Plesiotortrix edwardsi Rochebrune, 1884

Plesiotortrix edwardsi Rochebrune, 1884

( Fig. 1)

Plesiotortrix edwardsi Rochebrune, 1884: 156 .

TAXONOMIC HISTORY. — Plesiotortrix edwardsi Rochebrune, 1884 (new genus and species); [ Plesiotortrix edwardsi ] Rage & Augé 1993 (nomen dubium). Note that the “Taxonomic history” section here deals only with novel nomenclatural acts, new taxonomic combinations, or novel taxonomic renderings of the taxon, similarly to the style proposed in other recent reptilian papers (e.g. Joyce 2016, 2017; Georgalis & Joyce 2017; Georgalis et al. 2021c).

HOLOTYPE AND ONLY KNOWN SPECIMEN. — MNHN. F.QU16332 (formerly MNHN QU 332 ), three articulated trunk vertebrae ( Rochebrune 1884; pl. II.6, II.6a; this paper: Figs 1; 2) .

LOCALITY AND AGE. — Similar to most other fossil specimens collected during the 19th century, there are no precise locality data for the holotype of Plesiotortrix edwardsi , apart from the general information that it originates from the Phosphorites du Quercy. However, this is still vague, taking into consideration that the Phosphorites du Quercy include at least 170 fissure filling localities, distributed over a broad geographic area, encompassing large parts of the current departments of Lot, Tarn-et-Garonne, Tarn, and Aveyron, all in the administrative region of Occitanie ( Rage 2006; ( Sigé & Hugueney 2006; Georgalis et al. 2021a, c, 2023; Pelissié et al. 2021). They also stratigraphically span over a considerable time period, from the early Eocene (MP 8+9) until the Early Miocene (MN 3); however, most of the respective fossiliferous localities range between the late middle Eocene (MP 16) and the late Oligocene (MP 28) ( Rage 2006; Sigé & Hugueney 2006; Georgalis et al. 2021a, c; Pelissié et al. 2021), though still important finds have also been recovered from the older (early Eocene) and younger (Early Miocene) sites (e.g. Čerňanský 2023; Čerňanský et al. 2023a, b).

DESCRIPTION

The holotype and only known specimen of Plesiotortrix edwardsi consists of three articulated trunk vertebrae. The three vertebrae are relatively well preserved, but still have parts of their prezygapophyses and neural spines damaged and their paradiapophyses are eroded ( Fig. 1). The vertebrae are moderately small, with an average centrum length (CL) for each vertebra of approximately 3.3 mm and a neural arch width (NAW) of 3.9 (ratio CL / NAW approximately 0.8). In anterior view ( Fig. 1A), the zygosphene is straight and relatively thin. The neural canal is trapezoidal in shape. The prezygapophyses are only slightly inclined, but they clearly reach above the level of the floor of the neural canal. The cotyle is large and circular. Deep fossae are present on each lateral side of the cotyle; no paracotylar foramina are present. In posterior view ( Fig. 1B), the neural arch is moderately depressed, with a vaulting ratio (sensu Georgalis et al. 2021c) equal to 0.38. The parapophyseal portion of the paradiapophysis (fully preserved only in the right paradiapophysis of the third vertebra) extends ventrally below the level of the cotyle and condyle. The condyle is large and slightly elliptical. In dorsal view ( Fig. 1D), the zygosphene possesses two incipient lateral lobes and a wide median lobe. The neural spine commences slightly posteriorly from the level of the zygosphene; its base becomes wider and distinctly saddle-shaped towards the posterior half of the neural arch. The prezygapophyseal articular facets are massive and broad. The interzygapophyseal constriction is deep. In ventral view ( Fig. 1C), the centrum is wider than long. A distinct haemal keel runs almost throughout the midline of the ventral surface of the centrum. It is relatively thin and has the same width across its whole length; it commences anteriorly at around the ventral lip of the cotyle and terminates posteriorly right prior to the condylar lip. Prezygapophyseal accessory processes are present, with their tip only slightly projecting beyond the prezygapophyseal articular facets. The subcentral grooves are deep. Subcentral foramina are visible, situated next to the haemal keel. The paradiapophyses are eroded in the three vertebrae with the exception of the right paradiapophyses of the last vertebra, which is almost complete. There seems to be no clear differentiation into diapophyses and parapophyses; in the almost completely preserved right paradiapophysis of the third vertebra, it seems that the parapophyseal portion is slightly larger than the diapophyseal one. The postzygapophyseal articular facets are large and their posterior edge is somehow acute. In lateral view ( Fig. 1E, F), the neural spine is damaged and practically only its base is preserved. A small condylar neck is present. The subcentral ridges are slightly convex. The haemal keel slightly projects ventrally, with its dorsoventral height being higher towards its posteriormost section.

Judging from the absolute shape, the vaulting of the neural arch, the depth of the subcentral grooves, and the width of the haemal keel, I assume that these three vertebrae originate from the mid- or anterior posterior trunk region of the column. This slightly contradictsRochebrune’s (1884) original description, who treated them as “vertèbres de la région pelvienne”.