Finally, a Foothold on Land
Published in The EarthSphere Blog. Feature Image: Silurian Sea Scorpion (ArcheanArt)
Prologue
The Forgotten Origins series moves to the Silurian, where life gains a foothold on land.
A Reversal of Fortune
The Ordovician-Silurian mass extinction was one of the biosphere’s historic low points, so the Silurian Period came in on a down note. When the Silurian recovery started 443.8 million years ago, only 15 percent of the species present during the Ordovician remained. Fortunately, the tide turned for life and things were on the upswing. The ecological void left by a catastrophic mass extinction provided evolution with plenty of opportunities to innovate.
The Gondwana supercontinent still covered much of the southern hemisphere in-and-around the South Pole. Still, warmth slowly returned to the planet, causing the magnificent southern ice sheet to shrink. Rising temperatures and melting ice led to a significant sea-level rise.
The causes behind this reversal of fortune are uncertain. Changes in solar radiation levels are possible but undocumented. One suggestion points towards changes in the influx of galactic cosmic rays from nearby supernovae, but this also seems to be on the fringe of possible explanations.
In many warm periods, one historical culprit for rising global temperatures is degassing from massive flood basalt eruptions. Large quantities of carbon dioxide enter the atmosphere during these events creating a greenhouse effect. But a search of recognized flood basalt events places the earliest flood basalts at about 377 million years ago, long after the end of the Silurian at 419.2 million years.
Perhaps the massive chemical weathering from plants’ initial invasion of continental landmasses had run its course, reducing carbon dioxide uptake from weathering. Such a change may have led to a net accumulation of CO2 in the atmosphere. Silurian carbon dioxide levels were higher than our modern levels, with estimates ranging from three to ten times higher.
Despite the causes of warming, life benefited from Silurian environmental conditions and thrived in the warm shallow seas and on land.
Life at Sea
Mother nature was busy during the Silurian, building the foundation of ichthyology as fish rapidly evolved. Jawless fish were present at the beginning of the Silurian, and they continued to thrive, expanding their range and rapidly spreading around the planet. But fish evolution took two big steps forward during this period. The first freshwater fish appeared, and evolution equipped some newly evolved species with jaws. What better way to secure your dinner than clamping down on it so it can’t escape? This evolutionary adaptation was clearly an important step towards the production of the famous sea-horror movie ‘Jaws.’
But fish were not at the top of the food chain. This hallowed position was held by eurypterids, the apex predators of the Silurian shallow seas. These arthropods were like sea scorpions on steroids with jointed legs for walking, pinchers for grabbing, paddle-shaped appendages for swimming, and a tail with a venomous spike at the end. These sea monsters grew up to eight feet long, making them the largest arthropods to ever live on Earth.
Despite clear evolutionary progress during the Silurian, animal life was still mostly limited to the oceans. A stunning array of marine species lived in the Silurian shallow seas. Just as in the Ordovician, higher sea levels let the oceans spread over vast areas of low-lying continental topography, creating the perfect home for ancient marine life.
Atmospheric oxygen levels during the Silurian were lower than today, probably hovering between 35% and 65% of modern levels. Also, the climate was warmer by about 5 degrees C. Warm water holds less oxygen. So, between lower atmospheric oxygen and reduced capacity for absorbed oxygen in the seawater, life was generally limited to a depth of about 100 meters below the ocean surface. Waters deeper than 100 meters were often anoxic.
Life on Land
While life at sea was rapidly evolving, changes were also occurring along the shorelines. Life crept out of the ocean as vascular plants and the first macroscopic animal life appeared. Land plants provided food, and the precursors of spiders and millipedes followed the food chain, scavenging these first shoreline ecosystems, eating plants and decaying detritus, and preying on each other.
The mosses and hornworts of the Ordovician were joined by a more robust form of plant life that would eventually come to dominate our terrestrial ecosystems. Vascular plants held the potential to grow up and out, better exploiting free energy from the sun.
A mountain-building episode, which began in the Ordovician, picked up steam during the Silurian, creating the Scottish Highlands and associated mountain belts in Scandinavia and Greenland. Laurentia, which included North America and Greenland, Baltica (central and northern Europe and Scandinavia), and western Europe, then known as Avalonia, collided in a massive suturing together of the three plates. The Iapetus Ocean, which originally separated these three continents, disappeared.
This episode was part of the Caledonian Orogeny, an event describing a series of continental plate collisions occurring over a 100 to 200 million year period, depending on which researchers you speak with. So the Silurian collision was only part of a larger process.
Overall, the Silurian Period was a time of rapid evolutionary change. Earth’s ecosystems were maturing and actively shifting onto dry land. The food webs and energy chains of our modern biosphere gained a significant foothold during the Silurian.
ArcheanArt
Sources:
Evidence of nearby supernovae affecting life on Earth (by Henrik Svensmark; Royal Astronomical Society) —
On the ages of flood basalt events Sur l’âge des trapps basaltiques (By Vincent ECourtillot and Paul RRenneb; Science Direct)
Silurian Oceanic and Atmospheric Circulation and Chemistry (by Pat Wilde, William B. N. Berry, and Mary S. Quinby-Hunt) Marine Science Group, Berkley)
Silurian Period — 443.8 to 419.2 MYA (Source: National Park Service)
Silurian Period Facts: Climate, Animals & Plants (By Mary Bagley; Live Science)
Eurypterids, Giant Ancient Sea Scorpions (Source: Tale Peabody Museum)
Silurian Period (Source: Cal Poly Humboldt Natural History Museum)
