Download Late Paleozoic life & paleogeography

Paleogeography and
life of the Late
Paleozoic World
EPSC 233 Earth & Life History (Fall 2002)
Recommended reading:
STANLEY “Earth System History”
Chapter 15, pp. 414-431.
Keywords: Carboniferous (Mississipian,
Pennsylvanian), Permian, Ancestral Rocky
Mountains, coal cyclothems, Mazon Creek
concretions., seasonal climates, cordaites
(primitive gymnosperm trees), winged
insects, amphibians, amniote egg, reptiles,
pelycosaurs (fin-backed reptiles),
therapsids (mammal-like reptiles).
In Europe, the
Carboniferous system is
split in Early, Middle and
Late intervals. (The right
hand side names as
“stages” defined on the
basis of biozones.
In North America, the
Carboniferous system is
split in the Mississipian
and Pennsylvanian periods.
The Mississipian
strata are separated
from the younger
Pennsylvanian strata
by a 4-million year
gap.
This “discomformity”
is an interval of nondeposition. Bedding is
parallel in strata
below and above it.
Many genera of
crinoids and
ammonoids disappear
at that level.
Near the end of the Late Devonian ice age,
glaciers shrank.
Sea level rose, warm seas flooded continents,
and crinoidal limestones accumulated in shallow
continental seas.
Throughout all (or nearly all) of Carboniferous
time, however, large areas of Godnwanaland
were blanketed by ice sheets.
When glaciers shrank, during the Upper Carboniferous,
lycopods and seed ferns were replaced by spore ferns,
which were better adapted to moister conditions.
The reassembly
of continents
into the
supercontinent
“Pangea”
continued
throughout the
late Paleozoic,
giving rise to
orogens
(mountain
building events).
Most of these orogens are classic thrust-and-fold
belts but not the “Ancestral Rockies”.
Uplift and downfaulting in southwestern U.S.
took place along deep, nearly vertical faults.
This suggests that the crust was being
stretched rather than compressed. The exact
cause is unclear...
In Pennsylvanian (or Carboniferous) strata, coal beds
occur between other rock types. The different rock
types form remarkably repetitive cycles at any one
locality.
Throughout all (or nearly all) of Carboniferous
time, however, large areas of Gondwanaland
were blanketed by ice sheets.
(Gondwana seen from
the south pole during
the late Paleozoic)
Ice caps grew
and shrank
repeatedly...
Sea level
changes
caused the
cyclothems.
During the
Carboniferous,
Gondwanaland
joins
Euramerica.
Coal forms from plant
remains accumulating in
swamps found between
channels of river deltas.
Sea level
fluctuations
caused these
cycles
Predominantly
marine section
covers the coal.
Coal covers a
predominantly
non-marine
section
Part of the non-marine sequence within
cyclothems is due to the movement of delta
lobes, flooding the swamps with coarser
sediment (sands) carried by the meandering
river.
Climates became more seasonal at mid latitudes.
Yellow: evaporites Green: coal
Permian: drier conditions became more widespread.
Gymnosperms, “naked seed” plants (think
gymnasium and athletees in Antique
Greece), took over terrestrial
environments.
Cordaites, tall trees (up to 30 m high)
formed large woodlands resembling modern
conifer forests (pines, spruces, redwoods).
Joggins, Nova Scotia.
Cliffs are washed by the tides of the Bay of
Fundy. New material is constantly eroded and
exposed.
Lycopods (spore plants which today grow
mostly to sizes cm to < 1 m) grew to treesized plants.
Sphenopsids,
related to today’s
horsetails.
20 m tall
Glossopteris, a tree size
seed fern widespread
throughout Gondwanaland.
A 2-meter long
myriapod (as drawn
by Dawson, above),
left trackways (right)
at Joggins. The same
arthropod is known
from other localities.
This tree trunk was buried in
sediment on the floodplain.
Some tree trunks contain
complete skeletons of small
animals, like this Hylonomous.
Insects (another member of the phylum
Arthropoda) started to assume a very
important ecologic role in terrestrial
ecosystems.
Fossils of insect-like arthropods of late
Devonian age were wingless.
However, by Late Carboniferous time, many
types of insects had wings. Some of the
earliest winged insects had wings that they
could not fold away.
Today, mayflies and dragonflies share that
trait. Their fossils are found in Upper
Carboniferous deposits. Most were “normal”
size (a few cm) but one species is known to
have reached a wingspan of nearly a half
meter!.
Insects with foldable wings also appeared in
some Carboniferous beds.
Spectacular fossils of Carboniferous age
are also found south of Chicago, Illinois, in
an area named from a local river, Mazon
Creek. By 1979, 320 species had been
described from the deposit.
Most Mazon creek fossils can be assigned
to either a marine fauna or a freshwaterterrestrial fauna.
myriapod
“Tully monster”
The best fossils are
found by cracking open
siderite (FeCO3)
concretions that stick
out from the
weathered siltstone.
insect
The fossil-rich “coal balls” are rounded
masses of limestone or siderite.
These concretions form in coal beds
during early burial.
Carbonate mineral (CaCO3) is dissolved
and redistributed within the
sedimentary layers during compaction.
The carbonate mineral reprecipitates as
tiny crystals around an intact, dead
animal. This produces a hard envelope
and preserves delicate fossils from
being crushed during deeper burial.
The Carboniferous and Permian amphibians were
more diverse in size and shape than today’s
relatively small frogs, toads and salamanders.
Eryops was a stubby
alligator-like amphibian,
carnivorous, about 1.5
meter long.
Branchiosaurus is a
smaller,
salamander-like
amphibian.
There is a Carboniferous
“gap” in our knowledge of
the origin of the earliest
amphibians.
This corresponds to the
drop in sea level that
separates the Mississipian
and Pennsylvanian (the
disconformity).
During periods of low sea
level, continents are more
prone to erosion and our
knowledge of terrestrial
life is fragmentary.
We know that a key evolutionary breakthough
must have occurred in some Carboniferous
amphibians: the amniote egg. This innovation
would distinguish the earliest reptiles from their
amphibian ancestors if their remains were found.
The amniote egg provides the embryo with:
- food (yolk)
- 2 sacs:
- one containing the embryo
- the other one to collect waste products
- an outer shell to protect the embryo
Just as seeds allowed plants to reproduce
in drier environments, the amniote egg
freed some amphibian tetrapods from the
need of having their eggs and young
develop fully in aquatic environments.
Reptiles also evolved an advanced jaw
structure which could apply heavy
pressure, and, when combined with
blade-like teeth, slice food.
During the Permian period, reptiles started replacing
amphibians in various ecologic roles (carnivores,
scavengers, specialized herbivores).
Ophiacodon, a 3.5 meter-long Permian pelycosaur
(extinct order of reptile) from Texas, shown here
eating an amphibian.
Dimetrodon, a carnivorous pelycosaur, was a
jaguar-size carnivore with serrated teeth.
Its fin (characteristic of pelycosaurs) may have
helped regulate its body temperature, by soaking
up the sun or helping to lose excessive heat.
Edaphosaurus, another pelycosaur, was
herbivorous. Carnivores and herbivores develop
different types of dentition (size and shape of
teeth can be used to tell them apart).
Some Permian therapsids, nearly mammal-like...
Lycaenops: South
Africa, 1-meter (wolfsized) predator.
Titanosuchus: South
Africa, a 2.5 meter long
carnivore.
Moschops: South Africa,
a 5 meter-long herbivore.
Among reptiles, the therapsids, also gained in speed
and agility by evolving legs positioned more vertically
beneath their bodies.
Modern reptiles are ectothermic, i.e. cold-blooded
and had to rest frequently in order to soak up heat
from their environment.
Therapsids were clearly “built for action”: advanced
jaws, complex set of teeth, upright posture. They
may have been among the first tetrapods to become
endothermic, i.e. warm-blooded. By the Late Permian,
they had diversified into 20 or so families, while
carnivorous reptiles like the pelycosaurs were in
decline.