Thalattosauria in time and space: a review of thalattosaur spatiotemporal occurrences, presumed evolutionary relationships and current ecological hypotheses

Abstract

In the wake of the greatest mass extinction in Earth's history, the End-Permian Mass Extinction, the Triassic was a time of recovery and innovation. Aided by warm climatic conditions and favorable ecological circumstances, many reptilian clades originated and rapidly diversified during this time. This set the stage for numerous independent invasions of the marine realm by several reptilian clades, such as Ichthyosauriformes and Sauropterygia, shaping the oceanic ecosystems for the entire Mesozoic. Although comparatively less speciose, and temporally and latitudinally more restricted, another marine reptile clade, the Thalattosauriformes, stands out because of their unusual and highly disparate cranial, dental and skeletal morphology. Research on Thalattosauriformes has been hampered by a historic dearth of material, with the exception of rare material from Lagerstätten and highly fossiliferous localities, such as that from the UNESCO world heritage site of Monte San Giorgio. Consequently, their evolutionary origins and paleobiology remain poorly understood. The recent influx of new material from southwestern China and North America has renewed interest in this enigmatic group prompting the need for a detailed review of historic work and current views. The earliest representatives of the group may have been present from the late Early Triassic onwards in British Columbia. By the Ladinian the group had achieved a wide distribution across the northern hemisphere, spanning the eastern Panthalassic as well as the eastern and western Tethyan provinces. Distinct morphological and likely ecological differences exist between the two major clades of Thalattosauriformes, the Askeptosauroidea and the Thalattosauroidea, with the latter showing a higher degree of cranial and skeletal morphological disparity. In-group relationships remain poorly resolved beyond this bipartition. Overall, thalattosaurs may be closely related to other marine reptile groups such as ichthyopterygians and sauropterygians. However, their exact position within Diapsida remains elusive. Future focal points should utilize modern digital paleontological approaches to explore the many fragmentary specimens of otherwise poorly sampled localities.

Keywords: Biogeography; Ecology; Marine reptiles; Monte San Giorgio; Phylogeny; Thalattosauriformes; Triassic.

Fig. 1

Fossil remains of various thalattosaur taxa A Xinpusaurus suni (IVPP V 11860)*. B Paralonectes merriami (holotype, TMP 89.127.1). C Thalattosaurus alexandrae (USNM 10926) *. D XNGM WS-22-R5 *. E Anshunsaurus huangguoshuensis (IVPP V 11835) *. F Askeptosaurus italicus (PIMUZ T 5392) *. G Endennasaurus acutirostris (MBSN 5170) ***. *courtesy of T.M. Scheyer; **courtesy of Jun Chai and Da-Yong Jiang; ***courtesy of Annalisa Aiello. Scale bars B, C, D, G equal 2 cm

Fig. 2

Historic thalattosaur discoveries. A Thalattosaurus alexandrae (UCMP 9085, holotype) (Merriam, 1905). B Nectosaurus halius (UCMP 9124, holotype) (Merriam, 1905). C Askeptosaurus italicus (MSNM V3550, holotype) (Nopcsa, 1925). D Nectosaurus sp. (UCMP 9120) (Nicholls, 1999). E Hescheleria ruebeli (PIMUZ T 2469, holotype) (Peyer, 1936b). F Askeptosaurus italicus (PIMUZ T 4831) (Kuhn, 1952). G) Askeptosaurus italicus (MSNM V456) (Kuhn, 1952). Drawings are not to scale

Fig. 3

Spatiotemporal spread of thalattosaur species. Solid stars indicate described or recognized species; translucent stars represent postulated material that may be referable to Thalattosauriformes. Map modified from Spiekman and Scheyer (2019), for detailed information see Table 1

Fig. 4

Simplified phylogenetic hypotheses proposed for the relationships of Thalattosauriformes to other reptile groups. A based on Evans (1988); B Rieppel (1998); C Müller (2004); D Neenan et al (2013); E Motani et al. (2015); F Scheyer et al (2017). Figure modified from Sun et al (2020)

Fig. 5

The most up-to-date time-calibrated strict consensus tree showing the interrelationships within Thalattosauriformes. The color-coded lines represent the temporal ranges of the taxa; the dashed line reflects the uncertain age ranges. The color codes reflect the spatial paleo-distribution of the clades: red represents the Eastern Tethyan Province (southwestern China), green represents the Western Tethyan Province (Europe), and blue represents the eastern Panthalassan Province (the Pacific coastline of North America). Figure modified from Jiang et al (2023), using new temporal data of Anshunsaurus cf. huangguoshuensis from Chai et al. (2020b). SM Smithian, SP Spathian, A Aegean, BI Bithynian, P Pelsonian, IL Illyrian, FA Fassanian, LO Longobardian, JU Julian, TU Tuvalian, LA Lacian, AL Alaunian, SE Sevetian, RH Rhaetian

Fig. 6

Spatiotemporal distribution of thalattosaur localities and species during the Early and Middle Triassic. Yellow stars indicate thalattosauroid localities, red stars indicate askeptosauroid localities, green indicate indeterminate thalattosaurid remains. Transparent green indicates postulated indeterminate thalattosaurid fossils. Numbers indicate different localities. Modified from Scotese (2016). A Induan/Olenekian, 248.5 Ma (note these are putative Lower Triassic in age). (1) Meosin Mountain, B.C.; Meosin/Sulphur Mountain Formation, Smithian, Paralonectes sp., Agkistrognathus/Paralonectes sp., Thalattosauridae indet. (RTMP collections); (2) Wapiti Lake, British Columbia; Sulphur Mountain Formation, Early Triassic, Paralonectes sp., Wapitisaurus problematicus (TMP 86.153.14), Thalattosauridae indet. (RTMP collections) (Bastiaans et al., 2023; Nicholls & Brinkman, 1993). B Anisian, 241.5 Ma. (1) Augusta Mountains, Nevada; Favret Formation, Anisian, Thalattosauridae indet. (Sander et al., 1994). (2) Varese, Italy; middle Besano Formation, Anisian, Askeptosaurus italicus (Nopcsa, 1925; Kuhn, 1952; Kuhn-Schnyder, 1960, 1971; Müller, 2005). (3) Ticino, Switzerland; middle Besano Formation, Anisian, Askeptosaurus italicus (Kuhn, 1952; Kuhn-Schnyder, 1960, 1971; Müller, 2005; Nopcsa, 1925). (4) Val Porina, Ticino, Switzerland; middle Besano Formation, Anisian, Clarazia schinzi (Peyer, 1936a; Rieppel, 1987). (5) Val Porina, Ticino, Switzerland; middle Besano Formation, Anisian, Hescheleria ruebeli (Peyer, 1936b; Rieppel, 1987). C Ladinian, 232.9 Ma. (1) Wapiti Lake, British Columbia; Sulphur Mountain Formation, upper Anisian – early Ladinian and Ladinian, Agkistrognathus campbelli, Paralonectes merriami, Thalattosaurus borealis, Thalattosauridae indet (Nicholls & Brinkman, 1993; RTMP collections). (2) Xingyi, Guizhou Province, China; Falang Formation, (upper) Ladinian, Xinpusaurus xingyiensis, XNGM WS-22-R5 (Chai et al., 2020a; Li et al., 2016). (3) Luoping County, Yunnan Province, China; Gjiu Formation, upper Ladinian, cf. Askeptosaurus (Sun et al., 2005). (4) Guizhou Province, China; Falang Formation, (upper) Ladinian, Anshunsaurus huangnihensis, A. wushaensis, Anshunsaurus cf. huangguoshuensis (Rieppel et al., 2006; Cheng, 2007a, b, 2011; Chai et al., 2020b). (5) Germany; Upper Muschelkalk, Ladinian, Blezingeria ichthyospondylus (Fraas, 1896; Müller, 2002; Diedrich, 2015; Schoch, 2015. (6) Tarragona, Spain; Spanish Muschelkalk, Ladinian, Potential tail fragment thalattosaur (Rieppel & Hagdorn, 1998)

Fig. 7

Spatiotemporal distribution of thalattosaur localities and species during the Late Triassic. Yellow stars indicate thalattosauroid localities, red stars indicate askeptosauroid localities, green indicate indeterminate thalattosaurid remains. Transparent green indicates postulated indeterminate thalattosaurid fossils. Numbers indicate different localities. Modified from Scotese (2016). A Carnian, 222.6 Ma. (1) Humboldt County, Nevada, USA.; Natchez Pass Formation, lower Carnian, Claraziidae indet. (USNM collections; Storrs, 1991b; Sues & Clark, 2005); (2) Julian Alps, Italy; Predil Limestone Formation, lower Carnian, potential “Endennasaurus-like” anterior caudals. (Dalla Vecchia, 1993). (3) Guanling County, Guizhou Province, China; Xiaowa Formation, lower Carnian, Xinpusaurus suni, X. kohi, X. bamaolinensis, Concavispina biseridens (Cheng, 2003; He et al., 2023; Jiang et al., 2004; Liu et al., 2013; Maisch, 2014; Rieppel & Liu, 2006; Yin et al., 2000; Zhao et al., 2013). (4) Guanling County, Guizhou Province, China; Xiaowa Formation, lower Carnian, Anshunsaurus huangguoshuensis; Miodentosaurus brevis (Cheng et al., 2007a, 2007b; Liu, 1999; Rieppel et al., 2000; Wu et al., 2009; Zhao et al., 2010). (5) Guanling County, Guizhou Province, China; Xiaowa Formation, lower Carnian, Neosinasaurus hoangi (Chai & Jiang, 2021; Yin et al., 2000). (6) Guanling County, Guizhou Province, China; Xiaowa Formation, lower Carnian, Wayaosaurus geei (Chai & Jiang, 2021; Yin et al., 2000). (7) Guanling County, Guizhou Province, China; Xiaowa Formation, lower Carnian, Wayaosaurus bellus (Chai & Jiang, 2021; Yin et al., 2000). (8) Shasta County, California, USA; Hosselkus Limestone Formation, Carnian, Nectosaurus halius, Nectosaurus sp., Thalattosaurus alexandrae, T. perrini (Merriam, 1895, 1902, 1905, 1908; Nicholls, 1999). (9) Paulina, Oregon, USA; Vester Formation, upper Carnian, Brisbois Member thalattosaur (Metz, 2019). B Norian, 210 Ma. (1) Pink Mountain, British Columbia, Canada; Pardonet Formation, Norian, isolated skeletal remains Thalattosauridae indet. (Storrs, 1991b). (2) Williston Lake, Jewitt Spur, Hudson's Hope, British Columbia, Canada; Pardonet Formation, Norian, cranial and postcranial remains (incl. articulated material) (ROM collections). (3) Zogno, Bergamo, Italy; Zorzino Limestone Formation, Norian, Endennasaurus acutirostris (Müller et al., 2005; Paganoni & Pandolfi, 1989; Renesto, 1984, 2005). (4) Keku Islands, southeast Alaska, USA; Hound Island Volcanics, middle Norian, Gunakadeit joseeae, Claraziidae indet. (Adams, 2009; Druckenmiller et al., 2020). (5) Gaissau, near Salzburg, Austria; Kössen Formation, upper Norian-lower Rhaetian, indeterminate thalattosauroid (Müller, 2002, 2007). C Rhaetian, 201.6 Ma. (1) Úbočka near Čičmany, Strážovské Mountains (Slovakia); Fatra Formation, Rhaetian, potential “Edennasaurus-like” femur (Čerňanský et al., 2020). (2) Baden-Württemberg, Germany; Rhaetian, Pachystropheus rhaeticus (Cawthorne et al., 2024; Čerňanský et al., 2020; Huene, 1935; Quinn et al., 2024; Renesto, 2005; Renesto and Dalla Vecchia, 2017; Storrs, 1992, 1994; Storrs & Gower, 1993; Storrs et al., 1996). (3) Somerset, England; Rhaetian, Fissure Fills, Pachystropheus rhaeticus (Cawthorne et al., 2024; Čerňanský et al., 2020; Moreau et al., 2021; Huene, 1935; Renesto, 2005; Renesto and Dalla Vecchia, 2017); Storrs, 1992, 1994; Storrs & Gower, 1993; Storrs et al., 1996