module 8: histology growth 1

not constant

in dinosaurs, growth was …, it occurred in seasonal cycles

cyclical growth marks

cycles left ____ ___ ___ (CGMs) in the bone

• most common is lines of arrested growth (or LAGs)

since the cycles were yearly, counting the LAGs = a minimum estimate of the age of the dinosaur

• ontogenetic age

• growth rate & pattern

• metabolism

what can bone microstructure tell us?

a

growing fibro-lamellar bone (FLB) showing formation of primary osteons (a, b, or c?)

b

zonal bone with fibro-lamellar zones, lamellar annuli, & lines of arrested growth (LAGs)

c

zonal bone with lamellar zones & LAGs

subadult Edmontosaurus femur

• Alternating bands of reticular (faster growth) and circumferential (slower growth) FLB

• No real LAGs, but clearly cyclical growth variation

• Possibly recording periodic (seasonal?) nutritional stress?

• More faster growing tissue in the earlier (younger) growth bands

primary osteons

result from appositional bone growth

secondary osteons

result from bone modeling

• resorption & redeposition of bone tissue as part of mineral metabolism

• can overprint/obscure original bon growth patterns

• can identify through cross-cutting relationships

life history recorded on histology

• Cross-section of humerus from adult turkey vulture (Cathartes aura)

• M = medullary cavity

• Scale bar = 78 μm

• Fibrolamellar bone (FLB; fast-growing) from juvenile growth stage surrounded by periosteal (external) and endosteal (internal) lamellar bone indicating slowed growth and attainment of adult size

Suuwassea emiliae

• originally considered a subadult on body size & bone fusion characters in vertebrae & skull

• some characters that distinguish it from other Morrison Fm. sauropods are “primitive” characters that may also correlate w/ subadult status

• sauropod bones are difficult to numerically age bc of high dgrees of secondary remodeling tend to obscure growth marks

• histologists describes HOS that correlates with BOS and this allows relative aging of sauropods & identification of ontogenetic stages

HOS

histological ontogenetic stages

BOS

biological ontogenetic stages

• even if u have CGMs/LAGs, they only give minimum individual age estimates

• medullary expansion

  • bones don’t just get thicker thru ontogeny

  • earlier LAGs are resorbed & lost

  • smaller individuals can help reconstruct the early phases

why is the growth series of taxa critical to investigating dinosaur growth?

like modern mammals & birds, dinos appear to largely have has determinate growth

• Maiasaura: approx. 7-8 years

• Apatosaurus: approx. 15-20 years

• Tyrannosaurus: approx. 22 years

how long does it take to reach mature size?

determinate growth

growth stops after a certain point (usually, shortly after reproductive age is reached)

age & growth rates

• large dinos grew FAST

• Maiasaura: 500 kg (1100 lbs)/yr

• Tyrannosaurus: 840 kg (1825 lbs)/year

• get these #s by comparing age & size ^

growth in dinosaurs followed typical “S”-shaped curve

• growth starts slowly, accelerates rapidly through the major growth phase (pre-sexual maturity) then levels off after sexual maturity reached

what is the curve most dinosaurs follow in terms of age vs mass?

• T. rex reached maturity by ~22 years old and growth leveled off after that

• most T. rex specimen = young, averaging 18 years old

• T. rex died at ~30 years old

  • other Tyrannosaurids lived to similar ages BUT on this scale their growth curves are depressed, reflecting smaller sizes

Describe the growth in Tyrannosaurids.

• length

  • needs a full & complete skeleton, or the ability to extrapolate the effect of missing bones based on complete skeletons of closely related taxa

• most growth curve reconstructions use mass

What do we mean by size?

• we can’t directly

• mass of animal > mass of its skeleton

• so, volumetric models and/or linear regression used

How do we measure dinosaur mass?

volumetric model to estimate dinosaur mass

• water displacement

  • create a physical model

  • measure volume of water the physical model displaces

  • multiply volume by an “average” body density for archosaurs to find mass

• computer model

  • calculate fraction of body made up of various tissues & empty space

  • multiply by observed densities of tissues in modern animals

• only works for very completely known animals

linear regression model to estimate dinosaur mass

• how linear measurements in scale w/ body size in modern animals

  • femur circumference has tightest relationship

• develop allometric equation for the relationship between the two

• use linear measurements of dinosaur bone to calculate estimated body mass

• can be used for fragmentary remains

• extremely long growth period?

  • NO: histology doesn’t support this

• parasagittal limb stance?

  • mammals also have this, and yes, some elephant & rhino relatives get very large, but nowhere near the largest of dinos

• very efficient hearts & lungs?

  • well, YES, but crocodilians & modern birds have these too

• short food chains?

  • less total energy loss w/ fewer “steps”

  • maybe part of it? largest mammals tend to be in short food chains

• higher oxygen levels in mesozoic atmosphere?

  • some paleoclimate & geochemical models suggest higher oxygen levels than today

  • increase in primary productivity widens the base of the food energy pyramid, therefore you can have larger herbivores

Why did dinosaurs get so big?

Dreadnoughtus schrani

• among largest known of titanosaurs

• mass estimate 59.3 metric tons based on limb height & circumference

long bone histology

• inner cortex densely remodeled

  • black arrow & green line indicate out extent of dense secondary osteon formation

• outer cortex well-vascularized primary FLB

• no visible LAGs or EFS