Procolophonidae Cope, 1889

Säilä, Laura K., 2009, Alpha taxonomy of the Russian Permian procolophonoid reptiles, Acta Palaeontologica Polonica 54 (4), pp. 599-608 : 600-603

Family Procolophonidae Cope, 1889 Suchonosaurus minimus Tverdokhlebova and Ivakhnenko, 1994

Figs. 1 , 2 .

Holotype and only specimen: SGU 104 B/1326, an isolated right maxilla.

Type locality: Salarevo, Russia.

Type horizon: Sokolkovskii Subcomplex, Salarevskaya Svita, Vyatkian (uppermost Tatarian).

Emended diagnosis.— Suchonosaurus is distinguished from other procolophonids by the following three characters: (1) 11 or 12 conical, fairly large maxillary teeth that are taller anteriorly and have subcircular bases, (2) a bony lip that covers the entire lower half of the labial side of the maxillary dentition and (3) a distinctive, three−fold wear pattern of the maxillary tooth crowns.

Description.—The holotype, and only specimen, of Suchonosaurus minimus is a nearly complete right maxilla, free of surrounding matrix, and thus all sides can be seen ( Figs. 1 , 2 ). The porous surface of the specimen, however, might have obscured some details. The overall shape of the maxilla is very similar to that of basal procolophonid Coletta seca Gow, 2000 (see Modesto et al. 2002) but is slightly less tall. In lateral view, a depression can be seen behind the circular external naris ( Fig. 2A ) and its considerable depth is revealed in anterolateral view ( Fig. 1A ). A large foramen, identified as the “labial foramen of the infraorbital artery” by Bulanov (2002: 527), is situated behind the depression. The equivalent foramen in other reptilian taxa has also been called the “anterolateral maxillary foramen” ( Laurin and Reisz 1995: 186; Modesto et al. 2002: 885). Because of the erosion of the lateral surface, it is unclear how many other foramina opened on this surface but another, smaller, foramen can be seen near the posterior end of the maxilla ( Fig. 2A ). In lateral view, the teeth appear quite short and peg−like but this is because the lower half of the dentition is covered by a bony “lip”. This is not evident in lateral view, but can be seen clearly from the medial side where the tooth apices are exposed in their entire length and the lip is exposed where a tooth is missing ( Fig. 2B ). There is a two−fold facet on the anterior process of the maxilla, exposed medially. The two halves of the facet face dorsomedially and ventromedially ( Fig. 2B ), thus indicating either that the premaxilla had a maxillary process that fitted both of the facets or, more likely, that the dorsomedial facet was contacted by a septomaxilla. The anterior facets cannot be seen on the maxillae of Coletta but the shape of the anterior process in lateral view ( Modesto et al. 2002: fig. 1) is very similar to that of Suchonosaurus ( Fig. 2A ). In Coletta the process is contacted by both the premaxilla and the septomaxilla. Other notable features on the medial side of the maxilla of Suchonosaurus are a large foramen and a shelf ( Fig. 2B ) that were interpreted, respectively, as the “anterior foramen of the infraorbital artery” and a facet for the palatine attachment by Bulanov (2002: 527). However, a similar shelf or groove above the anterior maxillary dentition has been interpreted as an opening that carried nerves and/or blood vessels from the interior of the snout in pelycosaurs and the parareptile Colobomyter ( Vaughn 1958; Modesto 1999), making this an alternative hypothesis for the function of the shelf.

There are 11 tooth positions on the maxilla but the first and fifth teeth are missing. The first preserved tooth is the tallest, with the second to fourth becoming progressively shorter and the rest being of similar size to the fourth. A shallow, large pit, indicating a short root, can be seen where the first tooth should be ( Figs. 1C , 2C ). Based on the diameter of the pit, the first tooth was most likely shorter than the second. Because the bony lip continues behind the last tooth ( Fig. 2B ), it has been considered a sign of more teeth being present in an unbroken maxilla ( Tverdokhlebova and Ivakhnenko 1994; Bulanov 2002). The maxilla has, however, broken off very near the eleventh tooth ( Figs. 1 , 2 ), unlike Bulanov (2002: fig. 2) depicts. Additionally, the maxilla becomes very shallow towards the end of the row ( Fig. 2B ), indicating that it was nearing its posterior extent. The posterior tip of the maxilla is devoid of dentition in procolophonoids with similar maxillary shape ( Modesto et al. 2002; Reisz and Scott 2002; Modesto and Damiani 2007). Thus the tooth number of Suchonosaurus is considered as no more than 11 or 12 at most. A similar number of maxillary teeth is present in the basal procolophonids Coletta and in Pintosaurus magnidentis Piñeiro, Rojas, and Ubilla, 2004 . The teeth of Suchonosaurus were described as recurved by Tverdokhlebova and Ivakhnenko (1994) and depicted as totally straight by Bulanov (2002). In reality, the teeth are more or less straight, but the amount of wear on the mesial side of the tooth crowns has made some of them appear to be distally recurved ( Fig. 2B ). Most of the preserved teeth have three separate wear facets: the first on the mesolingual side of the tip, the second on the tip and the third on the distolingual side of the tip of the tooth crown ( Fig. 2B, C ). However, on the second and fourth preserved teeth the first two wear facets have fused together, making one large, mesolingually sloping facet that also touches the distal, third facet ( Fig. 2B ). The mesial wear facet is always the largest, except on the seventh preserved tooth where the distal wear facet is slightly larger. There are also distinct striations on the wear surfaces of some of the teeth ( Fig. 2B ). Similar, distinctive wear facets have not been previously reported on procolophonoids with conical dentition. There are also tooth replacement pits on the lingual sides of some teeth ( Fig. 2B, C ).

Comments.— Suchonosaurus was excluded from Procolophonoidea by Cisneros (2008a) because he regarded this genus as exhibiting “pleurodont dentition”, a mode of implantation that is unknown in procolophonoids. Cisneros (2008a) drew his conclusions on the basis of the illustrations of Tverdokhlebova and Ivakhnenko (1994), which show the maxilla in medial and lateral views. Bulanov (2002) also illustrates only these views. However, the assessment of pleurodont dentition for Suchonosaurus is mistaken. Pleurodont dentition is characterized by (1) no sockets/alveoli or roots, (2) the teeth sitting in a dental groove with a high labial wall (and possibly a low lingual wall), and (3) attachment mainly to the lingual side of the labial wall ( Motani 1997: fig. 1). The dentition of Suchonosaurus looks superficially pleurodont because it has a high labial wall or a bony “lip”. However, Suchonosaurus does not have a dental groove, and instead has individual alveoli and shallow roots for each tooth, even if the teeth are tightly packed. This is evident from the missing first tooth, which reveals the empty alveolus and no dental groove ( Figs. 1C , 2C ). Additionally, the labial “lip” of bone is very shallow next to the first, missing tooth, indicating it was not the primary attachment surface for the tooth. The pattern of tooth implantation of procolophonoids is poorly documented, but it is generally agreed that they have teeth that are firmly ankylosed to the bone, a condition labelled “protothecodont” by Cisneros (2008a: 17) and Small (1997: 676). However, the use of the term protothecodont is variable in the literature. A definition by Small (1997), modified from Benton (1984) and Bolt and DeMar (1975), states “protothecodont (= subthecodont) teeth have shallow or fairly deep roots, and are ankylosed into the socket by bone of attachment, with no space for a periodontal ligament or other soft tissue between the socket and the base of the tooth. A typical reptilian tooth replacement or a variation thereof occurs” ( Small 1997: 76). Other authors understand proto/subthecodonty to be comparable to pleurothecodonty, a condition where shallow sockets are within a dental groove that has low lingual and high labial walls ( Motani 1997; Romer 1956; Wild 1973). There is also an implantation type called “ankylosed thecodonty”, characterised by shallow roots that are ankylosed to the surrounding bone which is the same height on both sides of the teeth ( Edmund 1969; Motani 1997), and this definition has been applied to procolophonids by Sues and Olsen (1993). Small (1997) points out, however, that ankylosed thecodonty is also linked with non−reptilian or totally absent tooth replacement in rhynchosaurs ( Benton 1984; Chatterjee 1974), and while tooth replacement type is unknown in most procolophonoids, Libognathus ( Small 1997) displays the normal reptilian type with replacement pits on the lingual side of the teeth. Suchonosaurus has the same method of tooth attachment, by shallow roots ankylosed firmly to the bone, as do other procolophonoids, regardless of what the method of attachment is called. It also has the normal reptilian tooth replacement, indicated by the pits next to its second, fourth and tenth teeth ( Fig. 2B ).

In addition, however, Suchonosaurus has a bony “lip” covering approximately half of the tooth crown on the labial side. This feature has not been reported in any other procolophonoid, but a paratype maxilla of the Triassic Russian procolophonid Contritosaurus convector Ivakhnenko, 1974 , PIN 3357/2, appears to have an expansion of bone covering part of the labial side of its dentition ( Fig. 3A, B ), although labial foramen maxillary depression the “lip” is not quite as extensive as in Suchonosaurus . The tooth attachment and features of the medial side of the maxillary bone are also very similar in Suchonosaurus ( Fig. 2B ) and C. convector ( Fig. 3B ). However, C. convector has transversely expanded tooth bases ( Fig. 3C ) and the maxillary depression is extensive ( Fig. 3A ), whereas the teeth of Suchonosaurus retain the more rudimentary conical shape with subcircular bases and the maxillary depression is restricted to the immediate border of the external naris ( Fig. 2 ). Of other procolophonoids with conical dentition, tooth implantation is not specifically discussed ( Modesto et al. 2001, 2002, 2003; Reisz and Scott 2002; Cisneros et al. 2004; Piñeiro et al. 2004; Modesto and Damiani 2007). This is largely because most taxa are represented by skulls that are preserved in occlusion with the mandible. No “lip” has been reported on Coletta seca , which is most similar to Suchonosaurus with respect to the shape of the maxillary bone and tooth number. Additionally, the tooth bases of the conical dentition of Coletta (and also Pintosaurus , which shares some features with Suchonosaurus ) have been described as somewhat transversely expanded ( Cisneros 2008a), whereas in Suchonosaurus the tooth bases are subcircular. Conical teeth with subcircular bases are also present in all owenettids and the basal procolophonid Sauropareion anoplus Modesto, Sues, and Damiani, 2001 ( Modesto and Damiani 2007). Sauropareion , however, differs from Suchonosaurus in tooth number and the shape of the maxillary bone, and owenettids have a much higher number of much smaller teeth than Suchonosaurus . Additionally, Suchonosaurus is the only procolophonoid taxon that has straight, conical maxillary theet that is noticeably taller anteriorly.

Thus, Suchonosaurus does not possess pleurodont dentition or any other characters that would exclude it from Procolophonoidea . Furthermore, the circular shape of its external naris, the laterally visible maxillary depression, tooth number of 11 or 12, and size of its teeth support its inclusion in the procolophonoid family Procolophonidae . It is also not identical with any other known procolophonid, as evidenced by its several unique autapomorphies, and thus its status as a separate genus and species is upheld here.

Stratigraphic and geographic range.—Vyatkian (uppermost Tatarian) of European Russia.

Figures

Fig. 1. Procolophonid reptile Suchonosaurus minimus Tverdokhlebova and Ivakhnenko, 1994, Salarevskaya Svita, uppermost Tatarian; SGU 104B/1326 (holotype), right maxilla. In anterolateral (A), medial (B), and medioocclusal (C) views.

Fig. 2. Procolophonid reptile Suchonosaurus minimus Tverdokhlebova and Ivakhnenko, 1994, Salarevskaya Svita, uppermost Tatarian; SGU 104B/1326 (holotype), right maxilla. In lateral (A), medial (B), and occlusal (C) views.

Fig. 3. Procolophonid reptile Contritosaurus convector Ivakhnenko 1974, Vokhmian Gorizont, Induan, earliest Triassic; PIN 3357/2, partial right maxilla. In lateral (A), medial (B), and occlusal (C) views.