed at the Palaeontological Collection of Tübingen University, cataloged as GPIT 1793/1. This specimen comes from an unknown position in the Besano Formation in Switzerland. This skull is preserved on three shale slabs, and detailed preparation work was done using air abrasion by Fritz Lörcher. While the individual bones are generally fairly well preserved, the skull overall is crushed and strongly disarticulated. Determining it distinct after exhaustive comparisons with other ichthyosaurs, Maisch and Matzke named it the holotype of a new genus and species, Mikadocephalus gracilirostris. The genus name comes from the words Mikado and kephalos, referring to the way the skull was preserved, the authors likening its appearance to a game of pick-up sticks, while the species name comes from the Latin words gracilis and rostrum. However, the authors did not mention Besanosaurus or include it in their comparisons, likely due to them not being aware of its relatively recent publication at the time.
Sander argued that Mikadocephalus was probably a junior synonym of Besanosaurus in 2000, noting that they were anatomical similar. Additionally, he found the fact that Besanosaurus and Mikadocephalus would have lived in the same environment to be ecologically implausible given how many other ichthyosaurs had been reported from Monte San Giorgio. He also considered it possible that Wimanius, another fragmentary ichthyosaur named in 1998 by Maisch and Matzke, was a juvenile Besanosaurus rather than its own genus, though he also noted that it may instead belong to the same species as the unnamed medium-sized shastasaurid. However, Sander argued that the better-preserved material would need to be studied before definite assignments could be made. Later the same year, Maisch and Matzke defeneded the validity of Mikadocephalus, listing multiple differences between it and Besanosaurus. Additionally, they referred PIMUZ T 4376 to Mikadocephalus gracilirostris, and assigned another specimen to Besanosaurus, PIMUZ T 1895. This specimen, coming from the Cava Tre Fontane mine, consists of a skull associated with postcranial remains, with little limb and tail material. Preparation of this specimen remains incomplete.
In a 2003 book, Christopher McGowan and Ryosuke Motani listed both Mikadocephalus gracilirostris and Wimanius odontopalatus as a species inquirendae, and also considered the high diversity of ichthyosaurs reported from Monte San Giorgio suspicious. They considered both genera potentially synonymous with the much earlier-named Pessosaurus, though remained tentative as the material was not studied firsthand. While Pessosaurus polaris had previously been considered nondiagnostic and thus a nomen dubium, McGowan and Motani suggested that due to its wide historical recognition the name could be revived for some distinctive referred material, noting its similarity to that of the Swiss medium-sized shastasaurid. However, they argued that the medium-sized skeleton, as well as similar Chinese ichthyosaurs, would need to be described in greater detail before they could assess the validity of P. polaris. Maisch argued that Wimanius and Mikadocephalus were distinct and valid in 2010, stating that there were was no morphological or phylogenetic support for their synonymy, and kept Besanosaurus and Mikadocephalus separate as well. Otherwise, however, little further research was published on the taxonomy of Besanosaurus.
In 2021, a paper by Gabriele Bindellini and colleagues detailing the skull anatomy of Besanosaurus was published. The authors studied six shastasaurid specimens from Monte San Giorgio, namely the holotypes of Besanosaurus and Mikadocephalus, PIMUZ T 4376, PIMUZ T 4847, PIMUZ T 1895, and BES SC 1016, most of which had not previously had their skull anatomy described in detail. BES SC 1016 had not been previously studied at all, and is an incomplete, somewhat flattened, partially articulated skull preserved in dolomite, which was analyzed with CT scans. It was recovered from stratum 70 at Sasso Caldo, the same site at which the holotype of Besanosaurus was discovered. The authors reassessed the status of Mikadocephalus, finding many of the features initially used to distinguish it to be present in Besanosaurus as well, and were not able to find any distinguishing characteristics between the two genera. Therefore, they synonymized Mikadocephalus gracilirostris with Besanosaurus leptorhynchus, and referred the other four specimens to the latter species as well, since they also showed identifying features of Besanosaurus. The same team of authors continued their revision of Besanosaurus with a study of the postcranial anatomy published in 2024. Another study published earlier that year, led by Christian Klug, considered Wimanius to probably be a distinct genus from Besanosaurus, though noted that more research would be needed to confim this.