| Wimanius | |
|---|---|
 | |
| GPIT-PV-76272, theholotype specimen, with close-ups of the teeth (lower left) and back of the skull (lower right) | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Order: | †Ichthyosauria |
| Node: | †Hueneosauria |
| Family: | †Wimaniidae Maisch, 2010 |
| Genus: | †Wimanius Maisch & Matzke, 1998 |
| Species: | †W. odontopalatus |
| Binomial name | |
| †Wimanius odontopalatus Maisch & Matzke, 1998 | |
Wimanius is agenus ofichthyosaur from theMiddle Triassic ofSwitzerland, containing a singlespecies,Wimanius odontopalatus. It was described by Michael Maisch and Andreas Matzke in 1998 based on an incomplete skull fromMonte San Giorgio, a mountain on the Swiss-Italian border.Wimanius possesses teeth on its palate, though whether they were located on thepalatine orpterygoid is disputed. Other features ofWimanius include a largeorbit andjugals with two rami of similar lengths. Differentphylogenetic placements ofWimanius have been recovered by different studies, including it being amixosaurid relative or amerriamosaur, and amonotypicfamily,Wimaniidae has been named for it. However, itsvalidity has also been questioned, andsynonymy with various other genera has been proposed. The only specimen ofWimanius come from theBesano Formation. During theAnisian, this region was alagoon populated by a wide variety of marine life, including a variety of other ichthyosaurs.
Thetype and only specimen ofWimanius was first recognized as belonging to a newgenus by Michael Maisch and Andreas Matzke while examiningMixosaurus specimens in the Museum für Geologie und Paläontologie derUniversität Tübingen.[1] This specimen, now numbered GPIT-PV-76272,[2] formerly GPIT 1797, is a partial, crushed skull, consisting of a well-preservedlower jaw and upper part of the snout in addition to further elements from thetemporal region,braincase,palate, region around theorbits, and the very incompleteskull roof.[3][1] This skull originated fromMonte San Giorgio inSwitzerland, recovered from the Grenzbitumenzone of theBesano Formation. The specimen was found in the middle part of the Grenzbitumenzone, dating to the lateAnisian, near the border between the Anisian andLadinian stages of theMiddle Triassic.[3][4][5]: 62 The Middle Triassicdolomites andshales of Monte San Giorgio inItaly and Switzerland are rich in fossils and have yielded many marine reptiles, including the ichthyosaursBesanosaurus,Cymbospondylus, and the especially commonMixosaurus.[6][7][8][2] The genus nameWimanius honors the Swedish paleontologistCarl Wiman, known for his numerous works concerningTriassic ichthyosaurs. Thespecific nameodontopalatus refers to the presence of teeth on the palate, an unusual feature for an ichthyosaur.[3]
The foremost bones in the snout ofWimanius, thepremaxillae, are long and thin. Next to the tooth row, each premaxilla is marked with a palatal ridge on its inner surface.[3] The premaxillae would not have extended over theexternal nares, the openings for the nostrils.[1] Behind the premaxillae are themaxillae, each of which bear only nine teeth. The teeth do not extend below the weakly hooked upwards-projectingprocesses of the maxillae which form the external nares' back edges. The portions of the maxillae in front of these processes are short[1] and shallow, while those behind them are plate-like and tapering, and bear a ridge marking where thejugal articulated. Thenasal bones are mostly narrow but are broad and flat toward their hind ends.[3] They do not seem to have extended very far back on the skull roof.[1] A poorly preserved, curved bone was tentatively identified as apostfrontal by Maisch and Matzke in 1998,[3] but they backtracked on this identification the following year.[1]
The jugal is a thin bone, consisting of two rami of the same length[1] that intersect in a curved, 110-degree angle.[3] While this shape is like that of someleptonectids, no other Triassic ichthyosaur features such a shape.[1] The lower of the two rami bears a ridge along its top and its outer surface is bowed inwards. The other ramus rises vertically and is thick and keeled, with thepostorbital probably being located in front of it. Therefore, the rear portion of the orbit would have been formed by the crescentic postorbital.[3] Based on their reconstruction of the skull, Maisch and Matzke foundWimanius to have had very large orbits, proportionally the largest known among Triassic ichthyosaurs at the time, resulting in the back part of the skull being rather tall. However, they considered this possibly the result of the specimen being a juvenile. A circle of bony plates within the orbit known as thesclerotic ring would have supported the eye; while only two of these plates are known in the holotype,[3] Maisch and Matzke estimated that a complete ring would have 13 or 14 in total based on their reconstructed orbit size. These plates are quite large inWimanius, which gave the ring a rather small inner diameter.[1]
The portion of the skull behind the orbits is reduced in length, though this again may be due to the specimen being a juvenile.[1] Thesupratemporals aretriradiate bones in this skull region. Two of their processes contributed to the rim of thesupratemporal fenestrae, one process large, wide, quadrangular and reaching forwards, the other thin and extending towards the midline. The third, backwards-projecting process is reduced. A probablesquamosal is also known, but incomplete and poorly preserved.[3][1] Thequadratojugals are flat, roughly triangular bones in the postorbital region, with more or less straight rear and lower edges and bowed upper edges. Maisch and Matzke considered it probable that the jugal and the front lower portion of the quadratojugal were in contact in life. A distinct, constricted process on the quadratojugal would have articulated with thequadrate, the cranial bone involved in the jaw joint not preserved in the holotype.[3] Based on the quadratojugal's shape, Maisch and Matzke predicted that a large opening would have been present between these two bones in their 1999 reconstruction.[1]
Wimanius possesses palatal teeth, though what bone they are located on is disputed due to the poor preservation of itspalatal bones.[9] Maisch and Matzke considered the "flake-like" bone upon which these teeth are situated to be apalatine in their original description, citing the presence of teeth on this bone inGrippia.[3] In 1999, however, Ryosuke Motani argued that the tooth-bearing palatal bone was instead likely thepterygoid as inUtatsusaurus, noting that the supposed palatal teeth ofGrippia were instead additional rows of maxillary teeth.[10] Spread over an area about 17 millimetres (0.67 in) long, the palatal teeth are arranged in one or two rows in different regions, and oriented from front to back. The most complete of these teeth is only about 1 millimetre (0.039 in) tall, with a smooth-sided conicalcrown and a coarseroot.[3]
The bone Maisch and Matzke identified as a pterygoid has a straight inner edge, indicating that the pterygoids would have enclosed little space between each other when articulated. This bone possesses a robust process on the outer side interpreted as articulating with the quadrate,[3] and a prominent backwards projection on its inner side, the posteromedial process.[1]Wimanius has a quadrangularsupraoccipital, a midline braincase bone, which forms the top part of the opening of theforamen magnum, semicircular in this taxon. Theotic capsules would have been attached to the supraoccipital on well-developed facets. Maisch and Matzke tentatively identified a small, flat, and narrow bone as astapes. Unusually, this bone has a forked end, more typical of atemnospondyl than an ichthyosaur,[3] though such a morphology has been reported inPhalarodon.[1]
The lower jaw ofWimanius measures around 25 centimetres (9.8 in) in the holotype, indicating that the animal was probably rather small.[3] Thedentaries reach as far back along the lower jaw as the level of thecoronoid processes.[1]Wimanius has a low eminence situated on thesurangular in front of the jaw joint,[1] the surangulars being long and reinforced rear mandibular bones. The surangulars are upcurved and located directly above theangulars.[3] The teeth ofWimanius are all straight and cone-shaped, with each tooth being set into a distinct socket. The tooth crowns bear vertical striations, which are more strongly pronounced at their bases than at the tips, but lack cutting edges. This ornamentation is less pronounced on the known lower teeth than the upper teeth. In both jaws, tooth size is smaller at the front and larger at the back.[3]
In their 1998 paper namingWimanius, Maisch and Matzke noted that the fragmentary nature of the only known specimen made it difficult to determine how to classify it with much confidence. They noted that it did not seem to closely resemble eithermixosaurids orshastasaurids, though they considered it to be more closely related to the latter group, and, by extension, ichthyosaurs from theJurassic andCretaceous.[3] However, in the following year, the same authors restudied the skull and found it to more closely resemble a mixosaurid; therefore they proposed it to potentially be closely related to that group instead. However, they also considered it possible thatWimanius branched off earlier in ichthyosaur evolution. They compared it toMikadocephalus, another ichthyosaur from Monte San Giorgio, and considered the two to be very different in a variety of features.[1]
In 1999, Motani classifiedWimanius as Ichthyosauriaincertae sedis. Additionally, he noted that whileWimanius andMikadocephalus did differ in some aspects, they were quite similar in others, therefore arguging that further study was needed before the distinctiveness ofWimanius could be confirmed.[10] In 2000,Paul Martin Sander [de] also expressed doubt over whetherWimanius was valid and stated that further study would be needed on the ichthyosaurs of the region to determine this. In addition to noting its similarity toMikadocephalus, he also argued thatMikadocephalus was probablysynonymous withBesanosaurus. Sander classifiedWimanius as a shastasaurid and considered it possible that it represented a juvenileBesanosaurus.[11] However, Maisch and Matzke consideredWimanius to be a valid, diagnosable taxon in 2000, including it in theirphylogenetic analysis and finding it to be thesister taxon of Mixosauridae.[5]: 1, 62, 95
In a 2003 book with Christopher McGowan, Motani considered that bothWimanius andMikadocephalus might bejunior synonyms of the taxonomically problematicPessosaurus. However, as he had not personally observered the relevant specimens he refrained from formally synonymizing the taxa, instead listing them asspecies inquirendae.[12]: 127–128 In 2010, Maisch named a newmonotypicfamily, Wimaniidae, using the name Mixosauria for the group consisting of it and mixosaurids. Noting the distance betweenWimanius andMikadocephalus in this classification, he reiterated his argument thatWimanius was a distinct genus fromMikadocephalus, considering the evidence for this synonymy to be lacking.[13] The phylogenetic analyses of Ben Moon in 2017 found a different placement forWimanius, among the morederived groupMerriamosauria.[14]Wimanius was listed as a shastasaurid by Judith M. Pardo-Pérez and colleagues in 2020[4] and treated as potentially invalid by Ya-Lei Yin and colleagues in 2021, following McGowan and Motani's 2003 publication.[9] In 2024, in a review detailing ichthyosaurs discovered in Switzerland, Christian Klug and colleagues maintainWimanius as a distinct and valid ichthyosaur taxon pending re-evaluation.[2]
Cladogram following Maisch and Matzke, 2000.[5] | Cladogram following the preferred tree of Moon, 2017.[14]
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The only currently known specimen ofWimanius is recorded from theBesano Formation, also known as the Grenzbitumenzone in Switzerland. This formation is located in theAlps and extends from southern Switzerland to northern Italy, containing numerous fossils dating from between the end of the Anisian and the beginning of theLadinian.[7][6] This formation is one of a series of Middle Triassic units atop acarbonate platform at Monte San Giorgio, and measures 5–16 metres (16–52 ft) thick. During the time when the animal lived, 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.[15][6] Researchers estimate that this same lagoon would have reached between 30–130 metres (98–427 ft) deep.[6] The near-surface waters would have been well oxygenated and were inhabited by a wide range ofplankton andfree-swimming organisms.[15][16][17] However, water circulation within the lagoon was poor, resulting in typicallyanoxic water at the bottom, deprived of oxygen.[16][17] 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.[6] The presence of terrestrial fossils, such asconifers and land-dwelling reptiles indicates that the region would have been near land.[16]
Among the most common of the invertebrates from the Besano Formation is the bivalveDaonella.[18] Manygastropods are known from the Besano Formation; predominantly those that could have lived as plankton or on algae.[17] Cephalopods present includenautiloids,coleoids, and the especially common ammonites.[18] 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.[6][18]Arthropods known from the formation includeostracods,thylacocephalans, andshrimp. Other, rarer invertebrate groups known from the formation includebrachiopods andechinoids, which lived on the seabed.[18][16]Radiolarians andmacroalgae are also known in the formation, though the latter may have been washed in from elsewhere, as with many otherbottom-dwelling organisms.[18] A very large number of bony fish have been recorded in this formation.[18][19][20] Manybony fish have been recorded in this formation, withactinopterygians being quite diverse, including abundant small species as well as larger representatives likeSaurichthys, though rarersarcopterygiancoelacanths were also present.[19][20][18] Thecartilaginous fish of the Besano Formation are uncommon as well and mainly consist ofhybodonts.[21][18]
Aside fromWimanius, the other ichthyosaurs that have been discovered in the Besano Formation include the smallMixosaurus andPhalarodon as well as the imposingBesanosaurus andCymbospondylus.[22][6] Varioussauropterygians are known from the Besano Formation,[23] including the shell-crushingplacodontsParaplacodus andCyamodus[24] as well aspachypleurosaurs andnothosaurids. The pachypleurosaurOdoiporosaurus is known from the middle Besano Formation, while the particularly abundantSerpianosaurus did not appear until the upper portion of the formation, when ichthyosaurs were much less common.[25][18] Nothosaurids from the Besano Formation consist ofSilvestrosaurus buzzii,Nothosaurus giganteus, and an additional species ofNothosaurus, potentiallyN. juvenilis.[23] While rare,N. giganteus may have been an apex predator likeCymbospondylus.[6] Besides ichthyosaurs and sauropterygians, marine reptiles in the Besano Formation are represented by the thalattosaursAskeptosaurus,Clarazia, andHescheleria[26] in addition to thesemiaquatic, long-neckedTanystropheus.[27][18]
