Lecture 10: Invasion! 2: creeping, crawling onto land Flashcards
First (tiny) steps on land
trace fossils – trackways
500 Ma: amphibious arthropods
428 Ma: arthropod body fossils – millipedes
the first vertebrates to crawled onto land?
lobe-finned fish have traditionally been viewed as the first vertebrates to crawled onto land
fleshy, lobe-shaped fins joined to body by single bone
evolved limbs, became amphibian tetrapods…
But why colonize land?
left water either in search of oxygen (freshwater bodies poorly oxygenated)…
…or to find prey
…or to find water when the water body in which they lived dried out
…later evolved legs, lungs, etc.?
What is the evidence?
good fossil record of Devonian fish, including lobe-finned forms (i.e., aquatic)
good fossil record of land-dwelling Carboniferous tetrapods
Will the real ancestor please stand up?
lobe-finned fish?
divided into:
lungfish (the Dipnoi)
coelacanths
used to be viewed as “the fish which first walked on land
A red herring lungfish
freshwater lobe-fins gills (in most living forms almost lost) lung: modified swim bladder aestivate (hibernate over summer when water dries up) fin bones poorly differentiated thus not immediate ancestors
Coelacanths
last seen in the Cretaceous
1938: rediscovered in South Africa by Marjorie Courtenay-Latimer
(described as Latimeria)
populations found in deep waters of the Indian Ocean
‘living fossil’ lobe-finned fish
Another red herring coelacanth
not immediate tetrapod ancestors as:
never aquired shallow water characteristics
skull is laterally flattened with a narrow mouth
sideways-facing eyes
but fin bones are robust, differentiated
Problem: ‘gaps’ in the record
360-345 Ma almost no tetrapod fossils: recognized by Alfred Romer (Harvard), known as “Romer’s Gap”
no transitional forms from fin-to-limb – anti-Darwinists
why no fossils?
after end-Devonian extinctions – oxygen too low for land animals?
there but not preserved
just not yet found?
Bones built for life on land: the backbone
stronger backbone
vertebrae enlarge and link together
develop interlocking prongs to prevent twisting
vertebral spines enlarge: more muscle support
Bones built for life on land: the back end
strong legs/feet require strong attachments into skeleton
pelvic bones expand
pelvic skeleton becomes more robust and connected to the backbone
fins-to-limbs: ankle and digits develop (‘feet’)
Bones built for life on land: the front end
at the front the fins connect to shoulder bones
ribs enlarge to support shoulder and backbone
skull disconnects from shoulder bones, neck develops
fins-to-limbs: wrist and digits develop (‘feet’)
earbones develop from reduction of jaw bones
Fish have primitive ears but use them as
balance organs
sense pressure changes with ‘lateral line’ system
land dwellers developed the ears
one jaw-supporting bone (‘hyomandibula’) reduced in size
became an earbone (the ‘stapes’), which transmits changes in air pressure
The breath of life
water bodies have low concentrations of dissolved O2
fish pump H2O over their gills and extracting O2
ancestors of lobe-finned fish already had gills and simple lungs
facultative air breathers (can breathe air if need to)
Early tetrapods could use both gills and lungs
O2 is seasonally variable in shallow H2O, fish living here can gulp in air
fossil tetrapod skulls indicate they could do this too…
simliar to ‘buccal pumping’, the method amphibian tetrapods use to breathe today
We now know of a third group of lobe-fins:
lungfish
coelacanths
“tetrapodomorphs”
Tetrapodomorphs: getting closer
Eusthenopteron, >2000 specimens 385 Ma: Late Devonian fin bones reduced/differentiated distinct femur, tibia, fibula, etc., internal nostrils much misinterpreted FULLY AQUATIC
Getting less fishy?
Panderichthys, 90-130cm some features more like tetrapods than lobe-fins: ossified backbone skull flatter, wide at back, narrow at front tail unlike lobe-fins shoulder & longer humerus …but fins still fish-like fully aquatic – shallow water?
Tiktaalik: a ‘fishapod’
transitional between lobe-fin fishes and tetrapods
up to 2m, Canadian Arctic
body form strongly suggests life in shallow water
raised snout, closely spaced eyes on top of flat skull
no skeletal link between skull and pectoral girdle: first to have a neck!
Propped up
ribs stronger, overlap
trunk does not crush internal organs when out of water
first articulating wrists, but no digits (‘fingers’)
could prop trunk up on pectoral fins - to raise head out of water?
Fishy fingers: Acanthostega
360 Ma: Greenland
spine only suited for swimming
long limbs (NOT fins), but hips weakly attached to trunk
no wrists, no ankles
but first digits
however: 8 fingers, 8 toes!
for moving round in shallow water full of debris: a fish with legs
Sealed: Ichthyostega
374 Ma: ~1.5m long
formerly viewed as one of the first tetrapods
unlikely - recently revised
7 rear digits
very strong overlapping ribs to support trunk organs
poorly adapted to terrestrial locomotion
may have ‘beached’ like a seal, but mostly aquatic
Hynerpeton & Tulerpeton
~360 Ma
strong backbones, strong muscular shoulders/front limbs
front digits long: for use on land
gills have been lost
all point to considerable time out of water
Water to land transition now proven
an extensive fossil record of intermediates between lobe-fins and tetrapods
skulls, jawbones-earbones
fins-to-limbs
pelvic girdle
