Lesson 6: Attack and Defense PDF

Lesson 6: Attack and Defense Written by Victoria Arbour, W. Scott Persons, Matthew Vavrek, Philip Currie, and Eva Koppelhus Learning objective for lesson 6: Horns of buffalo and rhinos or the spiky quills of Students will learn about defensive porcupines make them d...

Lesson 6: Attack and Defense Written by Victoria Arbour, W. Scott Persons, Matthew Vavrek, Philip Currie, and Eva Koppelhus Learning objective for lesson 6: Horns of buffalo and rhinos or the spiky quills of Students will learn about defensive porcupines make them dangerous prey to attack. Even if predators succeed at killing such and offensive behaviors and prey, they may be seriously injured in the structures process. Weapons like horns and spikes, beyond their usefulness in defending prey when Learning objective 6.1: Identify potential attacked, are also deterrents. They discourage defensive and predatory adaptations in predators from choosing to attack in the first dinosaur skeletons. place. Armour, like the shell of a turtle, can also be a deterrent because it makes the animal difficult to eat, and therefore not worth the Although dinosaurs were certainly not the effort. Large size can be a defense entirely on its insatiable, blood-thirsty monsters that are own. Giant sauropods may have lacked horns commonly depicted by Hollywood, there is no and armor, but their sheer size would have doubt that dinosaurs lived with the threat or made them formidable prey. Like modern with the self-sustaining necessity of violence. elephants, giant sauropods could have trampled Herbivorous dinosaurs had to avoid being even their largest potential predators, and, caught and killed, and carnivorous dinosaurs although they were not armed with spikes and had to catch and kill prey. The killing tools of clubs, sauropods could have dealt severe blows carnivorous dinosaurs, like the bone-crushing with their massive tails. jaws of tyrannosaurs and the sickle-claws of dromaeosaurs, are among the most impressive Cursorial limbs are another obvious prey in the whole armory of the animal kingdom. defense. Being able to outrun and/or Such adaptations in carnivorous dinosaurs were outmaneuver potential predators keeps prey countered by an array of defensive adaptations safe and avoids a physical fight altogether. in herbivorous dinosaurs. Based on their hind limb proportions, ornithomimids and many small ornithopods are As have been previously described, many cursorially adapted, and it is likely that these ceratopsians evolved long horns, and dinosaurs made use of their speed when ankylosaurs and stegosaurs evolved an array of threatened. However, predators can also make spikes and armor. Having horns or spikes is a use of speed, and long limbs in a carnivore can common strategy used by modern animals. be seen as a predatory adaptation. University of Alberta - PALEO 200/201 Top left: Argentinosaurus, a sauropod, may have relied on its large size to deter predators. Top right: the horns and frill of Triceratops would have made an excellent threat display, and the horns could also have been used as weapons. Bottom left: Anodontosaurus, an ankylosaur, was covered in osteoderms and had a tail modified into a club, all of which would have been excellent defenses. Bottom right: The large osteoderm plates of Stegosaurus may not have provided much protection, but the spikes at the end of the tail could have been swung at attackers. Art by Joy Ang. Cryptic adaptations allow potential prey to go a step further and avoid even being seen by predators. Crypsis is the ability of an animal to avoid detection, and cryptic adaptations include camouflage color patterns, hiding behaviors, and odor-masking chemicals. Crypsis is difficult to judge from only fossil evidence. Because cryptic adaptations are widespread among modern animals, it is reasonable to assume that Ornithomimus had proportionately long legs, which cryptic adaptations were also wide spread would have made it a relatively fast runner. Art by among dinosaurs. However, relying primarily on Joy Ang. crypsis as a predator defense is more common among small animals, which are able to hide more easily behind environmental structures, 2 University of Alberta - PALEO 200/201 than among large animals, and it is unlikely that crypsis was the sole predator defense of any large herbivorous dinosaurs. It is difficult to demonstrate that any dinosaur species used cryptic adaptations like camouflage, because colour and colour patterns are not usually preserved in dinosaur fossils, and 'hiding' isn't something that can be fossilized. Like crypsis, many defenses, including chemical weapons and intimidating displays, are difficult to detect from fossil evidence. Some modern animals use bright colours or false eyes to The striped skunk (Mephitis mephitis) has dramatic warning colouration and scent glands that release a powerful odour – but neither of these defensive adaptations would be likely to fossilize. Image from ARKive. The potoo (Nyctibius grandis) is one of the most The sunbittern (Eurypyga helias) has vivid eyespots well-comouflaged birds, blending almost seamlessly on its outstretched wings which it uses to scare into the bark of the branches they roost on. Image larger animals away, but again, this threat display would not fossilize and colours rarely fossilize. Image from ARKive. from Stavenn via Wikimedia Commons and used under the CC-BY-SA license. 3 University of Alberta - PALEO 200/201 scare predators away, or to clearly label hypothesis that the tail cubs of ankylosaurs themselves as toxic or otherwise dangerous. were used as weapons. The reasoning behind Given the diversity of dinosaurs, it is more likely this study was that, if ankylosaur tail cubs were that some dinosaurs used such defenses than used as weapons, it is likely that the tail cubs that no dinosaurs did. were able to withstand large impact forces without breaking, whereas, if ankylosaur tail Some features of dinosaurs, like horns, spikes, clubs were not used as weapons, it would be and teeth, are similar to structures observed in very unlikely that the tail cubs were adapted in modern animals and have a form that make such a way as to withstand large impact forces. their function appear obvious. However, some The tail club of the ankylosaur Euoplocephalus features of dinosaurs are novel, without was digitally scanned, and this digital model comparable modern analogs, and sometimes was then imported into a finite element analysis appearances may be deceiving. program. The digital tail club model was given Finite element analysis is a technique that has material properties equivalent to that of bone. been used by paleontologist to help evaluate To simulate a tail club strike, the force of a hypotheses about the functions of many Euoplocephalus tail swing was estimated and dinosaur adaptations. Finite element analyses was applied to a point on the outer surface of are computer simulations that apply set the digital tail cub model. The results showed material properties to a digital object and that that the resulting stresses across the tail club report data on how stresses are dispersed were insufficient to damage to club. Thus, the through the object, when a force is applied at a study concluded that ankylosaur tail clubs were particular point. A recent finite element analysis capable of serving as weapons, and this carried out by University of Alberta researchers supports the hypothesis that weapons were the attempted to evaluate the function of the tail clubs. Determining how well a dinosaur could see, hear, and smell is difficult... but not impossible. The brain cases of dinosaurs offer some clues, because the sizes of different regions of the brain relate directly to the strength of specific senses. Eye size can be estimated from the size of the orbits, and the inner ear cavity of the skull offers many clues to the strength of a dinosaur’s hearing. Naturally, both predators and prey benefit from keen senses that can alert the one to the presence of the other. However, the sensory CT scans of tail clubs were converted into 3D models needs of predators and prey are not identical. and tested using finite element analysis. Image by For example, herbivores, who are concerned Victoria Arbour. with avoiding being snuck up on by predators, benefit from a wide field of view. For this 4 University of Alberta - PALEO 200/201 reason, herbivores often have eyes positioned acquire prey that would be too difficult to kill on the sides of their heads. This prevents the on their own, and we call these social field of vision of one eye from redundantly predators. Hunting behaviours can be diverse overlapping with the field of vision of the other even among closely related animals. For eye and maximizes how much of its surrounding example, tigers are solitary hunters, lions hunt an animal can see at one time. cooperatively, and cheetahs sometimes hunt alone and sometimes hunt in pairs or small Conversely, predators benefit from being able groups. to maximize their perception of a single target. Often, but by no means always, predators have Were any predatory dinosaurs capable of social eyes that are positioned near each other and hunting? In Dry Island Buffalo Jump Provincial that both face forward. This causes the field of Park in Alberta, a bonebed of the tyrannosaurid vision of both eyes to overlap and grants the theropod Albertosaurus preserves the remains predator stereoscopic vision. Stereoscopic of more than 20 individuals. This seems to vision allows an animal to see the same object suggest that Albertosaurus may have lived in with both eyes, and thus to see it from two groups rather than as solitary individuals. But if slightly different angles, which improves the you remember our lesson on taphonomy, there animal’s ability to judge depth. Other animals are other reasons why many individual besides predators may benefit from enhanced dinosaurs may have collected in one spot. depth perception, and stereoscopic vision is However, the geological and taphonomic also common among animals that fly and climb. evidence at Dry Island suggests that these Albertosaurus represented a single pack that met some catastrophic end. It appears that at Learning objective 6.2: Suggest predatory least some theropod dinosaurs formed groups, behaviours and styles in carnivorous and therefore may have been social hunters. dinosaurs. An incredible fossil, called the "Fighting Dinosaurs", from Mongolia preserves a Not all carnivores are alike, and there are many Velociraptor (a dromaeosaurid theropod) and a different hunting styles among extant Protoceratops (a ceratopsian) seemingly predators. Some hunters are ambush predators, interacting with each other. One hand of the which lie in wait until prey comes within striking Velociraptor is in the mouth of the distance – a good example of this kind of Protoceratops, and the other is holding on to hunting strategy is a crocodile, waiting for prey the frill of the Protoceratops. The Velociraptor's to enter the water, then lunging. Other hunters sickle-shaped claw is positioned in the throat of stalk and pursue prey, and may rely on stealth the Protoceratops. This amazing fossil pair to approach quietly and then strike. Still others, seems to be a snapshot of a Velociraptor in the like wolves, may pursue prey over long midst of killing a Protoceratops, and provides distances, using their endurance to tire out and support for the use of the sickle-shaped toe eventually overtake their prey. claw as a predatory adaptation. This is one of Predatory animals can hunt alone or in groups. the best examples of an interaction between Some hunters work cooperatively in order to two dinosaurs that we know of. Based on this 5 University of Alberta - PALEO 200/201 The Fighting Dinosaurs at the Mongolian Natural History Museum. Photos by V. Arbour. fossil, we might guess that Velociraptor was a special kind of agonistic behavior called solitary hunter. However, maybe other ritualized agonistic combat. Velociraptor were present, but were able to escape whatever killed the Fighting Dinosaurs. It has long been suspected that the thick domed skulls of many pachycephalosaurs were adaptations for agonistic head butting competitions. Finite element analyses of 6.3 Suggest potential intraspecific pachycephalosaur skulls have supported this behaviours in dinosaurs hypothesis. Like modern animals that engage in ritualized agonistic head butting, such as musk Violence is not always limited to interactions oxen and big horn sheep, pachycephalosaur between predators and prey or even between skulls were strong enough to withstand severe members of two different competing species. It impact forces and had special mechanical stress is common for members of the same species to reducing adaptations. fight over territory, mates, food resources, and for social rank within a group. Fighting and aggressive displays between members of the same species are called agonistic behaviors. Because agonistic behaviors are common, so are adaptations that facilitate them. The antlers of an elk are one example. Male elk use their antlers in head-to-head shoving competitions. This kind of competition that determines which of two individuals is the strongest without either combatant risking serious injury is a Stegoceras by Joy Ang. 6 University of Alberta - PALEO 200/201 The results of a finite element analysis on the dome CT scans of the Stegoceras skull UALVP 2 were of UALVP 2 – in the top image, we are looking at a converted into 3D models. From these, the relative sagittal (lengthwise) cross-section through the bone density can be shown as different colours, dome, and the cavity towards the bottom of the with red representing high density bone and blue skull is where the brain was housed. The graph representing low density bone. The bone in the below shows how much stress (pressure) there was dome also has a strut-like appearance, which is from the top of the dome to the brain, and the similar to the structure in modern head-butting absolute stress values were all low enough that animals. Bottom image from: Snively E, Theodor Stegoceras would not have given itself brain JM. 2011. Common functional correlates of head- damage from head-butting. Image from: Snively E, strike behavior in the pachycephalosaur Stegoceras Theodor JM. 2011. Common functional correlates validum (Ornithischia, Dinosauria) and combative of head-strike behavior in the pachycephalosaur artiodactyls. PLOS ONE 6:e21422. Stegoceras validum (Ornithischia, Dinosauria) and combative artiodactyls. PLOS ONE 6:e21422. to cooperatively bring down prey that is too 6.4 Identify other fossil evidence for dangerous or difficult to be attacked by an defensive, predatory, or other individual. It is usually more difficult to sneak up intraspecific behaviours in dinosaurs on an alert group than on a single alert individual. Prey that band together in a herd benefit from the additional sets of watchful There can be strength in numbers, and both eyes (and alert ears and noses). Some prey predators and prey may benefit from forming herds may also mount collective offenses groups. Predators that form packs may be able 7 University of Alberta - PALEO 200/201 against predators that would be too dangerous hadrosaurs, and tyrannosaurs (like the to challenge alone. Albertosaurus at Dry Island). Evidence of dinosaurs forming groups comes A healthy dinosaur ecosystem was filled with from a variety of social display adaptations and many different kinds of dinosaur species, and it adaptations relating to agonistic behaviors and is improbable that a large random fossil-sample from trackways and monospecific bonebeds. of any such ecosystem would yield multiple Some sauropod and ornithopod trackways show bones from only a single species. Monospecific many sets of footprints, all from the same bonebeds, therefore, are often interpreted as species of dinosaur and all heading in the same nonrandom samples, and an explanation is direction. These trackways suggest that the needed for why only one species is included. dinosaurs that made them were traveling One obvious explanation for this nonrandom together as a group. Monospecific bonebeds sample is that the particular species was are large accumulations of fossil bones that are traveling in a group and that the group all from multiple individuals of the same collectively met the fate that ultimately resulted species. Monospecific bonebeds are known for in their fossilization (such as dying in a flash many kinds of dinosaurs, including ceratopsians, flood, a mud or rock slide, or a massive volcanic ash fall). A Centrosaurus herd panics in a flood. Large numbers of Centrosaurus bones from many individuals have been found in localized areas in Dinosaur Provincial Park, which suggests that these animals may have travelled in groups. (Jan Sovak) 8 University of Alberta - PALEO 200/201 Many monospecific bonebeds are formed from more heavily-built adults. Perhaps young the disarticulated skeletons of many individuals, tyrannosaurs hunted smaller and faster prey, like the Edmontosaurus bonebed we studied in while adults were better able to attack larger lesson 2. However, sometimes we find and slower prey. Alternatively, the more articulated skeletons in close association with cursorial limbs of young tyrannosaurs may have one another, and this is very strong support for been a defensive strategy that helped the social behaviour. A locality called Bayan youngsters to avoid themselves becoming prey Mandahu in China preserved the articulated for adults. skeletons of juvenile ankylosaurs called Pinacosaurus, lined up next to each other like they had laid down to sleep. This is very good Another source of potential information about evidence that juvenile Pinacosaurus traveled in dinosaur inter- or intraspecific interactions is groups. the presence of pathologies. Palaeopathologies can include healed injuries or other evidence of diseases, such as infections, cancer, or arthritis. The cause of many pathologies can have multiple interpretations, so it is important to be cautious when extrapolating from evidence of pathologies. Nevertheless, they can provide some interesting clues about dinosaur behaviour. For example, several tyrannosaur skulls show signs of healed bite marks that can only have been made by other tyrannosaurs. Because the injuries had time to heal, these bite marks cannot have been formed by a Pinacosaurus skeletons at Bayan Mandahu. Photo tyrannosaur killing and feeding on the carcass by P. Currie. of another. Nonlethal face biting is a common Adult Pinacosaurus have never been found in agonistic behavior among modern carnivores, groups or bonebeds, which suggests that, while such as crocodiles, and so tyrannosaurs may juvenile Pinacosaurus were social dinosaurs, have engaged in a similar behaviour. adult Pinacosaurus lived solitary lives. Adaptations that serve in predator defense may Behaviours may change ontogenetically, and so also be used in agonistic behaviors. A study can skeletal adaptations for attack and defense. examining healed injuries on skulls of two Juvenile Pinacosaurus lacked the heavy armour different species of ceratopsians found that of adult ankylosaurs, which may be why they injuries on the squamosal bone were more lived in groups as juveniles. On the other side of common in Triceratops than in Centrosaurus. the dinosaur family tree, juvenile tyrannosaurs This provides some evidence that Triceratops may have employed different hunting strategies may have locked horns during intraspecific compared to adults. Juvenile tyrannosaurs had competitions similar to those of modern deer, proportionately longer legs, which would have cattle, and rhinos. The large orbital horns in made them more fleet-footed than the stockier, Triceratops would touch the squamosal of an 9 University of Alberta - PALEO 200/201 ankylosaurids definitely used their tails for tail clubbing. If they did use their tails for clubbing, we also can't say right now whether they used them mainly as defensive weapons against predators, or whether they used them in intraspecific combat. Generally speaking, the more lines of evidence you can find to support a given hypothesis about dinosaur behaviour, the more confident we can be in our conclusions about that behaviour. Studying the behaviours and adaptations of modern animals, doing biomechanicals tests, collecting evidence from bonebeds and trackways, and looking for pathologies in skeletons are all excellent ways to investigate the behaviour of dinosaurs and other extinct organisms. Frequency of healed injuries on the skull bones of Triceratops (top) and Centrosaurus (bottom). From: Farke AA, Wolfe EDS, Tanke DH. 2009. Evidence of Supplementary Materials. combat in Triceratops. PLOS ONE 4:e4252. Tetrapod Zoology: Heinrich's digitial Kentrosaurus. [Blog post] opponent, causing injuries. Centrosaurus has The Open Source Paleontologist: Triceratops smaller orbital horns, and there weren't as Combat? [Blog post] many healed injuries on the squamosal, which might mean that the orbital horns were too Laelaps: Gnarly fossil tells of a torn dinosaur small to cause injuries in that area, or that tail. [Blog post] Centrosaurus did not use its cranial ornamentation for fighting. Laelaps: Tracks hint at the social life of tyrant dinosaurs. [Blog post] Understanding dinosaur behaviours is one of the most interesting parts of palaeontology, but Laelaps: Social sauropods. [Blog post] it is important to understand the limits of the evidence we have available. While we can find evidence in support of many behaviours, we Videos of extant animal behaviours from ARKive: cannot 'prove' that a dinosaur definitely did or Pack-hunting in African wild dogs - these are highly did not do certain behaviours. For example, the social mammals that bring down prey as a team and anatomy of ankylosaur tails suggests they were share the food among themselves. adapted for tail-clubbing, and the results of Ambush predation in Nile crocodiles - at the other finite element analyses show that they could extreme, crocodiles tend to sit-and-wait for prey to use their tails for clubbing, but we can't say that approach, then lunge. Although many crocodiles are 10 University of Alberta - PALEO 200/201 hunting in this small area, they are not generally Oryx sparring – note the injuries inflicted by the working cooperatively, and so this is not considered horns! 'pack hunting'. Giraffe necking – perhaps not unlike ankylosaur tail Muskox head-butting – a possible analogue for clubbing, except using heads and necks instead of pachycephalosaurs. tails. Bighorn sheep head-butting - a possible analogue for Reindeer herding – just one example of many extant pachycephalosaurs. animals that form large herds for protection against predators. 11