In the last century, it was common read in popular books that sauropods were split in two large groups: spoon-toothed forms (i.e. Camarasaurus) and pencil-toothed forms (i.e. Diplodocus). This division was proposed in the 30s by Janesch, and was followed by most later authors, but it was based on a simplistic concept common to other classifications hailing from the 20th century: lumping or splitting animals on the basis of some similarities and ignoring the many differences. The shape of the teeth was the response to different kinds of plants and different ways of feeding on them, and pencil- or spoon-shaped teeth appeared several times in sauropod history.
What undermined the dichotomy pencil/spoon teeth was the discovery of sauropods which presented hybrid morphologies between the typical “pencil-toothed sauropod” and the typical “spoon-toothed sauropod”. Mamenchisaurus, for example, was considered closer to to Diplodocidae (pencil-toothed sauropods) because of the morphology of the caudal vertebrae, which have forked chevrons (as mentioned in Upchurch et al. 2004). Yet, Mamenchisaurus’ teeth are spoon-shaped and more similar to Camarasaurus’.
A curiosity: a restoration of Mamenchisaurus as a diplodocid by Mark Hallett as study for his famous painting “Crossing the Flat” was the base for its appearance in The Lost World – Jurassic Park.
Over the years, several species of Mamenchisaurus were described from formations dating back to the Middle to Terminal Jurassic of China, and in turn looked to be related to other genera such as Omeisaurus, many of which characterized by extreme elongation of the neck (up to 19 vertebrae!). However, once the diplodocid hypothesis was put apart, this and other Chinese forms did not seem to berelated to any other sauropod group outside China.
Could it be that, in the Jurassic and early Cretaceous, Eastern Asia was isolated from the rest of the world? The most accepted explanation for these faunal differences was a sea located east of the Urals: it would have isolated Asia, preventing widespread groups like Diplodocoidea and Megalosauridea from reaching it. This hypothesis is called Eastern Asia Isolation Hypothesis (EAIH).
Everything changed when Lingwulong was discovered.
Described in 2018, this sauropod was unearthed in the Zhiluo Formation (Ningxia Hui Autonomous Region). It was initially dated to the end of the Toarcian – beginning of Bajocian (late Early Jurassic/Early Middle Jurassic), but later publications (You, 2019) redated it to the Batonian-Callovian (Middle Jurassic). But, more importantly, Lingwulong is a Diplodocoidea (specifically, a Dicraeosauridae), the oldest member of the group, and it cames from an area that the Diplodocoidea, according to the EAIH, could not have reached.
Thus, Asia must have been connected to the rest of the world at least enough to allow Diplodocoidea to reach it. It could not have been completely isolated for the whole Jurassic. And the same thing is suggested by the discovery of species related to Mamenchisaurus outside Asia.
It was proposed a “light” version of the EAIH that suggests a shorter duration for the isolation (limited to the Upper Jurassic), aiming to explain how Lingwulong and his descendants are absent from the most famous formations of the Chinese Jurassic. However, said formations are located mainly in southern China, and therefore Diplodocoidea may be absent only locally. In addition, many of the formations that seem to be dominated by typically Chinese sauropods, such as Mamenchisaurus, have been redated, and this brings the two faunas closer together. Perhaps too much for such a radical wildlife change.
Lingwulong is an example of how the discovery of a single animal can mess with established hypotheses!
Bibliography:
Lambert D. Dinosaur Data Book: The Definitive, Fully Illustrated Encyclopedia of Dinosaurs. Avon Books, 1990, 320 pp
Hallett M. Bringing Dinosaurs to life in Dinosaurs past and present, Natural History Museum of L. A. County ed. Seattle and London, 1987, 149 pp
Suteethorn S.; Le Loeuff J.; Buffetaut E.; Suteethorn V.; Wongko K. (2012) First evidence of a mamenchisaurid dinosaur from the Upper Jurassic-Lower Cretaceous Phu Kradung Formation of Thailand. Acta Palaeontologica Polonica. 58 (3): 459–469
Upchurch P.; Barrett P.M.; Dodson P. Sauropoda. In The Dinosauria, 2nd edition. David Weishampel, Peter Dodson, and Halszka Osmólska (eds.). University of California Press, Berkeley, 2004, pp. 259–322
Wilson J. A. Overview of Sauropod Phylogeny and Evolution in The Sauropods: Evolution and Paleobiology. Kristina Curry Rogers (ed.), Oakland (CA), 2005, 358 pp
Woodruff D.C.; Carr T.D.; Storrs G.W.; Waskow K;, Scannella J.B.; Nordén K.K.; Wilson J.P. (2018) The smallest diplodocid skull reveals cranial ontogeny and growth-related dietary changes in the largest dinosaurs. Scientific reports, 8(1), 1-12.
You S.; Li Z.; Li Y. (2019) The stratigraphical characteristics and sedimentary environment of dinosaurs in Lingwu, Ningxia. Acta Geologica Sichuan. 39 (S1): 31–35