Tail-propelled aquatic locomotion in a theropod dinosaur

Abstract

Intensive research on non-avian dinosaurs in recent decades strongly suggests that these animals were restricted to terrestrial environments 1. Historical views proposing that some groups, such as sauropods and hadrosaurs, lived in aquatic environments 2,3 were abandoned decades ago 4,5,6. Recently, however, it has been argued that at least some spinosaurids, an unusual group of large-bodied Cretaceous theropods, were semi-aquatic 7,8 , but this idea has been challenged on anatomical, biomechanical, and taphonomic grounds and remains controversial 9,10,11. Here we present the first unambiguous evidence for an aquatic propulsive structure in a dinosaur, the giant theropod Spinosaurus aegyptiacus 7, 12. This dinosaur has a tail with an unexpected and unique shape consisting of extremely tall neural spines and elongate chevrons forming a large, flexible, fin-like organ capable of extensive lateral excursion. Using a robotic flapping apparatus to measure undulatory forces in physical tail models, we show that the tail shape of Spinosaurus produces greater thrust and efficiency in water than the tail shapes of terrestrial dinosaurs, comparable to that of extant aquatic vertebrates that use vertically expanded tails to generate forward propulsion while swimming. This conclusion is consistent with a suite of adaptations for an aquatic lifestyle and a piscivorous diet in Spinosaurus 7,13,14. Although developed to a lesser degree, aquatic adaptations are also found in other spinosaurids 15,16 , a clade with a near global distribution and a stratigraphic range of more than 50 million years 14 , documenting a significant invasion of aquatic environments by dinosaurs.

The cranial morphology ofTanystropheus hydroides(Tanystropheidae, Archosauromorpha) as revealed by synchrotron microtomography

“…On the contrary, long transverse processes/pleurapophyses are widespread in the anterior caudal vertebrae of aquatic reptiles exhibiting lateral undulation of the tail (e.g. extant crocodylians, nothosaurs, spinosaurids, Ibrahim et al, 2020;Rieppel, 2000) and they provide important muscle attachment sites for undulation. Furthermore, the slight expansion of the neural spines of the trunk and tail vertebrae in Tanystropheus possibly indicates the presence of supraspinous ligament of some sort, but it does not provide convincing evidence that the trunk and tail were highly stiffened.…”

Evaluating the ecology of Spinosaurus: shoreline generalist or aquatic pursuit specialist?

“…The spinosaurids are widely regarded as being at least partly piscivorous, and with potential affinities for aquatic habitats (e.g., Charig and Milner, 1995;Bertin, 2010;Cuff and Rayfield, 2013;Hone and Holtz, 2017;Aureliano et al, 2018;Hassler et al, 2018) although this does not exclude other prey or the occupation of other habitats (Bertin, 2010;Amiot et al, 2010a, b;Allain et al, 2012;Hendrickx et al, 2016), and likely took prey from the margins of aquatic habitats. The recent discovery of new and much more complete material has brought new data and with it new hypotheses about the biology of this animal (Ibrahim et al, 2014a(Ibrahim et al, , 2020a. Based on new finds, it has more recently been suggested that Spinosaurus was an aquatic specialist (Ibrahim et al, 2014a, 2020a, Arden et al, 2019Beevor et al, 2020) and pursuit predator of aquatic prey (Ibrahim et al, 2020a).…”

“…The recent discovery of new and much more complete material has brought new data and with it new hypotheses about the biology of this animal (Ibrahim et al, 2014a(Ibrahim et al, , 2020a. Based on new finds, it has more recently been suggested that Spinosaurus was an aquatic specialist (Ibrahim et al, 2014a, 2020a, Arden et al, 2019Beevor et al, 2020) and pursuit predator of aquatic prey (Ibrahim et al, 2020a). This stands in stark contrast to previous interpretations of spinosaurid biology and, if correct, would mark out Spinosaurus (and potentially other spinosaurines) as having a unique ecology for a nonavialian theropod.…”

Epidermal complexity in the theropod dinosaurJuravenatorfrom the Upper Jurassic of Germany

“…Although Juravenator does not exhibit any anatomical features indicative of an aquatic lifestyle as in Spinosaurus (Ibrahim et al . 2014, 2020) or Halszkaraptor (Cau et al . 2017), we note that its dental morphology is particularly close to that of small opportunistic predatory coelurosaurs such as Scipionyx , Microraptor and Anchiornis , which are known to have fed at least partially on fish (Dal Sasso & Maganuco 2011; Xing et al .…”