Dinosaurs 3 Several morphological features suggest dinosaurs relied on visual displays for communication, like modern animals



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Dinosaurs 3

  • Several morphological features suggest dinosaurs relied on visual displays for communication, like modern animals

  • Display behavior usually occurs when

  • Males display to court females

  • Males display to defend their territories

  • Males establish dominance heirarchies

  • Features that suggest display behavior:

  • Horns (Ceratosaurus, Carnatosaurus etc.)

  • Frills (ceratopsians)

  • Spikes (Stegosaurus)

  • Crests (hadrosaurs)

  • Thickened skulls (Pachycephalosaurus)

  • Stereotyped behavior centering on visual and vocal displays is very adaptive

  • Minimizes the risk of injury in sorting out mates, rank within the herd, territories etc.

  • Good circumstantial evidence that such structures are display structures

  • In sexually dimorphic species, well-developed structures are limited to one sex (presumably the male)

  • In species with large numbers of specimens, these structures only appear on mature individuals (as in modern animals)

  • The thickened skulls of Pachycephalosaurus are hard to explain in any other way

  • Seem to have evolved for good old-fashioned head butting…


  • Recent analysis of Triceratops frill bones shows ten times as many combat scars as analogous bones in Cetrosaurus (Farke 2009)

  • Suggests Triceratops horns and frill were not just ornamental, but regularly used in fighting


  • Frills of ceratopsians were structurally too weak to be an effective defense against large predators

  • More likely served in visual or aggressive displays to attract mates


  • The fancy plumage of Epidexipteryx, on the other hand, seem designed for display

  • Long feathers on this non-avian Jurassic dinosaur are not functional flight feathers, may predate origin of flight by millions of years

  • Earliest known example of display structures in dinosaurs


  • Dinosaurs had a limited set of facial muscles (typical of reptiles), so would have been very limited in their facial expressions

  • Would have added to the selective value of display structures in communication


  • Visual communication and species identification in modern animals is also helped by color patterns

  • Dinosaurs often reconstructed in bright colors

  • Colors are pure speculation until 2010


  • Many specimens of dinosaur skin

  • No colors preserved, pigments too delicate

  • There are visible patterns, however, in the size and type of scales or tubercules

  • Stripes and similar areas are apparent, so probably had color patterns

  • Melanins make the darker cryptic colors, basic pigment of feather coloration

  • Melanin is contained in tiny organelles called melanosomes

  • Recent analysis of melanosomes in fossil feathers shows true colors of feathers of fossil birds and dinosaurs


  • Certain hadrosaurs may have used their crests to generate sounds for vocal displays

  • Crests have extensive and elaborate nasal passages that may have acted as resonance tubes

  • David Weishampel has tried to duplicate these nasal passages with PVC pipes

  • “Plays” them to try to recover the lost song of the hadrosaur…

  • Others have worked with CAT scans and computer models to recover these sounds

  • Much of what we know about dinosaur behavior comes from the study of their nests and eggs (ichnology)

  • John Horner’s brilliant work in the Badlands of northern Montana tells a remarkable story about dinosaur behavior (Digging Dinosaurs)

  • Dinosaurs behaved very much like modern colonial waterbirds

  • Coloniality – often find groups of nests of similar type from unknown dinosaur species

  • All dinosaurs laid eggs, but only a handful of species have been found together with their nests

  • Includes Oviraptor, Troodon, Maiasaura, Hypacrosaurus

  • Horner’s site was a communal nesting ground for large herds of a new species of the herbivorous duck-billed dinosaurs

  • Maiasaura, which means “good mommy lizard”

  • Dinosaurs that lived 80 million years ago nested very much like gulls, terns, herons, egrets, and ibises today

  • For one thing, they returned to the same location year after year to nest

  • Horner burrowed down through several layers representing nests built atop nests

  • Dinosaurs showed site fidelity

  • Location modern birds select for nesting colonies is usually sheltered or protected

  • Find modern heron colonies in the middle of deep swamps, where ground predators can’t get at them

  • Except for snakes, the major nest predators of modern wading birds

  • Horner's dinosaur nesting sites were both on small islands in a shallow alkaline lake, Egg Mountain and Egg Island

  • The water was shallow enough to be easily crossed by adult dinosaurs, but deep enough to discourage some terrestrial predators

  • The spacing of the individual dinosaur nests is very regular, like that of modern waterbirds

  • Each nest is positioned just far enough away from other nests, out of reach of aggressive nesting neighbors

  • Condition of the eggshells in the nest also tell us that these dinosaurs were very similar in their behavior to modern birds

  • Some birds have young that are born “ready to party”

  • Certain types of ducks hatch out of the egg, hop out of the nest, and plunk into the water to swim away after mama

  • Precocial strategy reduces the time the young are exposed to nest predators

  • Other birds, like robins, or herons, are born naked and defenseless

  • Must be constantly fed and fussed over for several days or weeks before they are ready to leave the nest

  • Altricial strategy increases the amount of time the young are exposed to predators while in the nest

  • Also puts a big energetic burden on the parents…

  • But when the young finally fledge they are fully equipped and trained to survive

  • One nest held the remains of 15 baby dinosaurs, each ~3 feet long

  • Lots of eggshell fragments

  • Baby’s teeth were worn, indicating they had been feeding for some time

  • This suggested that the baby dinosaurs must have spent some time in the nest, being cared for by their parents

  • They were altricial

  • Horner later found nest with several intact embryos

  • Leg bones of hadrosaur embryos showed same ratio of ossified bone to cartilage as found in tissues of modern altricial birds

  • He also examined nests of a different species, a small hypsilophodont dinosaur

  • Horner found that the top parts of the eggs were broken into fragments, but the bottoms of the shells were intact

  • These newborn dinosaurs must have immediately left the nest

  • They were precocial

  • Horner’s amazing detective work shows us that we can often infer a great deal about how extinct organisms may have behaved

  • Dinosaurs were prey as well as predators

  • Predators included our very distant ancestors, the first mammals

  • Repenomamus robustus lived in China ~130 mya, resembled a small honey badger

  • Recent specimen was found with bones of a small dinosaur in its stomach!

  • Appear to be bones of a baby Psittacosaurus, ~ 14 cm long

  • Despite their superior adaptations, despite their dominance of almost every ecological niche for hundreds of millions of years, dinosaurs became extinct in the KT boundary event

  • But did dinosaurs really become extinct?

  • Evidence continues to mount that they did not disappear completely, but are still with us today

  • Birds are the direct descendants of carnivorous dinosaurs

  • The discovery of Archaeopteryx in 1861 burst like a thunderbolt on the Victorian scientific community

  • The first specimen was a single feather imprint in a slab of Solnhofen limestone, uncovered in a Bavarian quarry and acquired by Hermann von Meyer

  • von Meyer soon acquired a second specimen, a beautifully preserved small fossil reptile, captured in limestone so fine it was used to make lithographic plates

  • He named the specimen Archaeopteryx lithographica, meaning literally "ancient wing"

  • Thomas Henry Huxley, “Darwin’s Bulldog”, seized on the newly discovered Archaeopteryx

  • Concrete evidence of a critical “missing link” between birds and reptiles, proof from the fossil record that Darwin was correct

  • Was Archaeopteryx a bird or a reptile?

  • In many respects it resembled a small dinosaur

  • Two specimens of Archaeopteryx were initially misclassified as a pterosaur and a small theropod dinosaur, languished in museum drawers for decades before being recognized as fossil birds

  • T.H. Huxley was struck by the fact that Archaeopteryx, in many respects, resembled nothing more than a small carnivorous dinosaur

  • Archaeopteryx has many reptilian features:

  • blunt reptilian snout

  • full set of reptilian teeth

  • cranial skeleton is very reptilian

  • bones of the hand, pelvis, are separate (not fused, as in modern birds)

  • Archaeopteryx has many reptilian features:

  • ribs lack uncinate processes (small projections that link the ribcage of modern birds)

  • long, bony reptilian tail (bird tail is reduced to a short stump)

  • At the same time, Archaeopteryx has many avian features:

  • numerous feathers all over the body

  • long- feathered wings and tail

  • rudimentary furcula (wishbone) critical adaptation for avian flight

  • lightweight bones, lots of air spaces

  • Huxley’s theory held sway until the discovery in 1913 by Robert Broom

  • Broom studied a primitive South African reptile that he named Euparkeria

  • Claimed that these primitive reptiles (called thecodonts) were ancestral to both dinosaurs and birds

  • Gerhard Heilmann's Origin of Birds (1926) championed the thecodont theory

  • Was so well persuasive that the thecodont ancestry of birds became the prevalent theory for the next fifty years

  • In the 1970’s, John Ostrom revitalized Huxley's dinosaur origin theory, contending that birds evolved from small theropod dinosaurs

  • Ostrom has assembled an impressive amount of evidence to demonstrate that Huxley was correct

  • Recent work by Jacques Gauthier places the birds within the Coelurosauria, a diverse group of small, agile, bipedal, carnivorous dinosaurs

  • Huxley (1868):

  • “Surely there is nothing very wild or illegitimate in the hypothesis that the phylum of the Class of Aves has its foot in the Dinosaurian Reptiles - that these, passing through a series of such modifications as are exhibited in one of their phases by Compsognathous, have given rise to birds.”

  • Gauthier says that birds are not distant descendants of theropod dinosaurs, birds actually are theropod dinosaurs

  • Gauthier offers an impressive list of 83 shared derived characters (synapomorphies) that unite dinosaurs and birds

  • Includes 17 characters first proposed by Huxley




  • Birds share many traits with theropods, including

  • Lightweight bones (pneumatic bones)

  • Arrangement of toes - foot reduced from 5 to 3 toes

  • 1st toe held off ground - rotates backward for modern perching birds

  • 2/3/4 used for running, 5 reduced or vestigial

  • Birds probably evolved from the Dromaeosaurs, a group of agile carnivores that includes Velociraptor and Deinonychus

  • Starting in 1996 and 1997, several spectacular new discoveries of bird-like feathered dinosaurs from mainland China

  • These discoveries have provided convincing evidence that birds are the direct descendants of theropod dinosaurs

  • Is Archaeopteryx ancestral to modern birds?

  • Or is an evolutionary dead end, not directly related to modern birds at all?

  • There are many bird-like dinosaurs in the Triassic

  • Early coelurosaurs could well have given rise to birds early in the Mesozoic

  • Lineage of feathered birds gradually diverging from a lineage of early feathered theropods

  • Some of these feathered theropods may have been capable of flight

  • Gregory Paul has hypothesized that some maniroptoran dinosaurs were secondarily flightless!

  • Latest discovery was gleaned from a fragment of T. rex bone

  • Bone discovered by R.T. Bakker was too big to fit in the helicopter, so was broken into smaller pieces

  • Mary Schweitzer at North Carolina State put the fragment in an acid bath to dissolve the mineral matrix

  • Found small fragments of soft tissue, 68 my old!!

  • Found what appears to be collagen (organic component of bone)

  • Found blood vessels and red blood cells

  • Some of the red blood cells appear to have intact cell nuclei

  • Potential to recover actual genetic material

  • Asara (2007) examined the samples and extracted segments of 7 proteins, including 5 collagen fragments

  • Comparison with other vertebrate showed the collagen was closest to chickens

  • First solid molecular evidence linking birds and dinosaurs!

  • Further examination of the bone revealed something even more remarkable

  • The T. rex was female, and it was pregnant!

  • Bone tissue resembles that of modern flightless birds, like emus and ostriches

  • Medullary bone forms only in ovulating female birds, no other egg-laying animals

  • Further physiological link between dinosaurs and modern birds

  • Thus we have come full circle from Huxley's original interpretation of Archaeopteryx

  • Dinosaurs are not extinct, but very much alive and well

  • The idea that birds are descendants of dinosaurs has a romantic flair that appeals to our sense of evolutionary justice

  • In reality the dinosaurs have never left us

  • Whenever we see a heron foraging in a still pool, or hear the beak of a swallow snap shut on an insect, we are witnessing the survival of the evolutionary architecture of the dinosaurs




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