•Understanding the locomotion and other activity of dinosaurs is improved by studying biomechanics: reconstructing their actions using the principles of engineering and physics.
•Ichnology is the study of trace fossils.
•Ichnotaxa are named trace fossils: they technically are names for the objects themselves, not for the biological species which made them.
•The most commonly studied dinosaur trace fossils are footprints. Although it is often difficult to assign an ichnofossil to a particular biological species, we can often tell the larger clade to which it belonged.
•Trackways allow us to estimate the speed at which a dinosaur was moving.
•Land animals range from more graviportal ones (adapted to weight support) toward cursorial ones (adapted to running). Some adaptations of limb and girdle features can be used to recognize trends to one or the other of these behaviors.
•As animals grow, their proportions do not necessarily remain the same. More often, there is some form of allometry: differential growth in proportion.
•While gliding is very common among living things, powered flight is rare and only evolved in insects, pterosaurs, birds, and bats.
•Unlike bats and pterosaurs (and scansoriopterygids), bird and their outgroup did not primarily rely on skin for their flight surface, but feathers.
•Up through the beginning of the 21st Century, two rival models were proposed for the origins of flight: either from the trees-down of from the ground up.
•Discovery that modern birds use a series of wing-based non-flight behaviors (such as wing-assisted incline running (WAIR) and controlled-flapping descent (CFD) suggests that the trees-down vs. ground-up dichotomy is false.
DINOSAURIAN TERRESTRIAL LOCOMOTION
How can we figure out how dinosaurs walked, ran, fed, etc?
Fossils are our primary line of approach, both body fossils and trace fossils.
In the case of body fossils, a few traits (skeletal ones) can be read directly from fossils. Others (muscles, tendons, sense organs, and so forth) might have osteological correlates (that is, direct traces on the bone, even if they are not made of bone themselves.) Still others might be determined by biomechanical or other functional studies.
There are four main methods of inference in dinosaur biology:
Let's consider the issue of dinosaur locomotion. Dinosaurs are essentially striders: their hindlimbs were restricted to antero-posterior movement.
Ichnology: the study of trace fossils.
Most dinosaur ichnologists concentrate on footprints and trackway analysis.
An individual footprint represents:
Footprints give direct information about the soft tissues of the bottom of the foot, and about the natural position of the toes.
Trackways, however, give even more data. By measuring the stride length, and estimating hip height, the speed of the dinosaur at the time of that trackway can be calculated. These data tend to show dinosaurs walking around at speeds comparable to modern large-bodied mammals.
However, trackways do have some problems:
Footprints and trackways can, however, reveal the presence of dinosaurs not yet known by body fossils (such as Middle Jurassic North American dinosaurs).
An interesting new discovery in ichnology is that Mesozoic dinosaurs (like extant birds) did not have discrete gaits like walk-jog-run or walk-trot-canter-gallop. Instead, they had a continuous transition of speed, stride length, and step width.
There is a whole discipline of ichnotaxonomy: the naming of trace fossils. However, it must be remembered that these are sedimentological entities, not biological entities: the same animal can produce tracks given entirely different ichnotaxonomic names if it is walking slowly or running; on soft mud or hard mud; if adult or juvenile; etc.
One interesting note: almost no dinosaur trace fossil shows tail drag marks: this was some of the first evidence that dinosaurs held their tails up above the ground.
Trace fossils can help us understand something about group behavior. A recent study of Alaksan hadrosaurs showed many individuals moving in the same direction at the same time (based on the similar quality of footprint preservation.) Additionally, by counting up the trackways of different sizes, it helped to give an estimate of the relative fraction of the herd of different growth stages.
Deciding whether an animal is running or not is a trickier problem that it first sounds. The classic definition of running is "a gait in which there is a suspended phase" (that is, all feet of momentarily off the ground). We can generally approximate when a trackway has reached that point. However, the modern (or "kinetic") definition of running is "a gait in which the leg is at its most compressed at the middle of its contact with the ground" (like the spring on a pogo stick).
We can apply principles of functional morphology to understanding how dinosaurs operated: that is, work out the function of organisms by the shapes and structures of their body parts.
For instance, we recognize from their parasagittal limbs and digitigrade feet that dinosaurs were striders: they walked one foot directly in front of the other rather than with splayed limbs. Study of modern striders (mammals and birds) show a continuum from cursorial (running specialists) to graviportal (support specialist) forms. Cursorial animals tend to have:
while graviportal animals tend to have:
Examining these traits in dinosaurs, we find that some small ornithopods but especially many coelurosaurian theropods show cursorial adaptations relative to other dinosaurs, while stegosaur and ankylosaur thyreophorans and eusauropod sauropodomorphs show greater graviportal adaptations.
Of course, there are also issues of scaling: as body size increases, the mass supported by any surface (like the bottom of the foot) or cross-section (like the cross-section of the femur) increases faster, so the relative mechanical strength of the animal decreases. Body parts do not all grow at the same rate: that is, they often show allometry:
Some key terms:
In the history of life, only four groups of animals have evolved powered flight:
While there are only three groups of flying vertebrates, there are many modern gliding/parachuting ones: flying fish; flying frogs; flying lizards; flying snakes; sugar gliders (gliding marsupials); flying squirrels; colugos, or "flying lemurs". In addition, there were various extinct gliding reptiles and mammals in the Permian and Mesozoic.
Traditionally, paleontologists have considered two main scenarios for the origin of bird flight:
(NOTE: during the 1970s-1990s, this debate was tangled up with a scientifically separate debate; that is, where birds fit phylogenetically among the archosaurs. The media in particular made the equation "'arboreal model = non-dinosaurian origin of birds'; 'cursorial model = dinosaurian origin of birds'". But these were actually separate debates. Even among those who recognized the dinosaurian origin of birds, some argued for the trees-down model, and others for the ground-up.)
This debate was primarily waged prior to the new discoveries of Early Cretaceous feathered coelurosaurs, which greatly increased our knowledge of the anatomy (integumentary and skeletal) of the basal members of the coelurosaur clades. Additionally, important observations of modern birds revealed a very significant locomotory behavior, previously overlooked.
EVOLUTION OF FEATHERS:
Modern feathers are very complex structures, with a hollow shaft that branches, with those branches branching and the branches of the branches branching. Modern feathers are NOT just the big flight feathers of the wings and tail: there are body feathers (that insulate and streamline the body); down (that keeps pre-flying baby birds warm); display feathers; and others.
Until recently, the basalmost theropod known to have feathers was Archaeopteryx, although some researchers speculated that other theropods had them as well. And the feathers of Archaeopteryx were identical to the feathers of modern birds, so they didn't reveal much about the early phases of these structures. But fossils from lake sediments of the Early Cretaceous (and now the Middle Jurassic) of China have given us a better understanding of the distribution of feathers and protofeather structures.
Megalosaurs and basal coelurosaurs (compsognathids, tyrannosauroids) (and now primitive ornithischians) show a body covering of plumulose protofeathers: simple hair-like or down-like filaments. Based on the developmental biology of modern feathers, this is the expected earliest phases of feather evolution. Protofeathers obviously don't have a flight function, since they don't form an aerodynamic surface. However, they might serve other functions:
However, at least some of the protofeathers of compsognathids are pennaceous, and some of those of the tyrannosauroid Yutyrannus are long strap-like pennaceous feathers of the sort also found in maniraptorans. The poorly preserved feathers of ornithomimid arms and tails are also pennaceous, apparently broader than in compsognathids and tyrannosaurids but not as much as in pennaraptorans.
Note that the fuzz of the heterodontosaurid Tianyulong is very similar and some of the structures on Kulindadromeus are as well. (Kulindadromeus had other structures not yet found in any other dinosaur.) Whether these turn out to be formed by convergence with theropods or retention in both groups of some ancestral dino-fuzz is not yet certain. (If it is a homologous structure, this means that the concestor of all Dinosauria was fuzzy, at least in part!)
Plumolose protofeathers of this sort seem to be present as part of the body covering of therizinosaurs, oviraptorosaurs, dromaeosaurds, troodontids, and basal avialians. (In ornithothoracines, they are replaced by body feathers.)
Tyrannosauroids and therizinosaurs in addition have strap-like protofeathers intermediate in size and form between the downy protofeathers and true feathers. At present it is not certain what the exact structure of the integument of of alvarezsaurs was like. However, these pennaceous strap-like feathers (as well as fully modern feathers) are present in oviraptorosaurs.
All pennaraptorans for which the integument is preserved (oviraptorosaurs, scansoriopterygids, dromaeosaurids, troodontids, avialians) show true broad pennaceous feathers on the arms and the tail. In eumaniraptorans (dromaeosaurids, troodontids, and basal avialians) long broad true feathers are present on the hindlimbs as well. Additionally, these same dinosaurs show some major transformations of the forelimb: elongate arms; semilunate carpal allowing for folding the hands; laterally oriented shoulder joints allowing the arms to stick out sideways; enlarged sternum for more powerful arm muscles. In modern birds these adaptations are useful in the flight stroke.
Were all these dinosaurs fliers? It appears unlikely, given the anatomy of the non-eumaniraptoran forms especially! What other function could broad feathers have served?
Maniraptorans are known to have brooded their eggs (nests with parents on them are known for oviraptorosaurs, troodontids, and avialians). Broad feathers may have helped insulate and protect these eggs: increased arm length, better ability to stick arms out to the side; increased surface area of feathers allows greater coverage of eggs. So perhaps brooding was a selective feature in these adaptations?
Additionally, the carnivorous members of this clade used their forelimbs to capture prey: elongation of the forelimbs gave them a better range in which to strike, while modifications of the shoulder and wrist would let them fold the arms in tight rather than drag them along.
However, it is observations of modern birds that revealed another possible selective force in the evolution of the maniraptoran forelimb and feathers.
WING ASSISTED INCLINE RUNNING (WAIR) and CONTROLLED FLAPPING DESCENT (CFD):
In the early 2000s research by Ken Dial of the University of Montana's Flight Laboratory revealed a locomotory behavior in modern birds not previously realized. Birds (in this case chukar patridges) were discovered to run vertically up surfaces, aided by flapping their wings back and forth in order to generate traction against the surface. They called this behavior Wing Assisted Incline Running, or WAIR for short. They have since found many bird species with this behavior, even perfectly good fliers like pigeons:
Note that these birds are NOT climbing in the typical sense: they are literally running up the sides of trees. Dial and his team studied the ontogenetic (growth) changes in the ability for birds to use this behavior, and also experimented by trimming the feathers of birds to different lengths.
Additional work has shown that this behavior is widespread among modern birds.
The apparatus required to use WAIR is:
All these attributes are present in many pennaraptorans (oviraptorosaurs, dromaeosaurids, troodontids, basal avialians). Additionally, modern birds use WAIR to escape predators: certainly a selective factor present in the Mesozoic, too! Furthermore, there are net selective advantages to slight increases in the length and breadth of the feathered arm surface: the sort of material that can easily be increased by natural selection.
WAIR might represent a "stepping stone" or "behavioral missing link" in the origin of flight. Small (or juvenile) maniraptorans might have used this method to escape predators. Now that they had the ability to get up into trees and other high spots, some lineages of maniraptorans might become specialized for life up on these high spots. Additional natural selection could favor further development of wing size and shape as an aid for getting back down off of high places (controlled flapping descent), or (eventually) from branch-to-branch.
Thus, WAIR serves as a functional link between cursorial and arboreal models (and organisms). It is a cursorial model in that wings begin in part as an aid to running locomotion (just vertical running); it is an arboreal model in that once maniraptorans have an ability to get into the trees, evolution can further develop the forelimbs to get them back down to the ground. And all of these behaviors are still found in modern animals: no speculation of behaviors not currently seen needed.
A NEW SCENARIO FOR BIRD FLIGHT ORIGINS:
The various recent discoveries of the skeletal and integumentary anatomy of various coelurosaurs (including basal avialians) and the behavioral and biomechanical evidence of modern birds suggests a more complete possible scenario for bird flight evolution than the historical "ground up" or "trees down" versions. Note that as with all evolutionary scenarios this would be a simplification, but the following is consistent with our current evidence:
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