A vertebra from the Albian of Mesnil-Saint-Père (Aube, eastern Paris Basin), previously identified as the first caudal of a sauropod dinosaur, is shown to be a dorsal vertebra of a large pliosaur. The specimen resembles vertebrae from the Albian of England and eastern France that have been referred to the pliosaur Polyptychodon, a taxon in need of revision.
Vertebra;
Cretaceous; France; Plesiosauria; Sauropoda.
E., C., B., J.-L. (2005).- The "sauropod" from the Albian of Mesnil-Saint-Père (Aube, France): a pliosaur, not a dinosaur.- Carnets de Géologie / Notebooks on Geology, Brest, Letter 2005/01 (CG2005_L01)
Le "sauropode" de l'Albien de Mesnil-Saint-Père (Aube, France) est un pliosaure, non un dinosaure.-
Il est montré qu'une vertèbre provenant de l'Albien de Mesnil-Saint-Père (Aube, E du Bassin de Paris), précédemment identifiée comme une première caudale d'un dinosaure sauropode, est en fait une vertèbre dorsale d'un grand pliosaure. Le spécimen évoque des vertèbres de l'Albien d'Angleterre et de l'Est de la France qui ont été attribuées au pliosaure Polyptychodon, taxon qui a besoin d'être révisé.
Vertèbre ; Crétacé ; France ; Plesiosauria ; Sauropoda.
In
2000, et alii described an isolated vertebra from the Albian of Aube (eastern Paris Basin) as an anterior caudal of a sauropod dinosaur (see also the French translation: et alii,
2002). This identification was subsequently accepted by &
(2003), and on its basis
et alii (2004, p. 560) listed the occurrence of Sauropoda indet. in the Albian of Aube in their review of dinosaur distribution.
We have re-examined the specimen and found that
its identification as a sauropod vertebra is erroneous. The vertebra from Mesnil-Saint-Père in fact does not belong to a dinosaur; as shown below, it is clearly a dorsal vertebra of a large pliosaur.
As noted by et alii
(2000), the vertebra found by one of us (J.-L. P.) at Mesnil-Saint-Père apparently comes from clayey layers overlying the "Sables verts" (Greensand), and is considered to be
Early Albian in age. The specimen is in the J.-L. collection in Troyes and can be
examined through contact with the Association Géologique Auboise; a cast of it is kept at the Musée des Dinosaures, Espéraza, France.
The specimen
(Fig. 1 
) is generally well-preserved, although some abrasion of the surface of the bone probably indicates transport. It consists of a fairly complete
centrum: only the posterodorsal and right posterolateral regions are damaged and partly missing. Contrary to the description by et alii
(2000), the neural arch is not completely missing, for its pedicels are clearly visible in the anterior half of the specimen, enclosing the floor of the neural canal (the posterior part of the pedicels is largely missing). The anterior articular surface is nearly circular in outline, and very shallowly concave. It shows a well-marked central elevation or papilla. Similarly, the posterior articular surface, which is slightly more concave, appears to have been circular in outline, and bears a central elevation. The lateral faces of the centrum are concave anteroposteriorly and show no sign of a rib articulation or transverse process. The ventral face is concave anteroposteriorly, with a prominent posterior margin (exaggerated by deformation). There are no chevron facets. No large foramina are visible on the ventral and lateral surfaces of the centrum, although small, irregularly distributed foramina, can be seen. Those visible on the left side of the centrum appear as oval pits with a maximum diameter of 4 mm.
et alii
(2000) considered the specimen from Mesnil-Saint-Père
as the first caudal vertebra of a sauropod dinosaur. However, this
identification is untenable, for the simple reason that in sauropods the first caudal (like the other anterior caudals) bears well-developed transverse processes which extend over a large part of the lateral sides of the centrum (see, for instance, ,
1990, Fig. 16.7, p. 363, and et alii,
2004, Fig. 13.9, p. 286). Even if the neural arch was totally missing on the vertebra from Mesnil-Saint-Père, which is not the case, the inception of the transverse processes on the sides of the centrum would be visible. Instead, the lateral faces are smooth. According to et alii
(2000), the vertebra from Mesnil-Saint-Père shows very strong
similarities with the first caudal of Brachiosaurus brancai.
However, a comparison with 's description and figures (,
1950) shows that this is inaccurate: in Brachiosaurus brancai, as in other sauropods, there are well-developed transverse processes, inserting on the lateral faces of the centrum, down to nearly mid-height.
Nothing of the sort can be seen on the specimen from Mesnil-Saint-Père; its well-preserved left side clearly shows the absence of any process.
Consequently, there can be no doubt that the identification of the vertebra from Mesnil-Saint-Père as a sauropod caudal is erroneous. The characters of this vertebra do not fit in a sauropod vertebral column.
The specimen from Mesnil-Saint-Père can be identified as a dorsal vertebra of a large sauropterygian. The fact that there are no insertion facets for ribs on the lateral faces shows that it must have been located far posteriorly in the dorsal series, so that the rib facets were located on the now missing parts of the neural arch. The more or less cylindrical, although constricted, shape of the centrum is also in agreement with
its identification as a sauropterygian vertebra. Well-marked central papillae on the articular surfaces, bearing or not a notochordal pit, are a very frequent feature of sauropterygian vertebrae, whereas they are not common in sauropods. On the whole, the vertebra from Mesnil-Saint-Père closely resembles
the dorsal vertebrae of large Cretaceous representatives of the Plesiosauria. In size and shape, for instance, it compares well with a large vertebra from the Upper Cretaceous of Elbing, Prussia (today Elblag, Poland) described by
(1885) as Pliosaurus ? gigas (referred to as Plesiosauria indet. by & ,
1995).
One unusual feature of the vertebra from Mesnil-Saint-Père is the lack of large foramina on the ventral or lateral surfaces of the centrum. These "foramina subcentralia" (,
1991) are very common in the Plesiosauria, and their absence in the specimen from Aube may at first sight seem surprising. However, in some pliosaur taxa the foramina subcentralia are reduced or
missing altogether. As noted by
(1903, p. 62), in the Late Cretaceous Brachauchenius lucasii, "the ventral vascular foramina, so characteristic of plesiosaur vertebrae, appear to be wanting throughout the whole series". &
(2005) report the same lack of ventral vascular foramina in a pliosaurid from the Turonian of Kansas. According to
(2001), reduction of the foramina subcentralia is a derived character found,
in the family Pliosauridae, in the genera Liopleurodon, Pliosaurus and Brachauchenius. The lack of such foramina in the specimen from Mesnil-Saint-Père is therefore not a sufficient reason to exclude it from the Plesiosauria, and may suggest that it belongs to a member of the family Pliosauridae (a conclusion also supported by the large size of the vertebra).
Measurements:
Height of anterior articular face of centrum: 137 mm
Width of anterior articular face of centrum: 140 mm
Ventral length of centrum: 108 mm
Estimating the total length of a pliosaur on the basis of an isolated vertebra is fraught with difficulties. On the basis of the proportions of the mounted skeleton of Kronosaurus queenslandicus, from the Lower Cretaceous of Australia, described by &
(1959) as being 12.80 m long, the pliosaur from Mesnil-Saint-Père may have been about 10 metres long. However, the total number of vertebrae in that specimen of Kronosaurus queenslandicus is uncertain and its total length may have been overestimated.
(2003, p. 291) describes Kronosaurus as a "large-bodied pliosaurid up to and probably in excess of 9 m". This would suggest an approximate length of 7 metres for the pliosaur from Mesnil-Saint-Père. Moreover, the proportions of the pliosaur from Aube may have been different from those of Kronosaurus, so the above estimates must be considered as very rough approximations. Be that as it may, the pliosaur vertebra from Mesnil-Saint-Père clearly belonged to a very large animal.
The vertebra from Mesnil-Saint-Père is not the first pliosaur specimen to be reported from the Albian Greensand of the eastern Paris Basin.
(1875) noted the occurrence of Polyptychodon teeth at Grandpré (Ardennes) and Louppy (Meuse).
(1882) described and illustrated teeth from Grandpré, and in
1903 described a tooth and eight vertebrae from Varennes-en-Argonne (Meuse).
(1922) also mentioned Polyptychodon in the Albian of the Paris Basin and noted that the fossils from Varennes indicated an individual of an enormous size.
Polyptychodon was erected by
(1841) on the basis of isolated teeth, with two species, P. interruptus and P. continuus, which may in fact correspond to teeth from different positions in the tooth row (,
1903). Teeth and other remains from many mid-Cretaceous formations, ranging in age from Albian to Turonian, in Europe and North America ( & ,
1963), have been referred to that genus. Although and
(1995, p. 182) considered it as "systematically well defined", Polyptychodon remains a relatively poorly known taxon.
(2001) did not include it in his phylogenetic analysis and classification of the Plesiosauria. Nevertheless, and although they did not redefine the taxon, 's and 's conclusion ( & ,
1995, p. 183) that it is a "valid genus of Pliosauridae" is probably well-founded.
Although the type species of Polyptychodon was based on teeth, isolated vertebrae from deposits where such teeth have been found have been referred to this genus.
Thus,
(1876) described several vertebrae from the Cambridge Greensand as belonging to Polyptychodon. The Cambridge Greensand vertebrates are considered as
Late Albian, reworked into the basal Cenomanian (,
2001). Photographs of a dorsal vertebra (Sedgwick Museum, CAMSM B57396) from that collection kindly provided by Dr Leslie (Sedgwick Museum, Cambridge) show considerable
similarities with the specimen from Mesnil-Saint-Père, both in the shape of the centrum and in the absence of large foramina subcentralia (Fig.
2 ).
The vertebrae from Varennes-en-Argonne referred by
(1903) to Polyptychodon are of particular interest because they appear to be reminiscent of the specimen from Mesnil-Saint-Père both in their morphology (most of them are described as having more or less concave articular faces with a strong central papilla) and
in their size (vertebra "8" is described as being 140 mm high, 115 mm long and 135 mm wide), although most of them show costal facets on the sides of the centrum and must have
come from more anterior parts of the vertebral column. However, whether the vertebrae described by
(1903) can safely be referred to Polyptychodon is uncertain; there is no clear evidence that the tooth and the vertebrae he mentioned were from a single individual (associated skeletal elements seem to be rare in the Sables verts, although
(1882) did describe a partly articulated theropod skeleton). This applies even more to the isolated vertebra from Mesnil-Saint-Père: pending a revision of Polyptychodon in general, and of the pliosaur remains from the Sables verts in particular, it seems safer to refer to it as Pliosauridae indet. The
coexistence of large teeth referable to Polyptychodon and of large pliosaur vertebrae in the Albian Sables verts of the eastern Paris Basin does suggest, however, that they may belong to the same taxon.
Although sauropod remains do occur in the Albian of the eastern Paris Basin (see ,
1995, for a review), and have recently been reported from the Sables verts (,
2002), the vertebra from Mesnil-Saint-Père clearly does not belong to a dinosaur, and it obviously cannot be used to support the conclusions of et alii
(2000) on the possible occurrence of brachiosaurids in the Albian of Europe.
However, its identification as a dorsal vertebra of a large pliosaurid raises the question of the plesiosaurs and pliosaurs from the Sables verts of the eastern Paris Basin. A certain amount of material is present in the collections of various museums (notably in Lille, Nancy and Charleville-Mézières), but has
received little attention since the beginning of the
20th century. A revision of these marine reptiles is clearly needed.
We thank Dr Leslie (Sedgwick Museum, Cambridge) for information (including photographs) about the vertebrae referred to Polyptychodon by
(1876). Thanks also to the referees (Marie-Céline , Mike , Jean , Jean-Michel and Leslie ) for their useful comments.
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Click on thumbnail to enlarge the image.
Figure 1: Dorsal vertebra of a large pliosaur from the Albian of Mesnil-Saint-Père, Aube (J.-L. collection, Troyes), in anterior (A), left lateral (B), posterior (C) and ventral (D, anterior end to the left) views.
Click on thumbnail to enlarge the image.
Figure 2: Dorsal vertebra referred by (1876) to Polyptychodon, from the Cambridge Greensand (Sedgwick Museum, Cambridge, CAMSM B57396), in right lateral (A), anterior (B) and ventral (C, anterior end to the left) views. Photos courtesy of Dr Leslie (Sedgwick Museum).
