Lesson 5: Birth, Growth, and Reproduction

Question 1

What did eggs look like prior to the adaption of amniotic eggs?

Answer 1

all tetropods laid eggs that were similar to those of modern frogs and salamanders and could not retain water

such eggs would dry out and die if not laid in a wet, humid place

Question 2

What are amniotic eggs?

Answer 2

they have encapsulating membranes that are watertight

being able to hold in their own water, amniotic eggs can be laid in dry habitats

the membranes of amniotic eggs also became adapted to form tough leathery or hard shells

shells improved amniotic eggs’ ability to hold in water and also made the eggs more durable and less vulnerable to small predators

Question 3

What are amniotes?

Answer 3

animals that lay amniotic eggs

this adaptation allowed amniotes to colonize new terrestrial environments

mammals, birds, dinosaurs, and reptiles are all amniotes

although most extant mammals do not lay eggs, mammalian embryos still have membranes that cover them while in uterus

Question 4

How do the organisms inside amniotic eggs breathe?

Answer 4

as the living cells inside an egg grow and develop, they consume oxygen and produce carbon dioxide waste (just as animal cells do)

this carbon dioxide waste needs to go somewhere, and fresh oxygen needs to be constantly supplied

even hard eggshells are covered with tiny holes that permit gasses to be exchanged between the insides of the egg and the outside world

this need to breathe places a limit on how big eggs can be

Question 5

How does the cube-square law relate to amniotic eggs?

Answer 5

as any shape increases in size, its surface area increases more slowly than its volume

the largest dinosaur egg is only half a meter long and most are much smaller

eggs that are much larger than this are not possible, because the amount of oxygen that a dinosaur developing inside an egg requires is a function of its volume, while the rate at which oxygen can be exchanged is a function of the eggshell’s surface area

giant eggs would have a low ratio of surface area to volume and would die

Question 6

What are lines of arrested growth (LAGs)?

Answer 6

bone cells are called osteons

as animals grow their bones, they add osteons to their bones’ outer walls

but the rate at which osteons are added is not always the same and varies with changes in growth rates

during seasonal periods, when resources needed for growth are scarce, such as during winter or the dry season, growth may slow down

this creates rings inside the bones, analogous to those of a tree trunk

Question 7

What did the growth rate of dinosaurs look like?

Answer 7

it turns out that dinosaurs grew fast

it is estimated that a T. Rex grew to its adult size in only 20 years

even large sauropods only took 30 years to fully mature, and they are estimated to have gained an average of one to two pounds every day

Question 8

What are the characteristics of the bones of younger dinosaurs?

Answer 8

high vascularity: many blood vessels

a texture called lamellar bone

LAGs formed later, as dinosaurs grew

Question 9

What is the process of remodeling?

Answer 9

where the old bone cells were replaced by newer bone cells

this kind of bone is called Haversian, or secondary bone

Question 10

What is the external fundamental system (EFS)?

Answer 10

as growth slows and then finally stops, a closely spaced series of LAGs is formed

this presence of an EFS indicates that the dinosaur is skeletally mature and has stopped growing

Question 11

How did the appearance of newborn dinosaurs differ from the adults?

Answer 11

big heads and big eyes are common traits of young animals, baby dinosaurs also had relatively large heads and eyes

the crests of many hadrosaurs were not present in very young individuals, but grew gradually as the dinosaurs reach maturity

it seems that many baby ankylosaurs hatched with little or no armor and with no tail clubs and they did not grow until later in life

Question 12

What are ontogenetic changes?

Answer 12

changes in the form of an organism that occur as it matures

Question 13

What are non-isometric ontogenetic changes?

Answer 13

changes in the relative proportions of an animal as it grows, that are not simply changes resulting from a general increase in size

the changes in the relative lengths of the horns and frills of ceratopsians are examples

Question 14

What are isometric ontogenetic changes?

Answer 14

are changes in absolute size but not proportions

for instance, the length of ceratopsian hind legs changed proportionally as the animal grew

that is, the length of the tibia relative to the length of the femur of a baby ceratopsian was nearly the same as the length of the tibia relative to the length of the femur of a full-grown adult

Question 15

What is sexual dimorphism?

Answer 15

males and females of the same species are different

sexually dimorphic features of the skeleton are usually subtle but can be extreme

it is common for sexually dimorphic features to be ontogenetic changes

Question 16

Why is sexual dimorphism difficult to identify in dinosaurs?

Answer 16

for example, some paleontologists have suggested that the ceratopsian Protoceratops was sexually dimorphic, because some specimens have wider frills than others even though their heads and bodies are about the same size

however, it is hard to be certain that the wider-frilled Protoceratops are not simply older individuals or that they represent a different species

not all paleontologists agree with the sexual dimorphism interpretation

Question 17

What does a dinosaur skeleton being found near eggs tell us about the sex?

Answer 17

if a dinosaur skeleton was found with eggs preserved inside its body cavity, this would probably be pretty good evidence that the dinosaur was a female

however, we would have to rule out that the eggs are there because of other reasons: perhaps the dinosaur had eaten them, or they had washed in after death and the association is just a coincidence

Question 18

What does a dinosaur skeleton being found sitting on eggs tell us about the sex?

Answer 18

in some very rare and spectacular cases, dinosaurs have been fossilized while sitting on, (“brooding” or incubating) a nest of eggs

we often think that female birds do the brooding, but in fact many male birds spend a lot of time looking after eggs, and so a dinosaur sitting on a nest of eggs could be a male or a female

Question 19

What is the medullary bone?

Answer 19

laying eggs with hard shells requires a female to donate a large quantity of calcium

in preparation for this donation, female birds grow a medullary bone

medullary bone contains concentrations of calcium that are stored prior to eggshell development

because only female birds produce eggs, the presence of medullary bone shows that a particular specimen is female

Question 20

Why does the absence of a medullary bone not guarantee a specimen is male?

Answer 20

the application of this technique is limited, because medullary bone is only grown by females prior to egg production and is not present at other times

theropod bones without medullary bone therefore could be from a male, or from a female that was getting ready to lay eggs

Question 21

How do dinosaurs modern relatives offer insight into their parental care styles?

Answer 21

many birds, not only care for their eggs but also feed and protect their young after they hatch

crocodilians also tend to be good parents

female crocodiles guard their nests and, although they do not provide their hatchling with food, also protect their young for an extended period of time after they hatch

Question 22

What evidence shows that dinosaurs devoted considerable time and effort to parental care?

Answer 22

skeletons have been found of oviraptorosaurs (a kind of herbivorous theropod) positioned over top of their egg-filled nests

it appears that these dinosaurs were fossilized in the processes of incubating their eggs and it seems likely that they were also guarding their nests

often, the skeletons of young dinosaurs are found alongside the skeletons of adult dinosaurs, and this suggests that these dinosaurs lived together as a family group

Question 23

What evidence shows that dinosaurs avoided parental care completely?

Answer 23

although mother sauropods could not lay big eggs, they were able to lay a great many eggs

fossil nests of sauropods from Argentina show that herds of sauropods also laid their eggs all at the same time and at the same time and at the same place

these mass sauropod nesting grounds have the individual nests too close together for a mother sauropod to have attended to her eggs without stepping on the nest of a neighbor

Question 24

What is predator satiation?

Answer 24

to produce a new generation of sauropods, only a tiny fraction of the eggs that were laid needed to hatch and grow into adults

rather than investing time into guarding and rearing their young

sauropods simply produced so many offspring at one time that predators would not have been able to eat them all before they matured

the same strategy is used by many modern sea turtles