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

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

• Early mammals generally assigned to a nocturnal niche, granivorous or insectivorous

• Some now appear to have been a bit more ambitious…

• 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 speculated that Archaeopteryx was most closely related to small dinosaurs called Coelurosaurs

• 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