A new species of mixosaurid ichthyosaur from the Middle Triassic of Luxi County, Yunnan Province, South China

Acta Palaeontol. Pol. 69 (2): 263–280, 2024 https://doi.org/10.4202/app. 01133.2024 A new species of mixosaurid ichthyosaur from the Middle Triassic of Luxi County, Yunnan Province, South China YE-WEI FANG, ANDRZEJ S. WOLNIEWICZ, and JUN LIU Fang, Y.-W., Wolniewicz, A.S., and Liu, J. 2024. A new species of mixosaurid ichthyosaur from the Middle Triassic of Luxi County, Yunnan Province, South China. Acta Palaeontologica Polonica 69 (2): 263–280. Ichthyosaurs, an iconic lineage of Mesozoic marine reptiles, were an important component of recovering ecosystems after the Permo-Triassic Mass Extinction event. Mixosauridae, a clade of small, early-diverging ichthyosaurs, were of particular significance for this process, being abundant predators in Middle Triassic shallow seas. Despite the abundance of well-preserved mixosaurid specimens from South China, Mixosaurus panxianensis remains the only comprehensively described species, hindering our understanding of the variability, taxonomy and diversity of mixosaurids from this region. Here, we report a new species of Mixosaurus, Mixosaurus luxiensis, from Luxi County, Yunnan Province, South China. The wider postorbital skull portion differentiates the new species from Mixosaurus cornalianus and Mixosaurus kuhnschnyderi from central Europe. The non-durophagous dentition, composed of tiny piercing mesial teeth and robust but pointed distal teeth, resembles the dentition of M. cornalianus. However, the distal teeth of M. luxiensis sp. nov. are twice the size of the mesial ones, in contrast to M. cornalianus, in which the mesial and distal teeth are approximately equal in size. The forelimb exhibits a unique morphology, including a proportionally narrow radius, the presence of a peripheral notch on the ulna, and a large metacarpal V. A preliminary phylogenetic analysis suggests a close affinity of the new taxon with M. cornalianus from Western Tethys. Our study introduces important, new anatomical information on Mixosaurus from South China, useful for future studies of mixosaurid diversity. Key words: Ichthyosauria, Mixosauridae, marine reptile, phylogeny, biotic recovery, Guanling Formation, Anisian, Triassic. Ye-Wei Fang [[email protected]; ORCID: https://orcid.org/0000-0002-8150-6331 ] and Jun Liu [[email protected]. cn; ORCID: https://orcid.org/0000-0001-7859-5209 ] (corresponding author), School of Resources and Environmental Engineering, Hefei University of Technology, 193 Tunxi Road, Hefei 230009, Anhui, China. Andrzej S. Wolniewicz [[email protected]; ORCID: https://orcid.org/0000-0002-6336-8916 ], School of Resources and Environmental Engineering, Hefei University of Technology, 193 Tunxi Road, Hefei 230009, Anhui, China; Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK; Institute of Paleobiology, Polish Academy of Sciences, Twarda 51/55, 00-818 Warsaw, Poland. Received 8 January 2024, accepted 10 April 2024, published online 14 June 2024. Copyright © 2024 Y.-W. Fang et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License (for details please see http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Introduction The Permo-Triassic Mass Extinction (PTME) was the most severe extinction event in the Phanerozoic (Bambach 2006). Despite its devastating effect on biodiversity, Triassic ecosystems recovered within only a few million years after the PTME (Brayard et al. 2017; Dai et al. 2023). The richness of benthic communities increased rapidly in the early Middle Triassic (Anisian) and stabilized towards its end (Chen and Benton 2012; Friesenbichler et al. 2021), while pelagic ecosystems perhaps regenerated even earlier (Song et al. 2018; Qiao et al. 2022). Mesozoic marine reptiles, such as ichthyosaurs and sauropterygians, colonized the marine realm in the aftermath of the PTME and quickly became important predators in Triassic seas (Carroll 1997; Fröbisch et al. 2013; Liu et al. 2014; Li and Liu 2020; Sander et al. 2021). Because predators are important indicators of ecosystem complexity, obtaining a detailed knowledge of their taxonomic and ecological diversity throughout the Triassic is important for understanding the process of marine ecosystem recovery after the PTME (Benton et al. 2013; Scheyer et al. 2014; Motani et al. 2015; Liu and Sander 2019). Ichthyosaurs were one of the most successful groups of Mesozoic marine reptiles (Motani 2005a; Bardet et al. 2014). The earliest ichthyosaur fossils are known from the Early Triassic, but it is unclear if the clade originated in the latest Permian or earliest Triassic (Motani et al. 2017; Kear et al. 2023). Ichthyosaurs achieved a broad geographic distribu- 264 ACTA PALAEONTOLOGICA POLONICA 69 (2), 2024 tion and high ecological diversity by the Middle Triassic (Callaway 1989; Sander and Mazin 1993; McGowan and Motani 2003; Liu 2011). Mixosauridae was a short-lived, but abundant clade of ichthyosaurs from the Middle Triassic. They evolved a fusiform body plan and had powerful jaw muscles (Motani et al. 1996; Maisch and Matzke 2000; McGowan and Motani 2003; Renesto et al. 2020). Their fossils are known mainly from shallow water sediments, like the Muschelkalk in Germany and Poland (Maisch and Matzke 1998a), the Besano Formation in Italy/Switzerland (Brinkmann 1997; Renesto et al. 2020), the Vega-Phroso siltstone and the Llama members of the Sulphur Mountain Formation in Canada (Callaway and Brinkman 1989), and the Upper Member of the Guanling Formation in South China (Benton et al. 2013; Liu et al. 2013), but they are also found in more pelagic sediments, such as the Botneheia Formation in Spitsbergen (Maxwell and Kear 2013; Hurum et al. 2014) and the Fossil Hill Member of the Favret Formation in the western United States (Schmitz et al. 2004; Sander et al. 2021). Mixosaurids have a long history of research, dating back to the 19th century (Quenstedt 1852; Hulke 1873; Bassani 1886; Baur 1887) and are one of the better-known ichthyosaur groups. Their anatomy (Callaway 1997; Motani 1999b; Brinkmann 2004), histology (Kolb et al. 2011), reproduction (Brinkmann 1996; Miedema et al. 2023b), ontogeny (Kolb et al. 2011; Miedema et al. 2023a), intraspecific variation (Maisch and Matzke 1998b; Zhou et al. 2022), and even body outline (Renesto et al. 2020) have all been studied in detail. The monophyly of Mixosauridae is well-established and supported by several synapomorphies of the circumnarial area, skull roof, girdles, limbs, and vertebrae (Liu 2011; Ji et al. 2016; Moon 2019). However, the taxonomy of Mixosauridae remains incompletely understood. Mixosauridae are traditionally divided into two genera, Mixosaurus and Phalarodon, which differ from each other in narial shelf, dentition, humerus and caudal centrum morphology (Schmitz et al. 2004; Schmitz 2005; Jiang et al. 2006; Liu et al. 2013). Nonetheless, the taxonomic composition and phylogenetic topology within these genera differ between studies. Whereas the majority of ichthyosaur researchers follow the Mixosaurus–Phalarodon dichotomy (Jiang et al. 2006; Ji et al. 2016; Motani et al. 2017; Økland et al. 2018; Huang et al. 2019; Roberts et al. 2022), Maisch (2010) and Moon (2019) separated Mixosaurus panxianensis and Phalarodon atavus into distinct genera (Barracudasauroides and Contectopalatus, respectively). In addition, some phylogenetic analyses have failed to recover Mixosauridae resolved into a simple Mixosaurus– Phalarodon dichotomy (Liu 2011; Huang et al. 2019; Moon 2019). Therefore, it is possible that the taxonomy and phylogeny of mixosaurids are more complex than the current paradigm suggests. Over the past 20 years, abundant fossils of mixosaurids have been recovered from the Middle Triassic strata of South China (Motani et al. 2008; Jiang et al. 2009; Hu et al. 2011). Despite the abundance of fossils, the taxonomic composition of mixosaurids from South China remains only partly known. A total of six species of Mixosaurus has been erected based on Chinese materials (“Mixosaurus maotaiensis” from Renhuai City, early Anisian [Young 1965], “Mixosaurus guanlingensis” from Guanling biota, early Carnian [Yin et al. 2000], Mixosaurus panxianensis from Panxian fauna, Pelsonian, Anisian [Jiang et al. 2005, 2006], “Mixosaurus yangjuanensis” from Panxian fauna [Liu and Yin 2008], “Mixosaurus xindianensis” from Panxian fauna [Chen and Cheng 2010], and “Mixosaurus xinzhaiensis” from Luoping biota, Pelsonian, Anisian [Chen et al. 2016]). In addition, the occurrence of four additional mixosaurid species, known from Europe and the USA, was also reported from South China (Phalarodon cf. P. fraasi from Panxian fauna [Jiang et al. 2007], Phalarodon atavus from Luoping biota [Liu et al. 2013], “Mixosaurus cornalianus” from Panxian fauna [Liu and Yin 2008], and “Mixosaurus kuhnschnyderi” from Luoping biota [Chen and Cheng 2009]). Furthermore, three mixosaurid specimens, referable only to family or genus level, were also described from South China (Phalarodon sp. from Panxian fauna [Jiang et al. 2003], Mixosauridae gen. et sp. indet. from Luoping biota [Liu et al. 2011], Mixosaurus sp. from Fuyuan County, upper Ladinian [Chen et al. 2016]), but it is not clear whether these specimens represent new taxa or are referable to any of the currently known mixosaurid species. However, three of the Mixosaurus species reported from South China (“M. maotaiensis”, “M. guanlingensis”, and “M. yangjuanensis”), and one occurrence of an otherwise European taxon (“M. cornalianus”), were subsequently identified as invalid (McGowan and Motani 2003; Jiang et al. 2006, 2007, 2008a). Among the remaining species new to South China, “M. xindianensis” and “M. xinzhaiensis” were inadequately figured and only briefly described (due to inaccessibility of the holotype for study, the former was even regarded as species inquirendae by Liu et al. 2013). As a consequence, Mixosaurus panxianensis is the only species of Mixosaurus from South China that has been comprehensively described and well-illustrated (Jiang et al. 2005, 2006), but whether it represents a species of Mixosaurus (Jiang et al. 2006; Ji et al. 2016) or a separate genus (Maisch 2010; Moon 2019) remains a matter of debate. Incomplete knowledge of the anatomy and taxonomy of Mixosaurus species from South China hinders our understanding of the global diversity, phylogenetic interrelationships, and palaeobiogeography of mixosaurids. Here, we report a new species of Mixosaurus collected from a recently discovered shallow marine fauna in Luxi County, Yunnan Province, South China (Wen et al. 2020; Xu et al. 2022; Hu and Liu 2023; Lu and Liu 2023; Hu et al. 2024) (see Fig. 1 for localities of the eosauropterygians Nothosauroidea indet. (Hu and Liu 2023) and Dianmeisaurus mutaensis near Muta (Hu et al. 2024) and the tanystropheid Luxisaurus terrestris near Suomeiluo (Lu and Liu 2024). An anatomical description of the new species is provided, FANG ET AL.—NEW MIXOSAURID ICHTHYOSAUR FROM THE MIDDLE TRIASSIC OF CHINA 265 highlighting the most important differences and similarities with other species of Mixosaurus. A preliminary phylogenetic analysis of mixosaurid interrelationships, incorporating anatomical information from the new species, is also presented and discussed. Institutional abbreviations.—CCCGS, Chengdu Center of China Geological Survey, China; GMPKU, Geological Museum of Peking University, Beijing, China; HFUT, Geological Museum of Hefei University of Technology, China; PIMUZ, Department of Palaeontology, University of Zürich, Switzerland (formerly Paläontologisches Institut und Museum der Universität Zürich, Switzerland); PMU, Paleontological Museum, University of Uppsala, Sweden; SMNS, Staatliches Museum für Naturkunde Stuttgart, Germany; YIGMR, Yichang Institute of Geology and Mineral Resources (currently WCCGS, Wuhan Center of China Geological Survey), China. Nomenclatural acts.—This published work and the nomenclatural acts it contains have been registered in ZooBank: urn:lsid:zoobank.org:act:AF9FF8CE-A718-48E3-9132-A11 FA3392666 Material and methods HFUT HL-21-08-002 was collected from a quarry about one kilometer southeast of Huale Village, Luxi County, Yunnan Province, China (Fig. 1). It originates from the Upper Member (also known as Second Member or Member II) of the Guanling Formation (formerly part of the Gejiu Formation) and is Pelsonian (middle Anisian) in age (Wen et al. 2020; Xu et al. 2022; Hu and Liu 2023; Lu and Liu 2023; Hu et al. 2024), similar to the Luoping and Panxian faunas located nearby (Benton et al. 2013). The rock matrix of thinly lamFig. 1. A. Map of China showing location of the study area. B. The geological map of the area around Luxi, Yunnan Province, South China, showing the locality of HFUT HL-21-08-002 (modified after Hu and Liu 2022; Lu and Liu 2023; Hu et al. 2024). Abbreviation: Fm., Formation. 266 ACTA PALAEONTOLOGICA POLONICA 69 (2), 2024 inated micritc limestone with alternating interlayers indicates an intraplatform sedimentary facies, similar to the depositional setting of the Luoping Biota (Hu et al. 2011; Wen et al. 2020). HFUT HL-21-08-002 is accessioned in the collections of the Geological Museum of Hefei University of Technology in Hefei, Anhui Province, China (HFUT). It was prepared using mounted needles and pneumatic tools in the paleontological preparation laboratory of HFUT. Detailed observations of the specimen were taken under a LEICA S9i microscope. Canon EOS 80D and Nikon D7200 cameras with macro lenses were used for specimen photography. The phylogenetic analysis was based on a recently published, comprehensive data matrix for ichthyosaurs (Huang et al. 2019). HFUT HL-21-08-002 was added to this charactertaxon matrix. Characters 43(?), 50(?), 51(?), 175(?), 187(?) were re-coded for M. cornalianus to 43(0), 50(0), 51(0), 75(0), 187(0) (based on Miedema et al. 2023a). Character 43 was re-coded for M. kuhnschnyderi from 43(?) to 43(0) (based on PIMUZ T 1324; Brinkmann 2004). Character 112 was re-coded for M. panxianensis from 112(1) to 112(0) (based on GMPKU-P-1008 and GMPKU-P-1038 [holotype]; Jiang et al. 2005, 2006), for M. cornalianus from 112(1) to 112(0&1) (based on figures in Maisch and Matzke 1998b, Motani 1999a, Renesto et al. 2020, and Zhou et al. 2022), and for M. kuhnschnyderi from 112(1) to 112(?) (based on PIMUZ T 1324; Brinkmann 2004). The phylogenetic matrix (SOM 2, Supplementary Online Material available at http:// app.pan.pl/SOM/app69-Fang_etal_SOM.pdf) was edited in Mesquite 3.81 (Maddison and Maddison 2023) and analyzed in TNT 1.5 (Goloboff and Catalano 2016). All characters were treated as unordered and equally weighted. Max. trees was set to 10 000. Collapsing was set to “max. length = 0”, using rule 3-collapse. Traditional Search using 1000 Wagner tree replications was applied to obtain maximum parsimony trees. The Bremer support values were calculated using TBR branch-swapping from existing trees, retaining trees suboptimal by 100 steps. Iterative Positional Congruence Reduced (IterPCR) was used to prune unstable taxa in a second analysis, in order to obtain a better-resolved reduced strict consensus cladogram (Pol and Escapa 2009). Systematic palaeontology Ichthyosauria de Blainville, 1835 Mixosauridae Baur, 1887 Genus Mixosaurus Baur, 1887 Type species: Mixosaurus cornalianus Bassani, 1886; Monte San Giorgio, Italy and Switzerland; late Anisian–early Ladinian, Middle Triassic. Emended diagnosis (modified after Brinkmann 2004; Jiang et al. 2006; Liu 2011; Ji et al. 2016; and Renesto et al. 2020).—Centrum height/length ratio moderately increased in the mid-caudal region, but not exceeding a value of 3.0 (centrum height/length index significantly increased in the mid-caudal region to a value of over 3.0 in Phalarodon); interclavicle T-shaped with transversal bars perpendicular to the posterior process (interclavicle V-shaped, with posterior process triangular and broad, and transversal bars triangular, narrow and projecting anterolaterally in Phalarodon); humerus relatively broad with proximodistal length subequal to anteroposterior width (humerus narrow, proximodistally longer than anteroposteriorly wide in Phalarodon). Mixosaurus luxiensis sp. nov. Figs. 2–7; SOM 1: figs. S1, S2, tables S1, S2. ZooBank LSID: urn:lsid:zoobank.org:act:AF9FF8CE-A718-48E3-9132A11FA3392666 Etymology: In reference to the type locality in Luxi County. Holotype: HFUT HL-21-08-002, a nearly complete skeleton, with the postcranium mostly disarticulated. Type locality: Huale Village, Luxi County, Yunnan Province, China. Type horizon: Upper (Second) Member of the Guanling Formation, Pelsonian, Anisian, Middle Triassic. Diagnosis.—A species of Mixosaurus characterised by the following combination of character states: anterior terrace of supratemporal fenestra reaching only the most posterior part of nasal (anterior terrace reaches the level of the external naris in M. panxianensis, M. cornalianus, and M. kuhnschnyderi); postorbital broad and postorbital portion of the skull around half the length of the orbit (similar to M. panxianensis and “M. xindianensis”, proportionally longer than in M. cornalianus and M. kuhnschnyderi); jugal without posteroventral process (variable in M. panxianensis, absent in M. cornalianus); no distinct plicidentine (also absent in M. cornalianus, present in M. panxianensis); anterior teeth slender and remarkably small (similar to M. cornalianus, different from M. panxianensis and M. kuhnschnyderi, in which the anterior dentition is relatively larger); dentition weakly heterodontous, posterior teeth robust but pointed (similar to M. cornalianus, different from “M. xindianensis”, M. pan xianensis, and M. kuhnschnyderi, in which the posterior teeth are molariform and mesiodistally elongated); centrum height/length ratio varying from <1.5 (anterior dorsal) to 2.0 (postflexural caudal) (up to 2.0 for M. cornalianus [Schmitz et al. 2004; Schmitz 2005], higher than 2.0 in M. panxianensis [Zang 2014], M. kuhnschnyderi [Brinkmann 2004], and “M. xindianensis” [Chen and Cheng 2010]); radius narrow (proximodistal length/midshaft width ratio = 2.4; 1.5–1.9 for M. panxianensis and M. cornalianus), carrying two notches on the leading edge (present in M. panxianensis, variable in M. cornalianus, absent in M. kuhnschnyderi and “M. xindianensis”); single notch on the posterior margin of the ulna (present in “M. xindianensis”, rarely occurring in M. pan xia nensis, absent in M. cornalianus, M. kuhns chnyderi, and “M. xinzhaiensis”); metacarpal V larger in size than distal carpal IV and probably bearing a notch (small and without notch in other Mixosaurus species, but similar in size and morphology to the Luoping specimen of Phalarodon ata vus [Liu et al. 2013]); proximal phalanges in digit 1 well-emarginated at the leading edge of the forefin FANG ET AL.—NEW MIXOSAURID ICHTHYOSAUR FROM THE MIDDLE TRIASSIC OF CHINA 267 (anterior margin complete in M. cornalianus, also emarginated in M. panxianensis and “M. xinzhaiensis”). Except for displaying the diagnostic features of Mixosaurus, HFUT HL-21-08-002 possesses the following character states inconsistent with a referral to Phalarodon: narial shelf and buccal ridge absent; dental groove present posteriorly; dentary labial shelf absent; plicidentine absent. Description.—HFUT HL-21-08-002 comprises an almost complete, partially articulated skeleton exposed in right lateral view, with only the posterior caudals missing (Fig. 2). Fig. 2. The holotype of the mixosaurid ichthyosaur Mixosaurus luxiensis sp. nov. (HFUT HL-21-08-002) from Huale Village, Luxi County, Yunnan Province, China (Pelsonian, Anisian). Photograph (A1) and interpretative drawing (A2). Dashed lines indicate inferred margins. 268 ACTA PALAEONTOLOGICA POLONICA 69 (2), 2024 The humeral head of HFUT HL-21-08-002 is not convex and evident striations are present on its surface. On the other hand, the basioccipital has a midline ridge and lateral bulges developed, and lacks a ventral invagination. These features indicate the specimen likely represents an advanced juvenile or osteologically immature adult (Liu 2011; Liu et al. 2013; Miedema et al. 2023a). The estimated total length of HFUT HL-21-08-002 is around 75 cm (based on the mandibular length, following the formula provided by Brinkmann 2004), making M. luxiensis sp. nov. one of the smallest known mixosaurids, comparable in size to M. kuhnschnyderi (Brinkmann 1998b, 2004). The mandible of HFUT HL21-08-002 (160 mm in length) is about the same length as that of the holotype of M. kuhnschnyderi (PIMUZ T 1324) (mandible length 164 mm; Brinkmann 1998b), but its basioccipital and appendicular elements are larger than those of PIMUZ T 1324 (basioccipital length 15 mm vs. 12.5 mm; basioccipital width 12.6 mm vs. 11 mm; scapular length 46 mm vs. 40 mm; coracoid length 47 mm vs. 32 mm; humerus length 24.1 mm vs. 20.5 mm; radius length 19.9 mm vs. 15.6 mm) (Brinkmann 1998a, 2004). Skull: The skull is mediolaterally compressed and exposed from its right lateral aspect (Fig. 3). However, because the skull was separated into two contralateral halves, the medial surfaces of the left part of the snout and the left skull roof can be seen exposed above the right side of the skull. Around 20 mm of the anterior tips of the premaxillae are missing. The preserved portion of the skull (length 131.2 mm, height 45.0 mm) is shorter than the complete mandibular rami (length ~160 mm). The orbit (length 36.8 mm, height 25.0 mm) is approximately oval in outline, although its dorsal margin is straight. The postorbital part of the skull (cheek region) is longer than half of the orbit length (18.5 mm) and occupies 27% of the postnarial length of the skull (68.6 mm). The postorbital region of HFUT HL-21-08-002 is similar in proportions to the postorbital region of M. panxianensis, P. atavus, and P. callawayi (Schmitz et al. 2004; Jiang et al. 2005), but differs from the condition in M. cornalianus and M. kuhnschnyderi, in which the postorbital portion of the skull is markedly shorter (Brinkmann 2004; Schmitz et al. 2004; Renesto et al. 2020). The premaxillae constitute most of the length of the slender snout. Posteriorly, the premaxilla wedges in between the nasal and maxilla, contributing slightly to the anteroventral margin of the external naris, a feature common in mixosaurids (Schmitz et al. 2004; Liu et al. 2011, 2013). The external naris is anteroposteriorly elongated and slit-like. It is bounded by the maxilla ventrally, whereas the nasal constitutes its whole dorsal margin. The maxilla possesses a very long anterior process, extending anteriorly nearly as far as the nasal. The slender posteroventral process of the maxilla, together with the tapering posteroventral process of the lacrimal, contacts the jugal at the level of the anterior margin of the orbit. The postnarial process of the maxilla, which is slightly damaged, contacts the prefrontal and separates the external naris from the lacrimal and prefrontal. Some mixosaurids have large neurovascular foramina on the external surface of the maxilla (Maisch and Matzke 2001; Brinkmann 2004; Schmitz et al. 2004; Jiang et al. 2005), but in HFUT HL-21-08-002 these foramina seem to be absent. The lacrimal comprises the anteroventral margin of the orbit and contacts the prefrontal dorsally, forming prominent, dorsal extensions. A pronounced antorbital ridge extends from the anterodorsal to the posteroventral part of the lacrimal. The prefrontal produces several projections anteroventrally, which interlock with the corresponding projections of the lacrimal, forming a serrate suture. The narrow posterior part of the prefrontal comprises the anterior part of the dorsal margin of the orbit, whereas the extensive anterior portion contacts the nasal, lacrimal and the postnarial process of the maxilla. Posteriorly, the prefrontal seems to contact the postfrontal along an oblique suture, but because of bone surface damage, this cannot be discerned with confidence. The prominent supraorbital crest is formed by the raised dorsal margins of the prefrontal and the postfrontal. The dorsal part of the temporal region is slightly damaged, but the postfrontal seems to contact the supratemporal posteriorly, excluding the postorbital from participation in the upper temporal fenestra, like in other mixosaurids with well-preserved skulls (Motani 1999b; Maisch and Matzke 2001; Schmitz et al. 2004). Direct observation of the anterior extent of the anterior terrace of the supratemporal fenestra on the nasal is hindered by the mediolateral compression of the skull. However, contrasting the well-preserved and convex surface of the nasal with the crushed and collapsed surface of the frontal reveals that most parts of the nasal, except for its posterodorsal corner, likely did not contribute to the anterior terrace. The sagittal crest, which forms the medial wall of the anterior terrace, is tall and anteroposteriorly elongated. It is formed by the parietal, frontal and nasal, with the frontal comprising its majority. However, because of numerous surface cracks, the sutural contacts between these bones cannot be confidently determined. Anteriorly, the sagittal crest extends to the level of the posterior end of the external naris, being shorter than the sagittal crests in species of Phalarodon and M. panxianensis, which extend beyond the anterior end of the external naris (Merriam 1910; Maisch and Matzke 1998a, 2000; Schmitz et al. 2004; Jiang et al. 2005). The sagittal crest, which is raised markedly above the supraorbital crest in lateral view, decreases in height anteriorly and forms a smooth transition with the snout, in contrast to forming an apparent terminal “step”, which is present in species of Phalarodon (Maisch and Matzke 1998a; Schmitz et al. 2004). Posteriorly, the sagittal crest terminates at the parietal-supratemporal suture. The supratemporal is divided into a lateral, a medial and a ventral process. It forms the posterior rim of the skull roof and the concave, posterior margin of the upper temporal fenestra. The squamosal is taller than long and possibly produces an anteroventral process, but this is difficult to confirm because the bone is severely damaged and pos- FANG ET AL.—NEW MIXOSAURID ICHTHYOSAUR FROM THE MIDDLE TRIASSIC OF CHINA 269 sesses several surface cracks. The postorbital constitutes the posterodorsal margin of the orbit and contacts the adjacent supratemporal, postfrontal, squamosal, and jugal. It is a relatively broad element, similar to the postorbital in species of Phalarodon (Motani 1999b; Schmitz et al. 2004) and M. panxianensis (Jiang et al. 2005, 2006), but different from the slender postorbitals of M. cornalianus and M. kuhnschnyderi (Brinkmann 2004; Renesto et al. 2020). The jugal forms the ventral and posteroventral margins of the orbit. It is a slender bone, with an approximately straight and narrow anterior ramus and a proportionally broader postorbital ramus. In contrast to the holotype of M. panxianensis, Fig. 3. The skull and lower jaw of the mixosaurid ichthyosaur Mixosaurus luxiensis sp. nov. (HFUT HL-21-08-002) from Huale Village, Luxi County, Yunnan Province, China (Pelsonian, Anisian). Photograph (A1) and interpretative drawing (A2). Dashed lines indicate inferred margins. The ’ symbol indicates left elements. Abbreviations: an, angular; ar, articular; ax, axis; bo, basioccipital; cb, ceratobranchial; d, dentary; f, frontal; j, jugal; l, lacrimal; m, maxilla; mc, Meckelian canal; n, nasal; p, parietal; par, prearticular; pl, palatine; pm, premaxilla; po, postorbital; pof, postfrontal; prf, prefrontal; pt, pterygoid; q, quadrate; qj, quadratojugal; san, surangular; sc, scleral plate; sp, splenial; sq, squamosal; st, supratemporal. 270 ACTA PALAEONTOLOGICA POLONICA 69 (2), 2024 the jugal does not produce a distinct posteroventral process (Jiang et al. 2006). Some other cranial elements are disarticulated from the rest of the skull and displaced from their original position. The broad quadrate can be seen inside the orbit. It has a large rounded dorsal lamella, and a smaller, stout articular condyle, which bears a transverse groove. The quadrate possesses a small, triangular process, also present in Besanosaurus and Guanlingsaurus (Bindellini et al. 2021). The basioccipital is exposed in ventral view and is very similar to the basioccipital of M. cornalianus, with both possessing an extensive extracondylar area (Maisch et al. 2006; Miedema et al. 2023a). The convex basioccipital condyle is smooth and bears a subtle notochordal pit. The extracondylar area has a midline ridge and protruding lateral wings, with no ventral invagination present (Miedema et al. 2023a). A bone exposed in the lower temporal embayment immediately posterior to the jugal possibly represents the posterolateral part of the pterygoid. The large sheet-like bones visible inside the ventral part of the orbit probably comprise the pterygoids and palatines. Some extremely compressed scleral plates are preserved on the surface of the exposed palatal elements. Mandible: The two mandibular rami are disarticulated from each other. The left ramus is preserved in anatomical position, but the right ramus underwent a 180° rotation and lies ventral to the skull. As a result, both rami fully expose their medial sides. The Meckelian canal forms a longitudinal groove on the dentary, until it becomes posteromedially closed by the splenial. The splenial produces two long, prong-like processes along the posterodorsal and posteroventral margins of the Meckelian canal, which clearly contact the dentary. However, the sutural contacts between the splenial and the angular and surangular are indeterminate due to numerous surface cracks caused by the collapse of the medial wall of the Meckelian canal. The suture between the dentary and surangular is also indeterminate. The surangular is exposed in the posterodorsal part of the medial wall of the mandibular ramus, and bears a very small paracoronoid process (= coronoid process of Schmitz et al. 2004) (Miedema et al. 2023a). Posteriorly, the prearticular exposes its triangular medial surface. It wedges in between the dorsally positioned surangular and the ventrally positioned angular, and contacts the splenial anteroventrally. The articular comprises the most caudal part of each mandibular ramus. Anteriorly, it bears a shallowly concave articular surface for the quadrate, and possesses a smooth, saddle-shaped medial surface. Two bar-like bones lying between the mandibular rami are interpreted to be the ceratobranchials of the hyoid apparatus. They are straight and have a truncated anterior end. Dentition: As in most other mixosaurids (with the exception of P. atavus; Liu et al. 2013; Engelschiøn et al. 2023), heterodonty is clearly exhibited by HFUT HL-21-08-002. Because the anterior tip of the upper jaw is missing, the most mesial teeth are only preserved in the mandible, whereas middle and distal teeth are well preserved in both the upper jaws and the mandibular rami. The very tip of the lower jaw is edentulous, with the first tooth preserved 6.0 mm posterior to the tip of the left mandibular ramus. This is similar to the condition in Phalarodon fraasi, which was also reported to have an edentulous snout tip (Nicholls et al. 1999). The dentary teeth are set in a continuous dental groove, which has a labial wall evidently higher than the lingual wall, and is very pronounced posteriorly. Individual tooth alveoli seem to be absent and some teeth are spaced very closely to each other, so the tooth implantation mode in the dentary can be determined as aulacodonty or subthecodonty (if unexposed sockets are present at the bottom of the dental groove) (Motani 1997b; Bertin et al. 2018). The mode of tooth implantation in the upper jaws in not possible to infer. The most mesial teeth are extremely small, with the crown height of the first preserved tooth measuring only around 1.1 mm in height and 0.7 mm in basal width (height:width ratio 1.57). More distally, in the middle of the dentigerous region, the teeth become larger and more robust. The posterior dentition bears the largest teeth, with the largest of the fully exposed posterior teeth reaching a crown height of about 2.5 mm and a crown basal width of about 2.1 mm (height:width ratio = 1.19), being more than twice the size of the anterior teeth. The most posterior teeth are smaller than the preceding ones but are more robust, with their crowns being noticeably broadened (mesiodistally), with basal widths exceeding their heights (e.g., the last right dentary tooth has a crown height of 1.7 mm and a crown basal width of 1.8 mm, which gives a height:width ratio of 0.94). There is no shape difference between the corresponding upper and lower teeth. All teeth, including the posterior ones, have a pointed apex. There are no molariform teeth present, in contrast to M. kuhnschnyderi, M. panxianensis, P. callawayi, and P. fraasi (Brinkmann 1998b; Schmitz et al. 2004; Motani 2005a; Jiang et al. 2006; Chen and Cheng 2010). Longitudinal striations can be observed along the entire height of the crown surfaces. The roots have a matte surface texture, different from the shiny crown enamel surface. Many of the roots are mediolaterally flattened, which might reflect the crushing of pulp cavities. No furrows indicating the presence of dentine infolding can be observed in the roots of HFUT HL-2108-002, which resembles the condition in M. cornalianus (Maxwell et al. 2012a), but contrasts with the morphology in other mixosaurids, like species of Phalarodon and M. panxianensis, in which plicidentine is present (Merriam 1910; Nicholls et al. 1999; Schmitz et al. 2004; Jiang et al. 2006; Chen and Cheng 2010; Maxwell et al. 2012a). In general, the dentition of HFUT HL-21-08-002 is most similar to the dentition of M. cornalianus (e.g., PIMUZ T 2418), as both taxa share moderate heterodonty with tiny, mesial-most teeth and stouter, non-molariform posterior teeth (Fig. 4). However, the dentition of both taxa differs in that the distal teeth of M. cornalianus are about the same size as the mesial teeth, whereas in HFUT HL-21-08-002 the distal teeth are about twice the size of the mesial teeth. FANG ET AL.—NEW MIXOSAURID ICHTHYOSAUR FROM THE MIDDLE TRIASSIC OF CHINA 271 Axial skeleton: The majority of the vertebral column is disarticulated, but three cervical neural arches (2–4) and the middle caudal vertebrae are preserved as articulated series. Only the axial neural arch is articulated with its corresponding centrum (axis), whereas the two other neural arches are isolated. The neural spine of the axis is trapezoid in outline and bears a vertical groove on its lateral surface. It is 10 mm high and 8 mm wide at its base and is markedly broader than the subsequent cervical neural spines. All three preserved cervical neural arches bear suboval, dorsoventrally elongated diapophyses, which articulated with the tuberculum of the bicipital cervical ribs (SOM 1: fig. S1). This condition is similar to the one reported for Phalarodon callawayi, in which the diapophysis is also located on the cervical neural arch (Schmitz et al. 2004). Anteriorly projecting prezygapophyses and posteriorly projecting postzygapohyses are well-developed in the cevical neural arches. The amphicoelous posterior dorsal and anterior caudal centra are disarticulated and scattered in the posterior portion of the specimen and their corresponding neural arches are also disarticulated and often broken. Several hexagonal (in articular view) and mediolaterally compressed centra, representing the more posterior caudal centra, are also scattered in the same area. Most dorsal centra possess single rib facets, but some centra located close to the sacral region have double rib facets. In the middle dorsal region, the centra vary from about 8–9 mm in height and 6–7 mm in length, with their corresponding neural arches reaching up to 20 mm in height. In the posterior dorsal/anterior caudal region, centra achieve their maximum size, the largest being over 10 mm in height and around 7–8 mm in length. The longest neural spine from this region is about 21 mm tall. A series of 18 consecutive centra and two incomplete neural spines, measuring 112.9 mm in total length, comprises the most posterior part of the preserved vertebral column (SOM 1: fig. S2, table S2). Based on the relative length and orientation of the neural spines, as well as the disappearance of rib facets in the seventh centrum in the preserved series, these vertebrae can be confidently identified as representing the caudal peak and its immediate vicinity. 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