Anatomy and Paleobiology of Spinosaurid Dinosaurs

Illustration by Brian Engh depicting two Spinosaurus aegyptiacus doing what their family does best:
fishing, and swimming.

Recently, I have been speaking with a number of people online concerning the life habitats and paleobiology of the spinosaurid dinosaurs, which is a family of highly specialized theropods that had evolved aquatic habits and are known to eat fish. The information we've been discussing has certainly inspired me to do a post on these animals, and they're one of my personal favorites among theropods due to their bizarre appearance and specialized behaviors.

 

Spinosaurids were a family of dinosaurs within the megalosauroid radiation of tetanuran theropods which seem to have evolved during the late Jurassic. While original spinosaurids remain limited their range to North Africa, South America, and Europe in predominantly swampy and tropical paleoenvironments, the last five years has revealed remains from every continent (excluding Antarctica, although I bet we'll eventually find them there). An ungual phalanx from the famed Morrison formation is also thought to be from one of these animals, and a neck vertebrae from Australia has been found to come from a juvenile spinosaurid. The latter is from the same locality that we're currently finding a number of polar dinosaur species, proving that these animals weren't limited to swampy environments.

 

Combinations of stomach contents, shed teeth, and even embedded bones from prey within the jaws of these animals prove that they were piscivores, and fed on a diet of fish. A study of oxygen isotope levels in spinosaurid teeth have also shown that they were indeed spending the majority of their time submerged within water, and perhaps actively swimming after fish in their underwater world. However, the discovery of bones from a sub-adult Iguanodon within the European Baryonyx specimen, along with a pterosaur vertebrae with part of a spinosaur tooth embedded within it prove that despite their fishy diets, they were still capable of feeding on land-based prey. The fact they were aquatic may also suggest that some of these theropods were feeding on marine reptiles like plesiosaurs and icthyosaurs. We have found marine reptile bones and teeth in some of the same deposits, but this is all speculative until any evidence is found that they preyed upon these animals.

 

Body Builders of the Mesozoic?



Image of armature paleontologist William Walker with the thumb claw of Baryonyx walkeri in his hand.
The individual Baryonyx who left this claw was about 9 meters long, and it was not even fully grown!

 

A common trait seen in many, if not all members of the spinosauridae is their well-developed arms. Despite many people claiming that these animals had long arms, they aren't particularly long compared to other theropods, the obvious exceptions being tyrannosaurs and alvaresaurids among others. Instead they are well-muscled and very robustly built. The lower arm bones are relatively short, but the humerus length makes up for it. The fingers show a high degree of flexibility like many big theropods, and the thumb was tipped with an extremely large thumb claw.

 

Many have suggested that these arms were used for fishing, rather like how a bear will use its forelimbs to catch salmon. However, similar robust arms are also seen in earlier Megalosauroids, so perhaps their use was the same throughout the group. Megalosaurs are thought to have used their robust arms to help with dispatching large land-based prey animals, and we know currently that Spinosaurids at least occasionally fed on dinosaurs, so perhaps their arms were used to kill them as well. Still, they would have also served very effective for fishing when the time came.

 

Billboards, Fish Traps, or Giant Humps?



Skeletal reconstruction by Scott Hartman showing the full extent of some Spinosaurus vertebral spines
Wonder if I could advertise my blog on one of those things....

 

The most noticeable part of any spinosaur must be the elongated dorsal neural spine along their backs, producing a "sail" or ridge along the backbone. In some species these became very elaborate: Spinosaurus itself has neural spines which were over five feet tall, and Ichthyovenator had not one, but two sails along its back.

 

The evolution and purpose of these sails has been at the center of a hot debate. Some scientists think that they may have served a thermoregulatory function. Many of these creatures were living in some of the hottest environments the Mesozoic had to offer, and these vertebrae could serve as extra surface areas to cool down the animal during the hottest parts of the day. However, if the animals were really spending a lot of time submerged in water, this may not have been a problem for them, as the water would keep them cool. The sails may also have served as visual display organs. Similar structures are found in other dinosaurs and living animals, and may have served as billboards to advertise to other animals their age, size, gender, species, and overall health. The sheer number of sail shapes and forms found within the group supports the idea they were for some form of visual communication, and I agree that that is likely one of their functions.

 

Another theory I have heard be suggested is the fishing lure idea. Some living birds like herons use their wings to shade the water around them, which attracts fish to the animals where they can then be easily snatched up in their bill. Some have suggested the same thing was happening with spinosaurids, using their sails to shade the water and attract fish, but I personally find myself disagreeing. Many species have low ridges that could not serve such a function, and the sails are thought to appear as an offset to sexual maturity, further suggesting the visual idea.

 

Stromer had his own idea for the sail, however. He suggested that the sail may have supported a hump-like structure filled with muscles. While originally dismissed for the sail-backed animal, Jack Bailey resurrected the hypothesis in 1997, and reconstructed what looked like a giant terrestrial crocodile with a bison-like hump on its back (similar to the extinct archosaur Arizonasaurus). However, this posture and reconstruction now seems, at best, unlikely, but the hump theory still isn't out of the question. As both he and Stromer noted, the sail is made up of thick and strong vertebral extensions, a far cry from the thin extensions in some sail-backed animals, like Dimetrodon, but very similar to the thicker extensions in living hump-backed animals, like bison and camels.

 

Living bison use these muscular humps on their backs to power huge neck muscles for both plowing through snow and head-butting mates during the breeding season. Camels use their humps to store fat (NOT water) while traveling over large desert landscapes. Bears also have muscular humps over their shoulders to power huge arm muscles while hunting, digging, and moving large objects (in some cases even boulders weighing over a ton), but how would the spinosauridae possibly use such a structure?

 

Investigation into the neck vertebrae in spinosaurids suggests that, while they had rather weak side-to-side motion in the neck, they had well-developed muscles responsible for the up-down motions. Such a fast up-down motion might be good when catching fish and raising them out of the water while swimming, and any muscles in this sail may have assisted them while hunting. The well-developed arms of spinosaurs may also have been powered by muscles that extend into these structures, although as I mentioned about megalosaurs, they would have still been powerful without them.



 

One of the best and most used skull reconstructions of Spinosaurus I've seen on the net, with the rear proportions based off its relative Irratator.
The image is from Andrea Cau's Theropoda blog, although I don't know the exact origins of it (except it's modified by S. Maganuco).

 

Leaving their vertebrae, we of course come to their next most defining feature in spinosaurids: their heads. The skulls of spinosaurs were all low and elongated, similar to a crocodile, and due to their fishy diets it's no wonder why. However, despite this general shape, the skulls between different species can differ dramatically between the two subfamilies; the Baryonychinae and the Spinosaurinae.

 

The skulls of members of the baryonychinae are known for their moderately raised nostrils, rounded eye socket, numerous closely-spaced teeth, shallow lower jaw, and extremely small, but numerous serrations on the teeth. These animals include Baryonyx walkeri and Suchosaurus cultridens from Europe (although the latter may be dubious), the African Suchomimus tenerensis, possibly Ostafrikasaurus crassiserratus (if it's not more primitive), and Cristatusaurus lapparenti (again, the latter may be dubious), the Asian Ichthyovenator laosensis, and presumably the newly discovered Australian specimen, Morrison specimen, and a Campanian tooth from China all belong to members of the the baryonychinae.

 

The defining skull features members of the spinosaurinae share is their extremely high nostrils (half way up the skull), raised teardrop-shaped eye sockets, widely-spaced and relatively longer teeth, deeper lower jaw, and lack of serrations. These members so far include the African Spinosaurus aegyptiacus, the South American Irritator challengeri, the extremely fragmentary Oxalaia quilombensis, and possibly Siamosaurus suteethorni (since its teeth closely resemble those of Spinosaurus).

 

Both groups have raised nostrils, which helps them to breathe while submerged. In the spinosaurinae they are especially high up, which allows them to still breathe in deeper water, and the eyes are also partially raised, allowing them to also see while submerged. This could possibly indicate that the spinosaurinae were more aquatic than the baryonychinae, and the large sizes of some members of the spinosaurinae (both Spinosaurus and Oxalaia could get larger than Tyrannosaurus) may have been supported by the weightlessness of this lifestyle.

 

The differences in jaw morphology may also reflect different lifestyles among the two groups, most notably spaces between their teeth. The teeth of the baryonychinae were uniformly-spaced, while the teeth of the spinosaurinae had spaces which varied. The former arrangement is more similar to living fish-eating gharials, which have interlocking teeth to allow them to more successfully catch small slippery fish. However, the latter spinosaurinae have an arrangement that is more similar to other types of crocodilians, and might suggest that like them, their diet was more varied. Alternatively, varied spaces between teeth are also seen in crocodilians which feed on larger prey, so it is also possible that the spinosaurinae were feeding on larger prey items than the baryonychinae.

 

Comparisons between the skulls of different types of spinosaurids have been going on for a while now. Donald M. Henderson started it off in 2002 when he examined Suchomimus's skull and found that it was weaker in comparison to the skulls of the much smaller dinosaurs like Dromeosaurus. This was due to the skull's weaker build compared to other theropods, which have taller skulls in order to counter-act stresses while biting. In 2007 Emily Rayfield made similar findings in the closely related Baryonyx walkeri. However, paleontologist Andrea Cau posted on his blog Theropoda about the skull of Spinosaurus as well as members of the spinosaurinae, and suggested that unlike the baryonychinae, it may have had a much stronger bite.

 

He noted numerous adaptations in the skull which could be responsible for a more powerful bite force, and I quote:

 

"How powerful was that bite? A precise quantification is not possible, however, it should be noted that increasing the length of the rostrum, increases the moment of the force exerted by the muscles mandibular end of the rostrum. The long skull of a Spinosaurus was adapted to withstand the forces exerted by its own bite? The cross section of the rostrum, triangular, the presence of the long nasal dorsal ridge which served as a point of discharge of the forces and the presence of the long secondary palate (a mechanical expedient to resist torsion), all adaptations are intended to support the intense stresses caused bites very powerful. A further indication of this capability is given, at least in Spinosaurus, from the retracted position of the nostrils: this odd fact that the entire region was placed in front of the nostrils consists of only compact bone, with no cavities or soft part, and then allowed to exercise precisely at the level of the rostrum of the considerable pressures without the risk of damaging vital parts such as the nostril."

 

This suggested to me that unlike members of the baryonychinae, the spinosaurinae were better adapted to feeding on a range of animals from fish, to pterosaurs, to dinosaurs with their more powerful bites. However, while writing this article I came across yet another paper concerning the strengths in spinosaur skulls published in 2011 (again by Emily Rayfield). This study was similar to the Baryonyx one in that she digitalized the skulls of 7 different theropod species, including Suchomimus and Spinosaurus, and put them under stress in a 3D computer program to see which ones were most effective at distributing it. The results were surprising.

 

Out of all the dinosaurs in the study, the three largest theropods, Acrocanthosaurus, Carcharodontosaurus, and Spinosaurus had the highest levels of stress in the skull. Spinosaurus especially showed levels off the main curve, and was likely unable to feed on larger dinosaurs, being restricted to smaller prey. Ironically, this study found that despite Henderson's stating that the skull of Suchomimus was incredibly weak, this study found that it was in fact just as good at distributing stresses as the other theropods graphed, and the skull was overall much stronger than Henderson thought.

 

I am, however, reluctant to make these seem like conclusions since Emily herself stated that they could not be 100% sure that the results were accurate based on the resources she had. She instead suggested that the study should be considered more of an educated prediction, and should be tested with the offset of better 3D models in the future. I personally can't wait until such advances are made and we can more accurately find out how these skulls worked, and thus learn how these creatures behaved.

 

Why, what big teeth you have!



Image of the jaw of Baryonyx from Emily J. Rayfield's paper reguarding its skull
As you can clearly see from image B, spinosaurid teeth were extremely long.

 

But of course, one of the most interesting things I'd like to get into is these animal's teeth. The teeth of all spinosaurs show some of the greatest examples of convergence within the theropods, greatly resembling crocodilian teeth to the point that we have trouble telling isolated examples of the two apart. Both croc and spinosaurid teeth are nearly conical in shape and appearance, but in the baryonychinae, these teeth still held onto serrations along the front and back like other theropods, but are so small that they can only be seen with a microscope. The spinosaurinae teeth have become truly conical and have no serrations present, but also tend to be thicker as well as larger than baryonychinae teeth. Both groups also show large amounts of wear on their teeth. Similar wear is seen in crocodilians, marine reptiles, as well as some toothed whales, further evidence that these were aquatic animals.

 

Oxalaia is also unique from all other theropods in one regard. The tooth in the third row has shown not one replacement tooth behind the functioning one, but two. This is unusual as replacement teeth tend to grow one at a time in most theropods. For those that don't know, both dinosaurs and crocodiles replace their teeth by resorbing the existing tooth at its base to make room for the replacement. The replacement eventually moves to a position within the functional tooth, and when resorption is largely complete, the existing tooth falls away, allowing the new tooth to take its place.

 

The fact that Oxalaia has two replacement teeth within one socket means that it would've been resorbing and producing two teeth at the same time within one another, almost like Russian nesting dolls. Due to the fact we only have one extremely fragmentary Oxalaia specimen, we can't tell if this was a "freak" individual or a trait the species had as a whole, but it certainly shows these animals were weird in their own ways. However, I haven't even gotten to one of the most significant things about these teeth in my opinion: their sheer size.

 

Many people just don't realize that how big spinosaurid teeth were. At first glance the teeth of spinosaurids may seem about the same size as those of other theropods, despite their odd shape. But CAT scans of the skull have shown that their roots were extremely deep, sometimes embedded half-way within the socket. Similarly deep roots are also seen in, again, crocodilains, but in Spinosaurus itself these teeth may have been over 10 inches long, rivaling T-rex's banana-sized 12 inch teeth for the longest theropod tooth! It has surprised me just how little documentation exists about the size of spinosaurid teeth, and very few people I've talked to know how long they actually are. Many scientists have referenced the length of these teeth in the past, such as Thomas Holtz, who has talked about their length in the documentary Monsters Resurrected  as well as while answering questions online.

____________________________________________________________________________________

 

So that's it. Thanks to everyone who I've been talking to about these fascinating creatures, and hopefully talking about these crocodile mimics will get me back in the mood to write about actual crocs....

 

Eh-hem....

 

As always, feel free to ask any questions you may have. But to make it seem a bit more exciting around here, you get extra points if you understand the Holtz reference. Until next time, stay sharp and see ya around the net. ;)