Saturday, December 3, 2016

The only known dinosaur adapted to life in water, Spinosaurus aegyptiacus swam the rivers of North Africa about 95 million years ago. Image credit: © Davide Bonadonna / National Geographic magazine.

Spinosaurus (meaning “spine lizard”) is a genus of theropod dinosaur that lived in what now is North Africa, during the lower Albian to lower Cenomanian stages of the Cretaceous period, about 112 to 97 million years ago. This genus was known first from Egyptian remains discovered in 1912 and described by German paleontologist Ernst Stromer in 1915. The original remains were destroyed in World War II, but additional material has come to light in recent years. It is unclear whether one or two species are represented in the fossils reported in the scientific literature. The best known species is S. aegyptiacus from Egypt, although a potential second species, S. maroccanus, has been recovered from Morocco.

Back to size, here is a comparison with Spinosaurus that is actually reasonable. Author: terra raptor

Spinosaurus was among the largest of all known carnivorous dinosaurs, possibly larger than Tyrannosaurus and Giganotosaurus. Estimates published in 2005, 2007, and 2008 suggested that it was between 12.6–18 metres (41–59 ft) in length and 7 to 20.9 tonnes (7.7 to 23.0 short tons) in weight. A new estimate published in 2014 and based on a more complete specimen, supported the earlier research, finding that Spinosaurus could reach lengths greater than 15 m (49 ft). The skull of Spinosaurus was long and narrow, similar to that of a modern crocodilian. Spinosaurus is known to have eaten fish, and most scientists believe that it hunted both terrestrial and aquatic prey; evidence suggests that it lived both on land and in water as a modern crocodilian does. The distinctive spines of Spinosaurus, which were long extensions of the vertebrae, grew to at least 1.65 meters (5.4 ft) long and were likely to have had skin connecting them, forming a sail-like structure, although some authors have suggested that the spines were covered in fat and formed a hump. Multiple functions have been put forward for this structure, including thermoregulation and display.

Recent fossil evidence shows Spinosaurus was the first dinosaur that was able to swim, and likely spent most of its life in the water, according to a study published September 2014 in the journal Science. “Spinosaurus had short hind limbs (like early whales and other animals that spent more and more time in the water), dense and compact bones (penguins show a similar bone profile in cross section), wide and flat claws and feet (possibly used in paddling), and a long and slender snout with conical teeth (perfect for catching fish),” said Nizar Ibrahim, a University of Chicago vertebrate paleontologist and lead author of the study.

Life reconstruction of Spinosaurus aegyptiacus based on the recent description of a new specimen. Artwork by Durbed

Function of the sail

There has been much scientific debate regarding the evolution and purpose of Spinosaurus‘ sail. Because of its size, this dinosaur did not have many predators, but the sail could have been used to ward off enemies, as the dinosaur would have appeared to be twice its size with the sail fully extended. The dinosaur’s upper spine was fairly flexible, and its vertebrae had ball-and-socket joints, meaning it was likely able to arch its back to a point. It may have been able to spread the sail when threatened or looking to attract a mate.

Size of Spinosaurus

Spinosaurus is now often regarded as the biggest known meat eating theropod dinosaur (herbivores like large sauropods were of course bigger),‭ ‬however the actual size is really just an estimate extrapolated from an educated guess.‭ ‬What is clear is that Spinosaurus was a very large animal but herein lies the problem as the larger animals get,‭ ‬the less complete their remains tend to be because it takes so much more material to bury them and protect the body from scavengers and as well as the full ravages of nature.‭ ‬The more an animal is exposed upon death means the less complete long term remains like fossils will be.

Still with a smaller estimate of just over twelve and a half meters,‭ ‬Spinosaurus would have been comparable to Tyrannosaurus,‭ ‬and only just smaller than Giganotosaurus‭ (‬it needs to be remembered that even though Giganotosaurus has a size estimate of thirteen meters,‭ ‬it would still only be marginally larger than the largest known Tyrannosaurus‭)‬.‭ ‬Comparison to other smaller spinosaurids that were consequently scaled up to the same size as the Spinosaurus material points to sizes that approach the larger length estimate as indeed being possible.‭

Tyrannosaurus, Giganotosaurus, Carcharodontosaurus, Acrocanthosaurus, Saurophaganax. Artwork by PrehistoricWildlife.com

The lifestyle of Spinosaurus

Since its discovery, Spinosaurus has been a contender for the longest and largest theropod dinosaur. Both Friedrich von Huene in 1926 and Donald F. Glut in 1982 listed it as among the most massive theropods in their surveys, at 15 meters (49 ft) in length and upwards of 6 t (5.9 long tons; 6.6 short tons) in weight. In 1988, Gregory Paul also listed it as the longest theropod at 15 meters (49 ft), but gave a lower mass estimate of 4 tonnes (3.9 long tons; 4.4 short tons).

The‭ ‬2014‭ ‬reconstruction of Spinosaurus by Ibrahim et al also supported the idea that Spinosaurus would readily enter the water and actually swim about.‭ ‬Not only did they cover isotope analysis which confirmed a great deal of exposure to aquatic environments,‭ ‬they also noted that Spinosaurus had particularly dense bones.‭ ‬This is a common feature in animals that spend a lot of time swimming in the water as the greater bone density helps the animal with buoyancy issues so that it can actually swim under the surface is necessary.‭ ‬Ibrahim et al also noted that the claws on the feet were flat-bottomed,‭ ‬which would be a further aid in pushing against the water while swimming.‭ ‬This also strongly suggests that the main swimming propulsion was provided by the rear legs.

The possibility also remains that Spinosaurus may have hunted ‬land animals,‭ ‬although no fossil evidence is known that strongly supports this.‭ ‬In South America a pterosaur bone was found with a spinosaurid tooth stuck into it,‭ ‬and recovery of the related Baryonyx revealed the presence of Iguanodon bones inside of the area that its gut would have been.‭ ‬Still these may have been cases of scavenging rather than attempted hunting.‭ ‬Baryonyx also revealed the partially digested remains of the fish Lepidotes,‭ ‬further supporting the fish specialisation hypothesis.

Because Spinosaurus disappears from the fossil record well before the end of the dinosaurs sixty-five million years ago,‭ ‬it must have succumbed to something else other than the established extinction theories that ended the dinosaurs once and for all.‭ ‬Perhaps the easiest explanation for its demise is that it simply became far too specialised,‭ ‬and when the ecosystem it was living in changed to be drier the rivers systems dried up,‭ ‬removing the prey source that Spinosaurus was best equipped to deal with.‭ ‬In the face of competition with more generalist theropods,‭ ‬Spinosaurus just could not compete with their success and was eventually driven to extinction.‭

Sail or hump,‭ ‬and more importantly why‭?

Very tall neural spines growing on the back vertebrae of Spinosaurus formed the basis of what is usually called the animal’s “sail”. The lengths of the neural spines reached over 10 times the diameters of the vertebral bodies from which they extended. The neural spines were slightly longer front to back at the base than higher up, and were unlike the thin rods seen in the pelycosaur finbacks Edaphosaurus and Dimetrodon, contrasting also with the thicker spines in the iguanodontian Ouranosaurus.

Reconstruction of the holotype fossils. Author: Ryan Somma

Spinosaurus sails were unusual, although other dinosaurs, namely the ornithopod Ouranosaurus, which lived a few million years earlier in the same general region as Spinosaurus, and the South American sauropod Amargasaurus, might have developed similar structural adaptations of their vertebrae. The sail may be an analog of the sail of the Permian synapsid Dimetrodon, which lived before the dinosaurs even appeared, produced by convergent evolution.

Spinosaurus gives its name to the Spinosauridae family of dinosaurs, which includes two subfamilies: Baryonychinae and Spinosaurinae. The Baryonychinae include Baryonyx from southern England and Suchomimus from Niger in central Africa. The Spinosaurinae include SpinosaurusIrritator from Brazil, and Angaturama (which is probably synonymous with Irritator) from Brazil. The Spinosaurinae share unserrated straight teeth that are widely spaced (e.g., 12 on one side of the maxilla), as opposed to the Baryonychinae which have serrated curved teeth that are numerous (e.g., 30 on one side of the maxilla)

2 – Baryonyx, 3 – Ichthyovenator, 4 – Oxalaia, 5 – Suchomimus, 6 – Irritator. Artwork by PrehistoricWildlife.com


The skull had a narrow snout filled with straight conical teeth that lacked serrations. There were six or seven teeth on each side of the very front of the upper jaw, in the premaxillae, and another twelve in both maxillae behind them. The second and third teeth on each side were noticeably larger than the rest of the teeth in the premaxilla, creating a space between them and the large teeth in the anterior maxilla; large teeth in the lower jaw faced this space. The very tip of the snout holding those few large anterior teeth was expanded, and a small crest was present in front of the eyes. Using the dimensions of three specimens known as MSNM V4047, UCPC-2, and BSP 1912 VIII 19, and assuming that the postorbital part of the skull of MSNM V4047 had a shape similar to the postorbital part of the skull of Irritator, Dal Sasso et al. (2005) estimated that the skull of Spinosaurus was 1.75 meters (5.7 ft) long. The Dal Sasso et al. skull length estimate was questioned because skull shapes can vary across spinosaurid species.

Skull of a privately owned reconstructed skeleton of a subadult. Skull of Spinosaurus aegyptiacus ; Morroco ; Private collection Stage : Albian-Cenomanian (112-93 Ma) Size : 1,20 m. Author: Didier Descouens

A 2013 made study performed by scientists Andrew R. Cuff and Emily Rayfield showed that Spinosaurids like Spinosaurus had relatively poor resistance in their skulls for torsion compared to other members of this group (Baryonyx) and modern alligators, thus showing Spinosaurus preyed more regularly on fish than it did on land animals, although considered predators of the former too.