Paleogeography and life of the Late Paleozoic World EPSC 233 Earth & Life History (Fall 2002)

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 non-deposition. 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)

During the Carboniferous,Gondwanaland joins Euramerica.

Ice caps grew and shrank repeatedly... Sea level changes caused the cyclothems.

Coal forms from plant remains accumulating in swamps found between channels of river deltas.

Coal covers a predominantly non-marine section

Predominantly marine section covers the coal.

Sea level fluctuations caused these cycles

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 tree-sized plants.

20 m tall

Sphenopsids, related to today’s horsetails.

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 freshwater-terrestrial fauna.

myriapod

insect

The best fossils are found by cracking open siderite (FeCO3) concretions that stick out from the weathered siltstone.

“Tully monster”

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.

Ophiacodon, a 3.5 meter-long Permian pelycosaur (extinct order of reptile) from Texas, shown here eating an amphibian.

During the Permian period, reptiles started replacing amphibians in various ecologic roles (carnivores, scavengers, specialized herbivores).

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).

Lycaenops: South Africa, 1-meter (wolf-sized) predator.

Titanosuchus: South Africa, a 2.5 meter long carnivore.

Some Permian therapsids, nearly mammal-like...

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.