Neosaurus cynodus ( Gervais, 1869 )

Falconnet, Jocelyn, 2015, The sphenacodontid synapsid Neosaurus cynodus, and related material, from the Permo-Carboniferous of France, Acta Palaeontologica Polonica 60 (1), pp. 169-182 : 171-172

Neosaurus cynodus ( Gervais, 1869)

Figs. 1 , 2 . 1858 Protorosaurus Speneri ; Coquand 1858: pl. 1: 1, 2.

1869? Geosaurus cynodus Gervais : 222, figs. 29, 30.

1907? Geosaurus cynodus ; Case 1907: 67, fig. 20 [copied from Gervais 1869: fig. 30].

1910? Stereorachis cynodus ; Thévenin 1910: 57, pl. 8: 5 [countercast of HN004 2009-00-1B].

1923 Neosaurus cynodus ; Nopcsa 1923: 5.

1969 Stereorachis ; Heyler 1969: pl. 52: 6 [countercast of HN004 2009- 00-1B].

Holotype: HN 004 2009-00-1, partial tooth-bearing left maxilla and counter-part impression in a red, micaceous sandstone, here respectively referred to informally as HN 004 2009-00-1A and HN 004 2009- 00-1B for convenience. Collected by Henri Coquand (accompanied by Charles Grenier) between 1854 and 1856. It was housed in the collections of the Faculté des sciences de Besançon, where he was teaching at the time. It was then successively loaned to Gervais (1869) and Thévenin (1910). The holotype was re-discovered in 2007 by Jocelyn Falconnet in the MNHN collections, where it is still housed today. Type locality: Les Gorges, Moissey Commune, Jura Department, Franche-Comté Region, Eastern France. The type locality was misspelled “Moissy” on p. 31 but correctly on p. 320 by Romer and Price (1940). Eberth (1985: 37) and Berman et al. (1997: 129) each indicat- ed an incorrect locality for Neosaurus (“near Paris” and “Besançon”, respectively).

Type horizon: Red, micaceous sandstones, unnamed stratigraphic unit ( Campy et al. 1983); late Gzhelian–Asselian. The macroflora recovered in the Moissey area indicates a late Autunian age ( Corsin and Devaux 1959).Although the validity of the Autunian regional stage has been challenged, a typical Autunian floral assemblage found in the Donets Basin was correlated to the Late Gzhelian–Asselian interval using biostratigraphic markers present in marine intercalations ( Broutin et al. 1999; Izart et al. 1998a, b; see details in Falconnet 2014).

pre caniniforms post

Description

Preservation.— The holotype of Neosaurus cynodus is preserved in two small blocks. HN004 2009-00-1A comprises a left tooth-bearing maxilla exposed in lateral view ( Fig. 1A, B ). The area anterior to the level of the caniniform is missing and the anterior half of the preserved portion is damaged. The tooth row is still present, but the overlying dorsal lamina is incomplete. Most of the teeth are broken, but some of them are preserved in the counterpart impression. Several roots are visible laterally, revealing fine details about their inner structures. Fortunately , further data can be gathered from HN004 2009-00-1B on the lateral impression of the maxilla and its teeth—including the anterior area, which is missing in HN004 2009-00-1A ( Figs. 1C, D , 2 ). The dorsal lamina margin is nevertheless missing. Its upper half is preserved only as an impression at the level of the first postcaniniforms, whereas its lower half is partly missing above the precaniniforms only. In addition, the impression of the dorsal lamina is concealed between the level of the caniniform and the precaniniforms by a small detached piece of the maxilla .

Maxilla.—The maxilla consists of a thick tooth-bearing alveolar ridge, convex ventrally and with a low step anterior to the single caniniform tooth. Dorsally the maxilla extends into a thin vertical lamina. On HN 004 2009-00-1B, the surface shows a number of vessel scars parallel to the tooth row that narrows posteriorly. The position of the caniniform root is indicated laterally by a slight swelling of the dorsal lamina of the maxilla and medially by the thickening and deepening of the alveolar ridge just posterior to the caniniform ( Fig. 1A, B ). These features indicate the presence of a supracaniniform buttress, the development and extent of which is unknown.

Dentition.—There are 13 tooth impressions on HN 004 2009- 00-1B: four precaniniforms, one caniniform, and eight postcaniniforms. In addition, HN 004 2009-00-1A displays also an empty space mesial to the caniniform and a root without a crown and two empty spaces distal to the caniniform. This suggests there were nine to 11 postcaniniforms ( Fig. 2 ; see also Discussion).

All the teeth are roughly triangular in outline and labiolingually compressed, with moderately developed but unserrated mesiodistal cutting edges. Their surface shows a slight longitudinal fluting on the apical two-thirds. The base, squarish in cross section, displays a low V-shaped median longitudinal groove, on both the labial and lingual surfaces. This groove results from the labiolingual constriction of the roots, giving them an hourglass-like cross section. The respective crowns acquired consequently an almost similar shape, as displayed by the broken postcaniniforms, though with some variations. The labiolingual groove narrows as it extends apically from the base, extending up to the two-thirds of the length of most teeth, but only to the first third on the caniniform. This groove is not observable on the last postcaniniform, but this is probably because of its small size.

The size of the single preserved caniniform tooth is a little more than twice that of the largest postcaniniform tooth. As far as preserved, the precaniniforms are of subequal size, whereas postcaniniforms increase slightly in size from the caniniform tooth to the midlength of the maxilla, then diminish rapidly posteriorly. The non-caniniform teeth are much stouter than the narrow, sharply pointed caniniform. A pronounced recurvature is exhibited by the caniniform and most postcaniniforms, whereas it is nearly absent in the precaniniforms and the two last postcaniniforms.

Figures

Fig. 1. Sphenacodontid synapsid Neosaurus cynodus (Gervais, 1869), holotype, from Moissey, Late Gzhelian–Asselian. A. Left maxilla (HN004 2009- 00-1A) in lateral view. B. Corresponding impression (HN004 2009-00-1B) in lateral view. Drawings by Peggy Vincent.

Fig. 2. Sphenacodontid synapsid Neosaurus cynodus (Gervais, 1869), holotype (HN004 2009-00-1), from Moissey, Late Gzhelian–Asselian. Schematic comparison between the structure preserved in bone and impression.