Dianmeisaurus mutaensis, Hu & Li & Liu, 2024

Hu, Yi-Wei, Li, Qiang & Liu, Jun, 2024, A new pachypleurosaur (Reptilia: Sauropterygia) from the Middle Triassic of southwestern China and its phylogenetic and biogeographic implications, Swiss Journal of Palaeontology (1) 143 (1), pp. 1-15 : 3-10

publication ID	https://doi.org/ 10.1186/s13358-023-00292-4
DOI	https://doi.org/10.5281/zenodo.12795192
persistent identifier	https://treatment.plazi.org/id/8A24D705-FFB3-BC0A-E6E3-FA5FD95F6BC6
treatment provided by	Felipe
scientific name	Dianmeisaurus mutaensis
status	sp. nov.

Treatment

Dianmeisaurus mutaensis sp. nov.

Holotype

HFUT MT-21-08 - 001 , a complete and articulated skeleton exposed in dorsal view (part and counterpart).

Ontogenetic evaluation

There are currently three known specimens of Dianmeisaurus , among which HFUT MT- 21-08-001 represents the smallest individual with a total body length of 99.2 mm. It is much smaller than the two published specimens of Dianmeisaurus gracilis (315 mm for the holotype, see Shang & Li, 2015; 250 mm for IVPP V 17054, see Shang et al., 2017). Several morphological characters indicate that HFUT MT- 21-08-001 is skeletally immature. First, the skull is poorly ossified. The skull has fontanelles, which generally indicates that the individual is at an early ontogenetic stage (Lin & Rieppel, 1998; Piñeiro et al., 2012; Rieppel, 1992a, 1992b, Rieppel, 1993a; Wise et al., 2009). Secondly, the distal end of the humeri is incompletely ossified since the entepicondylar groove is still present. The entepicondylar groove starts close and turns into a foramen when the individual becomes more mature (Currie & Carroll, 1984; Sander, 1989). Thirdly, among the carpals and tarsals of HFUT MT- 21–08-001, only the astragalus is ossified, also indicating an early ontogenetic stage of the individual (Fröbisch, 2008; Rieppel, 1992b; Sander, 1989). All these lines of evidence strongly support the conclusion that HFUT MT- 21-08- 001 is skeletally immature.

Type locality

Muta Village, Luxi County, Yunnan Province, China.

Type horizon

Upper Member of Guanling Formation, Anisian, Middle Triassic.

Etymology

Named after Muta village where the holotype was collected.

Diagnosis

A pachypleurosaur with following autapomorphies among pachypleurosaurs: 23 cervical vertebrae, 20 dorsal vertebrae, and two sacral vertebrae; postfrontal extending posteriorly to a level beyond the middle of parietal; last dorsal rib stout and shorter than the first sacral rib; phalangeal formula of manus and pes 2-3-4-4-2 and 1-2- 3-4-3 respectively. In addition to the above-mentioned autapomorphies, Dianmeisaurus mutaensis also differs from D. gracilis in the following morphological characters: maxilla enters the external naris; anterior process of the frontal does not extend beyond the anterior margin of the orbit; postfrontal excluded from the upper temporal fenestra; coronoid process absent.

Description

The skeleton, embedded in the dark-grey micritic limestone, consists of a part and its counterpart. The specimen is well-preserved, with a total length of 99.2 mm. Adjacent to the specimen, there are scattered limb and rib bones from other individual(s), but the limited information available prevents further identification.

Skull

The skull of HFUT MT- 21-08-001 is dorsoventrally compressed and slightly distorted ( Fig. 2 View Fig ). The surface of the dermatocranial bones shows weak sculpturing. The preorbital region of the skull is slightly shorter than the postorbital region. The snout is very short and round anteriorly. The occipital portion is plate-like without an occipital crest.

The paired premaxillae constitute the short snout in front of the external nares and the anterior margin of the external nares. The posterodorsal processes of the premaxilla extend backward along the midline, separating the anterior part of the nasal. Each posterolateral process of the premaxilla contacts the maxilla around the lateral margin of the external naris, where the snout constriction is absent as in most pachypleurosaurs ( Rieppel, 2000), but a depression is developed.

The maxilla forms the anterolateral margin of the orbit. The anterior process of the maxilla runs into the lateral corner of the external naris where it contacts the premaxilla. Dorsally, the ascending process of the maxilla is wedged between the nasal anteromedially and the prefrontal posteromedially. It almost reaches the level of the midpoint of the anterior margin of the orbit. A small pit is located at the maxilla-prefrontal suture, which is close to the anterolateral corner of the orbit. The posterior process of the maxilla abuts the lateral margin of the jugal and reaches the posterior margin of the orbit.

The external naris is located anteriorly, quite close to the tip of the snout, as is also the case in Dianmeisaurus gracilis and Panzhousaurus (Jiang et al., 2019) . The length from the tip of the snout to the anterior margin of the external naris divided by the condylobasal skull length is 0.08. The longitudinal diameter of the external naris is less than its transverse diameter and the longitudinal diameter of the orbit. The lateral corner of the external naris shows an acute angle.

A pair of roughly triangular nasal bones meet along the midline, with the contact length comprising 3/4 of the total nasal length. Anteriorly, the nasal forms the posterior and part of the dorsal margin of the external naris. Anterodorsally, the paired nasals embrace the posterior processes of the premaxillae. Anterolaterally, the nasals are not well ossified, leaving an open gap with the maxilla and prefrontal. This gap is interpreted as a morphological feature related to the early ontogenetic stage of the specimen, i.e., a fontanelle. The posterior processes of the nasal separate the posterior processes of the prefrontal and taper backward to overlie the frontal, almost reaching the midpoint of the medial margin of the orbit, which is an autapomorphy of the species. The surface of the nasal shows a few deep pits.

The circular orbit is large, about twice longer than the upper temporal fenestra. The interorbital septum is extremely narrow. The minimum width of the interorbital septum is distinctly shorter than the minimum distance between the external nares, a synapomorphy of Dianmeisaurus . The prefrontal forms the anterodorsal margin of the orbit. Laterally, the prefrontal contacts the maxilla. The posterior process of the prefrontal meets the frontal. The paired frontals form the dorsal margin of the orbit. The anterior process of the frontal almost extends to the midpoint of the medial margin of the orbit, which is convergently present in Dianopachysaurus among pachypleurosaurs (Liu et al., 2011). The anterior process of the frontal in other pachypleurosaurs extends very close to or beyond the anterior margin of the orbit (e.g., Cheng et al., 2016; Jiang et al., 2019; Klein et al., 2022; Shang et al., 2011; Xu et al., 2022). The well-developed posterolateral processes of the frontals are widely separated from the upper temporal fenestra and enter between the postfrontal and parietal.

The postfrontal forms the posterodorsal margin of the orbit. It has a roughly triradiate shape. The lateral margin of the postfrontal is distinctly constricted, which is also present in Dianmeisaurus gracilis , Anarosaurus , Honghesaurus , and Prosantosaurus among pachypleurosaurs (Klein, 2009; Klein et al., 2022; Shang et al., 2017; Xu et al., 2022). The postfrontal is separated from the upper temporal fenestra by the postorbital and parietal, which is otherwise only seen in Honghesaurus (Xu et al., 2022) among pachypleurosaurs. The posterior process of the postfrontal is embraced by the parietal and extends beyond the midpoint of the skull table, a synapomorphy shared with Panzhousaurus (Jiang et al., 2019) among pachypleurosaurs.

The postorbital defines the lateral and the entire anterior margin of the upper temporal fenestra. The lateral process of the postorbital contacts the jugal. The dorsal process of the postorbital narrowly meets the parietal, separating the postfrontal from the upper temporal fenestra. The posterior process of the postorbital contacts the squamosal with an interdigitated suture.

The boomerang-shaped jugal constitutes the posterolateral corner of the orbit. The ventral margin of the jugal contacts the maxilla. The posterodorsal process of the jugal covers the postorbital, being separated from the squamosal by the postorbital.

The parietals are partly fused. A distinct suture is present in front of the pineal foramen, dividing the paired parietals, but the parietal is fully fused behind the pineal foramen. The parietal is very broad. Anteriorly, the interdigitated parietal–frontal suture is located anterior to the posterior margin of the orbit. A large unossified gap between the frontal and parietal indicates the existence of another fontanelle. Laterally, the parietal constitutes the posterodorsal margin of the upper temporal fenestra. Posteriorly, the parietal contacts the squamosal and the supraoccipital. The circular pineal foramen is in the middle of the parietal table.

Due to the postmortem alteration, the right upper temporal fenestra is completely covered by the right squamosal, and the left fenestra is incomplete. Even so, it is conclusive that the upper temporal fenestra is distinctly shorter than the orbit.

The large squamosal is irregular in shape due to its postmortem alteration. The forked anterior process of the squamosal forms half of the supratemporal arch. The lateral process of the squamosal caps the quadrate. Medially, the squamosal contacts the dorsal margin of the supraoccipital.

The quadrate is partially covered by the squamosal. The condylar portion of the left quadrate is exposed in lateral view. The quadrate shows a concave region on the right side of the skull.

The supraoccipital is exposed horizontally without a medial crest. The supraoccipital contacts the parietal anteriorly with a V-shaped suture and the squamosal laterally. Posteriorly, it meets the exoccipital-opisthotic complex. The basioccipital is located at the same level as the mandibular articulations.

The relationships between bones of the lower jaw are indeterminate due to the poor preservation of the exposed surface. However, a coronoid process is certainly absent, which is different when compared to Dianmeisaurus gracilis , Diandongosaurus and Keichousaurus (Holmes et al., 2008; Shang et al., 2011, 2017). The dentary extends posteriorly to the midpoint of the orbit and contacts the angular. Medially, the angular meets the surangular. Both of them contribute to the lateral and dorsal margins of the lower jaw. The articular shows a distinct trough in dorsal view, forming the dorsal part of the well-developed retroarticular process. The prearticular is disarticulated from the articular and constitutes the floor of the retroarticular process.

The dentition of the right side is better preserved than the dentition of the left side. Thus, the following description is based on the dentition of the right side. All teeth have a pointed apex. Two premaxillary teeth are visible and are similar in size. No premaxillary and maxillary fangs are present. Five maxillary teeth are visible, of which the second is distinctly smaller than the others, likely because it represents a replacement tooth.

Postcranial skeleton

Vertebrae HFUT MT- 21-08-001 comprises 23 cervical vertebrae, 20 dorsal vertebrae, only two sacral vertebrae, and at least 40 caudal vertebrae ( Fig. 2 View Fig ). All zygapophyses are pachyostotic and no intercentra are present. The atlas is dislocated and covered by the basioccipital. Two triangular atlas arches are disarticulated and well exposed ( Fig. 3 View Fig ). The cervical vertebrae have low neural spines. The neural spines of the dorsal region are also low, and there is no elongated transverse process on the dorsal region. The caudal vertebrae become smaller posteriorly.

Ribs The ribs are slender ( Fig. 3 View Fig ). The distal end of the dorsal rib becomes flat and slightly expanded. The first dorsal rib is almost twice as long as the last cervical rib. The last (20th) dorsal rib is short and robust. It is shorter than the sacral ribs and all other dorsal ribs, which represents an autapomorphy of the species among Pachypleurosauria. The distal end of last dorsal rib shows an expansion that is slightly more obvious than the distal expansion of other dorsal ribs. However, the last dorsal ribs are too short to articulate with the ilium and do not extend toward the ilium. Additionally, there is no evidence of dislocation for the last dorsal ribs ( Fig. 4 View Fig ). So they can not be sacral ribs, but at the best be called transitional ribs ( Romer, 1956). The left side of the sacral ribs are completely exposed ( Fig. 4 View Fig ). The distal end of the sacral ribs is slightly expanded. The slender caudal ribs all taper to a point. The first caudal rib extends perpendicularly to the body axis. The third caudal rib is the longest among all caudal ribs. From the third caudal rib to the sixth, the length of the caudal rib is reducing gradually. The 1st–6th caudal ribs of this specimen are prominent. In Dianmeisaurus gracilis , the 1st–9th caudal ribs are prominent, while in Panzhousaurus , prominent caudal ribs are present on 1st–11th caudal vertebrae.

Pectoral girdle Among the pectoral girdle elements

( Fig. 4 View Fig ), the interclavicle and coracoid are completely covered by ribs and vertebrae. The left scapula is exposed in the lateral view, while the right scapula is exposed in the medial view. As in all other sauropterygians (Klein et al., 2022; Rieppel, 2000), the posterior margin of the clavicle is connected to the medial surface of scapula, a synapomorphy of sauropterygians. The posteriorly directed dorsal wing of the scapula is rod-like and tapers to a blunt tip, a synapomorphy of eosauropterygians ( Rieppel, 2000).

Forelimb The right forelimb is preserved completely

( Fig. 5 View Fig ). The humerus is curved as in all sauropterygians. Owing to the weakly developed deltopectoral crest, the preaxial margin of the humerus is slightly angulated. The distal end of the humerus is slightly broadened. Due to its early ontogenetic stage, an entepicondylar groove can be seen in this specimen, instead of the entepicondylar foramen ( Sander, 1989). The ulna is shorter than the radius. The preaxial margin of the ulna is smoothly concave. Both ends of the ulna are slightly expanded. The radius is straight, with its proximal part slightly wider than the distal end. The proximal end and mid-shaft of the radius are approximately as broad as those of the ulna.

No carpal element is ossified. Metacarpal 1 is distinctly shorter and stouter than metacarpals 2- 4, of which metacarpal 3 is the longest. The phalangeal elements are tightly connected. The phalangeal formula of the manus is 2-3-4-4-2.

Pelvic girdle The pelvic girdle is partially exposed in dorsal view ( Fig. 6 View Fig ). The dorsal blade of the ilium is reduced to a simple stub. The pubis and ischium are flat bones, thickened dorsoventrally at the lateral margin. The ischium shows the concave postaxial margin.

Hindlimb The hind limb is well preserved except for the distal portion of the left femur ( Figs. 3 View Fig , 7 View Fig ). The femur is long and sigmoidally curved, and the ratio of femur length divided by humerus length is 1.39. The anterior and posterior femoral condyles are subequally extended. The internal trochanter is absent. The fibula and tibia are equal in length, but the flat tibia is much more broadened than the fibula. The small round astragalus is the only ossified tarsal bone. Metatarsal 1 is the shortest and stoutest element of the metatarsals, while others are long and slender. Metatarsals 3 and 4 are approximately the same length, slightly longer than metatarsals 2 and 5. The phalangeal formula of the pes is 1-2-3-4-3.

Phylogenetic analysis

To assess the phylogenetic position of Dianmeisaurus mutaensis among eosauropterygians, we compiled a new data matrix consisting of 203 characters, among which 182 are informative, for 43 taxa. The matrix was based on a revised version of the one presented by Li and Liu (2020), and many new characters were added through a comparative study. We also coded several new taxa of eosauropterygians discovered in recent years, including Qianxisaurus (Cheng et al., 2012) , Odoiporosaurus ( Renesto et al., 2014) , Dianmeisaurus (Shang & Li, 2015; Shang et al., 2017), Dawazisaurus (Cheng et al., 2016) , Panzhousaurus (Jiang et al., 2019; Lin et al., 2021), Honghesaurus (Xu et al., 2022) , and Prosantosaurus (Klein et al., 2022) . In our phylogenetic analysis, Araeoscelidia, Younginiformes, Archosauromorpha, and Placodus were still selected as consecutive outgroups as in Li and Liu (2020).

Heuristic searches of the new data matrix (only the 182 informative characters were included for the analysis) found five most parsimonious trees (tree length = 797, consistency index = 0.3049, retention index = 0.6139). Dianmeisaurus mutaensis forms the sister group to D. gracilis . The strict consensus tree recovered Dianmeisaurus as the sister group to Panzhousaurus . The clade consisting of Dianmeisaurus and Panzhousaurus occupies the basal-most position of Pachypleurosauria ( Fig. 8 View Fig ). Meanwhile, the monophyly of Eusauropterygia is collapsed. Pistosauroidea, Majiashanosaurus , and Hanosaurus constitute the consecutive sister group to a monophyletic clade including Pachypleurosauria and Nothosauroidea. Our phylogenetic analysis recovered a monophyletic Pachypleurosauria clade, which is supported by six unambiguous synapomorphies: bones in the dermatocranium relatively smooth (character 1: 1); the ratio of longitudinal diameter of upper temporal divided by that of orbit is between 0.5 and 1 (character 45: 2); presence of a trough on the dorsal surface of retroarticular process (character 82: 1); anterolaterally expanded corners of clavicles present (character 128: 1); anterior preaxial margin of shaft of radius rather straight (character 190: 2); pes ungual phalanges extremely expanded (character 194: 1).