Placosaurus sp.
|
publication ID |
https://doi.org/ 10.5252/geodiversitas2021v43a9 |
|
publication LSID |
urn:lsid:zoobank.org:pub:11D0D852-39D7-449C-9EB3-C3D804114556 |
|
DOI |
https://doi.org/10.5281/zenodo.4721466 |
|
persistent identifier |
https://treatment.plazi.org/id/03A1633B-FFBA-FFDC-3421-F8F8FB363D83 |
|
treatment provided by |
Felipe |
|
scientific name |
Placosaurus sp. |
| status |
|
( Figs 19-22 View FIG View FIG View FIG View FIG )
REFERRED SPECIMENS. — Three presacral vertebrae (NHMW 2019/0046/0001,NHMW 2019/0046/0002,andpossiblyalsoNHMW 2019/0046/0008); one caudal vertebra (NHMW 2019/0046/0009) .
DESCRIPTION
Presacral vertebrae ( Figs 19-21 View FIG View FIG View FIG )
The vertebra NHMW 2019/0046/0001 is relatively large (centrum length = 9.8 mm) ( Figs 20 View FIG ; 21 View FIG ), while the cervical vertebra (NHMW 2019/0046/0002) is also large, with a centrum length of 8.5 mm ( Fig. 19 View FIG ) ( Appendix 1). NHMW 2019/0046/0001 is primarily characterized by the presence of a median ridge running throughout the ventral surface of the centrum ( Figs 20 View FIG ; 21 View FIG ). This median ridge is prominent and sharp, being slightly thicker at its anteriormost portion. Nevertheless, in the single known, cervical vertebra (NHMW 2019/0046/0002), this median ridge is substituted by a thick hypapophysis ( Fig. 19 View FIG ). The base of the hypapophysis in this specimen is broad and extends throughout the entire midline surface of the centrum. In this cervical vertebra, the neural spine is relatively thick in dorsal view, while in lateral view it is inclined much posteriorly and develops in height mostly in the posterior portion of the neural arch. In NHMW 2019/0046/0001, the neural spine is rather high in lateral view, whereas in anterior view, it possesses a distinctive dorsal thickening. The neural spine augments in height only at the posterior half of the neural arch, but its base extends, in the shape of a longitudinal ridge, much anteriorly, reaching the anteriormost edge of the neural arch. The prezygapophyses are much dorsally inclined. The postzygapophyses are small. The synapophyses are massive and elongated. The cotyle and the condyle are dorsoventrally depressed. Similarly to the vertebrae of Palaeovaranus described below, the cotyle is rather anteroventrally directed, so that its dorsal level can be observed even in ventral view of the specimens. Unlike the melanosaurine and palaeovaranid presacral vertebrae described below, there are no anocotylar foramina (i.e., foramina above the cotyle). NHMW 2019/0046/0008 is a rather fragmentary specimen and its size cannot be fully evaluated, though it is certainly a much smaller specimen than the other two. The median ridge in this specimen is in fact a much wide surface and not sharp; the fact that the posterior portion of the ventral surface of the centrum is totally missing does not even permit to identify this specimen as a cervical or post-cervical vertebra. The referral of NHMW 2019/0046/0008 to Placosaurus should be considered as rather tentative.
Caudal vertebra ( Fig. 22 View FIG )
The single available caudal vertebra NHMW 2019/0046/0009 is relatively large in size, with a centrum length of 9.5, but its outline is rather slender ( Fig. 22 View FIG ). The transverse processes extend laterally in dorsal view. The neural spine is
much broken but seems to have been developed only at the posterior half of the neural arch, although, similarly to the presacral vertebrae, its base was extending also anteriorly until the anteriormost edge of the neural arch, in the shape of a thin longitudinal ridge. The prezygapophyses are strongly dorsally inclined in anterior view. Both cotyle and condyle are dorsoventrally depressed. The haemapophyses are not fused to each other and appear almost parallel in ventral and posterior views; they are situated rather near the level of the condyle. The autotomic septum is evident in the anterior portion of the ventral surface of the centrum, at the level of the transverse processes.
REMARKS
The main distinction of the (post-cervical) presacral vertebrae of Placosaurus from those of Palaeovaranus appears to be the presence of a median ridge in the ventral surface of the centrum of the former genus ( Rage 1978; see Discussion below). We accordingly refer this material to Placosaurus , highlighting the overall resemblance it shares with similar vertebrae described and figured by Rage (1978) from the late Eocene of Sainte Néboule, also in Quercy. The cervical vertebra NHMW 2019/0046/0002 possesses a rather thick hypapophysis. Such thick hypapophyses are present also in cervical vertebrae of Palaeovaranus (see Rage 1978: fig. 4), however, the latter specimens are slenderer and more elongated in dorsal view, while their hypapophyses do not extend throughout the centrum and are not so broad. Additionally, the single caudal vertebra described herein is strongly reminiscent to the one described and figured also by Rage (1978: fig. 2B) from the late Eocene of Sainte Néboule.
As it is also highlighted in the Discussion below, an identification of isolated vertebrae to the genus Placosaurus should be considered as tentative and is apparently primarily based on the existing figures and documentation of that genus in the literature ( Rage 1978). Frustratingly, there is no associated articulated cranial material or osteoderms of Placosaurus in this (or any other) collection that would enable a more confident identification. Moreover, the types of all known species of Placosaurus (i.e, the type species Placosaurus rugosus , as well as Placosaurus estesi Sullivan & Augé, 2006 , “ Placosaurus ” europaeus [ Filhol, 1876], and? Placosaurus ragei Sullivan, Augé, Wille & Smith, 2012 ) are cranial remains. Note that other species that have originally established as species of Placosaurus , are now considered to belong to different genera. These are: Placosaurus leenhardti Leenhardt, 1926 (currently considered a junior synonym of Paraplacosauriops quercyi ), Placosaurus lugdunensis Depéret, 1917 (never figured), and Placosaurus waltheri Weigelt, 1929 (a nomen dubium). In any case, these are also typified by cranial material, with the exception of the latter taxon which is based on isolated osteoderms. Nevertheless, these glyptosaurine vertebrae from the NHMW collection appear to be much distinct from the below described melanosaurine ones.
No known copyright restrictions apply. See Agosti, D., Egloff, W., 2009. Taxonomic information exchange and copyright: the Plazi approach. BMC Research Notes 2009, 2:53 for further explanation.