INTRODUCTION

Estes (1964) established the family Parasaniwidae for a number of Late Cretaceous taxa from North America, but later (Estes, 1983) synonymized it under Necrosauridae Hoffstetter, 1943 (the type genus of this group was established from the Paleogene of Europe) in the superfamily Varanoidea. Representatives of Parasaniwidae and Necrosauridae are characterized by platynotan-like structure of the lower jaw and dentition. Distinctions concern primarily the structure of osteoderms known in them; in Parasaniwidae, they are polygonal or circular, while in Necrosauridae, they are carinate and overlap each other. The two families are also distinguished by the geological and geographical ranges, which contradict synonymization.

The skull structure of Parasaniwidae has become known from three taxa from the Barun Goyot Formation (Middle–Late Campanian) of southern Mongolia: Proplatynotia longirostrata, Parviderma inexactum, and Gobiderma pulchrum. In the original descriptions, all of them were assigned to the “necrosaurian grade,” but only Proplatynotia was placed in the family Necrosauridae sensu Estes, 1983 (Borsuk-Białynicka, 1984). Later, Gobiderma and Parviderma (Lee, 1997) were combined in the family Gobidermatidae. However, it is hardly probable that this is a separate family.

The Parasaniwidae from Mongolia lack certain characters of modern Platynota, such as the shortened occipital process of the maxillae, the frontals participating in the border of the nares, the closed olfactory canal, and streptognathy. The fossil group is characterized by fusion of osteoderms with membrane bones of the skull roof, multiseriate palatal teeth, and complete postorbital and upper temporal arches.

In cladistic studies, the Parasaniwidae are treated as basal members of the clade Monstersauria (Norell and Gao, 1997; Conrad, 2008), in which the Helodermatidae are regarded as the terminal group, or, based on the example of Gobiderma, taken for basal Platynota (Gauthier et al., 2012). Based on the presence of osteoderms, we admit that the families Parasaniwidae and Necrosauridae are monophyletic kins (Alifanov, 2012) related to Helodermatidae and all of them can be classified in the composition of the superfamily Helodermatoidea and microorder Platynota (Anguimorpha). Judging from the osteoderms in the area of the head (Maisano et al., 2002), this lineage is close to modern Lanthanotus (Lanthanotidae). The group of nonosteodermal Platynota includes modern Varanidae (Varanoidea; Varaniformes: Conrad, 2008).

In the composition of Parasaniwidae, Proplatynotia Borsuk-Bialynicka, 1984 represents the most archaic lineage. Its frontals and maxillae are connected, the nasals are large, and maxillary teeth are numerous. Proplatynotia is probably close to Colpodontosaurus Estes, 1964 from the Maastrichtian of North America. In other Parasaniwidae, the number of maxillary teeth is reduced. Parviderma Borsuk-Bialynicka, 1984 is distinguished by the unpaired frontal and large supraorbitals. Probably, Parasaniwa Gilmore, 1928 and Paraderma Estes, 1964 from the Maastrichtian of North America and also Chianghsia Mo et al., 2012 from the Early Campanian of China are close to Gobiderma Borsuk-Bialynicka, 1984, which is known from the skull. Judging from Gobiderma, these taxa lack a connection between the maxilla and frontal and their osteoderms are often fused with the maxillae.

Primaderma Nydam, 2000 is the earliest Parasaniwidae, which comes from the Albian–Cenomanian of North America. In Asia, the earliest record of this family comes from the Turonian of Uzbekistan (Ekshmer bissektaensis Nessov, 1981, nomen dubium; Parasaniwidae gen. indet.). The latest Asian Parasaniwidae have been recorded in the Late Campanian (Nemegt Formation) of Mongolia (Necrosauridae gen. indet.: Alifanov, 2000, text-fig. 30d; Parasaniwidae gen. et sp. nov.: Alifanov, 2012, p. 42).

During the Dzhadokhta–Barun Goyot stage of the historical development of tetrapods, the Parasaniwidae of Central Asia competed with representatives of the extinct varanoid family Cherminotidae Alifanov, 2000 (Cherminotus Borsuk-Bialynicka, 1984; Estesia Norell et al., 1992; Aiolosaurus Gao et Norell, 2000; Ovoo Norell et al., 2007; Late Cretaceous of southern Mongolia; Asprosaurus Park et al., 2015; Late Cretaceous of southern Korea). The possible co-existence in time and space of Parasaniwidae and Varanidae remains an open question. The last family appears in the fossil record of Central Asia only in the Early Eocene.

Tsagansaurus nemegetensis gen. et sp. nov, an Early Cenozoic representative of Parasaniwidae is described below. It is the second Paleocene Platynota of Central Asia, if the material from the Dou-Mu Series (Late Paleocene) in China (Anhui), identified as Varaniformes gen. et sp. indet. (Dong et al., 2016), is taken into account. The material of the new taxon was collected in 1987 by a party of the Joint Soviet–Mongolian Paleontological Expedition headed by V.Yu. Reshetov in the Tsagan Sair locality (southern Mongolia), at the top of the Zhigden Member (Naran Bulak Formation). This member contains abundant remains of small mammals, including Multituberculata (Badamgarav and Reshetov, 1985; Lopatin, 2006). Lizards, along with Parasaniwidae, are represented by the families Agamidae and Changjiangosauridae.

  • SYSTEMATIC PALEONTOLOGY

  • Infraorder Anguimorpha

  • Microorder Platynota

  • Superfamily Helodermatoidea Gray, 1837

  • Family Parasaniwidae Estes, 1964

  • Genus Tsagansaurus Alifanov, gen. nov.

Necrosaurus sp.: Alifanov, 2000, p. 71, text-figs. 31c–31e; Alifanov, 2012, p. 44, text-fig. 16.

Etymology. From Tsagan Hushu.

Type species. Tsagansaurus nemegetensis sp. nov.; Late Paleocene of southern Mongolia.

Diagnosis. Large osteodermal scutes not fused with maxilla surface. Apex of dorsal process of maxilla falling on third quarter of bone extent. Its pointed frontal process well developed and positioned in line with fourth functional tooth. Tooth apices narrow and curved posteriorly. Enamel folds at tooth base numerous. At least seven teeth present. Centra of dorsal vertebra relatively short. Neural spine high and extended craniocaudally. Articular surface of prezygapophyses positioned at angle of 45° to horizontal. Vertical axis of synapophyses almost perpendicular to long axis of vertebral centrum. Opening of neural canal relatively large and subtriangular in shape.

Species composition. Type species.

Comparison. The new genus differs from the majority of genera of the family in the absence of fusion between large osteoderms and maxillae, the greater number of teeth (except for Proplatynotia and Colpodontosaurus), the presence of a pointed frontal process of maxillae (except for Proplatynotia and Parvidema), the larger angle between the horizontal and articular plane of the prezygapophyses, the vertically positioned synapophyses and wide opening of the neural canal. In addition, it differs from Gobiderma in the presence and pointed shape of the frontal process of the maxillae.

Remarks. Parasaniwidae vertebrae are poorly known. They are investigated by us based on specimen PIN, no. 4216/204 (Parasaniwidae gen. indet.) from the Nemegt Formation in Mongolia. They have a low neural spine, an inclined synapophysis, and a small foramen of the neural canal. The new taxon is assigned to Parasaniwidae based on the presence of the osteodermal sculptures on the external surface of the maxilla, the small condyle, and the development of a superficial interprezygapophyseal notch, which are characteristic of representatives of the family.

The new taxon is phylogenetically close to the parasaniwid group Proplatynotia Borsuk-Bialynicka, 1984. It is also similar to the platynotan lizard (“Varaniformes” gen. indet.) from the Dou-Mu Series (Late Paleocene of China, Anhui) in numerous teeth (not more than ten) and vertical articular planes of pre- and postzygapophyses (Dong et al., 2016). The Mongolian taxon differs from the last in the relatively short centra of the dorsal vertebra and the vertical synapophyses.

  • Tsagansaurus nemegetensis Alifanov, sp. nov.

  • Plate 14, figs. 1–7

Etymology. From the Nemegetu Ridge in southern Mongolia.

figure a

Explanation of Plate 14

Figs. 1–7 .Tsagansaurus nemegetensis sp. nov., Parasaniwidae (Platynota, Anguimorpha); Mongolia, South Gobi Aimag, Tsagan Sair locality; Upper Paleocene, Naran Bulak Formation, Zhigden Member: (1) holotype PIN, no. 4758/1, right maxilla, labial view; (2) holotype PIN, no. 4758/1, fourth maxillary tooth, lingual view; (3) specimen PIN, no. 4758/2, left dentary fragment, lingual view; (4–7) specimen PIN, no. 4758/8, dorsal vertebra: (4) dorsal, (5) ventral, (6) lateral, and (7) cranial views. Scale bar, 5 mm.

Holotype. PIN, no. 4758/1, right maxilla; Mongolia, South Gobi Aimag, Nemegt Depression, Tsagan Hushu, dry river channel of the Tsagan Sair River; Upper Paleocene, Naran Bulak Formation, Zhigden Member.

Description. Laterally, the maxillae have foramina scattered over the entire surface. The bone also has well-developed tubercles and grooves which are usual, as the osteodermal scutes are formed (Pl. 14, fig. 1). Transition from the premaxillary to dorsal processes of the maxilla is poorly pronounced. The medial part of the premaxillary process is large and raised upwards. The notch between the medial and lateral parts of the last process is superficial. The dorsal process of the bones is relatively low. Their supradental crest is narrow.

The maxillary teeth are narrow and curved at the end. The number of enamel folds at their bases reaches ten. The number of teeth preserved in the specimen is six. The total number of teeth was undoubtedly greater (eight or nine), since the maxilla has lost the middle and occipital parts (Pl. 14, figs. 1, 2). The largest teeth are located in the anterior part of the tooth row.

The dentary is very fragmentary. In the specimen in our collection, the margins of the Meckel’s canal are distinctly narrowed and displaced vertically. Dentin deposits connected with the tooth bases are extensive (Pl. 14, fig. 3).

The centra of the dorsal vertebrae are relatively short and narrowed distinctly occipitally. In ventral view, it is gently widened in the middle part. The condyle is one-third as wide as the cranial part. The zygapophyses are large. The articular surfaces of the prezygapophyses are oval. The neural spine is high and wide. Its lateral axis coincides with the middle of the vertebra. The interprezygapophyseal notch is wide (Pl. 14, figs. 4–7).

Material. In addition to the holotype, specimen PIN, no. 4758/6, left maxilla; specimen PIN, no. 4758/2, fragmentary left dentary; specimen PIN, no. 4758/8, dorsal vertebra; type locality.

Measurements. Maxilla (holotype PIN, no. 4758/1): preserved (and reconstructed) length, 15 (20–22); depth, 7; height of the largest teeth, 4.5; vertebra (specimen PIN, no. 4758/8): length, 7; height, 9; width, 10.