Wikipedia

Besanosaurus

Besanosaurus(meaning'reptile fromBesano') is aextinctgenusofMiddle TriassicichthyosaurfromMonte San GiorgioofItalyandSwitzerland,containing the single speciesB. leptorhynchus.Besanosauruswas named byCristiano Dal Sassoand Giovanni Pinna in 1996, based on the nearly complete flattened skeleton BES SC 999, theholotypespecimen. This skeleton is preserved across multiple thin rock slabs spanning 3.5 by 4 metres (11 by 13 ft) when assembled and took thousands of hours toprepare.Additional specimens from Monte San Giorgio that have previously been considered separate genera, including a partialskullnamedMikadocephalusand a well-preserved, largely complete skeleton, have been reinterpreted as additional specimens ofBesanosaurus.Putative specimens ofBesanosaurushave been discovered in theNorwegianarchipelagoofSvalbardandGermany,although their attribution to this genus remains disputed.

Besanosaurus
Temporal range:Middle Triassic,latestAnisian
Cast of BES SC 999, theholotype(top) and interpretative drawing (bottom)
Scientific classificationEdit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: †Ichthyosauria
Family: †Shastasauridae
Genus: †Besanosaurus
Dal Sasso& Pinna,1996
Type species
†Besanosaurus leptorhynchus
Dal Sasso & Pinna, 1996
Synonyms
  • †Mikadocephalus gracilirostrisMaisch & Matzke,1997

As an ichthyosaur,Besanosaurushadflippersfor limbs and afin on the tail.Besanosaurusis a large ichthyosaur, with the largest known specimen estimated to measure about 8 metres (26 ft) long. It has a long, slender body with a small head and long tail. The snout ofBesanosaurusis long and thin, and contains numerous small pointed teeth. In the upper jaw, the teeth are mostlyset into socketsbut the rearmost teeth are implanted in a groove. The lower jaw bears enlargedcoronoid processesfor the anchorage of jaw muscles. There are 61vertebraein 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 thehumeriare round and squat. Thephalanges(finger and toe bones) are elliptical in the forelimbs but constricted in the hindlimbs.

While it is understood to be ashastasaurid-type ichthyosaur, how exactlyBesanosaurusand other members of this group are related to each other is unclear. The skull bones ofBesanosaurusindicate that it would have possessed strong jaw muscles, but its delicate snout suggests it would have fed on smallfishandcoleoidcephalopods,which it could have caught with rapid, snapping bites. Ichthyosaursgave birth to live young,and the holotype may contain the remains of anembryoin its chest cavity. All definite specimens ofBesanosauruscome from theBesano Formation.During theAnisian,this region was alagoonpopulated 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

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Holotype and naming

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Skull and front part of the skeleton of BES SC 999, theholotypeofBesanosaurus

In 1985,paleontologicalwork began at a site known as Sasso Caldo (meaning'warm stone')[1]: 140 on theItalianside ofMonte San Giorgio.[2]This site is located about 800 metres (2,600 ft) above sea level, and was fairly easy to reach. The rocks of this quarry were unmodified byfaulting[1]: 140 and consisted of layers ofoil shaleanddolomitepertaining to the geological unit known as theBesano Formation.[3]The excavation at Sasso Caldo is the longest-lasting excavation that was conducted at Monte San Giorgio,[1]: 140 and was part of an operation organized by theMilan Natural History Museum,with the excavation performed by the Gruppo Paleontologico di Besano, avolunteergroup.[2]In the spring of 1993, while extracting a block of dolomite containing the skeletons of a pair ofmixosauridichthyosaurs,a piece ofshalefrom the layer below broke loose. Preserved within this piece of shale from bituminous level n. 65[2]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 usingsledgehammers,wedges, and ajackhammerto 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.[1]: 140–141 

The large ichthyosaur skeleton was deposited in the Milan Natural History Museum, where it was given the specimen number BES SC 999.[2]To determine the extent of the skeleton, the museum collaborated with a hospital in the city to scan the slabs usingX-rayanalysis. Eachradiogramcovered an area of 35 by 45 centimetres (14 by 18 in), and 145 were needed to show the entire specimen, which was ultimately revealed to be spread across a total of 33 slabs.[2][1]: 144 While the slabs are only about 2.7 centimetres (1.1 in) thick, when put together, they cover an area of about 3.5 by 4 metres (11 by 13 ft). 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 1 centimetre (0.39 in). 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.[2]

By February 1996, 2500 hours ofpreparationhad been performed by three[1]: 146 four preparators on the specimen, exposing the skull andappendicular skeleton.While the preparation was not yet complete, a preliminary report on this large ichthyosaur byCristiano Dal Sassoand 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 othershastasaurs,and made it theholotypeof a newgenusandspecies,Besanosaurus leptorhynchus.The genus name means'reptilefromBesano',[1]: 17 referencing a village inVarese Province,Lombardy,while theGreekwordsleptòsfor'thin'or'slender'andrhynchosfor'snout'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.[2]It ultimately took 16500 hours spread over 5 years in total to fully reveal the skeleton, which was concealed under 21,000 square centimetres (3,300 sq in) of rock.[1]: 146 The holotype subsequently entered storage as a total of 25 slabs, while a cast of the entire specimen put on display in 1999.[1]: 146 BES SC 999 remains the most complete specimen ofBesanosaurus.[3]While P. Martin Sander and Christiane Faber in 1998 considered it possible thatBesanosaurusrepresented another specimen of the relatedShastasaurus,[4]Besanosaurushas otherwise been accepted as avalid taxon.[5][6][7][3]

Further specimens andMikadocephalus

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Referred specimen PIMUZ T 4376, together with aMixosaurus(top) preserved on the same slab

The holotype ofBesanosauruswas not the first shastasaur known from Monte San Giorgio. Two shastasaurid specimens fromSwitzerlandwere deposited in the collections of Paläontologisches Institut und Museum derUniversität Zürichin 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,[8]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,[9]and poorly preserved and disarticulated, missing the tail and limbs.[3]Sander and Jean Michel Mazin considered both of these specimens to represent a distinct genera in 1993.[10]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.[8]Dal Sasso and Pinna considered this specimen similar toCalifornosaurusand proportionally distinct fromBesanosaurusin their description of the latter genus, following discussion withRobert Appleby.[2]However, Cook's study of PIMUZ T 4376 did not lead to a paper, and the specimen remained incompletely known.[3]

Disarticulated skull GPIT 1793/1, the holotype specimen ofMikadocephalus gracilirostris

In 1997, Michael Maisch and Andreas Matzke described an ichthyosaur skull housed at the Palaeontological Collection ofTübingen 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 usingair abrasionby 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 wordsMikadoandkephalos(Greek for "head" ), referring to the way the skull was preserved, the authors likening its appearance to a game ofpick-up sticks,while the species name comes from the Latin wordsgracilis(slender) androstrum(snout).[11]However, the authors did not mentionBesanosaurusor include it in their comparisons, likely due to them not being aware of its relatively recent publication at the time.[5][3]

Sander argued thatMikadocephaluswas probably ajunior synonymofBesanosaurusin 2000, noting that they were anatomical similar. Additionally, he found the fact thatBesanosaurusandMikadocephaluswould 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 thatWimanius,another fragmentary ichthyosaur named in 1998 by Maisch and Matzke, was a juvenileBesanosaurusrather 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.[5]Later the same year, Maisch and Matzke defeneded the validity ofMikadocephalus,listing multiple differences between it andBesanosaurus.Additionally, they referred PIMUZ T 4376 toMikadocephalus gracilirostris,and assigned another specimen toBesanosaurus,PIMUZ T 1895.[6]: 16, 27, 68–69 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.[3]

Referred specimen PIMUZ T 4847 on display at the Paleontological Museum of Zurich

In a 2003 book, Christopher McGowan and Motani listed bothMikadocephalus gracilirostrisandWimanius odontopalatusas aspecies 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-namedPessosaurus,though remained tentative as the material was not studied firsthand. WhilePessosaurus polarishad previously been considered nondiagnostic and thus anomen 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 similarChineseichthyosaurs, would need to be described in greater detail before they could assess the validity ofP. polaris.[7]: 127–128 Maisch argued thatWimaniusandMikadocephaluswere distinct and valid in 2010, stating that there were was no morphological or phylogenetic support for their synonymy, and keptBesanosaurusandMikadocephalusseparate as well.[12]Otherwise, however, little further research was published on the taxonomy ofBesanosaurus.[3]

In 2021, a paper by Gabriele Bindellini and colleagues detailing the skull anatomy ofBesanosauruswas published. The authors studied six shastasaurid specimens from Monte San Giorgio, namely the holotypes ofBesanosaurusandMikadocephalus,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 withCT scans.It was recovered from stratum 70 at Sasso Caldo, the same site at which the holotype ofBesanosauruswas discovered. The authors reassessed the status ofMikadocephalus,finding many of the features initially used to distinguish it to be present inBesanosaurusas well, and were not able to find any distinguishing characteristics between the two genera. Therefore, they synonymizedMikadocephalus gracilirostriswithBesanosaurus leptorhynchus,and referred the other four specimens to the latter species as well, since they also showed identifying features ofBesanosaurus.[3]The same team of authors continued their revision ofBesanosauruswith a study of the postcranial anatomy published in 2024.[13]Another study published earlier that year, led by Christian Klug, consideredWimaniusto probably be a distinct genus fromBesanosaurus,though noted that more research would be needed to confim this.[14]

Putative specimens from Svalbard and Germany

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Historically, a fairly rare[4]species of large ichthyosaur was reported from the units informally termed the "Upper Saurian Niveau" ofSvalbard,spanning from theLadinianstage of the Middle Triassic to theCarnianstage of theLate Triassic.[15]The first remains of this taxon were a total of 11 vertebrae with some associated rib fragments found during the 1860s, and namedIchthyosauruspolarisbyJohn Hulkein 1873.[16]The species was subsequently suggested to belong toShastasaurusorCymbospondylusinstead,[4]beforeCarl Wimannamed a new genus,Pessosaurus,to contain the species.[17]Wiman assigned many additional specimens toP. polaris,comprising all large ichthyosaur specimens from the Upper Saurian Niveau.Pessosauruswas subsequently recognized by many authors,[7]: 128 but its validity was questioned by Sander and Faber in 1998. Specifically, these authors noted that there were not enough differences preserved in the material to distinguishPessosaurusfrom other shastasaurids, and that it was too fragmentary to establish a species on. Additionally, they questioned Wiman's assumption that only one large ichthyosaur was present in the Upper Saurian Niveau, noting that multiple species of large ichthyosaurs were known to coexist elsewhere. Therefore, they consideredPessosaurusto be anomen dubiumand indeterminate shastasaurid.[4]

Diagram of the specimen assigned toPessosaurusthat was suggested to have affinities withMikadocephalus

Maisch and Matzke in 2000 agreed with the taxonomic position of Sander and Faber, but argued that PMU 24584 (formerly cataloged as PMU R176), a "Pessosaurus"specimen consisting of associated shoulder and forelimb material, including ahumerus,was diagnostic. They found it to be very similar to PIMUZ T 4376, the medium-sized shastasaurid from Monte San Giorgio. They observed some differences between the two, but argued that they could be growth-related. As they considered PMIUZ T 4376 to belong toMikadocephalus,they tentatively assigned the Svalbard specimen toMikadocephaluscf.gracilirostris.[6]: 85–86 While Motani had previously agreed withPessosaurusbeing anomen dubium,he took a different stance in his 2003 book with McGowan, noting that the morphology of the humerus was unique, as was that of the bones associated with it. Like Maisch and Matzke, he recognized its similarity with the specimen in Zürich, but instead argued thatPessosaurusshould be reinstated as the name for the material due to its historical significance, in which case it could be the senior synonym ofMikadocephalus.Since PIMUZ T 4376 had a femur similar to those historically assigned toPessosaurus,Motani argued that Wiman was probably right that only one large ichthyosaur was present in the Upper Saurian Niveau.[7]: 128 

In their 2013 reassessment of Triassic ichthyosaurs of Svalbard, Erin Maxwell and Benjamin Kear argued in favor of Sander and Faber's assessment ofPessosaurusas anomen dubium,since the eightcentra(vertebral bodies, some of which were subsequently lost) making up its holotype were nondiagnostic. While they acknowledged the distinctiveness of the forelimb and shoulder material, they argued it was too poorly known to make any definite taxonomic statement on it, and that there was insufficient evidence to assume only one was present in the Upper Saurian Niveau. Due to the Svalbard specimens not matching those from Monte San Giorgio in age, Maxwell and Kear argued against the assignment of the former to species from the latter location.[15]In 2024, Bindellini and colleagues determined that the similar femur anatomy of PIMUZ T 4376 andPessosauruswas the result of the femora being incomplete. They also noted that the elements preserved in PMU 24584 showed differences from those ofBesanosaurus,indicating that it pertained to a different ichthyosaur, although similar or even related.[13]

In 1916,Friedrich von Hueneerected a second species ofPessosaurus,P. suevicus,on the basis of a single vertebra discovered in theMuschelkalkof theBlack Forestin thestateofBaden-Württemberg,Germany.[18]: 33 Although this taxon has been widely recognized as anomen dubiumsince the late 20th century,[4][7]: 136 Maisch and Matzke noted in 2000 the possibility that the fossil material came fromBesanosaurusor a similar ichthyosaur, but agreed that it could not be distinguished from other representatives of the group.[6]: 85 

Another large ichthyosaur from Svalbard,Pessopteryx nisseri,comes from theEarly TriassicVendomdalen Member of theVikinghøgda Formation,was named based on various specimens by Wiman in 1910.[15][17]However, the material Wiman named this species based on is chimaeric, comprising both ichthyopterygian remains and jaw material ofomphalosaurids,a group of crushing-toothed marine reptiles.[15][19][7]: 135–137 This resulted in temporary disagreement over whether the namePessopteryxshould be used for the ichthyosaur or omphalosaurid material, and the namesRotundopteryxandMerriamosauruswere sometimes employed ifPessopteryxwas considered to be the omphalosaurid.[19][15]However, consensus subsequently emerged thatPessopteryxis indeed the correct name for the ichthyosaurian material.[12][15]In 2003, McGowan and Motani argued thatPessopteryx nisseriwas anomen dubium,and that the ichthyopterygian remains referred to it also represented multiple species. They noted that some elements were comparable to those ofBesanosaurusandIsfjordosaurus,specifically various limb bones in the case of the former, considering the differences observed to be growth-related.[7]: 135–136 Maisch, however, in 2010, argued against this synonymy, noting that his previous publications with Matzke had found various differences betweenPessopteryxandBesanosaurus,and recovered the two of them as not being particularly close relatives. He also maintained thatPessopteryxwas a valid genus.[12]Maxwell and Kear in 2013 doubted the assignment of some of the material considered to bePessopteryxby Maisch and Matzke, but agreed that it was distinct fromBesanosaurusand considered it to be potentially valid.[15]Bindellini and colleagues in 2024 also consideredPessopteryxto be distinct fromBesanosaurus,and concluded that there was no evidence ofBesanosaurusfrom Svalbard.[13]

Description

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Life restoration

Ichthyosaurs likeBesanosaurusweremarine reptiles,withflippersfor limbs and afin on the tail.[5][20]While later ichthyosaurs developed fish-shaped body plans,Besanosaurusis more elongate,[21][20]and resemblesCymbospondylusin overall shape. The skull ofBesanosaurusis 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 ofBesanosaurusis formed by its very long tail.[2]The holotype specimen measures 5.065 meters (16.62 ft) long from its snout tip to tail tip. The specimen PIMUZ T 4847 was estimated by Bindellini and colleagues in 2021 to have a total length of 8 meters (26 ft), and is the largest known specimen ofBesanosaurus,indicating thatBesanosaurusis the largest known ichthyosaur in its environment.[3][13][14]

Skull

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Skull reconstruction in oblique (A), rear (B), top (C), side (D), and bottom (E) views

About two-thirds of the skull ofBesanosaurusis occupied by its very long, thin snout lined with small teeth. This distinctive snout is strongly demarcated from the rear part of the skull by a constriction.[2][3]Well-preservedBesanosaurusskulls show drawn-out slotlikeexternal nares,openings which housed the nostrils in life. Behind the external nares are the ellipticalorbits,somewhat longer than tall. Within the orbits are thesclerotic rings,circles of bony, quadrangular plates that supported the eyeballs. Bindellini and colleagues estimated a total of between 15 and 17 of these plates per ring inBesanosaurus.While large relative to the orbits, the rings are smaller relative to the animal's overall length than typical forparvipelvianichthyosaurs. Further back, the top of the skull is perforated by another pair of openings, thesupratemporal fenestrae,which are rather small inBesanosaurus.Extending forwards from each supratemporal fenestra is a depression, forming roughly elliptical terraces in front of the fenestrae. The region behind the orbits is short, occupying 13% of the skull's length,[2]though it is still long compared to some parvipelvians.[3]

In the upper jaw, the snout is formed by the elongation of the tooth-bearingpremaxillae.The front ends of these bones are blunt, while their rear ends are forked, with processes extending both above and below the external nares. The back edges of the external nares are formed by the upwards-directed processes of themaxillae,gracile, triradiate bones behind the premaxillae.[3]While initially interpreted as not forming part of the narial border in the holotype,[2]further specimens revealed that thenasals,which run along the top of the snout, do participate in the upper back corners of the external nares. Along the midline of the skull, between the front ends of the temporal fenestrae, is a small opening called thepineal foramen,whose border is formed by thefrontalsin front andparietalson the sides and back. Asagittal crestalso runs along these two pairs of bones. The frontals are long, broad,[2]and flat bones involved in the supratemporal terraces, though they do not form any part of the borders of the supratemporal fenestrae. The parietals bear a variety of prominent surfaces for jaw muscle attachment, including the sagittal crest and their contributions to the rims of the supratemporal fenestrae.[3]

Photograph and diagram of the skull of PIMUZ T 4376

The front portions of the orbits are formed by a pair of crescentic bones calledlacrimals.Theprefrontalsandpostfrontalsmake up the top of the orbital rim, where they are thickened. The prefrontals also extend forwards, where they contact the ascending processes of the maxillae. The back edges of the orbits are formed by thepostorbitals,which also reach upwards and enter the supratemporal fenestrae. On the lower regions of the sides of the skull, wide portions of the postorbitals are visible, which Bindellini and colleagues considered potentiallyautapomorphicof the genus. Thejugalsare thin, strip-like bones bent at right angles. The articulations with other bones holding it in place were rather weak, and resultingly the jugals were often disarticulated from the skull during fossilization. ThesquamosalsofBesanosaurusare large and quadrangular, as is typical for ichthyosaurs with longer cheek regions, and seen in many other shastasaurids.[3]

The jaw joint was formed by the robust, kidney-shapedquadratesin the cranium. Each quadrate has a triangular eminence on the lower part of its inner margin. In 1997, Maisch and Matzke considered this to be a unique feature ofMikadocephalus,as at the time it was only known in the holotype skull of that taxon;[11]however, Bindellini and colleagues identified this on multipleBesanosaurusspecimens in 2021, as well as a specimen ofGuanlingsaurus,and therefore suggested that it may be a fairly widespread feature of shastasaurids.[3]The vertebral column is articulated with a convex eminence on the back of the skull known as theoccipital condyle.[3][2]Above this, thespinal cordentered through an opening called theforamen magnum,which Bindellini and colleagues reconstructed as rounded and roughly pentagonal inBesanosaurus.[3]ThestapedesofBesanosaurus,a pair of bones in thebraincase,are broad-ended but slender-shafted.[3][2]The openings between thepterygoidsin the rear part of the palate was interpretted as wide by Maisch and Matzke in their description and reconstruction of GPIT 1793/1;[11][22]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.[3]The pterygoids possess long processes for articulation with the quadrates.[11][3]

Most of thelower jawis formed by the long, thin toothbearingdentaries.Behind the dentaries, the largesurangularsform the top portion of the rear part of the lower jaw. The upper margin of each surangular bears two eminences, a taller, pointed one immediately in front of the jaw joint and another, lower one further forwards. Based on comparisons withChaohusaurus,Bindellini and colleagues interpretted the taller of the two projections as thecoronoid process,formed through the fusion of thecoronoidto the surangular. The coronoid process is massive inBesanosaurus,only rivaled in its development by that ofPhantomosaurus.The teeth ofBesanosaurusare all similar in shape, being small, cone-shaped, and acutely pointed. Thetooth crownsare lined with vertical ridges, though lack cutting edges, while thetooth rootsbear even stronger ridging. The teeth are separated from each other by a rather wide amount of space and interlock when the jaw is closed. The frontmost teeth in the upper jaw are the longest, and somewhat curved, while the back teeth are shorter and thicker, with a similar pattern of size and shape also present in the lower jaw. How the teeth are implanted varies through the tooth row; throughout most of the upper jaw, the teeth areset into sockets,with the exception of the rear 30% of the maxillary teeth,[11]which are implanted in a groove. Conversely, in the lower jaw, the front teeth are located in a groove while the back teeth are set into sockets.[3]

Vertebrae and ribs

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The holotype ofBesanosauruspreserves 201 vertebrae, 61 of which are located in front of the hips. These vertebrae, termed presacral vertbrae, consist of 12neck vertebrae,located in front of the shoulders, and 49 dorsal vertebrae, located in the trunk.[13]The neck vertebrae have prominentzygapophyses(bony projections of the vertebrae) and large, robustneural spineswith roughly round cross-sections. The roughly rectangular neural spines of the trunk vertebrae are also tall, and are more than half as wide from side to side as they are from front to back.[2]The vertebral bodies, known ascentra,of the presacral vertebrae are circular in outline.[13]The dorsal centra are at most half as long as tall. The front and back faces of all the centra are concave, being bowed inwards to a very thin layer of bone if not an opening.[2]The articular surfaces for theribson the sides of the dorsal centra extend forwards to the front edges of the centra, but they are not truncated.[23][13]The frontmostcervical ribs(those attached to the neck vertebrae) are double-headed, unlike the other, single-headed cervical ribs behind them. The dorsal ribs bear grooves along their shafts.[2]The heavily built[2]gastralia(belly ribs) form a basket on the underside of the torso, with each gastralium composed of a midline element flanked by two addition elements on each side. Behind the presacral vertebrae, the holotype preserves at least two hip vertebrae, which bore ribs with wide ends that may have articulated with the hip bones.[13]

The tail of the holotype makes up more than half of the animal's length[2]and contains at least 138 vertebrae.[13]While the frontmost caudal (tail) centra are roughly equidimensional like the presacral centra, those in the middle and end of the tail are proportionally narrower from side to side.[2][13]In their initial description, Dal Sasso and Pinna were not able to determine if the holotype ofBesanosaurushad a tailbend due to how the tail was preserved, though they suggested the tail would have been fairly straight.[2]However, PIMUZ T 1895 preserves five wedge-shaped caudal centra, which formed an approximately 30° bend in the tail. The neural spines located in front of the bend are angled backwards, those above the bend are vertical, and those behind the bend are angled forwards. By comparing the anatomy of these vertebrae with those of the holotype, Bindellini and colleagues identified the tail bend as occurring from the 56th to 60th tail vertebrae. Only the first six tail vertebrae bear ribs, which are rather short. However, there are surfaces for the articulation of caudal ribs on the centra all the way down to the tail bend, indicating that there may have been cartilaginous ribs associated with these vertebrae. Beneath the tail vertebrae are thechevrons,V- or Y-shaped bones, present beneath the fifth or sixth tail vertebra and onward.[13]

Appendicular skeleton

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Reconstructed forelimbs and shoulder girdle (left) and hindlimbs and pelvis (right) of theBesanosaurusholotype, seen in (from top to bottom) oblique, front, and lower views. Known elements in white

Thesickle-shapedscapulaeofBesanosaurusare wide and flat, bearing enlarged backwards extensions that produce very broad upper ends. Another pair of shoulder bones, thecoracoidsare shaped likeaxeheads, with strongly concave front edges, weakly concave back edges, and rounded inner edges.[2][13]InCymbospondylus,each coracoid is pierced by aforamen;however, no such openings are present in those ofBesanosaurus.Theclavicles(collarbones) ofBesanosaurusare thin and broadest at their middles,[2]and a triradiate bone called theinterclavicleis located between them on the midline.[13]The upper hip bones ofBesanosaurus,theilia,are large and rather wide, with their upper ends enlarged and directed inwards and their lower ends more heavily expanded. Thepubic bones(front lower hip bones) are vaguely circular in shape. They each bear a well-developed but narrow notch extending to the back[13]edge of the bone, rather than an enclosed foramen.[2]This notch is well-developed in PIMUZ T 4376, but it is almost closed in the holotype.[13]

The forelimbs ofBesanosaurusare longer than its hindlimbs, and mostly composed of rounded bones.[2][13]There are five digits in each forelimb, corresponding to the second to fifth fingers and an additional accessory digit behind them.[2]The humeri are very short, being slightly wider than they are long, and have a somewhat rounded profile.[2][13]Both the front and back edges of the humeri are bowed inwards, though this emargination is more notch-like on the front edges.[2]The lower arm bones as wide as they are long and about two thirds the length of the humerus,[13]with theradiibeing larger than theulnae.The roughly quadrangular radii have constricted middles, while the ulnae behind them are rounded and somewhat smaller.[2][24]Thecarpals,metacarpals,andphalanges(finger bones) are also rounded, the latter two types having the shape of circles.[2]The phalanges in the forelimb are well-spaced, indicating that extensivecartilagewas present in the forelimbs during life.[13]

The hindlimbs ofBesanosaurusare roughly 70% the length of its forelimbs. There are only four digits in each hindlimb, corresponding to the second to fifth toes.[13]Thefemoraare longer than wide, though only by a by a factor of 1.22 and are resultingly still quite stout. The femora are narrowest midshaft, broadening at their upper ends. Thetibiaeandfibulaeare 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.[2]They are also packed together, indicating that the hindlimbs had less cartilage than the forelimbs and may also have been more rigid.[13]

Classification

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While superficially resemblingCymbospondylus(top),Besanosaurusis more closely related toShastasaurus(bottom)

WhileBesanosaurushas generally been considered a shastasaurid,[25]how it is related to other shastasaurids is unclear due to the poorly understood phylogeny of the group.[3]To determine the phylogenetic affinities ofBesanosaurus,Dal Sasso and Pinna coded it into a data matrix created that was created by Jack Callaway in 1989. They found shastasaurids to be divisible into two main groups, termed Cymbospondylinae and Shastasaurinae by Callaway, with the former being agradeleading up to the latter.[2][6]: 4–5 Besanosauruswas placed in an intermediate position between these two subfamilies by the analysis. While Dal Sasso and Pinna noted thatBesanosaurussuperficially resembledCymbospondylusin overall body shape, they found it to compare more favorably to the shastasaurines in the fine points of its anatomy, and thus tentatively assigned it to Shastasaurinae.[2]The phylogenetic analysis of Dal Sasso and Pinna was one of the earlier of such studies on ichthyosaurs, and subsequent publications have criticized it for a variety of reasons. Key among these issues is that their analysis did not test their assumption that Shastasauridae was anatural group,as their matrix did not include any non-shastasaurid ichthyopterygians other thanGrippia.The usage of Cymbospondylinae for aparaphyleticgroup and the inclusion of highly fragmentary taxa was also criticized.[6]: 4–5 [7]: 144 

The first comprehensive, in-depth study of overall ichthyosaur relationships was done by Motani in 1999.[12][7]: 144 The traditional concept of Shastasauridae was not recovered by his analysis, with some "shastasaurids" such asCymbospondylusrepresenting earlier branches of ichthyosaur evolution while others likeCalifornosauruswere found to be more closely related to the later, post-Triassic ichthyosaurs. Some shastasaurids, however, were found to form a natural group in his analysis,Besanosaurusamong them. Based on the results of his analysis, he placedBesanosaurusin Shastasauridae, but outside Shastasaurinae, formed byShastasaurusandShonisaurus.Merriamosauriawas named for the group including the shastasaurids and later ichthyosaurs. Motani noted that some parts of the phylogeny were unstable, and that many traditional shastasaurids were "problematic" due to their incompleteness.[26]Another large phylogenetic study conducted by Sander was published the following year,[25]focusing on the more completely known ichthyosaurs. Unlike Motani, Sander found the three shastasaurids he included in his analysis to form a natural group, withBesanosaurusbeing thesister taxonofCymbospondylusandShonisaurusbeing basal to the two.[5]This analysis was criticized by subsequent work for only using a small sample of ichthyosaurs and encoding proportional characters based on arbitrary divisions. Concerns were also raised about the reproducibility of his results and the extremely high amount ofhomoplasy(convergent evolution of features by unrelated groups) implied by his cladogram.[12][7]: 145–146 

A third major analysis of ichthyosaur relationships was done by Maisch and Matzke later in 2000, featuring many similarities to Motani's analysis but based on a larger data set.[6][12]Like Motani, Maisch and Matzke found the traditional shastasaurids to not be a natural group, however, they also found Motani's concept of Shastasauridae to be a grade leading up to the post-Triassic ichthyosaurs and their relatives, withBesanosaurusas the first of these taxa to branch off. As it was considered separate at the time,Mikadocephaluswas also coded into their analysis, and found to be more closely related to the post-Triassic ichthyosaurs thanBesanosaurus.[12][6]: 96–97, 100 They had previously found a similar result in a smaller study they conducted in 1997, which foundMikadocephalusto be more closely related to the post-Triassic ichthyosaurs.[11][12]However, the phylogeny of Maisch and Matzke (2000) has subsequently been criticized for using a hypothetical ancestor, rather than actual non-ichthyosaurian taxa, as anoutgroup.[24][7]: 146 McGowan and Motani consideredBesanosaurusto fall outside of Merriamosauria in 2003, and named a newmonotypicfamily,Besanosauridae, to contain it.[7]: 70 Maisch also used this family forBesanosaurusin 2010, though he considered it to be within Merriamosauria. He classifiedMikadocephalusin a separate family,Guanlingsauridae,alongsideGuanlingsaurusin the same paper.[12]

Skeleton ofGuizhouichthyosaurus,which may be a close relative ofBesanosaurus

In 2011, Philippa Thorne and colleagues ran an analysis based on an updated version of Motani's 1999 data matrix. Their analysis foundBesanosaurusto be part of the shastasauridclade(natural group).[27]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, withBesanosaurusas the earliest diverging member.[24]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 notBesanosauruswas a shastasaur.[25]In their 2021 study, Bindellini and colleagues updated the codings forBesanosaurusand 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,Besanosauruswas found to be the earliest diverging member, while within the clade it was either in a similar position or in a smaller nested group withGuizhouichthyosaurusand"Callawayia" wolonggangense.Due to this instability, Bindellini and colleagues considered shastasaurid relationships to still be ambiguous.[3]

The following cladograms depict the three hypotheses recovered by the analysis of Bindellini and colleagues, 2021.[3]

Paleobiology

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Skull of theBesanosaurusholotype

Ichthyosaurs likeBesanosauruswere very well-adapted to their marine existence, spending their entire lives in the water and showing reduced levels ofossificationof their bones. Nevertheless, ichthyosaurs still would have breathed air. Ichthyosaurs were active animals with highmetabolic rates,able tomaintain their body temperatures.[5][28][29]The large eyes of ichthyosaurs indicate that vision would have been an important sense for them.[5][7]: 148 Besanosaurusis the earliest known long-snouteddiapsidthat attained a large body size. Bindellini listed many potential advantages of large size, including it allowing various feeding techniques, being ananti-predator adaptation,and resulting in more efficientthermoregulation.[3]A 2020 survey of ichthyosaurs from Monte San Giorgio by Judith Pardo-Pérez and colleagues found that shastasaurids likeBesanosauruswere more likely to show skeletal injuries than the smaller mixosaurids, but showed less frequent injuries thanCymbospondylus,thought to have preyed upon larger animals. The holotype ofBesanosaurusexhibitsankylosed(pathologicallyfused) metatarsals.[21]

Locomotion

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Intermediate-grade ichthyosaurs likely usedanguilliform(eel-like) orsubcarangiformlocomotion, swimming by undulating their entire body from side to side, owing to their long tails without strong bends and their long, flexible trunks. This differs from the more tail-driven locomotion presumed for the morespindle-shapedparvipelvians.[30][21][5][13]The body shape ofBesanosaurusclosely resembles that ofCymbospondylus,however, the shape of its tail fin is more similar to that ofGuizhouichthyosaurus.Bindellini and colleagues in 2024 interpretted this as evidence of a swimming style forBesanosaurusintermediate between these two taxa. The ribs ofBesanosauruswere predominantly single-headed, with double-headed ribs limited to the neck. Since single-headed ribs are less firmly braced against the vertebrae than double-headed ribs, this would have resulted in a flexible body. However, the rib articulations on the dorsal vertebrae are better developed inBesanosaurusthan inCymbospondylus,implying the former had a less flexible trunk than the latter. Conversely, the greater abundance of double-headed ribs inMixosaurusand parvipelvians indicates these ichthyosaurs were stiffer-bodied thanBesanosaurus.[13]

The tailbend ofBesanosauruswould have supported a tail fin. While this fin is not preserved in any known specimen, Bindellini and colleagues predicted that it would have beenheterocercal,with a much smaller upper lobe than lower. While this shape of tail fin indicates slower swimming speeds, it allows for more maneuverability.[13]The limbs ofBesanosauruswere particularly long, which would have helped stabilize the animal while swimming,[13]though they would not have been used for propulsion.[5]While some ichthyosaurs, such asMixosaurus,possessed adorsal finto keep the animal steady, the long limbs ofBesanosaurusmay have stabilized the animal enough that it did not need such a structure.[13]

Diet and feeding

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The long snout ofBesanosaurusresembles those ofgharialsandriver dolphins[3]

Dal Sasso and Pinna consideredBesanosaurusto likely have been specialized on feeding oncoleoidcephalopods,based on the shape and small size of its teeth.[2]Bindellini and colleagues elaborated on the tooth anatomy ofBesanosaurus,noting that the slender teeth in front would have been suited for piercing small prey while the slightly blunter ones further back suited for catching cephalopods by grasping. Direct evidence ofBesanosaurusfeeding on cephalopods is present in the form of aPhragmoteuthis-like hooklet preserved as stomach contents in the holotype.[3]However, following the discovery of a specimen ofGuizhouichthyosaurus,a supposed cephalopod predator, with a considerable portion of a largethalattosaurin its stomach region, Da-Yong Jiang and colleagues speculated in 2020 that other large ichthyosaurs adapted for consuming cephalopods may have been able to take on large prey. The researchers noted that despite the rather delicate construction of the skull ofBesanosaurus,aMixosauruswould have been able to fit inside its mouth.[31]However, Bindellini and colleagues doubted thatBesanosauruswould have been a predator of large animals, citing its long, slender snout, arguing that a diet of small animals was more likely.[3]

The large coronoid processes and rough, concave outer faces of the surangulars would have anchored strong jaw musculature. While the thin jaws ofBesanosauruswould not have been able to support a powerful bite, Bindellini and colleagues noted that it could instead have allowed the jaws to rapidly close. The length of the jaws would have caused their tips to move quite quickly during biting. Additionally, the long, thin snout could have been swung efficiently through the water, allowing quick up and down or side to side movements. Altogether, this configuration is ideal for feeding on prey items that are small and agile. Long, thin snouts haveconvergently evolvedmultiple times in aquaticamnioteswith similar feeding habits. As modern long-snouted predators such as somedolphinsarepiscivorous,Bindellini and colleagues suggested thatBesanosaurusmay have consumed fish as well as coleoids. Bindellini and colleagues also noted that the large size ofBesanosauruscould have helped with prey capture as well by providing moreinertia,with the head being able to have broad movement while the body stayed in place. The authors considered the animal's large size to have potentially resulted from its foraging method being particularly efficient.[3]

Reproduction and growth

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The small vertebrae preserved within the holotype's rib cage

Dal Sasso and Pinna discovered three or four small series of vertebrae within the holotype's chest through radiography. They interpretted the remains as pertaining toembryos,noting that a placement so far forwards in the body was not unheard of among ichthyosaurs. They also considered it doubtful thatBesanosauruswould have been able to take on mixosaurid-sized prey.[2]In a 2021 abstract, Feiko Miedema and colleagues regarded the embryonic remains as pertaining to one embryo.[32]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.[33]However, embryos are not the only interpretation of these remains. Jiang and colleagues in 2020 were doubtful about the inability ofBesanosaurusto 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.[31]

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.[3]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 distinguishMikadocephalusfromBesanosaurus.[6]: 34–35 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 ofBesanosaurus,the researchers found the orbit size and mandible length to increase at the same rate, and the tooth anatomy ofBesanosaurusto not change with ontogeny.[3]

In 2024, Bindellini and colleagues mention that the ontogeny ofBesanosaurusand the probable fetus present in the holotype's chest are subjects for future research.[13]

Palaeoenvironment

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Map of whereBesanosaurusspecimens have been found (yellow diamonds) at Monte San Giorgio
A view of Monte San Giorgio

All known definite specimens ofBesanosauruscome from the Besano Formation (alternatively called the Grenzbitumenzone), a unit composed of oil shale, laminated dolomite, andtuff.This formation is one of a series of Middle Triassic units atop acarbonate platformat Monte San Giorgio, and measures 5–16 metres (16–52 ft) thick. TheBesanosaurusspecimens 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 theN. secedensisZone, and dates to the latestAnisian,meaning thatBesanosaurusis the oldest definite shastasaurid genus. Noting that other Middle Triassic ichthyosaur genera were very widespread, Bindellini and colleagues considered it likely thatBesanosaurustoo was a wide-ranging genus, despite all definite specimens being known from this unit in theAlps.[3]

In the Triassic, when the Besano Formation was being deposited, the region where Monte San Giorgio is would have been a marinelagoon,located in a basin on the western side of theTethys Ocean.[3][34]This lagoon is estimated to be 30–130 metres (98–427 ft) deep.[3]The upper waters of the lagoon contained abundantoxygen,and were inhabited by a diverse array ofplanktonandfree swimmingorganisms.[34][35][36]However, water circulation within the lagoon was poor, resulting in typicallyanoxic waterat the bottom, deprived of oxygen.[36][35]The lagoon bottom would have been quite calm, as evidenced by the fine lamination of the rocks, and there is little evidence ofbottom-dwelling organisms modifying the sediment.[3]The disarticulation seen in the holotype ofBesanosaurusis minimal and random, which also indicates undisturbed waters.[2]The presence of terrestrial fossils, such asconifersand land-dwelling reptiles indicates that the region would have been near land.[36]

Contemporaneous biota

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Among the most common of theinvertebratesfrom the Besano Formation is thebivalveDaonella.[37]Manygastropodsare known from the Besano Formation; predominantly those that could have lived as plankton or on algae.[35]Cephalopods present includenautiloids,coleoids, and the especially commonammonoids.[37]The coleoids from the Besano Formation are not particularly diverse, but this may be due to their remains not readily fossilizing, with many of their known remains being preserved as stomach contents within the bodies of ichthyosaurs.[3][37]Arthropodsknown from the formation includeostracods,thylacocephalans,andshrimp.Other, rarer invertebrate groups known from the formation includebrachiopodsandechinoids,which lived on the seabed.[37][36]Radiolariansandmacroalgaeare also known in the formation, though the latter may have been washed in from elsewhere, as with many otherbottom-dwelling organisms.[37]Manybony fishhave been recorded in this formation, withactinopterygiansbeing quite diverse, including abundant small species as well as larger representatives likeSaurichthys,though rarersarcopterygiancoelacanthswere also present.[38][39][37]Thecartilaginous fishof the Besano Formation are uncommon as well and mainly consist ofhybodonts.[40][37]

The head shape ofBesanosaurus(top) compared with that of the contemporaneousMixosaurus(middle) andCymbospondylus(bottom)

Besanosaurusis one of a variety of ichthyosaurs from the Besano Formation. These different species would have had different feeding strategies to avoid competition.Cymbospondylus buchseriis known to have fed on coleoids, but its strong snout indicates that it could have taken larger prey with powerful bites, meaning it may have been anapex predator.The stomach contents ofMixosaurus cornalianusshow the remains of small coleoids and fish, suggesting that it would have gone after smaller prey than its larger relatives.[3]The rarer mixosauridsMixosaurus kuhnschnyderiandPhalarodonare also known from Monte San Giorgio; both possess broad crushing teeth.M. kuhnschnyderiis understood to have consumed coleoids, while the larger teeth ofPhalarodonmay have been suited for crushing prey items with external shells.[41]The abundance of ichthyosaurs in the middle Besano Formation correlates with when the lagoon was deepest.[37]

Varioussauropterygiansare known from the Besano Formation,[42]including the shell-crushingplacodontsParaplacodusandCyamodus[43]as well aspachypleurosaursandnothosaurids.The pachypleurosaurOdoiporosaurusis known from the middle Besano Formation, while the particularly abundantSerpianosaurusdid not appear until the upper portion of the formation, when ichthyosaurs were much less common.[44][37]Nothosaurids from the Besano Formation consist ofSilvestrosaurus buzzii,Nothosaurus giganteus,and an additional species ofNothosaurus,potentiallyN. juvenilis.[42]While rare,N. giganteusmay have been an apex predator likeCymbospondylus.[3]Besides ichthyosaurs and sauropterygians, marine reptiles in the Besano Formation are represented by the thalattosaursAskeptosaurus,Clarazia,andHescheleria[45]in addition to thesemiaquatic,long-neckedTanystropheus.[46][37]

See also

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References

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  1. ^abcdefghiDal Sasso, C. (2004).Dinosaurs of Italy.Bloomington, Indiana: Indiana University Press.ISBN978-0-253-34514-1.
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