Thoughts on paper by D'Emic (2023) regarding evolution of huge body size in sauropods

For generations, sauropod dinosaurs have starred in the popular imagination as the biggest land animals to ever walk the Earth, with interest in giant sauropods going back to the 1970s and 1980s with the discovery of the derived titanosaur Argentinosaurus and the diplodocids Diplodocus (=Seismosaurus) hallorum and Supersaurus (the giant rebbachisaurid Maraapunisaurus was seen in some publications as possibly the biggest-ever sauropod, although the missing nature of the holotype dorsal vertebra made size estimates of 190 feet guesswork, but Carpenter [2018] revised the size estimate of this genus to 105 feet, which makes sense because the initial size estimates of 170 feet for Diplodocus  hallorum was later revised to 110 feet). While children and adults alike marvel at the immense size of sauropods compared to elephants, rhinoceroses, and the extinct paraceratheres, I have not forgotten the fact that some sauropods living on island arcs in southern Europe during the Late Cretaceous which today are part of France, Spain, and Romania exhibited insular dwarfism, and that not all sauropods approached the body sizes attained by Apatosaurus, Brachiosaurus, Brontosaurus, Diplodocus, Galeamopus, Giraffatitan, and Supersaurus. Recently, D'Emic (2023) published a new paper illuminating the evolution of body size among sauropod taxa, relying on measurements of the circumferences of sauropod limb bones to propose that massive body size evolved convergently in multiple sauropod clades, while noting that select taxa of sauropods showed decreasing trends in body size. Having had the chance to read this paper, there are some noteworthy observations from the study that I wish to emphasize in detail. 

As admitted by D'Emic (2023), the patchy sauropod fossil record has constrained the focus on sauropod body size evolution to less than half of the approximately 250 described sauropod taxa, but the fact that Franz Nopcsa's interpretation of Magyarosaurus as a dwarf taxon has been vindicated by several studies (e.g. Benton et al. 2010; Stein et al. 2010) along with the small size of the short-necked dicraeosaurid Brachytrachelopan and the dwarf brachiosaurid Europasaurus shows that diminutive body size was present in more than one eusauropod clade. Although Cope's rule says that tetrapod lineages tend to increase in body size over time, it should be noted that when Edward Drinker Cope and Othniel Charles Marsh described to science the sauropod fauna of the Morrison Formation in western North America, they did not foresee that a few sauropod taxa of diminutive body size would be named in a handful of publications long after their deaths, and Magyarosaurus would be the first sauropod to be described that deviates from Cope's rule when it comes to body size in sauropods. With respect to the argument by D'Emic that the evolutionary cascade hypothesis put forward by Sander (2013) to interpret gigantism in some sauropods as being driven by anatomical and physiological innovations stemming from a nested array of historical prerequisites is best explained by ecological and life-history factors rather than any shared morphological traits, I should emphasize that the co-existence of dwarf and gigantic titanosaurs in the Late Cretaceous of Romania, Spain, and France (Stein et al. 2010; Vila et al. 2022) lends support to D'Emic's conclusion that ecological and life-history factors influenced the degree of dwarfism or gigantism among sauropod clades by showing that the insular nature of the ecosystems of the Ibero-Armorican island and Hațeg Island didn't necessarily translate into uniform dwarfism among titanosaur taxa from the Late Ceretaceous of Europe. For instance, Vila et al. (2022) note that the holotype of Abditosaurus kuehnei is substantially larger than specimens of other titanosaur taxa from the Ibero-Armorican Island as well as titanosaur specimens from the Hațeg Island, indicating that Cope's rule might apply to the evolution of body size in titanosaurs over the course of the Maastrichtian in southern Europe.

Phenogram of sauropod body size evolution over the course of the Mesozoic (from D'Emic 2023). Note that the lineages of the six eusauropod lineages that surpassed the maximum mammalian body mass are denoted by longitudinal lines highlighted in different colors (light green=non-neosauropod eusauropods; dark green=turiasaurians; blue=diplodocoids; orange=basal macronarians and non-titanosaur somphospondyls; purple=brachiosaurids; red=titanosaurs).

According to D'Emic, a total of 36 sauropod lineages independently evolved huge body sizes greater than those of other terrestrial tetrapods from the Mesozoic and Cenozoic, 32 of them belonging to the clade Neosauropoda. The discovery of isolated postcranial remains from the Bajocian-Callovian age Dongdaqiao Formation of Tibet belonging to a eusauropod over 66 feet (20 meters) long (Wei et al. 2023), two gigantic eusauropod cervical vertebrae from the Shishugou Formation of Xinjiang indicating a eusauropod 115 feet (35 meters) in length (which are referred to Mamenchisaurus sinocanadorum by Paul [2019] but considered to be of indeterminate taxonomic placement by Moore et al. [2023]), and a maximum size estimate of 105 feet (32 meters) for the Middle Jurassic mamenchisaurid Xinjiangtitan given by Wu et al. (2013), indicate that gigantic body size evolved in more than one clade of non-turiasaurian, non-neosauropod eusauropods during the Middle Jurassic, although the limited amount of material known for Mamenchisaurus sinocanadorum renders the exact size of this taxon unclear. The gigantic size of Turiasaurus relative to that of other turiasaurs makes me feel tempted to suggest that the rate of body size evolution in Turiasauria is inconsistent with Cope's rule because of the medium body size of the Early Cretaceous turiasaurs Mierasaurus and Moabosaurus and small body size of the Late Jurassic form Amanzia from Switzerland (Schwarz et al. 2020), the minor age difference between  Turiasaurus and the Yellow Cat Member of the Cedar Mountain Formation yielding Mierasaurus and Moabosaurus could suggest large-bodied turiasaurs might survived into the Valanginian-Barremian interval and that the small size of Amanzia can be best explained as a result of insular dwarfism given that much of western and central Europe was covered by seas during the Late Jurassic.

References:

Benton, M.J., Csiki, Z., Grigorescu, D., Redelstorff, R., Sander, P.M., Stein, K., and Weishampel, D.B., 2010. Dinosaurs and the island rule: The dwarfed dinosaurs from Haţeg Island. Palaeogeography, Palaeoclimatology, Palaeoecology 293 (3): 438–454. doi:10.1016/j.palaeo.2010.01.026

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.

D'Emic, M. D., 2023. The evolution of maximum terrestrial body mass in sauropod dinosaurs. Current Biology 33 (9): R349–R350. doi:10.1016/j.cub.2023.02.067

Paul, G.S., 2019. Determining the largest known land animal: A critical comparison of differing methods for restoring the volume and mass of extinct animals. Annals of the Carnegie Museum 85 (4): 335–358. doi:10.2992/007.085.0403

Sander, P.M., 2013. An Evolutionary Cascade Model for Sauropod Dinosaur Gigantism - Overview, Update and Tests. PLoS ONE 8(10): e78573. https://doi.org/10.1371/journal.pone.0078573

Schwarz, D., Mannion, P.D., Wings, O., and Meyer, C.A., 2020. Re-description of the sauropod dinosaur Amanzia (‘Ornithopsis/Cetiosauriscus’) greppini n. gen. and other vertebrate remains from the Kimmeridgian (Late Jurassic) Reuchenette Formation of Moutier, Switzerland. Swiss Journal of Geosciences 113: 2. https://doi.org/10.1186/s00015-020-00355-5

Stein, K., Csiki, Z., Curry Rogers, K., Weishampel, D.B., Redelstorff, R., Carballidoa, J.L., and Sander, P.M., 2010. Small body size and extreme cortical bone remodeling indicate phyletic dwarfism in Magyarosaurus dacus (Sauropoda: Titanosauria). Proceedings of the National Academy of Sciences of the United States of America 107 (20): 9258–9263. doi:10.1073/pnas.1000781107

Vila, B., Sellés, A., Moreno-Azanza, M., Razzolini, N.L., Gil-Delgado, A., Canudo, J.I., and Galobart, A., 2022. A titanosaurian sauropod with Gondwanan affinities in the latest Cretaceous of Europe. Nature Ecology & Evolution 6: 288-296. doi:10.1038/s41559-021-01651-5.

Wei, X.-F.; Wang, Q.-Y., An, X.-Y., Wang, B.-D., Zhang, Y.-J., Mou, C.-L., Li, Y., Wang, D.-B., Ma, W., and Kundrát, M., 2023. New sauropod remains from the Middle Jurassic Dongdaqiao Formation of Qamdo, eastern Tibet. Palaeoworld: in press. doi:10.1016/j.palwor.2023.02.002

Wu, W.H., Zhou, C.F., Wings, O., Sekiya, T., and Dong, Z.M., 2013. A new gigantic sauropod dinosaur from the Middle Jurassic of Shanshan, Xinjiang. Global Geology 32: 437–446.