Geoscience Reference
In-Depth Information
Peninsula and from western New Brunswick to Newfoundland. Rivers and streams deposited great loads of
gravel, sand, and mud into this basin. In some places, deep lakes were formed and in other places the rivers
fanned out to form deltas. The muddy environments of these ancient lakeshores and river deltas were ideal for
the preservation of fossil trackways.
In 1841, Sir William Logan, who was to become the first director of the Geological Survey of Canada, was
intrigued by some building stone on the wharf at Windsor, Nova Scotia. His inquiry about its origins led him to
Horton Bluff, overlooking the Avon River estuary. There he discovered a set of footprints made by an amphi-
bian 350 million years earlier. It was a radical discovery, because, until then, it was believed that fishes were
the only vertebrates of the Carboniferous period and that they did not crawl onto land until the Permian period,
some 50 million years later. Logan's discovery furnished the first evidence of a tetrapod's successful foray
onto terra firma.
In 1964, some 120 years after Logan's groundbreaking discovery, an even more impressive set of trackways
was uncovered at Horton Bluff. The twenty-seven footprints spanned a 20-meter (65-foot) stretch of beach,
formed of 350-million-year-old sediments that showed distinct ripple marks made by wave action on a shallow
lake bed. At the time, they were the oldest vertebrate footprints in the fossil record, and they were of unpreced-
ented size— 0.3 meters (1 foot). They were also deep, with raised edges where the track-maker had sunk into
the mud. Obviously they had been made by a large, heavy animal. The absence of claws and the width of the
trackway marked the creature as an amphibian. No bones large enough to match the tracks were found, but a
likely candidate for the trackmaker was an embolomere, an order of extinct amphibians more related to cro-
codiles than to living frogs and salamanders. It probably had fangs and was the most feared predator of the
Carboniferous swamp. The size of the fossil also meant that amphibians had already been around for a consid-
erable time.
Amphibians did not actually rule the Earth, being only partially adapted to life on land; they still required
water to breed, just as frogs do today, laying and fertilizing their eggs externally in water. But amphibians were
the dominant vertebrate group during the Carboniferous period.
THE VARIOUS CONTINENTAL
plates had now fully merged, and land plants had fully taken hold, especially in
the equatorial regions, which included much of the eastern seaboard. The giant club moss trees, such as Sigil-
laria and Lepidodendron, grew 30 to 40 meters (100 to 130 feet) tall. Giant tree ferns flourished in the under-
story, and bamboo-like horsetails, Calamites, fringed the riverbanks. Most plants bore spores rather than pol-
len. There were also many seed-bearing plants (gymnosperms) spreading across the land, as well as the ances-
tral conifer, Cordaites, that broadcast its seed as pollen. When fossilized, this luxuriant plant growth furnished
the coal beds that give their name to this geological period—the Coal Age or Carboniferous period.
These lush Coal Age forests often grew in lowland swamps and along riverbanks, which were subject to
periodic flooding. Such inundations buried the forests in sediment, where, over time, they were compressed in-
to coal seams. As living forests, these environments were home to a host of invertebrates that reached gigantic
sizes. The giant arthropod Arthropleura grew to 2 meters (6 feet) in length and left its caterpillar-like tracks in
the muds of the forest floor, where it fed on the rich supply of dead wood. It probably most resembled an over-
size sowbug, or wood louse, and played an important role in recycling the abundant litter on the forest floor.
As the largest land creature in the Coal Age forest, it likely had few enemies. Its aerial counterpart was a giant
dragon fly, Meganeura, whose nearly 1-meter (3.2-foot) wingspan was comparable to that of modern birds of
prey. This ancient insect was equipped with formidable mandibles, or jaws, and it could swoop down and pick
off its prey in its long, basket-like legs. All of these Coal Age giants are represented in the coal beds at Jog-
gins, Nova Scotia, which was recently given World Heritage site status for the preservation of a Coal Age eco-
system in its seaside cliffs along the shores of the Bay of Fundy.
