The Chroniosuchians were an enigmatic clade of non-Amniotic Tetrapods with uncertain phylogenetic position. This group can be divided into two families, Bystrowianidae and Chroniosuchidae. The earliest representatives of both families are known from the middle Permian Dashankou Fauna of Gansu Province, China. The latest representative of Chroniosuchidae is known only from a single Triassic taxon, Madygenerpeton pustulatus, from the Ladinian or Carnian of Kyrgyzstan; while those of Bystrowianidae are known from Middle Triassic of Russia and Germany.
This group was first reported from the late Permian Jiyuan fauna in China, i.e. Bystrowiana sinica; later, two more species were named and referred to Bystrowianidae for this fauna (Jiyuanitectum flatum and Dromotectum largum). Some postcranial bones were identified as Bystrowianid from the Permo-Triassic Guodikeng Formation of Jimusar in Xinjiang Province, China, but no definite Triassic Chroniosuchian is known from China up to now. Recent fieldwork has shown that Chroniosuchians were a diverse and persistent group in Xinjiang: they survived from the late Permian to at least the Early Triassic here.
In a paper published in the journal Vertebrata PalAsiatica in October 2020, Liu Jun of the Key Laboratory of Vertebrate Evolution and Human Origins at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, the Center for Excellence in Life and Paleoenvironment, and the College of Earth and Planetary Sciences of the University of the Chinese Academy of Sciences, reports three new Chroniosuchian specimens from Xinjiang, including the first definite Triassic Chroniosuchian from the Jiucaiyuan Formation.
The first specimen described, IVPP V 26539, is a left femur from the upper part of the middle Permian Quanzijie Formation at the Dalongkou section in Santai in Jimusar, Xinjiang.
This femur is slightly dorsoventrally compressed, and is not so curved as the femur of IVPP V 23295. It is slender and waisted as other Chroniosuchians. The anterior margin is more concave than the posterior margin. The length of the bone is 40 mm, and the width is 13, 15 mm respectively for the proximal and distal ends. Both expanded ends are incompletely ossified. The unfinished proximal articular surface is convex in outline dorsally and slightly concave ventrally when viewed mesially. It extends anteroventrally, forms a continuous surface with the unfinished internal trochanter. The proximal articular surface decreases in dorsoventral height backwards and forms a pointed angle posteriorly.
The dorsal surface of the femur bears striations on the proximal side. The dorsal surface of the distal end of the femur is divided by a relatively broad intercondylar groove. The anterior condyle extends slightly wider than the posterior one. Due to the incomplete ossification, the anterior condyle projects distally similar to the posterior condyle. The anterodorsal surface of the anterior condyle, near the middle, bears distinct longitudinal ridges for ligaments.
The adductor blade is low, directed posteroventrally, forming the anterior margin of the large intertrochanteric fossa. The anterior surface of the adductor blade is rugose. Distal to the adductor blade, a low but distinct adductor crest is slightly curved, runs distally near the posterior margin of the shaft on the ventral surface, and ends by a crack. Distal to the crest, there is a low ridge to a point close to the triangular popliteal area. However, it is likely separated from the adductor crest. Posterior to it, another crest runs on the posterior side of the shaft ending on the distal posterior corner. Two crests frame the small fibula fossa on the posteroventral corner of the distal end.
The shape of this femur is primitive among Tetrapods. However, it is slender, similar to that of Temnospondyls like Trimerorhachis or Chroniosuchians, rather than robust as in many other basal Tetrapod groups. As in IVPP V 23295 and Bystrowiella schumanni, the adductor crest follows a diagonal course, ending distally close to the posterior margin of the shaft. In most Permian Tetrapods, adductor crest generally runs towards, not posterior to the popliteal area. This could be a diagnostic character of Chroniosuchians.
IVPP V 26539 also shares the following features with other chroniosuchian femurs (V 23295 and SMNS 96948): the adductor blade is directed posteroventrally, and the posterior end of the proximal articular surface is pointed. So this specimen can be referred to Chroniosuchia.
This specimen has unfinished ends, as in the much longer femur of SMNS 96948, may indicated juvenile state of both specimens. Meanwhile, a medium sized femur of V 23295 is well-ossified, indicated that species has a smaller adult size.
The second specimen described, IVPP V 26540, comprises four vertebrae, one rib, and several scutes, from the base of the upper Permian Guodikeng Formation at the Taoshuyuan (Taoxigou) section, at Turpan in Xinjiang.
There are four articulated and one isolated vertebrae, two of them are nearly complete. They are identified as the vertebrae around sacral region. The vertebrae have a basic shape of Bystrowianidae.
Only two articulated intercentra are exposed. The anterior one shows smooth periosteal bone on ventral and lateral surfaces. Its lateral surface bears part of the parapophyses for articulation with the capitulum near the posterior margin, and ventral surface has no haemal arch. In lateral view, its length relative to the pleurocentrum is similar to IVPP V 23295, much narrower than in Chroniosaurus dongusensis.
The lengths of the pleurocentra are approximately 10 mm, while the heights are about 9 mm. The pleurocentra are massive with a round cross-section, and they are not perforated by the notochord. The ventral surface is relatively flat (slightly convex on the anterior one, slightly concave on the posterior one) with faint ventral ridges on three articulated larger pleurocentra. Two longitudinally aligned, low ridges are clear on the narrow ventral face of the smaller pleurocentrum of the isolated vertebra. This feature has been proposed as a Bystrowianid characteristic. The neural arches are fused to the pleurocentra, but their suture is clear; so this specimen looks like in a younger stage than V 23295, in which the suture is absent. This suture runs across the diapophysis in three articulated vertebrae, indicating the pleurocentrum participates in formation of the diapophysis. On two posterior pleurocentra, the anterior half of the ventrolateral surface participates in formation of the parapophysis, which is separated from the diapophysis by a narrow groove. So they are identified as the sacral and the first caudal vertebrae, as in Kotlassia and Proterogyrinus.
The transverse processes are very short and massive, directed ventrolaterally. The facets of the diapophyses are strongly enlarged in the sacral and first caudal. In the isolated vertebra, the transverse processes are completely formed by the neural arch. It is probably derived from the anterior tail region.
The neural spines are only nearly complete in the sacral vertebra and one isolated caudal vertebra. The neural spines are anteroposteriorly slightly shorter than the pleurocentrum. In the sacral, its anterior and posterior margins are nearly parallel. Its height is less than twice the height of the pleurocentrum. Its dorsal tip is connected with the osteoderm by interdigitating sutures. In the isolated caudal, the neural spine widens dorsally near the tip then narrows. Its dorsal tip does not carry an osteoderm.
A curved incomplete rib measures more than 5 cm in original length. The proximal side is not preserved.
There are two pieces of impression of the dermal surface of the osteoderms, one with three osteoderms, and the other with two osteoderms. Neither of the osteoderms is complete, so it is unsure on the width of the osteoderms. The distinct dermal sculpturing is composed of ridges and depressions. A mid-ridge is preserved. The pits have different sizes on two sides of the ridge, and they likely arrange in posterolaterally radiated line. No prominent parasagittal ridge is observed.
This specimen is diagnosed as a Bystrowianid chroniosuchian for the presence of the sculptured dorsal osteoderms which are sutured with the neural spine, paired deep paraneural canals on anterior and posterior surfaces of the neural arch. It cannot be further diagnosed for the poor preservation of the osteoderms.
The final specimen described, IVPP V 26541, is an incomplete dorsal osteoderm from the Lower Triassic Jiucaiyuan Formation, at a location 4 km east to the Dalongkou section at Santai in Jimusar, Xinjiang.
This single osteoderm is slightly convex dorsally. Most of the margins are broken other than right anterolateral margin The complete width should be approximately 3 cm. The dermal sculpture is not symmetric, and it is dominated by different-sized polygonal pits formed mainly by longitudinally oriented and oblique ridges. Transversely elongated sculptural depressions are only present near the posterior area. The sculpture has no axial crest or any parasagittal ridge, as in Synesuchus muravjevi or Dromotectum spinosum. The right anterior wing is preserved its posterior portion, with a smooth dorsal facet for the facies alaris (ventral facets) of the preceding osteoderm. The right accessory process is also partially preserved. The maximum width of one anterior wing is estimated as 9 mm, slightly smaller than the width of accessory processes.
The incomplete median articular plate measures 10 mm in width. It lies posteriorly and slightly ventral to the sculptured dorsal plate. The median crista decreases in width backwards and is triangular in shape in dorsal view. The broad lateral cristae are longer than and located at the same level as the median crista. Most parts of these cristae are formed by tapered extensions of the posterior margin of the sculptured dorsal plate. As most Bystrowianids, two deep longitudinal grooves (sulci articulares) lies between the median and lateral crests, for accommodation of the accessory processes and associated ligaments of the successive osteoderm. The area marginalis (marginal zone) is incompletely preserved on the right side. It extends anteriorly ventral to the dorsal plate as in Bystrowiana and Jiyuanitectum.
On the ventral surface, the shallow median depression, between the anterior wings back to the base of the ventral process, is preserved for most of the right side and nearly posterior half of the left side. The ridge extends from the right accessory process is distinct. The sulcus medius (median groove) should be narrow. A broad, low but distinct crista obliqua (oblique crest) extends along the ventral surface of the osteoderm posteromedially from the anterior wings toward the region of the ventral process, and continuous to the margin zone of the median articular plate. Lateral to the ventral margin, a shallow depression on the posterior part of the ventral surface is the ventral facet that overlapped the dorsal facet of the anterior wing of the succeeding osteoderm.
The base of the ventral process is located in the posterior half of the osteoderm, and its posterior margin extends posteriorly beyond the boundary of the sculptured surface of the osteoderm. The process is ovate in transverse section and lacks anterior and posterior extensions.
IVPP V 26541 can be referred to Bystrowianidae based on the unpaired posterior articular plate and weakly expanded plate bearing broadly separated anterior facets. IVPP V 26541 represents a new taxon closely related to Dromotectum. However, it is not named by Liu, who feels it should be named from more complete material. Its close relationship with Dromotectum is not surprise, because Dromotectum existed in the upper Permian of Henan, China.
The new specimens described here increase the diversity of Permo-Triassic Tetrapods. Previously, only one Tetrapod species, Kunpania scopulusa, was reported from the top of the Quanzijie Formation. Also, only one Chroniosuchian specimen was reported from the Guodikeng Formation. Although Chroniosuchian should have existed from middle Permian in Xinjiang based on their known distributions, this is the first evidence which confirmed its existence from the Quanzijie Formation. The bystrowianian Chroniosuchian specimens from the base and the top of the Guodikeng Formation and the Jiucaiyuan Formation demonstrated that this group survived in the end-Permian mass extinction here in Xinjiang, together with Lystrosaurus.
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