Mixosaurus luxiensis, Fang & Wolniewicz & Liu, 2024

Fang, Ye-Wei, Wolniewicz, Andrzej S. & Liu, Jun, 2024, A new species of mixosaurid ichthyosaur from the Middle Triassic of Luxi County, Yunnan Province, South China, Acta Palaeontologica Polonica 69 (2), pp. 263-280 : 266-275

Mixosaurus luxiensis sp. nov.

Figs. 2–7 ; SOM 1: figs. S1, S2, tables S1, S2.

ZooBank LSID: urn:lsid:zoobank.org:act:

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. panx ianensis , 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. panx iane nsis, absent in M. cornalianus , M. kuhnsc hnyderi , 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 atav us [ Liu et al. 2013]); proximal phalanges in digit 1 well-emarginated at the leading edge of the forefin (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 ). 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 HL-21-08-002 ( 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 possesses 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 , 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-21- 08-002, 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.

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. The centrum height/length ratios in the centra located immediately posterior to the caudal peak are only around 2.0, which is also characteristic for M. cornalianus Nicholls et al. 1999 ; Schmitz et al. 2004; Schmitz 2005). On the dorsal side of the articulated caudal series, most of the neural spines located posterior to the caudal peak are clearly visible. In the posterior part of the articulated caudal series, the slender haemal arches (chevrons) articulate between the adjacent centra.

The ribs in the anterior trunk region are largely fractured and fragmented. The posterior ribs are more completely preserved, the longest measuring up to 100 mm in chord length. The dorsal ribs of HFUT HL-21-08-002 possess a single head, are slightly expanded at their distal ends, and possess a longitudinal furrow on their anterior/posterior surfaces. Clusters of slender, disarticulated gastral elements are preserved in the ventral part of the specimen. Several boomerang-shaped median gastral elements can be recognised, some of them bearing a short anterior process. The lateral gastral elements are needle-like and mostly straight.

Appendicular skeleton: The elements of the pectoral girdle and forelimbs are partly disarticulated and expose their flattened surfaces ( Fig. 5 ). The left forefin and the left coracoid are partially overlapped by their right counterparts, whereas the outline of the glenoid process of the left coracoid is embossed on the right coracoid. The right coracoid measures 47 mm in length, whereas the width of the left coracoid equals 27.3 mm. The right scapula is of similar size to the coracoids, measuring 46 mm in length and 27.5 mm in width. The coracoids and scapula are fan-shaped, like in other mixosaurids and other basal ichthyopterygians ( McGowan and Motani 2003). The scapula is approximately symmetrical, whereas the glenoid processes of the coracoids are situated much more posteriorly, making the coracoids asymmetric. The interclavicle is preserved in dorsal view, attached to the anteromedial margin of the left coracoid. Its anterior edge bears articular facets for the clavicles. The triradiate interclavicle possesses broad transversal bars and a relatively shorter posterior process. The general shape of the interclavicle resembles the interclavicles of M. kuhnschnyderi , M. cf. cornalianus Type A, and some Chinese mixosaurids (e.g., CCCGS LPV 30986, GMPKU P-1065; Liu 2011; Zang 2014) more than the interclavicle of M. cornalianus Type B (posterior process longer than the transversal bars, e.g. PIMUZ T 2420 [ neotype]) Brinkmann 1998a, 1999, 2004; Jiang et al. 2006). The clavicles are slender, gently curved, rod-like bones, partly covered by the right scapula and right coracoid. The outlines of the clavicles are also partly embossed on the coracoids. The left clavicle is preserved in articulation with the interclavicle, whereas the contralateral clavicle is disarticulated and displaced anteriorly.

The right forelimb of HFUT HL-21-08-002 is exposed from its dorsal side. The length of the humerus is 24.1 mm, and its maximum width at the distal end is 19.6 mm. Compared with the relatively more elongated humerus in species of Phalarodon ( Motani 1999a; Brinkmann 2004; Schmitz et al. 2004; Jiang et al. 2006; Liu 2011), it is more similar to the humeri in other species of Mixosaurus ( Maisch and Matzke 1998b; Kolb et al. 2011; Zhou et al. 2022). The radius is 19.9 mm long, 11.4 mm wide proximally and 12.5 mm wide distally. The slenderness of the radius (proximodistal length/midshaft width ratio = 2.4) differs from other Mixosaurus species, in which the length/width ratio of the radius <2 ( Brinkmann 2004; Zhou et al. 2022). Anteriorly, the leading (anterior) edge of the radius bears two shallow notches located proximally and distally, with the middle portion situated between the notches forming a convex margin. The maximum mid-length width of the radial shaft is 8.4 mm, whereas its narrowest width at the notches is 7.6 mm. The two notches on the leading edge of the radius were regarded as a unique feature of M. panxinensis ( Jiang et al. 2006; Maisch 2010), but they also occur in Mixosaurus from the Luoping Biota (e.g., CCCGS LPV 30986, YIGMR SPCV-0810, and YIGMR SPCV-0831, Chen and Cheng 2009; Liu 2011; Chen et al. 2016) and several specimens of European Mixosaurus (e.g., SMNS 54068, Zhou et al. 2022; the fetus of PIMUZ T 2262, Miedema et al. 2023b). The approximately lunate ulna bears a prominent notch on its trailing (posterior) edge. The posterior notch of the ulna is rare among mixosaurids, but was reported in referred specimens of M. panxianensis (GMPKU P-1038, GMPKU P-1065, Zang 2014; Zhou et al. 2022), “ M. xindianensis ” (YIGMR SPCV-0732, Chen and Cheng 2010) and Phalarodon callawayi (PMU 45386 [= PMU R 191 in Motani 1999a; Schmitz et al. 2004]). The ulna has a proximodistal length of 18.3 mm, a proximal width of 11.4 mm, a distal width of 13.0 mm and its narrowest width at the notch equals 8.3 mm.

The proximal portion of the right manus is disarticulated to some extent, whereas the distal parts of both the left and right manus are preserved in articulation. The right manus is completely preserved, but some of its elements are covered by disarticulated gastralia and its trailing edge is largely obscured by gastralia and ribs. Four proximal carpals including a round pisiform can be discerned, and the distal carpals are preserved in close association with the corresponding proximal carpals. The pisiform is partly obscured by the ulna. The pentagonal intermedium comprises the largest carpal. It is wider than long, having a width of 9.6 mm and a length of 8.0 mm. The distal carpals are smaller than the proximal carpals. Distal carpals I, III, IV are quadrangular in outline, whereas distal carpal II is pentagonal. The metacarpals and proximal phalanges are hourglass-shaped and are similar to each other, except for metacarpals I and V. These two metacarpals are broader, metacarpal I being subquadrangular in outline, and the dislocated metacarpal V being suboval and probably bearing a notch. HFUT HL-21-08-002 shares the presence of a large (much larger than distal carpal IV) and notched metacarpal V with some mixosaurids from Luoping (e.g., CCCGS LPV 30872, 30986, Liu 2011; Liu et al. 2013). However, whereas the notch is located anterodistally in metacarpal V of mixosaurids from Luoping, metacarpal V in HFUT HL-21-08-002 is disarticulated and the element could have undergone rotation, so it is not possible to determine its exact orientation in the latter. The elements in digit I are shorter than the corresponding elements in digit II, which is clearly visible in the third phalangeal row. All phalanges (except the most distal ones) in digit I have a notched leading edge, a feature that is also typical for M. panxianensis but absent in M. cornalianus ( Maisch and Matzke 1998b; Motani 1999a; Renesto et al. 2020; Zhou et al. 2022). The most distal phalanges do not possess a midshaft constriction and appear rounded in shape. The distal part of the left forelimb is partially visible ventral to the right forelimb and exposed in ventral view. Its phalangeal morphology is consistent with that of its right counterpart.

The bones comprising the pelvic girdle and hindlimbs are scattered in the posterior part of the specimen ( Fig. 6 ). The plate-like pubes are partially preserved and exposed in ventral or dorsal view. One pubis is located near the right hindlimb and is relatively complete, whereas the other lies near the ribcage, is partially damaged, and encloses an obturator foramen. A single identifiable ischium is fan-shaped and is located in the ventral part of the fossil. Both femora are exposed in ventral view with the prominent ventral process clearly visible, extending from the femoral head to the mid-shaft region ( Maxwell et al. 2012b). The expanded distal end is wider than the rounded proximal head and has clearly demarcated tibial and fibular facets. The right femur (length 16.6 mm, distal width 11.5 mm) lies closely to the more completely preserved pubis and is loosely connected with the zeugopodium, whereas the left femur is disarticulated and dislocated from the remainder of the left hindlimb. The length of the right tibia equals 13.5 mm, and the widths of its expanded proximal and distal ends are 9.4 mm and 9.9 mm, respectively. The distal end of the fibula (length 13.2 mm) is distinctly expanded. The left zeugopodium, which is similar in shape and size to its right counterpart, is located near a cluster of indeterminate, flattened bones. These bones probably represent some of the other elements of the pelvic region but cannot be identified with confidence.

The right tarsals are loosely connected to the zeugopodium—the astragalus, calcaneum and three distal tarsals can be discerned. The more distal metatarsals and phalanges of the right hindlimb are partly concealed by the scattered ribs and centra, but these elements are well-preserved in the left autopodium. Among the left tarsals, the subcircular calcaneum and polygonal astragalus are the most conspicuous. Metatarsal V is oval in outline and has a small anterior notch. Distally, the metatarsals and phalanges in the left hindfin are heavily abraded and extremely compressed, so that the individual elements are very difficult to discern. The elements in digits III and IV are longer than the corresponding elements in digit V.

Stratigraphic and geographic range.— Type locality and horizon only.

Figures

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.

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.

Fig. 4. A comparison of dentition in species of the mixosaurid ichthyosaur Mixosaurus. A. Mixosaurus panxianensis Jiang et al., 2006, from Panzhou, China, middle Anisian, GMPKU P-1039. B. Mixosaurus kuhnschnyderi Brinkmann, 1998b, from Monte San Giorgio, Italy and Switzerland, upper Anisian–lower Ladinian, PIMUZ T 1324. C. Mixosaurus cornalianus (Bassani, 1886) from Monte San Giorgio, Italy and Switzerland, upper Anisian– lower Ladinian, PIMUZ T 2418. D. Mixosaurus luxiensis sp. nov.from Luxi, China, middleAnisian, HFUT HL-21-08-002. B and C depict the mandibular dentition only. Scale bars 10 mm.

Fig.5.The pectoral girdleand forefins of the mixosaurid ichthyosaurMixosaurus luxienis sp.nov.(HFUT HL-21-08-002) from Huale Village,LuxiCounty, Yunnan Province, China (Pelsonian, Anisian). Photograph (A1) and interpretative drawing (A2). Dashed lines indicate inferred margins. Abbreviations: cgp, glenoid process of coracoid; dc 1–4, distal carpals 1–4; i, intermedium; icl, interclavicle; mc I–V, metacarpals I–V; pi, pisiform; ra, radiale; sgp, glenoid process of scapula; ul, ulnare.

Fig. 6. The pelvic girdle and hindfins of the mixosaurid ichthyosaur Mixosaurus luxiensis sp. nov. (HFUT HL-21-08-002) from Huale Village, Luxi County, Yunnan Province, China (Pelsonian, Anisian). A. Right pubis and right hindlimb elements. B. Left pubis. C. Left femur in ventral view. D. Left ischium and left hindlimb. Dashed lines indicate inferred margins; the ’ symbol indicates left elements. Abbreviations: ast, astragalus; cal, calcaneum; dt, distal tarsal; fe, femur; fi, fibula; is, ischium; mt, metatarsal; of, obturator foramen; pb, pubis; ti, tibia; vp, ventral process of femur.

Fig. 7. The phylogenetic position of Mixosaurus luxiensis sp. nov. within Ichthyosauromorpha. A. Strict consensus tree, with Bremer values larger than 1 indicated at relevant nodes. B. The topology of Mixosauridae obtained in the 50% majority rule consensus tree, with the percentages of MPTs in which a particular clade is recovered given at relevant nodes. C. The reduced strict consensus tree of Mixosauridae obtained through IterPCR, with Bremer values larger than 1 indicated at relevant nodes. See Material and methods for analytical procedure.