Rebbachisaurids are a clade of diplodocoids largely known from South America and Africa, with minor occurrences in western Europe and Croatia (Upchurch et al. 2004; Fernández-Baldor et al., 2011; Fanti et al. 2015; Wilson and Allian 2015; Taylor 2018). Recently, the temporal and geographical distribution of Rebbachisauridae was extended to the Late Jurassic of western North America when Carpenter (2018) reassigned the mega-sauropod Amphicoelias fragillimus Cope, 1878 to Rebbachisauridae and renamed it Maraapunisaurus. Now, however, Averianov and Sues (2021) have described a new rebbachisaurid from the Late Cretaceous (Turonian) Bissekty Formation of Uzbekistan, Dzharatitanis kingi, on the basis of an isolated anterior caudal vertebra. The description of Dzharatitanis renders this genus the first occurrence of a rebbachisaurid from Asia, considering that diplodocoids were basically unknown from the Asian landmass until the description of the dicraeosaurid Lingwulong by Xu et al. (2018), and rebbachisaurids are known only in Laurasia from western North America, Spain, England, and Croatia.
The holotype of Dzharatitanis kingi, USNM 538217, was originally placed as Titanosauria indeterminate by Sues et al. (2015) in their discussion of titanosaur material collected from the Bissekty Formation of Uzbekistan. Subsequently, Averianov and Sues (2017) noted that USNM 538217 was most similar to the anterior caudals of Baotianmansaurus henanensis is being anteroposteriorly short but differed in being taller. They cautioned that the neural arch of the only known anterior caudal in Baotianmansaurus is incomplete, making it unclear if it had additional fossae like those present in Dongyangosaurus and USNM 538217. The decision taken by Averianov and Sues (2021) to reclassify USNM 538217 as a new taxon of rebbachisaurid thus raises a number of questions as to why it was originally classified in Titanosauria before being removed from this clade: how does Dzharatitanis differ from macronarian sauropods if it was initially interpreted as being similar to Baotianmansaurus?
In their description of USNM 538217, Sues et al. (2015) noted that this specimen possesses a transverse process (caudal rib) rising from both the centrum and the neural arch just above the mid-height of the centrum, and that it is triangular and its dorsal margin extends ventrolaterally, unlike flagellicaudatans and some rebbachisaurids. and that the neural spine of the Dzharatitanis kingi holotype has a large pneumatic cavity with a complex, internal structure comprising variously sized chambers as in the lognkosaurian titanosaur Futalognkosaurus. A prominent, deep postzygapophyseal spinodiapophyseal fossa in USNM 538217 was described as being similar to that of Dongyangosaurus in possessing four foramina on the left and three openings on the right side, with two of the foramina being situated at the base of the fossa just dorsal to the postzygodiapophyseal lamina. The paper by Averianov and Sues (2021) lists several characters to distinguish Dzharatitanis from titanosauriforms and support a referral to Diplodocoidea and specifically to Rebbachisauridae: (1) slightly convex or flat anterior articular surface of the first caudal centrum; (2) absence of the hyposphenal ridge; (3) absence of pleurocoels; (4) triangular lateral process on neural spine; and (5) composite lamina on the dorsal part of the lateral surface of the neural spine formed by the fusion of the SPRL and SPOL. It is highly probable that the authors took note of the varying presence or absence of pleurocoels on the first caudal vertebrae among rebbachisaurids and the presence of pleurocoels in many titanosauriforms to express second thoughts about USNM 538217 belonging to Titanosauria, because they identify the holotype as having slight opisthocoely and lacking chevron facets, which they note as being present among rebbachisaurids. The presence of opisthocoely in non-titanosauriform neosauropods is noteworthy because anterior caudal vertebrae with procoely have been documented for both titanosaurs and non-titanosaur sauropods (Wilson and Upchurch 2003).
When placing the age and location of Dzharatitanis into a broader contextual pattern of rebbachisaurid biogeography, it is important to note that almost all other rebbachisaurid taxa except Demandasaurus, Maraapunisaurus, and Xenoposeidon are present in Gondwana only, and that Katepensaurus is about the same geologic age as Dzharatitanis, hailing from the Cenomanian-Turonian Bajo Barreal Formation of Chubut Province, Argentina. All Early Cretaceous rebbachisaurids from Europe are of Valanginian to early Aptian age, and no rebbachisaurid has yet been reported from the Berriasian to Cenomanian of East Asia. Since Dzharatitanis is younger than other Cretaceous rebbachisaurids found in Laurasia, it is possible that a number of rebbachisaurids entered central Asia from Europe, or that Dzharatitanis was descended from an as-yet-undiscovered Early Cretaceous rebbachisaurid from East Asia, because a land connection existed between Europe and Asia during the Berriasian-Cenomanian (Baraboshkin et al. 2003) and the record of Laurasian rebbachisaurids is very scanty.
Update: A new paper by Lerzo et al. (2021) questions the rebbachisaurid placement of Dzharatitanis and instead re-assigns this genus to the titanosaur clade Lognkosauria based on comparisons with the caudal vertebrae of derived titanosaurs, including members of Colossosauria. In tandem with the recent paper by Averianov et al. (2021) classifying the genera Normanniasaurus and Volgatitan in the clade containing members of Colossosauria, the formal reclassification of Dzharatitanis as a titanosaur sheds new light on the early biogeography of Colossosauria, given that this clade was long restricted to Gondwana.
References:
Averianov, A.O., and Sues, H-D., 2017. Review of Cretaceous sauropod dinosaurs from Central Asia. Cretaceous Research 69:184–97.
Averianov, A., and Sues, H-D., 2021. First rebbachisaurid sauropod dinosaur from Asia. PLoS ONE 16(2): e0246620.
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Carpenter, K., 2018. Maraapunisaurus fragillimus, N.G. (formerly Amphicoelias fragillimus), a basal Rebbachisaurid from the Morrison Formation (Upper Jurassic) of Colorado. Geology of the Intermountain West. 5: 227–244. doi:10.31711/giw.v5i0.28.
Fanti, F., Cau, A., Cantelli, L., Hassine, M., and Auditore, M., 2015. New Information on Tataouinea hannibalis from the Early Cretaceous of Tunisia and Implications for the Tempo and Mode of Rebbachisaurid Sauropod Evolution. PLoS ONE 10(4): e0123475.
Lerzo, N.L., Carballido, J.L., and Gallina, P.A., 2021. Rebbachisaurid sauropods in Asia? A re-evaluation of the phylogenetic position of Dzharatitanis kingi from the Late Cretaceous of Uzbekistan. Publicación Electrónica de la Asociación Paleontológica Argentina. 21 (1): 18–27. doi:10.5710//PEAPA.24.03.2021.389
Sues, H-D,, Averianov, A.O., Ridgely, R.C., and Witmer, L.M., 2015. Titanosauria (Dinosauria: Sauropoda) from the Upper Cretaceous (Turonian) Bissekty Formation of Uzbekistan. Journal of Vertebrate Paleontology 35(1):e889145.
Taylor, M.P., 2018. Xenoposeidon is the earliest known rebbachisaurid sauropod dinosaur. PeerJ 6:e5212 https://doi.org/10.7717/peerj.5212
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