Opisthocoelicaudia, Borsuk-Bialynicka, 1977

Averianov, Alexander O. & Lopatin, Alexey V., 2019, Sauropod diversity in the Upper Cretaceous Nemegt Formation of Mongolia-a possible new specimen of Nemegtosaurus, Acta Palaeontologica Polonica 64 (2), pp. 313-321 : 318-319

Opisthocoelicaudia

PIN 3837/P821 is similar with Opisthocoelicaudia in size, in having a relatively low neural arch in middle-posterior dorsal vertebrae, small neural canal, and apparent lack of hyposphene–hypantrum articulations. The low neural arch of PIN 3837/P821 and Opisthocoelicaudia is narrowest anteroposteriorly at the junction with the centrum and widens dorsally, while in many other sauropods the higher neural arch has a narrowest point at the midheight. Wilson (2002) considered presence of the pcpl in middle and posterior dorsal neural arches as an autapomorphy of Opisthocoelicaudia . This lamina is well developed at least in dorsal 6 ( Borsuk-Białynicka 1977: fig. 3A 6). In PIN 3837/P821 the pcpl is present on dorsals 6 and 8 and it is confluent with acpl on dorsals 7 and 9. D’Emic (2012) listed divided pcpl on middle dorsal vertebra as an autapomorhy for Opisthocoelicaudia , but we see no evidence for this.

In Opisthocoelicaudia the dorsoventral flattening of the centra increases in the posterior part of dorsal series due to the broadening of the centra in that region and constant centrum height ( Borsuk-Białynicka 1977: 9). In dorsals 6–8 the centrum height to centrum width ratio is 0.60–0.64. This ratio is 0.47 in dorsal 9 and 0.52 in dorsal 10. In PIN 3837/P821 the posterior centrum width is constant while the posterior centrum height increases from dorsal 6 to dorsal 9 ( Table 1). This result in decreasing of the dorsoventral flattening from dorsal 6 to dorsal 9 (PCH/PCW changes from 0.80 to 1.05; Table 1).

In Opisthocoelicaudia the ventral concavity of the centrum in dorsal vertebrae is delimited by two ventrolateral crests and divided by a midline ridge ( Borsuk-Białynicka 1977: 9). This concavity becomes increasingly deeper towards the posterior dorsals. In PIN 3837/P821, the ventral profile of the centrum is much less concave, with a strong ventral ridge flanked by ventrolateral ridges and depressions between them. Thus, the construction of the ventral centrum surface in dorsals of Opisthocoelicaudia and PIN 3837/P821 is quite similar. The difference is that the ventral concavity is emphasized in Opisthocoelicaudia and ventral ridge is emphasized in PIN 3837/P821. This results in a much less concave ventral profile of the centrum in PIN 3837/P821. A concave ventral surface that bears a ventral midline keel in dorsal and sacral vertebrae has been considered a diagnostic character of Opisthocoelicaudia ( Upchurch et al. 2004; D’Emic 2012).

In Opisthocoelicaudia the pcdl is anterodorsally oriented in all dorsals. In PIN 3837/P821 this is true for the dorsal 6 and 7 while in the dorsals 8 and 9 pcdl is almost vertical.

In Opisthocoelicaudia the podl is absent and pocdf is open dorsally. The absence of podl has been considered an autapomorphy of Opisthocoelicaudia ( Wilson 2002) . In PIN 3837/P821 this lamina is well developed at least in dorsal 6 (this region is damaged in other dorsals), roofing the pocdf.

In PIN 3837/P821 the pacprf on the anterior aspect of the neural arch, bordered by cprl, acpl, and prpl, is undivided (this side can be seen only in dorsal 6). This is true for the dorsal vertebrae in Opisthocoelicaudia except the dorsal 6, where pacprf is divided by a middle accessory lamina which is perpendicular to the prezygapophysis and acpl ( Borsuk-Białynicka 1977: fig. 3A 3).

The accessory laminae within the pacdf, well pronounced in PIN 3837/P821, are poorly developed in Opisthcoelicaudia. The ventral accessory lamina can be seen only on the left side of cervical 7 ( Borsuk-Białynicka 1977: pl. 3: 1). The other accessory laminae are absent.

Although PIN 3837/P821 shares some common features with Opisthocoelicaudia , it clearly belongs to a different genus which is characterized by less dorsoventrally flattened dorsal centra, shallow ventral concavity of dorsal centra in lateral view, vertical pcdl in dorsals 8 and 9, presence of podl that closes pocdf dorsally, and strongly developed accessory laminae within the pacdf.