Besanosaurus

Holotype and naming In 1985, paleontological work began at a site known as Sasso Caldo (meaning ) By February 1996, 2500 hours of preparation had been performed by three 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. Besanosaurus has otherwise been accepted as a valid taxon. Further specimens and Mikadocephalus '' (top) preserved on the same slab 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 poorly preserved and disarticulated, missing the tail and limbs. 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, 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. 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. 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. Putative specimens from Svalbard and Germany Historically, a fairly rare The first remains of this taxon were a total of 11 vertebrae with some associated rib fragments found during the 1860s, and named Ichthyosaurus polaris by John Hulke in 1873. The species was subsequently suggested to belong to Shastasaurus or Cymbospondylus instead, Wiman assigned many additional specimens to P. polaris, comprising all large ichthyosaur specimens from the Upper Saurian Niveau. Pessosaurus was subsequently recognized by many authors, Although this taxon has been widely recognized as a nomen dubium since the late 20th century, Maisch and Matzke noted in 2000 the possibility that the fossil material came from Besanosaurus or a similar ichthyosaur, but agreed that it could not be distinguished from other representatives of the group. ==Description==

Ichthyosaurs like Besanosaurus were marine reptiles, with flippers for limbs and a fin on the tail. and resembles Cymbospondylus in overall shape. The skull of Besanosaurus is proportionately quite small, making up less than 10% of the animal's total length in the holotype. In addition to having an elongate trunk, about half of the total length of Besanosaurus is formed by its very long tail. however, Bindellini and colleagues in their redescription argued that this was not necessarily the case due to the specimen's disarticulation, and noted that the width of the interpterygoid vacuity was of typical width for a shastasaurid in BES SC 1016. The carpals, metacarpals, and phalanges (finger bones) are also rounded, the latter two types having the shape of circles. The phalanges in the forelimb are well-spaced, indicating that extensive cartilage was present in the forelimbs during life. The hindlimbs of Besanosaurus are roughly 70% the length of its forelimbs. There are only four digits in each hindlimb, corresponding to the second to fifth toes. The femora are longer than wide, though only by a factor of 1.22 and are resultingly still quite stout. The femora are narrowest midshaft, broadening at their upper ends. The tibiae and fibulae are also constricted at their middles and narrower than the radii and ulnae. The tibiae are the longer of the two pairs of lower leg bones, and the fibulae have heavily expanded lower ends. Compared with the corresponding elements of the forelimb, the tarsals, metatarsals, and phalanges of the hindflippers are less robust. Furthermore, while round, the phalanges of the pes are oblong and constricted. They are also packed together, indicating that the hindlimbs had less cartilage than the forelimbs and may also have been more rigid. ==Classification==

While Besanosaurus has generally been considered a shastasaurid, Another large phylogenetic study conducted by Sander was published the following year, focusing on the more completely known ichthyosaurs. Unlike Motani, Sander found the three shastasaurids he included in his analysis to form a natural group, with Besanosaurus being the sister taxon of Cymbospondylus and Shonisaurus being basal to the two. The results of another comprehensive analysis by Cheng Ji and colleagues were published in 2016, and was based on a much larger data set than previous studies. A large, well-supported clade of shastasaurids was recovered by their study, with Besanosaurus as the earliest diverging member. Another series of very large analyses were run by Benjamin Moon in 2017, created by revising and compiling previous analyses. The analyses, however, were unable to clearly elucidate the relationships of many early merriamosaurs, making it unclear whether or not Besanosaurus was a shastasaur. In their 2021 study, Bindellini and colleagues updated the codings for Besanosaurus and some other shastasaurids in the analysis of Ji and colleagues, preferring this study over Moon's larger one as it was constructed with more direct observation of the specimens. They found multiple different configurations of shastasaurids, with the group either being a grade or a clade, with the former arrangement found with marginally more frequency. Within the grade, Besanosaurus was found to be the earliest diverging member, while within the clade it was either in a similar position or in a smaller nested group with Guizhouichthyosaurus and "Callawayia" wolonggangense. Due to this instability, Bindellini and colleagues considered shastasaurid relationships to still be ambiguous. The following cladograms depict the three hypotheses recovered by the analysis of Bindellini and colleagues, 2021. Cladogram showing paraphyletic Shastasauridae: }} Cladogram showing monophyletic Shastasauridae: |1=Callawayia |2= }} }} }} Cladogram showing monophyletic Shastasauridae: }} ==Paleobiology==

Ichthyosaurs like Besanosaurus were very well-adapted to their marine existence, spending their entire lives in the water and showing reduced levels of ossification of their bones. Nevertheless, ichthyosaurs still would have breathed air. Ichthyosaurs were active animals with high metabolic rates, able to maintain their body temperatures. The large eyes of ichthyosaurs indicate that vision would have been an important sense for them. However, Bindellini and colleagues doubted that Besanosaurus would have been a predator of large animals, citing its long, slender snout, arguing that a diet of small animals was more likely. In 2023, Miedema and colleagues interpretted the embryo as being oriented so that it would have been born tail first, as typical in merriamosaurs. Historically, this orientation was thought to have evolved in ichthyosaurs to reduce the likelihood of the newborns drowning during birth; however, after surveying different vivaporous amniotes, Miedema and colleagues argued that the evidence for such an adaptation was lacking, and instead proposed that birth orientation was related to which way was easier to push the fetus through the birth canal or which way made affected the mobility of the pregnant adult less. However, embryos are not the only interpretation of these remains. Jiang and colleagues in 2020 were doubtful about the inability of Besanosaurus to have consumed mixosaurids, and argued that it could not be ruled out that the supposed embryos may be stomach contents instead, noting that the vertebrae were the right size for an immature mixosaurid, and were more ossified than would be expected in an embryo. As ichthyosaurs grow, their skulls typically become proportionately smaller. In PIMUZ T 4376, the skull is roughly half the length of the trunk; whereas it is only about a third as long as the trunk in the holotype. This difference was considered too extreme to be the result of ontogeney by Maisch and Matzke in 2000, arguing the specimens were too similar in size. Thus, they used this feature to distinguish Mikadocephalus from Besanosaurus. Bindellini and colleagues, however, did not find ontogeny to be an unreasonable explanation for this proportional difference in their 2021 study, instead finding the six specimens they studied to fit into a growth series when ordered by size. Unlike the skull length-body length ratio of Besanosaurus, the researchers found the orbit size and mandible length to increase at the same rate, and the tooth anatomy of Besanosaurus to not change with ontogeny. In 2024, Bindellini and colleagues mention that the ontogeny of Besanosaurus and the probable fetus present in the holotype's chest are subjects for future research. ==Palaeoenvironment==

All known definite specimens of Besanosaurus come from the Besano Formation (alternatively called the Grenzbitumenzone), a unit composed of oil shale, laminated dolomite, and tuff. This formation is one of a series of Middle Triassic units atop a carbonate platform at Monte San Giorgio, and measures thick. The Besanosaurus specimens all were likely recovered from the middle portion of the Besano Formation, though the exact localities of some specimens are unknown. The middle Besano Formation is also known as the N. secedensis Zone, and dates to the latest Anisian, meaning that Besanosaurus is the oldest definite shastasaurid genus. Noting that other Middle Triassic ichthyosaur genera were very widespread, Bindellini and colleagues considered it likely that Besanosaurus too was a wide-ranging genus, despite all definite specimens being known from this unit in the Alps. This lagoon is estimated to be deep. However, water circulation within the lagoon was poor, resulting in typically anoxic water at the bottom, deprived of oxygen. The abundance of ichthyosaurs in the middle Besano Formation correlates with when the lagoon was deepest. as well as pachypleurosaurs and nothosaurids. The pachypleurosaur Odoiporosaurus is known from the middle Besano Formation, while the particularly abundant Serpianosaurus did not appear until the upper portion of the formation, when ichthyosaurs were much less common. Nothosaurids from the Besano Formation consist of Silvestrosaurus buzzii, Nothosaurus giganteus, and an additional species of Nothosaurus, potentially N. juvenilis. While rare, N. giganteus may have been an apex predator like Cymbospondylus. in addition to the semiaquatic, long-necked Tanystropheus. ==See also==