Feeding Adaptations and Strategies of the Spinosauridae

The Spinosauridae is a small family of basal tetanuran neotheropod dinosaurs. Although the group is only known from the Cretaceous period, its origins most likely lie in the Late Jurassic. Spinosauridae is not a diverse clade, consisting of between four and six genera, and possibly as few as three. The group's rarity may in fact be due to its specialized lifestyle. The long, superficially crocodilian snout that defines Spinosauridae has been suggested to support a largely piscivorous diet. Gut contents from the type specimen of Baryonyx walkeri, however, indicate not only that these animals ate fish, but larger terrestrial prey as well. A recent study of Baryonyx's snout indicates that, like the modern gharial, spinosaurs may not have been able to attack large prey with their mouths without risking signficant injury. Rather, I suggest that spinosaurs subsisted mainly on fish but could have easily taken small prey. Spinosaurs also seem to be well adapted to scavenging, especially the more inclusive Spinosaurinae.

The taxonomic history of the group is fairly convoluted, with many taxa based on extremely fragmentary remains. For simplicity's sake, I will here by following Sues et al. (2002) in referring to Suchomimus tenerensis (Sereno et al. 1998) as Baryonyx tenerensis. Further, I agree with Dal Sasso et al. (2005) and Sereno et al. in considering Spinosaurus maroccanus (Russell, 1996) a junior synonym of Spinosaurus aegypticus (Stromer, 1915). Many workers have considered Angaturama limai (Kellner & Campos, 1996) as a junior synonym of Irritator challengeri (Sues et al. 2002) and that, indeed, the two taxa may simply belong to the same specimen (Charig & Milner, 1997; Sereno et al., 1998; Buffetaut & Ouaja, 2002). Thus, I will only discuss three genera and four species herein: Baryonyx walkeri, B. tenerensis, Irritator challengeri, and Spinosaurus aegypticus.

When Baryonyx walkeri was first described in 1986 (Charig & Milner), the comparison to gharials was immediately clear. In fact, Taquet (1984), investigating Stromer's myserious Spinosaurus, made a similar analogy regarding the giant theropod's dentary. Charig & Milner suggested that Baryonyx led a mostly piscivorous lifestyle, stalking riverbanks for fish. The gut contents of Baryonyx included fish teeth and scales, the humerus of a juvenile Iguanadon, and a small phalanx. Clearly, Baryonyx was at least partially piscivorous, but the ornithopod remains indicate that it also consumed larger terrestrial prey. But the question is whether it actively hunted that prey or scavenged it. The fragmentary nature of B. walkeri did not help to illuminate its lifestyle. Only in 1998, when Sereno et al. described B. tenerensis, did Baryonyx's anatomy, as well as Stromer's Spinosaurus aegypticus', become more clear.

Skull of B. tenerensis in lateral view based on Sereno et al. 1998

The vast majority of B. tenerensis' snout was recovered, showing for the first time just how long and narrow the structure really is. Additionally, the external nostril is recessed relative to its position in other theropod dinosaurs. The "terminal rosette" at the distal end of the snout is more obvious and better preserved in B. tenerensis. Interestingly, the giant gharial-like crocodilian Sarcosuchus imperitor (Broin & Taquet, 1966) lived alongside B. tenerensis and may have competed with it for food.

Sereno et al. (1998) divides the Spinosauridae into two distinct groups: the Baryonychinae, which includes the genus Baryonyx, and the Spinosaurinae, which includes the genera Irritator and Spinosaurus.

Skull of Irritator challengeri in lateral view based on Sues et al. (2002) and Dal Sasso et al. (2005).

Despite sharing obvious similarities, the Baryonychinae and Spinosaurinae are surprisingly different. Baryonyx has more teeth in its jaw than either Irritator or Spinosaurus, and those teeth are smaller and more recurved. In fact, spinosaurine teeth conical, tapering evenly toward their apex, and lack serrations. Baryonyx has very fine serrations on the anterior and posterior keels. The "terminal rossette," first pointed out in B. walkeri, is far more obvious and accented in spinosaurines. The spinosaurine premaxilla has fewer, but larger, teeth, and the matching area of the dentary has similarly large and reduced dentition. While Baryonyx's teeth continue along the maxilla nearly to its terminal end at regular intervals, the maxillary teeth of Irritator are small, unevenly spaced, far reduced in number, and end cranially to the antorbital fenestra. While it would be tempting to say that the Spinosaurinae is a derivation of the Baryonychinae rather than a sister group, the occurance of Irritator in Lower Cretaceous strata casts doubt on this hypothesis. Whatever their relationship, the two families were clearly pursuing different methods of obtaining food.

Skull of Spinosaurus aegypticus in lateral view based on Dal Sasso et al. (2005).

Spinosaurus shares more in common with Irritator than with Baryonyx, while developing some unique features of its own. The snout's terminal rosette is extremely long. The secondary bony palatte, a bony structure evolved independantly among spinosaurs, is large and dominates the ventral portion of the premaxilla and maxilla. Spinosaurus' external nostril is unusually small, not boardered at all by the premaxilla, and set far back on the snout. While the anterior portion of Irritator's skull rises sharply toward the orbits, Spinosaurus' skull remains on a fairly steady and shallow upward slant toward the orbits. The large crest of Irritator, which may have been a soft tissue structure, is underdeveloped in Spinosaurus, whose nasal bones fuse and fan upwards, producing a fluted crest. In the Spinosaurinae, the eyes were set high on the skull, while in the Baryonychinae, the eyes were set along the same plane as the external nostrils.

Premaxilla/maxilla in Spinosaurus (top) and Baryonyx (bottom) in ventral view, showing distinct secondary palate (dark blue). Redrawn from Sereno et al. (1998) and Del Sasso et al. (2005). Contact between left and right halves of the palate in Baryonyx extended a bit farther than restored above.

I mentioned previously the notion that spinosaurs have traditionally been considered crocodile analogues. Indeed, as you will recall, the type specimen of Baryonyx walkeri was found to have fish teeth and scales in its belly. When B. tenerensis was originally described, Sereno et al. named it "Suchomimus," literally, "crocodile mimic." Certainly the long and narrow snout of Baryonyx led to such comparison. And yet, until just recently, no test had been done to determine the authenticity of such intuition. Rayfield et al. (2007) used a comparative biomechanical analysis to test stresses applied to the skulls during biting for three taxa and one generalized theropod dinosaur. The type of Baryonyx walkeri was used, as was a modern gharial and alligator. The bite force analysis for B. walkeri was almost exactly the same as for the gharial. The authors suggested that spinosaurs evolved their own secondary palate to resist torsion stresses. The reduction and recession of the antorbital fenestrae also confer greater strength to the snout. In other words, the crocodile analogue is quite appropriate, although we should perhaps being calling them "gharial-mimics."

Gharials are, of course, specialized piscivores. Their extemely narrow jaws, complete with interlocking teeth, are easily whipped through the water to snatch a fish or swimming tetrapod. However, larger prey, especially struggling prey, poses a risk to gharials, whose thin snouts can be damaged by too much stress and torsion. Alligators and crocodiles, whose snouts are far broader, are able to tackle much larger prey without risk to their skulls. The convergent gharial-like shape of the snout in spinosaurs is almost certainly an adaptation to piscivory, although to what degree is contentious. Also, the mode of prey capture is mysterious. As Charig & Milner pointed out, B. walkeri contains no obvious aquatic adaptations. Aside from its head and hypertrophied thumb claw, the body of B. walkeri is that of a basal terrestrial tetanuran. The more complete remains of B. tenerensis confirm this. While not much of Spinosaurus' postcranial skeleton is known, its skeleton, too, seems distinctly "theropodian." Of note, Spinosaurus displays unusually lengthened (and widened) neural spines along its dorsal vertebrae.

Many illustrators, including Paul (1988) have restored B. walkeri as a bear-like piscivore, standing on the shoreline and using its enlarged thumb claw to bat at and "spear" fish from the water, as well as darting its gharial-like snout into a group of fish. The exact manner in which spinosaurs obtained their fish will probably never be known, but there are other possibilities. Despite not having any obvious skeletal adaptations for an aquatic lifestyle, many vertebrates, including bears, stalk their slippery prey in other ways. Bears do not simply stand on the shore and wait for a fish to jump toward them. Rather, bears will often run headlong into a stream or lake to actively pursue salmon. This is especially true during yearly salmon runs in Alaska and Canada. One can easily imagine a group of Baryonyx crowding around and in a river while a high concentration of fish swam through it. One questions whether or not Baryonyx would have been able to pluck a fish from its giant thumb claw with its jaws. Its arms, while robust, were not exceptionally long. Bears use their large paws to bat fish from the water onto land. The giant thumb claw of spinosaurs may have had other uses, such as defense from predators (more on that later) or intraspecies conflict.

Many modern fish-eating birds, such as herons, storkes, and pelicans stalk fish by standing motionless in the water, their heads held just above the water surface. When a fish swims close by, the bird snaps its head into the water and plucks the fish up. With their serpentine necks and long snouts, one can easily imagine a spinosaur doing the same thing. Some birds create shadows over the water with their wings, which fish are drawn to. While spinosaurs did not have wings, their size, and perhaps the sails of spinosaurines, may have helped in this regard. One wonders if perhaps spinosaurs hunted fish like modern gharials do--by lying in the water and scooping up fish as they pass. However, crocodilians have the advantage of telescopic eyes and noses, which at least Baryonyx lacks. Baryonyx's naris, while recessed, is fairly close to the end of the snout. Where its nose higher or farther back on the skull, the animal could leave its mouth in the water, agape, and still breathe. This was not the case, however, and more likely Baryonyx a more avian or ursine approach to its piscivory.

But what about larger vertebrate prey? Again, the original B. walkeri material contained a small Iguanadon humerus in the gut. Whether the carnivore actually killed the ornithopod will never be known, and the fact that it was a juvenile Iguanadon is more problematic. Could Baryonyx kill a juvenile but not an adult? Could Baryonyx's long skull withstand the stress and torsion of a struggling genusaur? And why was only a humerus found? I am not sure how capable a predator Baryonyx would have been--at least among large tetrapods. The long snout and terminal rosette of Baryonyx certainly would have its advantages for scavenging carcasses--the entire head need not be lodged in the prey's body cavity, and the premaxillary notch may have been useful in plucking meat from the carcass with a minimum of tough bones coming along with it.

But as we've seen, Baryonyx was clearly doing something far different than Irritator and Spinosaurus, so what of them? If we peg Baryonyx as a mainly piscivorous, potentially scavenging aimal, what can we say of the spinosaurines?

A collection of line drawings showing Spinosaurus aegypticus intimidating rival predators from a carcass (top), scavenging up to its nostrils (bottom left), and engaging in a unique mode of piscivory (bottom right).

What is immediately apparent to me of the spinosaurine skull is that the orbits have recessed caudally, but moved upward. While less severe in Spinosaurus, the skull slopes upwards from the snout moreso than in Baryonyx. Furthermore, the number of teeth is reduced, and those teeth tend to be larger and more spike-like in the spinosaurines. The snout of a spinosaurine is long, but not as long proportionatelly in relation to the total length of the skull as in Baryonyx. The external naris is recessed and shrunk down, moreso in Spinosaurus than in Irritator. The large distal portion of the dentary is nearly square in lateral view, and beyond that, the entire lower jaw is deeper than Baryonyx. The spinosaurines were clearly doing things differently than Baryonyx, but how?

First, I believe that spinosaurines would have been very effective scavengers. While they certainly had more bite force than Baryonyx, I doubt that even Spinosaurus would have been very effective in taking down the resident sauropods and ornithopods that shared its habitat. At least, not the adults. For predatory purposes, Spinosaurus and Irritator may have hunted sick, injured, or juvenile animals, but may have preyed on much smaller fauna in between carcasses. The huge sail and body size of Spinosaurus may be explained as a way to intimidate other carnivores away from a carcass. Even the large carcharodontosaurs that shared Spinosaurus' habitat must have reconsidered a kill after seeing an even larger carnivore approach it, complete with an enoromous, flash sail on its back! Spinosaurus was well-equipped (moreso than Irritator) to empty a dead animal. With its recessed nostrils, Spinosaurus would have been able to explore a dead body without worrying about breathing. The eyes, which were high on the head, would have been able to scan the horizen for potential threats while feeding.

Surely Irritator and Spinosaurus had not abandoned their piscivorous ancestry, and both animals show further adaptations for fish-feeding. Unlike Baryonyx, the recessed external naris of both genera would have allowed the animals to stalk their aquatic prey with their mouths partially in the water. Should a fish swim in between their teeth--SNAP--free meal! Perhaps a long pink tongue would have come in handy for attractive fish. Snapping turtles do it; why not Spinosaurus? Wouldn't it be an awesome sight to see several adult Spinosaurus back-deep in a river, with only the tops of their heads and giant sails in sight? It would be good to know whether spinosaurs displayed any degree of osteosclerosis, as that could indicate how much time it spent in water. If Spinosaurus, Irritator, or Baryonyx were bottom walkers, it would be much easier to decipher their feeding strategies.

Spinosaurs seem to have evolved to take over a small, but successful niche during the Cretaceous period. They may have lived in areas where crocodilians of similar habit did not exist, thus allowing such a specialization to occur. At least one giant crocodilian, Sarcosuchus imperitor, lived alongside Baryonyx tenerensis, but whether they hunted the same prey may never be known. While the Baryonychinae may have hunted much the way modern bears do and preferred piscivory over terrestrial prey, the Spinosaurinae seems equally suited to both piscivory and scavenging. Both groups probably hunted smaller vertebrate prey as well. In fact, Buffetaut et al. (2004) described a pterosaur vertebra with a spinosaur tooth embedded within it. Although the group does not appear to have been terribly diverse, further study of known specimens, as well as discovery of new ones, will surely elucidate more information on these strange and wonderful animals.

References

Charig, A. J. & Milner, A. C. (1986). Baryonyx, a remarkable new theropod dinosaurs. Nature 324: 359-361.

Sereno, P. C., et al. (1998). A long-snouted predatory dinosaur from Africa and the evolution of spinosaurids. Science 282: 1298-1302.

Sues, H. D., Frey, E., Martill, D. M. & Scott, D. M. (2002). Irritator challengeri, a spinosaurid (Dinosauria: Theropoda) from the Lower Cretaceous of Brazil. Journal of Vertebrate Paleontology 22(3): 535-547.

Dal Sasso, C., Maganuco, S., Buffetaut, E. & Mendez, M. A. (2005). New information on the skull of the enigmatic theropod Spinosaurus, with remakrs on its size and affinities. Journal of Vertebrate Paleontology 25(4): 888-896.

Rayfield, E. J., Milner, A. C., Xuan, V. B. & Young, P. G. (2007). Functional morphology of spinosaur 'crocodile-mimic' dinosaurs. Journal of Vertebrate Paleontology 27(4): 892-901.

Buffetaut, E., Martill, D. M. & Escuillie, F. (2004). Pterosaurs as a part of a spinosaur diet. Nature 430: 33.

Russell, D. A. (1996). Isolated dinosaur bones from the middle Cretaceous of the Tafilalt, Morocco. Bulletin du Museum National d'Histoire Naturelle, Paris 4(18): 349-402.

Buffetaut, E. & Ouaja, M. (2002). A new specimen of Spinosaurus (Dinosauria, Theropoda) from the Lower Cretaceous of Tunisia, with remarks on the evolutionary history of the Spinosauridae. Bulletin de la Societe Geologique de France 173: 415-421.

Kellner, A. W. A., & Campos, D. A. (1996). First Early Cretaceous theropod dinosaur from Brazil with comments on Spinosauridae. Neues Jahrbuch fur Geologie und Palaontologie, Abhandlungen, Stuttgart 199: 151-166.

Stromer, E. (1915). Too long to type. Das Original des Theropodes Spinosaurus aegyptiacus...too long to type 30: 1-22.

Charig, A. J. & Milner, A. C. (1997). Baryonyx walkeri, a fish-eating dinosaur from the Wealden of Surrey. Bulletin of the Natural History Museum, London, Geology Series 53: 11-70.

Taquet, P. (1984). Une curieuse specialisation du crane de certains Dinosaures carnivores du Cretace: Le museau long et etroit des Spionsaurides. Comptes Rendus de l'Academie des Sciences, Paris 2(299): 217-222.

Broin, F. & Taquet, P. (1966). Decouverte d'un Crocodilien nouveau dans le Cretace inferieur du Sahara. Comptes Rendus de l'Académie des Sciences à Paris, Série D(262):2326-2329