Are Qijianglong and Mamenchisaurus anyuensis of Middle Cretaceous age?

In a couple of scientific papers, Liu et al. (2017) and Wang et al. (2019) considered the Suining Formation (which overlies the Shangshaximiao Formation) to be of Aptian age based on U-Pb radiometric dating of detrital zircons from this unit, rather than Late Jurassic as stated by Dong et al. (1983) and Peng et al. (2005) based on tenuous biostratigraphic correlations. This conclusion potentially threw a wrench into the evolution of the eusauropod clade Mamenchisauridae, because the ages that Liu et al. (2017) and Wang et al. (2019) obtained for the Suining Formation indicated that mamenchisaurids survived until the middle Cretaceous. As the mamenchisaurid Qijianglong guokr and some referred specimens of Mamenchisaurus anyuensis are known from the Suining Formation (the holotype of M. anyuensis hails from the overlying Penglaizhen Formation), it occurred to me that any suggestion of the Penglaizhen and Suining Formations being much younger than the Shangshaximiao Formation once more reinforced the long-overdue need for a revision of Mamenchisaurus by implying that anyuensis would have to be assigned a new genus. In the meantime, however, Huang (2019) disputed the conclusion by Liu et al. (2017) regarding the age of the Suining Formation, arguing that the dating obtained from detrital zircons was affected by metamorphism and that the Suining Formation was not as young as concluded by Liu et al. (2017), asserting instead that the Suining Formation straddles the Tithonian-Berriasian boundary.

To begin addressing the question of whether or not Mamenchisaurus anyuensis and Qijianglong are of Middle Cretaceous age, it is imperative to analyze the available biostratigraphic evidence cited by Huang (2019) to make the case that the Suining and Penglaizhen Formations are older than asserted by Liu et al. (2017) and Wang et al. (2019) as well as state-of-the-art knowledge of mamenchisaurid evolution. Although Deng et al. (2015) and Wang & Gao (2012) date the Qigu Formation to 157-167 million years (late Callovian-early Oxfordian) based on U-Pb radiometric dating of detrital zircons from that unit, Huang (2019) asserts that the zircons used to determine this age estimate were likely recycled from the Xishanyao Formation, as volcanic rocks and tuffs have yet to be found in the Xishanyao Formation. As noted by Huang (2019), fossils of the ostracods Darwinula and Timiriasevia (which are of Late Jurassic age) appear in the lower parts of the Qigu and Suining Formations, whereas the Early Cretaceous ostracod Djungarica is present in the upper parts of the Qigu and Suining Formations but also the Penglaizhen Formation, indicating a latest Kimmeridgian-early Berriasian age for the Suining Formation and a late Berriasian-early Valanginian age for the Penglaizhen Formation. As Wang et al. (2019) admit, the traditional ages assigned to mamenchisaurid taxa from the Shangshaximiao, Suining, and Penglaizhen Formations by Dong (1980), Dong et al. (1983), and Peng et al. (2005) were based on tenuous stratigraphic correlations, and the age they assign to all Mamenchisaurus species from the Shangshaximiao Formation is largely consistent with the Oxfordian-Kimmeridgian age advocated for the Shangshaximiao by Huang (2019). Since the U-Pb radiometric dates obtained from detrital zircons from the Qigu and Suining Formations by Deng et al. (2015) and Wang et al. (2019) appear to have been subject to extraneous geologic factors like metamorphosis and recycling, it is highly reasonable to not rule out the possibly that the Suining and Penglaizhen Formations are of latest Kimmeridgian to Berriasian age rather than latest Aptian-Albian because the Cangxi Formation (located in the same basin as the Suining and Penglaizhen Formations) is of Berriasian-Valanginian age based on biostratigraphy (Hou et al. 2020).

At the current time of writing, an indeterminate cervical vertebra reported by Suteethorn et al. (2013) from the latest Tithonian-early Berriasian Phu Kradung Formation of northeastern Thailand is the only record of a mamenchisaurid from the Cretaceous, and if Wang et al. (2019) are correct, there would a be vast temporal gap between the Phu Kradung material and the mamenchisaurid taxa from the Suining and Penglaizhen Formations. No mamenchisaurid fossils have been found in the Valanginian-middle Aptian interval so far, even though basal eusauropod clade Turiasauria is present in Early Cretaceous deposits. Some of the fossils of Mamenchisaurus anyuensis have been found in the upper part of the Suining Formation, so the age of the Penglaizhen and upper Suining Formations would place M. anyuensis in the Berriasian-Valanginian interval. Since the youngest turiasaur fossils are from the Barremian-age Yellow Cat Member of the Cedar Mountain of Utah and the Cangxi Formation is slightly younger than the Penglaizhen Formation, a Berriasian-Valanginian or Hauterivan-Barremian age for Mamenchisaurus anyuensis and Qijianglong cannot be discounted, because no radiometric dates have been obtained for the Penglaizhen Formation.

Based on an analysis of the biostratigraphic evidence cited by Huang (2019) for the age of the Suning and Penglaizhen Formations as well as the age of geologic units underlying and overlying both the Suning and Penglaizhen Formations, along with prior knowledge of basal eusauropod evolution during the Early Cretaceous, it can be prudent to conclude that the Middle Cretaceous age proposed for Qijianglong and Mamenchisaurus anyuensis by Wang et al. (2019) is less likely than that concluded by Huang (2019) because of the age of the Qigu Formation and the presence of the earliest Cretaceous ostracod Djungarica in the upper Suining and Penglaizhen Formations. Even if Mamenchisaurus anyuensis and Qijianglong are not as young as proposed by Wang et al. (2019), the earliest Cretaceous age of these taxa doesn't diminish their significance, because they would still be younger than other nominal Mamenchisaurus species, and thus about the same age as the Phu Kradung mamenchisaurid, in which case Mamenchisaurus anyuensis would need a new generic name.

References:

Deng, S., Wang, S., Yang, Z., Lu, Y., Li, X., Hu, Q., An, C., Xi, D., and Wan, X., 2015. Comprehensive study of the Middle-Upper Jurassic strata in the Junggar Basin, Xinjiang (in Chinese). Acta Geosciences Sinica 36: 559–574.

Dong, Z.M. 1980. The dinosaurian faunas of China and their stratigraphic distribution. Journal of Stratigraphy 4: 256-263.

Dong, Z., Zhou, S. & Zhang, Y. 1983. Dinosaurs from the Jurassic of Sichuan. Palaeontologica Sinica 162: 1-151.

Hou, X.W., Shi, Z.J., Sun, Z.X., Tan, Z.Y., & Tian, X.S., 2020. The sporopollen assemblages in the Early Cretaceous red sediments in Cangxi area, northern Sichuan Basin and their geological significance. Geological Review 66 (3): 727-738. https://doi.org/10.16509/j.georeview.2020.0 3.014

Huang, D., 2019. Jurassic integrative stratigraphy and timescale of China. Science China Earth Sciences 62 223–255.

Liu G, Dong S, Chen X, Cui J. 2017. Detrital zircon U-Pb dating of Suining Fm. sandstone from the Daba Mountains, northeastern Sichuan and its stratigraphic implications. Palaeoworld 26: 380–395. 

Peng, G.Z., Ye, Y., Gao, Y.H., Shu, C.K. & Jiang, S. 2005. Jurassic Dinosaur Faunas in Zigong. People’s Publishing House of Sichuan, Chengdu, China.

Suteethorn, S., Le Loeuff, J., Buffetaut, E., Suteethorn, V., and Wongko, K. 2013. First evidence of a mamenchisaurid dinosaur from the Upper Jurassic–Lower Cretaceous Phu Kradung Formation of Thailand. Acta Palaeontologica Polonica 58 (3): 459–469.

Wang, J., Norell, M. A., Pei, R., Ye, Y. and Chang, S.-C., 2019. Surprisingly young age for the mamenchisaurid sauropods in South China. Cretaceous Research 104: 104176.