Final Fantasy: when dinosaurs decided to swim

Updated: Jul 29, 2020

    Breaking news: the team of Ibrahim et al. [1] bring for the first time to the world a strong evidence of a semi-aquatic lifestyle in a dinosaur, namely the Spinosaurus aegyptiacus (the big croc-like dude in Jurassic Park 3 that snaps the neck of T. rex, if you wonder). New vertebrae excavated from 2015 to 2019 revealed bone specialisations towards improved swimming abilities of the tail (Fig. 1). Fasten your belt and let's dive into deep waters, where Spinosaurus is waiting for its new prey.

Figure 1. Drawing of the new tail discovered in Morocco from 2015 to 2019, with vertebrae actually recovered (including their fragments) in orange, and missing elements in white. Above: dorsal view; bottom: lateral view. In dorsal view, you can clearly see how the vertebrae define a flat surface that could be used as a kind of flipper. Modified, from [1].

Bizarre dinosaur, but also quite incomplete

     Spinosaurus aegyptiacus is a quite weird theropod dinosaur (group of saurischian dinosaurs, meaning 'reptile-hipped', grouping mostly carnivores), discovered in Egypt by Stromer back in 1915 [2]. It belongs to the Spinosauridae, a specialised family dating back possibly from Early-Middle Jurassic to the mid Cretaceous (ca. 147 to 85 My), almost spread worldwide [2]. This species is characterised by robust forelimbs, a huge thumb claw, a crocodile-like skull equipped with crocodile-like conical teeth, and most importantly, very tall neural spines (dorsal part of a vertebra, typically shaped as a slender bony extension projecting dorsally) defining an amazing dorsal sail [3]. This theropod family is definitely bizarre, unique within dinosaurs, however, this clade is also quite well-known for the incompleteness of its fossil record [4] despite its geographic and temporal ranges [2]. Spinosaurus aegyptiacus is in turn the kind of dinosaur species everybody knows, and that everybody does not really know at the same time. Ironically, its first, century-old fossils ever recorded were even destroyed during World War II [5] to make sure the little we knew was lost forever... Looks like a challenge stating in bold 'You wanna understand this strange dino? You gotta deserve it God dammit!'

    Nowadays, the neotype (the new material of reference for a given species) of Spinosaurus aegyptiacus has been built upon a partial subadult skeleton discovered in Morocco, but also upon bones from other specimens of Spinosaurus aegyptiacus possibly of different ages - even from other spinosaurid species (keep that in mind) - from previous excavations in the Kem Kem beds (so-called isolated and surrogate bones) and even Stromer's drawings of the first Spinosaurus excavated in 1915, that needed 'scaling' to meet the actual size of the skeleton (Fig. 2) [6]. Incomplete, you said it sir.

Figure 2. The neotype of Spinosaurus aegyptiacus. Red bones are the actual sudadult skeleton discovered in Morocco, at the base of the neotype definition; orange bones are original bones dug out by Stromer; yellow bones are isolated bones found in the Kem Kem beds; green bones are bones 'borrowed' from other spinosaurids; blue bones are inferred bones from the ones surrounding them. Notice the typical tail for a theropod still inferred for Spinosaurus in 2014. Modified, from [6].

Spino the not-so-hippo: life in semi-aquatic conditions?

    For quite a long time, Spinosaurus and its relatives have been hypothesised as dinosaurs more or less closely linked to water bodies [2]; even more, there could be a grading tendency to live as semi-aquatic animals within spinosaurids [3]. During the description of the neotype by the team led by Nizar Ibrahim in 2014 [6], several skeletal charateristics pointing towards adaptations for aquatic locomotion have been observed: retraction of the fleshy nostrils to a position near the mid region of the skull, elevated orbits, an elongate neck and trunk shifting the center of mass to the knee joint, short pelvic girdle and hindlimbs, foot modification for aquatic propulsion, convergent jaw morphology linked to fishery activity, and so on. Histologically, there are also other adaptations that are the closing of the medullary cavity and the pachyostosis of long bones (thickening of the bone cortex to make it heavier, notably for ballast applications). Even deeper, isotopic signatures of Spinosaurus aegyptiacus teeth reveal that this species was used to live in an aquatic environment [3-4;6-9].

    However, due to the heavy incompleteness of the skeleton of Spinosaurus aegyptiacus, the semi-aquatic habitat for that dinosaur is subsequently heavily questioned by the palaeontologic community [2], even though a diet based on fishery (but not only) is water clear since the 90's [10]. Actually, Hone and Holtz [11] did in 2019 deconstruct Arden and colleagues' [3] interpretative work on Spinosaurus aegyptiacus (presented in substance in the last paragraph); basically, some of the skeletal structures used to infer semi-aquatic habitat are flawed to the former authors. For instance, the orbits are not positioned dorsally enough to use them as periscope while the skull is mostly underwater, seeking for preys like hippos and crocodiles do, and even though some bones of Spinosaurus aegyptiacus are definitely pachyostotic, the overall pneumatisation of its skeleton (empty 'pockets' in the bone design in order to lighten it and make its mass sustainable from a biologic point of view) would outreach the ballast effect of some bones, resulting in low diving skills. Even isotopic signatures could be not definitely pointing towards an aquatic organism, as Spinosaurus signal is comparable to other terrestrial theropods but also not-so-semi-aquatic animals [12]. In sum, when many of the skeletal advantages for semi-aquatic behaviour are taken to a higher comparative level, they fall to actually defend more aquatic habits than the average dinosaur [11], leaving Spinosaurus aegyptiacus' feet dry on the shore line for good. Or not?

    Well, after the first hit from Ibrahim et al. [6] on the subject that left scientists undecided whether spinosaurids swam or not, their recent work [1] on new spinosaurid caudal vertebrae spins palaeontologists like a record baby right round round round. Those vertebrae are belonging to the same single tail (around 80% of that tail, with minimal taphonomic deformations), which is supposed to be the other half of the neotype described in 2014 (Fig. 3) [1]. Quite elevated neural spines and long chevrons (bone extensions on the ventral side of caudal vertebrae) characterise those caudal vertebrae, meaning the tail itself had a large lateral surface and was thus a powerful propulsive tool for aquatic locomotion. Moreover, those vertebrae lack long zygapophyses (vertebral bone extensions allowing connections between vertebrae) unlike other terrestrial theropods, which allow in turn a higher flexibility of the tail, reinforcing the possibility of easier, large lateral movements of the tail of Spinosaurus aegyptiacus [1].

Figure 3. Latest reconstruction of Spinosaurus aegyptiacus, including its tail designed for swimming. Vertebrae displayed in green are the ones recovered for this study, which represent in turn almost the entire tail. Modified, from [1].

"Dinosaurs were supposed to walk only!"

    Basically, to the light of this new striking discovery, a life more or less heavily spent water is assumed for Spinosaurus aegyptiacus. This clashes the longstanding view of dinosaurs walking only on Earth, with only maniraptorans going to the air [13-14]. Even though the tail itself has a morphology really indicative of aquatic propulsion, there are many problems related to the neotype that should be taken into consideration.

    Actually, the neotype on which everything is based today about Spinosaurus aegyptiacus is a chimera, i.e. a mix of different skeletons. Besides the subadult bones discovered during the 2007-2008 expeditions, and the new tail described in [1], all other bones constituting the skeleton come from other individuals, as written some paragraphs above (Fig. 2): isolated bones for the Kem Kem beds, surrogate bones that are "borrowed" from other spinosaurids, bones recreated after Stromer's original drawings of the first skeleton of Spinosaurus, and bones that are inferred from surrounding ones (i.e. never found). So, first issue but not the less important one: some of the bones added to the neotype belong potentially to other individuals of Spinosaurus aegyptiacus of different ontogenetic stages, and some others come potentially from other spinosaurid species, all of them not being adapted to water life as Spinosaurus could be [3].

    Some bones were thus digitalised and rescaled to fit the neotype, with underlying allometric (differential growth of skeletal elements with time, shape changes, etc.) problems. For instance, pedal bones of Spinosaurus seem to grow isometrically [15] (bone growth that conserves overall bone shape) with age, so there is no problem to add them and to rescale them to a larger specimen, but this is probably not the case with other bones of Spinosaurus. That issue is also echoing other spinosaurid bones added to the neotype, as each species may have its own allometries, resulting in different adult sizes, bone characteristics and shapes, etc. Another example, complete forelimbs and hindlimbs were not reported in close association with the neotype of Spinosaurus (see additional data of [1]), so their reconstructions only rely on other specimens. Are they actually reflecting what limbs of Spinosaurus did look like? Not necessarily. This is why quadrupedality as depicted in [6] has been questionned by [2]. Some other studies including the dorsal sail's hydrodynamics [16] are purely speculative as they rely mainly on hypotheses that could not be validated or at least supported by some pieces of evidence [2]. And so on. Figure 4 below, published by Ibrahim et al. [6] shows that they did not investigate further this issue in my very own opinion, leading to an idea of overinterpretation of the palaeobiologic signal, at the base of many contentions on the subject (disclaimer: they did explain in the caption of this figure the collection of origin of each bone, but all grouped under 'known bone' label, albeit there are probably not all belonging to Spinosaurus aegyptiacus). If one assume the neotype as depicted here to fairly represent one single individual of Spinosaurus aegyptiacus, more than one interpretation of osteologic (and histologic) features will be flawed at some extant.

    However, what is water-clear is that Spinosaurus aegyptiacus is the very first dinosaur for which prolonged aquatic habits are mostly accepted due to its palaeoenvironment and diet, its cranio-dental anatomy, isotopic studies applied to spinosaurid materials, and the new tail from Morocco. Many other features (more posterioraly placed nares, large thumb claw, etc.) could be linked to semi-aquatic habits, but not mandatorily.

Figure 4. The neotype (and its known bones in red) of Spinosaurus aegyptiacus as depicted by Ibrahim et al. in 2014 [6].

My two cents: palaeontologic philosophy

    Even though this unique tail discovered in almost direct connection with the rest of the skeleton is an amazing discovery, the chimeric nature of the neotype raises question. When lower levels of resolution are already quite blurry, interpretations of higher levels of resolution may seem unstable. Hopefully here, the tail gives indubitable insights into Spinosaurus ecology, but what about the rest? If different species and ontogenetic stages are clustered into one skeleton, what are the significance of each trait? Are they only representative of Spinosaurus?

    This is likely the engine of the global controversy around this particular dinosaur, why many palaeontologists out there do agree with the tail, but maybe not with the whole picture given by the new definition of Spinosaurus aegyptiacus. However, palaeontologists do with what Mother Nature gives them, and this is cruelly exemplified with specific fossil species that are rarely preserved, and why not even lost during bombardments. Maybe that stating clearly the limits of the interpretation would have helped scientists to accept more the new bodyplan of Spinosaurus. Not like facts but more like plausible hypotheses.

     Such study is a good example on how to use fossil data, what are the ranges of inference you can do with one given bone, etc. Each and every bones of the neotype framework were well studied and interpreted by Ibrahim and his colleagues, but as standalone pieces. Together, however, palaeobiologic signals drop heavily because of the incompleteness nature of spinosaurids and the chimeric reconstructions of their skeletons that hinder the good read of their palaeobiology.


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