Pan-Shinisaurus indet

Augé, Marc Louis, Dion, Michaël & Phélizon, Alain, 2021, The lizard (Reptilia, Squamata) assemblage from the Paleocene of Montchenot (Paris Basin, MP 6), Geodiversitas 43 (17), pp. 645-661 : 655-657

Pan-Shinisaurus indet.

MATERIAL EXAMINED. — MNHN. F.MTC240-MTC242, MTC243, nearly fifty osteoderms, a few complete, most more or less severely damaged by digestive processes or post-burial damages ( Fig. 9 ).

DESCRIPTION

Measures (mm): maximum width (w) of osteoderms ranges between 1.1 and 3.9 mm. The osteoderms, when complete, are oval, suboval or subrectangular elements with more or less irregular margins. Gliding surfaces are absent. All bear a prominent medial keel that extends the full length of the osteoderm. A slight concavity on the underside of some osteoderms reflects the form of the keel. From the keel a pattern of deep pits or grooves and marked ridges radiates. The dorsal surface of most osteoderms is flat but a few have a rather strongly vaulted shape which certainly reflect different positions on the body.

COMPARISONS AND DISCUSSION

Some characters are traditionally used to separate anguimorph taxa (and more specifically anguid genera) by their osteoderms ( Hoffstetter 1962a; Meszoely 1970; Bochaton et al. 2015, 2016), including the presence of a gliding surface and of a keel. For example, Gauthier (1982) considered keeled body osteoderms to be the plesiomorphic state for Anguimorpha . Referral of the osteoderms from Montchenot to Pan - Shinisaurus (sensu Smith & Gauthier 2013) follows from the combination of the features described above. These osteoderms are similar in shape to those of other fossil pan-shinisaurs, particularly Provaranosaurus fatuus Smith & Gauthier, 2013 ( Smith & Gauthier 2013, early Eocene of the Wasatch Formation, Wyoming, United States), Merkurosaurus ornatus Klembara, 2008 ( Klembara 2008, early Miocene, Orleanian, MN3, Bohemia) and an indeterminate pan-shinisaur from Messel ( Smith 2017, middle Eocene, Germany). Crocodile-tailed lizards (Chinese crocodile lizard) or shinisaurs are represented by a single living species, Shinisaurus crocodilurus Ahl, 1930 . It is worth noting that similar osteoderms have already been reported in the European Paleocene and early Eocene, in particular in the localities of Cernay (MP6, Hoffstetter 1943), Dormaal and Le Quesnoy (early Eocene, MP7, Hecht & Hoffstetter 1962; Augé 1990) and perhaps at Rivecourt- Petit Pâtis ( Smith et al. 2014). Hecht & Hoffstetter (1962) and Augé (2005) suggested that these osteoderms could be attributed to the genus Necrosaurus as they are also similar to those of Palaeovaranus cayluxi (Ex Necrosaurus ), see figs. in Rage 1978; Estes 1983; Augé & Smith 2009; Klembara & Green 2010. However, the taxonomic status and phylogenetic affinities of these lizards are a complex matter. Georgalis (2017) pointed out that the name Necrosaurus , as established by Filhol (1876) is a nomina nuda and that Zittel (1887 -1890) was the first author to make the name Palaeovaranus cayluxi available. The phylogenetic affinities of Palaeovaranus are a moot point: briefly, McDowell & Bogert (1954) noted significant morphological differences between Palaeovaranus and members of the Platynota (sensu Pregill et al. 1986) and they referred it to xenosaurid lizards, an option first adopted by Hoffstetter (1954). Later this author returned Palaeovaranus to the Platynota ( Hoffstetter 1962b). Lee (1997) rejected this taxon as paraphyletic. The phylogenetic position of Palaeovaranus is still a matter of discussion, although several derived characters suggest Platynotan relationships (see discussion in Smith 2017). In contrast, the attribution of Provaranosaurus fatuus and specimen SMF ME 11403 (an autotomized tail) from Messel to pan-shinisaur is a settled matter as they show no Platynotan derived characters ( Smith & Gauthier 2013; Smith 2017). The fossils from Dormaal (osteoderms, vertebrae and an undescribed dentary) previously attributed to Necrosaurus (Palaeovaranus) show no Platynotan features and are nearly identical to the material of Provaranosaurus fatuus described by Smith & Gauthier 2013. In particular, Provaranosaurus has both rectangular and oval osteoderms, as in the material from Monchenot, while Palaeovaranus bears only ovoid osteoderms. On the basis of these resemblances, the osteoderms from Monchenot may be referred to pan- Shinisaurus and the presence of rectangular osteoderms seems to exclude an attribution to Palaeovaranus .

Figures

FIG. 9. — Pan-Shinisauridae,osteoderms,MNHN.F.MTC240-MTC242,specimen in:A, dorsal view;B, dorsal view;C, dorsal and ventral views.Scale bar:1 mm.