Middle Triassic

Middle Triassic

During the Middle Triassic , ecosystems slowly stabilized and complex trophic networks were established with some of the highest diversity of marine reptiles of the entire Mesozoic (Bardet, 1994; Kelley et al., 2014; Neenan et al., 2015; Benton, 2016; Widmann et al., 2020; Jiang et al., 2023). By the (late) Middle Triassic , a global transgression allowed thalattosaurs to radiate and achieve a near-cosmopolitan distribution ( Fig. 6B, C), inhabiting the low-latitude tropical shallow marine environments along the eastern (southern China) and western (central Europe) Tethyan and eastern Panthalassic (western North America) margins (Bardet et al., 2014; Druckenmiller et al., 2020; Rieppel et al., 2000; Sun et al., 2020). How thalattosauroids crossed from the eastern to the western margin of the Panthalassa remains unclear ( Sun et al., 2020). Te absence of an Atlantic passage between mid to low latitudes only allows for two remaining hypotheses: (I) a coastal or coastal-pelagic migration along the northern margin of Laurasia, through the colder, high latitude waters (“Boreal route” of e.g. Hallam, 1994 in: Bardet et al., 2014); or (II) a direct pelagic dispersal between the east and west coast of Panthalassa via equatorial refuges has occurred but remains of such widespread taxa are still to be recovered (Bardet et al., 2014; Müller, 2002; Rieppel et al., 2000; Sun et al., 2020). Dispersal through the high latitude environments seems unlikely due to physiological (see Motani, 2009) and climatic constraints ( Rieppel, 2000; Müller, 2002). Talattosauroids may not have been able to sustain adequate body temperatures in these colder waters in contrast to pelagic Triassic ichthyosaurs (Müller, 2000; Sander, 2000). Te climatic conditions in this area during the Early Triassic, however, may have been much warmer than previously assumed, allowing for the early dispersal of ichthyosaurs, sauropterygians and thalattosaurs between the Panthalassic and Tethyan Provinces (Brayard et al., 2007 in Bardet et al., 2014).

Anisian By the Middle Triassic many fossil assemblages, such as the Sulphur Mountain Formation (British Columbia), Upper Saurian Level ( Svalbard), Panxian ( China) and the Favret Formation (Nevada) show an explosive diversification of, especially durophagous, marine reptiles (Benson et al., 2010; Schmitz et al., 2005). During this time marine transgressions eased faunal interchanges between the eastern and western Tethyan realms ( Jiang et al., 2023; Liu et al., 2013; Neenan et al., 2013, 2015; Rieppel, 2019; Sun et al., 2020). Faunal interchanges between the eastern Tethyan ecosystems and the eastern Panthalassic faunas remained quite an undertaking with only taxa adequately adapted to pelagic or coastal-pelagic migrations, such as ichthyosaurs, plesiosaurs, and pistosauroids seemingly being able to easily cross ( Neenan et al., 2013).

Talattosaurs, as opposed to sauropterygomorphs and protorosaurs, do not show a wide peritethyan distribution during the Middle Triassic and are mainly restricted to the Anisian beds of the Besano Formation and Ladinian deposits in southwestern China ( Mazin, 2001; Rieppel, 2001; Müller, 2002, 2005; Motani, 2009; Neenan et al., 2013, 2015; Bardet et al., 2014; Neenan, 2014; Renesto & Dalla Vecchia, 2017; Rieppel, 2019; Spiekman & Scheyer, 2019; Druckenmiller, 2020; Klein et al., 2022). To date, no Anisian or Early Triassic remains of thalattosaurs have been recovered from the eastern Tethyan province and robust phylogenetic analyses are necessary to test hypotheses on the specific paleobiogeographic patterns of the clade.

During the late Anisian the western Tethyan basins were inhabited by the 2–3 m long Askeptosaurus italicus , and the smaller (~ 1 to 1,5 m long) thalattosauroids Clarazia schinzi and Hescheleria ruebeli ( Fig. 6) (Bardet et al., 2014; Kuhn, 1952; Kuhn-Schnyder, 1960, 1971, 1988; Müller, 2005; Peyer, 1936a, 1936b; Rieppel, 1987, 2019). In the very latest Anisian the fauna slightly changes, Mixosaurus and thalattosaurs disappear and pachypleurosauroids and nothosaurids diversify ( Peyer, 1936a, 1936b; Müller, 2002; Rieppel, 2019; Spiekman & Scheyer, 2019; Bindellini & Sasso, 2022). At least two genera are present in the eastern Panthalassic province during this time, namely Agkistrognathus (TMP 1995.115.1) and Talattosaurus borealis (holotype, TMP 89.126.1). Additional remains from Cirque T (upper Anisian-lower Ladinian) of Wapiti Lake in British Columbia indicate the potential presence of Paralonectes (or T. perrini according to Nicholls, 1999) (TMP 1989.126.2) and other indeterminate thalattosaurs (e.g. TMP 1991.123.2) alongside various ichthyosaurs (Callaway & Brinkman, 1989; Nicholls & Brinkman, 1993, p. 264; Sander & Mazin, 1993; McGowan, 1997). A potential small thalattosaurian interclavicle (FMNH PR 1803) from the Fossil Hill Member of the Favret Formation in the Augusta Mountains may indicate their presence also in Nevada ( USA) during the Anisian ( Sander et al., 1994).

Ladinian Despite the lack of unequivocal Ladinian thalattosaurian remains in Europe, their presence may be indicated by putative remains assigned to “ Blezingeria " from the Germanic basin and a tail fragment from the Spanish Muschelkalk ( Fraas, 1896; Rieppel & Hagdorn, 1998; Schoch & Wild, 1999; Schoch et al., 2015). Blezingeria ichthyospondylus ( Fraas, 1896) from the Muschelkalk/Lettenkeuper likely represents a wastebin taxon comprised of isolated vertebral, pelvic, rib and limb remains of various marine reptiles. Re-examination of multiple specimens by Müller (2002: 122–123) could neither ascertain nor refute the possibility of some of these remains representing thalattosaurs. Te 45-cm-long tail fragment from the upper Ladinian Alcover unit of the Spanish Muschelkalk in Tarragona, Spain was postulated to belong to a thalattosaur on the basis of its “elongate and slender neural spines and haemal arches" and close resemblance to caudal remains from the Carnian of Italy that was previously assigned to thalattosaurs (Bardet et al., 2014; Dalla Vecchia, 1993; Rieppel & Hagdorn, 1998). If these putative thalattosaur remains are corroborated by the discovery of additional remains, that may indicate that thalattosaurs diversified in the southern Alpine region and subsequently spread to the western and Peritethys ( Rieppel & Hagdorn, 1998). During this time many of the alpine Triassic ecosystems show a highly reduced vertebrate diversity which may partly be correlated to hypersaline conditions ( Müller, 2002).

Te earliest representatives of thalattosaurs in the eastern Tethys come from the upper Ladinian Zhuganpo Formation (previously Zhuganpo Member of the Falang Formation) near Xingyi City, Guizhou Province (Benton et al., 2013; Jiang et al., 2023; Lu et al., 2018). Te Ladinian in South China is marked by a distinct turnover from predominantly ‘Tethyan’ coastal to more pelagic taxa with close affinity to taxa of the Panthalassic ecosystems (Benton et al., 2013; Jiang et al., 2023; Lu et al., 2018; Rieppel, 2019).

A recent discovery of an isolated antorbital cranial fragment of a thalattosauroid from the lower assemblage of Xingyi shows a strongly ventrally deflected premaxillary rostrum (XNGM-WS-22-R5, Chai et al., 2020a, 2020b). Its cranial morphology resembles that of, and shares potentially close affinity with, Hescheleria ruebeli from the western Tethys (Chai et al., 2020a, 2020b; Jiang et al., 2023). Its similar paleoenvironment and presumed ecological resemblance and affinity with the claraziids of Monte San Giorgio and the relationship between Anshunsaurus and Askeptosaurus , again illustrate the close faunal links between the eastern and western Tethyan provinces during the (late) Middle Triassic (Chai et al., 2020a, 2020b; Cheng et al., 2007a, 2011; Jiang et al., 2023; Müller, 2005, 2007; Rieppel et al., 2000, 2006).

Finds from the upper Ladinian deposits of the fourth member of the Gejiu Formation (equivalent to the lower assemblage of the Falang Formation of Guizhou Province) around Niubudai Village , Banqiao , Luoping County in Yunnan Province, include a partial caudal vertebral series and articulated right hindlimb of a 1.5– 2 m long askeptosauroid thalattosaur (cf. Askeptosaurus ) (Benton et al., 2013; Sun et al., 2005). Tis contrasts the absence of askeptosauroids in the lower assemblage of Xingyi, however, overall the faunal composition of both localities are indistinguishable (Benton et al., 2013) .

Talattosaurs show relatively modest taxic diversity and low morphological disparity in the fauna of the upper assemblage of Xingyi, with at least two genera and potentially four species present, all of which with relatively elongate and straight rostra: three species of the askeptosauroid Anshunsaurus ( A. wushaensis (Rieppel et al., 2006) ; A. huangnihensis, ( Cheng et al., 2007a) ; Anshunsaurus cf. A. huangguoshuensis, (Chai et al., 2020b)) and a single species of Xinpusaurus , X. xingyiensis ( Li et al., 2016) (Benton et al., 2013; Rieppel, 2019). All are roughly 2–3 m in length (Benton et al., 2013; Chai et al., 2020b; Cheng et al., 2007a, 2007b; Rieppel, 2019; Rieppel et al., 2006).

Towards the end of the Middle Triassic volcanism, a shift to a warm-humid climate and major changes in global sea level and oceanic chemistry (e.g. possible acidification) restructured global marine ecosystems (Bardet, 1994; Benton et al., 2013; Lu et al., 2018; Zhang et al., 2021). A tectonically-driven change into largely pelagicdominated assemblages in the eastern Tethyan ecosystems is well-illustrated by the Guanling biota, which includes very large ichthyosaurs and thalattosaurs and a lack of benthic and endobenthic taxa (Benton et al., 2013; Lu et al., 2018; Zhang et al., 2021).

Carnian Te Carnian marks a time of substantially decreased terrestrial and marine diversity and significant biological turnover amidst major climatic upheavals, changes in the hydrological cycling and perhaps extensive volcanism (Bardet, 1994; Corso et al., 2020). Over onethird of marine genera and> 50% of marine reptile families were lost during this time, followed by emergences and radiations of new clades (Bardet, 1994; Corso et al., 2020). Troughout the Carnian period, thalattosaur diversity seems to experience a significant peak worldwide. Tis may partially or largely be influenced by lagerstätten effects, despite the high degree of geological sampling for the Carnian to Norian (Benson et al., 2010). Within various ecosystems in China and North America, a total of 9–12 confirmed thalattosaur species, with an additional 2 species currently under study, have been identified. Interestingly, ichthyosaurs may have experienced a relative decrease in diversity ( Renesto & Dalla Vecchia, 2017). Whether this decrease is genuine, and due to an abiotic cause or perhaps due to competition for niches with the increasingly disparate and diverse thalattosaurs is currently unclear. Te high diversity and disparity of Carnian thalattosaurs may inversely also be a consequence of the relative decline in ichthyosaur diversity, if again it reflects a genuine pattern, or perhaps of changes in feeding ecology in the latter group. Te last non-plesiosaurian eosauropterygians likely go extinct during the early or middle Carnian which may also have allowed thalattosaurs to greatly diversify and attain a greater paleoecological diversity (Bardet et al., 2014; Renesto & Dalla Vecchia, 2017).

At least four to six genera and six to nine species of thalattosaur have been recovered from the Carnian fossiliferous beds of the Xiaowa Formation (previously Wayao Member of the Falang Formation) in Guanling county, Guizhou Province, southwestern China: Xinpusaurus suni , X. kohi , X. bamaolinensis , Concavispina biseridens , Miodentosaurus brevis , Neosinasaurus hoangi , Wayaosaurus bellus , W. geei and Anshunsaurus huangguoshuensis , the latter of which may have been present in the late Ladinian already (see Chai et al., 2020b) ( Liu, 1999; Rieppel et al., 2000; Yin et al., 2000; Liu et al., 2001; Liu & Rieppel, 2001; Luo & Yu, 2002; Cheng, 2003; Jiang et al., 2004; Liu & Rieppel, 2005; Rieppel & Liu, 2006; Cheng et al., 2007a, 2007b; Wu et al., 2009; Zhao et al., 2010, 2013; Benton et al., 2013; Liu et al., 2013; Maisch, 2014; Li et al., 2016; Rieppel, 2019). Te Guanling biota represents a remarkably and unparalleled varied assemblage of contemporaneous thalattosaur species, surpassing any known assemblage from around the globe ( Rieppel, 2019). Numerous specimens of the genera Xinpusaurus and Anshunsaurus have been recovered, making them some of the most abundant faunal components in these Carnian ecosystems (Benton et al., 2013; Liu et al., 2013). Te poorly preserved and partially prepared remains of Neosinasaurus and Wayaosaurus may prove to be closely related or even synonymous to known askeptosauroid thalattosaurs (e.g. M. brevis ), thereby possibly further increasing/decreasing the thalattosaur diversity by up to two genera and three species (Chai & Jiang, 2021; Chai et al., 2023; Wu et al., 2009; Yin et al., 2000). Te faunal similarities between the Ladinian ecosystems from the Upper Assemblage at Xingyi and the Carnian Guanling biota are much greater than previously acknowledged with the genera Xinpusaurus and Anshunsaurus displaying a longevity of perhaps several millions of years (Benton et al., 2013; Li et al., 2016).

More than 75% of the vertebrate fauna at Guanling is comprised of medium- to large-bodied marine reptiles, which is more than three times as much as during the Spathian (Benton et al., 2013). Taxa are also significantly larger compared to the Lower Triassic Chaohu Fauna, possibly indicating a fully recovered ecosystem (Benton et al., 2013). However, while taxic diversity and disparity are very high among marine reptiles during this time, very little is known about the functional diversity and functional uniqueness, richness and specialization within these ecosystems, in particular with regards to thalattosaurs. Towards the end of the Carnian thalattosaurs such as Concavispina , Miodentosaurus and Anshunsaurus huangguoshuensis attained body sizes in excess of 3 m and may have occupied apex predatory niches ( Cheng et al., 2007a, 2007b; Liu, 1999; Liu et al., 2013; Rieppel, 2019; Rieppel et al., 2000; Wu et al., 2009; Zhao et al., 2013).

An articulated caudal series from the lower Carnian “scisti ittiolitici di Raibl” may indicate the presence of small-sized Endennasaurus -like thalattosaurs in the Alpine Triassic during this time ( Dalla Vecchia, 1993). Tis may indicate the persistence of askeptosauroids in the western Tethys after the Anisian. However, the post- Anisian fossil record for thalattosaurs of this region is too incomplete and the assignment of this specimen to Talattosauriformes is too uncertain to make unequivocal interpretations.

In the eastern Pacific, numerous cranial and postcranial remains of thalattosauroids have been collected from the lower Carnian inferior member of the Natchez Pass Formation of Humboldt County, northwestern Nevada ( Sues & Clark, 2005). Although currently under study, at least one new genus of “claraziid” has already been identified based on an articulated cranium and referred basicranial remains (Sues, H-.D. pers. Communication; Storrs, 1991b; Sues & Clark, 2005). Tis thalattosauroid strongly resembles Hescheleria ruebeli and XNGM-WS-22-R 5 in that its premaxillary rostrum is strongly ventrally deflected (beyond 90° relative to the horizontal plane) (Chai et al., 2020a, 2020b; Peyer, 1936a, 1936b; Rieppel, 1987; Sues & Clark, 2005). However, it differs from these taxa in that its premaxillary dentition consists of pseudodont osseous outgrowths as has been suggested for Talattosaurus alexandrae ( Nicholls, 1999; Sues & Clark, 2005). Detailed anatomical and phylogenetic work needs to establish its position and interrelationships in Talattosauriformes, but its presence may hint at faunal connectivity and faunal interchanges between all three Provinces during the Middle and early Late Triassic ( Rieppel, 2019; Sues & Clark, 2005).

Tree to four additional taxa are identified from the upper Carnian deposits of California and Oregon: Talattosaurus alexandrae , Nectosaurus halius , an undescribed taxon referred to here as the Brisbois Member taxon and if considered a valid taxon, T. perrini ( Merriam, 1905; Metz, 2019; Metz et al., 2016; Nicholls, 1999).

Te Hosselkus Limestone of Shasta County, California ( USA), is an extremely rich faunal deposit with small (<2 m, e.g. Torectocnemus), medium- and large-sized (> 7 m shastasaurids) ichthyosaurs, and various thalattosauroids ( Merriam, 1902, 1904, 1905, 1907, 1908; Nicholls, 1999). Talattosaurus alexandrae is a medium-sized early diverging thalattosauroid measuring between 2 and 3 m in length ( Druckenmiller et al., 2020; Merriam, 1905; Nicholls, 1999). Another species of Talattosaurus may be present in the Shasta County fauna, T. perrini , however, this poorly described taxon had been lost for over a century and has only recently been recovered ( Nicholls, 1999; Pers. Obs.). Tis genus seemingly is particularly long-lived, potentially spanning the Olenekian or at least the Ladinian to the Carnian, ( Müller, 2002; Nicholls & Brinkman, 1993). Te second genus of thalattosaur in these deposits is the smaller (~ 1 m) Nectosaurus , however, much larger referred specimens exist (e.g. Nectosaurus sp. , UCMP 9120) ( Merriam, 1905, 1908; Naish, 2023; Nicholls, 1999). Although only represented by fragmentary cranial and postcranial material, this taxon likely possessed a strongly ventrally deflected rostrum and heterodont dentition ( Nicholls, 1999). Given its position as the sister taxon to a clade including all Tethyan thalattosauroids in the most recent phylogenetic analyses ( Druckenmiller et al., 2020; Jiang et al., 2023), this taxon may be very important in determining the paleobiogeographic history of these faunas and the development of ventrally deflected rostra.

Multiple (≥ 5) individuals of different ontogenetic stages of a large early diverging thalattosauroid genus have been recovered from a calcareous conglomerate nodule of the Brisbois Member of the Vester Formation of central Oregon (Metz et al.,; Metz, 2019). Tis shallow nearshore setting in the forearc of the Izee Terrane additionally produced fossils of hybodontid sharks, colobodontid fish, indeterminate marine tetrapod (perhaps another thalattosaur), an archosaur and an ichthyosaur (Metz et al.,; Metz, 2019). Tis taxon shows a moderately ventrally deflected rostrum slightly more pronounced than that observed in Talattosaurus and sharp, relatively homodont conical teeth ( Metz, 2019).

Norian and Rhaetian Towards the end of the Triassic several marine reptiles seem to show a prominent decrease in disparity and diversity or perish altogether, which defined the macroevolutionary history for surviving groups for the remainder of their existence (Torne et al., 2011; Kelley et al., 2014; Renesto & Dalla Vecchia, 2017; Moon & Stubbs, 2020). Substantial transgression between early and late Carnian was followed by a major regression, which led to the lowest quantity of flooded continental shelf during the Mesozoic, from the late Carnian onwards. Tis resulted in a progressive loss of shallow marine habitats, and consequently many durophagous taxa, towards the end of the Triassic (Benson & Butler, 2011; Benton et al., 2013; Kelley et al., 2014; Stubbs & Benton, 2016; Renesto & Dalla Vecchia, 2017; Druckenmiller et al., 2020). A short marine transgression during the middle to late Norian submerged large parts of the Alpine realm, which has led some authors to suggest that this area could have served as refugium for the last northwestern Tethyan coastal and coastal pelagic marine reptiles, such as placodonts and thalattosaurs ( Müller, 2002; Kelley et al., 2014; Renesto & Dalla Vecchia, 2017). Te rifting of the North Atlantic Ocean and outgassing of the Central Atlantic Magmatic Province resulted in a distinct climatic shift towards arid and hot conditions and several widespread pulses of extinction (Benton, 2005, 2014, 2016; Cawthorne et al., 2024). Talattosaurs, although still present after the Carnian, seem to show very little cranial disparity, with the sole presence of taxa with straight, elongated, ichthyosaur-like crania. If representing a genuine signal, this pattern may reflect relatively narrow ecological diversity for remaining thalattosaur taxa ( Druckenmiller et al., 2020; Müller, 2007; Müller et al., 2005; Sander, 2000). However, relative taxic diversity may be underestimated due to the relatively poor Carnian, and especially the Norian-Rhaetian fossil records for marine reptiles (Benson & Butler, 2011; Benson et al., 2010; Moon & Stubbs, 2020; Torne et al., 2011), as may also be illustrated by the widespread occurrence of thalattosaurs in Alaska, Canada, Italy and Austria during that time ( Druckenmiller et al., 2020; Motani, 2009; Müller, 2007; Müller et al., 2005; Paganoni & Pandolfi, 1989; Renesto, 1984, 2005; Storrs, 1991b).

Te deep-water ecosystems of the Pardonet Formation of British Columbia along Williston Lake and in the Pink Mountain preserves a rich marine reptile fauna, including numerous ichthyosaurs and rare thalattosaurians ( McGowan, 1997; Nicholls & Manabe, 2004; Wignall et al., 2007). Besides partial cranial remains and articulated skeletal material from Jewitt Spur at Williston Lake (Pers. Obs. RTMP), isolated remains of limbs, caudal vertebrae and ribs have been recovered from the Pink Mountain in northeastern British Columbia ( Storrs, 1991b). Talattosaurs, seem to have remained relatively restricted in length with partial mandible lengths of <10 cm, possibly indicative of body sizes between 1 and 2 m. Te mandibular dentition of the better-preserved ROM 46211 seems to resemble that of Gunakadeit joseeae from the middle Norian of the Keku Islands, southeastern Alaska, in both size and morphology, being small, relatively homodont, with slender, conical and recurved teeth ( Druckenmiller et al., 2020). Te material from the Pardonet Formation such as the partial dentaries show a similar crown size and tooth density along the jaw margin as observed in G. joseeae . If proportionately comparable, these indicate an even smaller body size of roughly 1 m. Towards the end of the Triassic, thalattosaurs may display relatively reduced morphological disparity. Whether this pattern is genuine and reflects similar trends as seen in contemporaneous ichthyosaur disparity ( Moon & Stubbs, 2020; Torne et al., 2011), and thus hint at specific selective pressures, specialization in a few ecological niches, or selective extinction, remains unknown. To date, insufficient thalattosaur remains have been recovered from Norian deposits to unequivocally determine their diversity and disparity. Te mandibular fragments and two partial skeletons (ROM, Toronto, Canada, temporarily not available to the author) from the Pardonet Formation remain undescribed and may provide crucial insight in the thalattosaur fauna of British Columbia during the Late Triassic and its affinity to contemporaneous forms such as Gunakadeit joseeae .

G. joseeae is a relatively small (75–90 cm) early diverging thalattosauroid from the Hound Island Volcanics (middle Norian, Alaska) ( Druckenmiller et al., 2020). Additional remains from the middle Norian of the Keku Strait region include 19 remains of indeterminate claraziids (cf. Nectosaurus sp. ), making it the most abundant occurrence from the Hound Island deposits (Adams, 2009). Te Hound Island Volcanics material represents some of the youngest thalattosaurian remains worldwide ( Druckenmiller et al., 2020). G. joseeae shows close affinity to Spathian-Middle Triassic taxa from British Columbia (Talattosaurus borealis and Paralonectes ) and the latest Anisian European thalattosauroids Hescheleria and Clarazia ( Druckenmiller et al., 2020; Jiang et al., 2023). Tese close relationships require a ghost lineage that equals or exceeds roughly 20 million years and indicate that thalattosauroids acquired aquatic adaptations very early in their evolutionary history, allowing for trans-hemispheric dispersal ( Druckenmiller et al., 2020). Druckenmiller et al., (2020) further postulates the likelihood that multiple cross-Panthalassan dispersal events have taken place throughout thalattosauroid evolution.

Another small (~ 1 m) early diverging thalattosaur from the Norian is the askeptosauroid Endennasaurus acutirostris from the Zorzino Limestone near Zogno, Lombardy, northern Italy ( Müller, 2002; Müller et al., 2005; Paganoni & Pandolfi, 1989; Renesto, 1984, 1992, 2005). As is the case for Gunakadeit , the ghost lineage of the common ancestor of Askeptosaurus italicus and Endennasaurus acutirostris is also extensive, spanning at least 25 million years, between the upper Norian and latest Anisian. Tis too likely indicates a substantially lacking (northwestern) Tethyan fossil record for askeptosauroid material.

Te Kössen Formation thalattosauroid from the late Norian-early Rhaetian of the northern Calcareous Alps of Austria, inhabited a shallow lagoonal or intrashelf basin along the northwestern margin of the Tethys ( Müller, 2007). Te fauna has a similar composition to that of the slightly older Zorzino Limestone ( Müller, 2007). Tis relatively small (~ 1 m) thalattosauroid is often excluded from phylogenetic analyses due to its incomplete nature, however, it may be closely related to Xinpusaurus and/ or Nectosaurus ( Müller, 2007) . Tis may again indicate a trans-Pacific migration prior to the late Ladinian-early Carnian, and the emergence of Xinpusaurus and Nectosaurus , and thus a substantial ghost lineage of almost 30 million years ( Müller, 2007). It may also hint at multiple re-invasions of the western Tethys given the young age of the Kössen specimen and the lack of affinity with other western Tethyan taxa ( Müller, 2002, 2007). Te Kössen specimen represents the youngest definitive thalattosaur worldwide and indicates their presence up to or into the early Rhaetian.

Te enigmatic Pachystropheus may have survived into the late Norian or early Rhaetian ( Huene, 1935; Storrs, 1992, 1994; Storrs & Gower, 1993; Storrs et al., 1996; Renesto, 2005; Renesto & Dalla Vecchia, 2017; Čerňanský et al., 2020; Moreau et al., 2021; Cawthorne et al., 2024; Quinn et al., 2024). Whether these reports of putative Rhaetian thalattosaurs such as Pachystropheus rhaeticus and Pachystropheus sp. from Germany and the UK, as well as a potential Endennasaurus -like femur from the Fatra Formation of Slovakia represent true thalattosaur occurrences remains to be seen ( Huene, 1935; Storrs, 1992, 1994; Storrs & Gower, 1993; Storrs et al., 1996; Renesto, 2005; Renesto & Dalla Vecchia, 2017; Čerňanský et al., 2020; Moreau et al., 2021; Cawthorne et al., 2024; Quinn et al., 2024).

Talattosaurs may have gone extinct prior to the Triassic-Jurassic boundary, as has been suggested for various non-plesiosaurian sauropterygians, non-parvipelvian ichthyosaurs and marine archosauromorphs ( Renesto & Dalla Vecchia, 2017; Gere et al., 2020). However, intensive sampling is needed to clarify their diversity patterns during the latest Triassic.