The Dawn of Earth chapter 2: Life Galore

Period: Early Cambrian (Terreneuvian, Cambrian Stage 2)

Date: 521,000,000 BC

Location: Coastal waters near northwestern Gondwanaland.




The Gondwanaland beach is completely barren as the waves lap its edge. During the Cambrian, the lands are comparatively barren of life, with nothing more complex than a microbial soil crust near the ocean edges and a few primitive Mollusks that occasionally emerge to browse on the microbial biofilm during the hide tide. Most of the continents are dry and rocky due to a lack of vegetation. Shallow seas flank the margins of several continents created during the breakup of the supercontinent Pannotia. The seas are relatively warm, and polar ice shall be absent for much of the period. Earth is generally cold during these early days of the Cambrian due to Gondwanaland covering the South Pole and cutting off polar ocean currents. However, average temperatures are 7 degrees Celsius higher than the distant future.
In the oceans, diverse lifeforms prosper. The Cambrian not only marks the beginning of the Paleozoic era and the Phanerozoic eon, it also marks a profound change in life on Earth; prior to the Cambrian, the majority of living organisms on the whole were small, unicellular and simple; the plantlike Charnia, from the previous Ediacaran period of the Neoproterozoic era in the Proterozoic eon, being exceptional. Complex, multicellular organisms have gradually become more common in the millions of years immediately preceding the Cambrian, but it was not until this period that mineralized (hence readily fossilized) organisms became common. The rapid diversification of lifeforms in the Cambrian, known as the Cambrian explosion, has produced the first representatives of all future Animal phyla. During the Cambrian radiation, Metazoans (Animals) evolved monophyletically from a single common ancestor: flagellated colonial protists similar to present and future Choanoflagellates.
In this particular coastal era of water, there are sea level changes and tectonic activity. The region is a shallow sea with a muddy bottom. There are also periodic turbidity currents here. The local fauna is primarily benthic. There is a variety of macroscopic marine Plants and algae, such as the Chlorophyte green alga Margaretia. Arthropods are by far the most dominant Animals in the ocean, including the Trilobites, which make up only a minor part of the total Arthropod diversity. What makes Trilobites so apparently abundant is their heavy armor that is reinforced by calcium carbonate, which will one day be fossilized far more easily than the fragile more purely chitin exoskeletons of other Arthropods.
Trilobites, which will be among the most successful of all Paleozoic Animals, have many lifestyles; some move over the seabed as primary predators, scavengers, or filter-feeders, and some swim above the seabed, feeding on plankton. Most lifestyles expected of future marine Arthropods are seen in Trilobites, with the exception of parasitism. Some Trilobites have even evolved a symbiotic relationship with sulfur-eating Bacteria from which they derive food.
Above their heads, shoals of primitive Jellyfish move along with the currents thanks to their bells which can pulsate to acquire propulsion and blind locomotion.
Near a collection of primitive coral, some basic-looking, pale-colored, 1.3-foot-long Trilobites are feeding from the sandy floor. These are Redlichia, and they are one of the countless Trilobite genera that exist and that are still to come. The headshield (or cephalon) is a semicircular, about ⅓× as long as the body, with clear genal spines that are a smooth continuation of the border, that extend backward and outward and curving to be near parallel near their tips, which typically extend to the back half of the articulated thorax.
Foraging alongside them are some Hallucigenia. This is a 0.5 to 3.5 centimeter long tubular Animal with seven or eight pairs of slender legs (depending on the species), each terminating with a pair of claws. Above each leg is a rigid conical spine. The elongated head end extends some distance beyond the legs and often droops down to reach the substrate. The extended head has two simple eyes, a mouth with radial teeth, and pharyngeal teeth within the front of the gut. Interestingly, these are near relatives of both Arthropods and the yet to emerge Velvet Worms.
All of a sudden, a dark shadow passes overhead. The Hallucigenia are protected by their long spines from almost any attack from above. Lucky for them, though, this bigger Creatures has its sights set on the Redlichia, literally zeroing in with, on his large head, a single pair of large, compound eyes on stalks comprising approximately 16,000 lenses – many sorts of predators and prey have developed eyes. This is an Anomalocaris, and this male snatches one of the unsuspecting Trilobites with his claw-like great appendages, crunching it to pieces and passing the remains to his disk-like mouth.
This predator propels itself through the water by undulating the flexible lobes on the sides of its body. Each lobe slopes below the one more posterior to it, and this overlapping allows the lobes on each side of the body to act as a single “fin”, maximizing the swimming efficiency. This mode of swimming is intrinsically stable, so Anomalocaris does not need a complex brain to manage balance while swimming. The body is widest between the third and fifth lobe and narrows towards the tail.
The mouth of Anomalocaris is composed of 32 overlapping plates, four large and 28 small, resembling a Pineapple fruit ring with the center replaced by a series of serrated prongs. The mouth can constrict to crush prey, but never completely close, and the tooth-like prongs continue down the walls of the gullet. The two large great appendances (up to 18 centimeters in length when extended) with barb-like spikes are in front of the mouth. The tail is large and fan-shaped, and along with undulations of the lobes is used to propel the Creature through Cambrian waters. During the Cambrian, Anomalocaris is gigantic, reaching up to 1 meter long. This apex predator is part of a group closely related to the Arthropods, also possessing exoskeletons, called the Dinocaridids.
After feeding, the Anomalocaris goes back to patrolling the area. This section of ground near a collection of tall boulders is his territory, and competition for spaces like this with abundant amounts of prey can be fierce. Just then, another male Anomalocaris appear. This rival immediately challenges the resident for the territory, first by head-butting. They then start swimming in tight circles around each other in order to overwhelm the other and to quickly give a painful “go away” bite with their great appendages. The resident male eventually bites hard into the bottom of his opponent’s head with his great appendages and makes the rival begin to bleed dark blood. The wounded intruder is forced to leave.
Even though exoskeletons are generally tough, they can also be rigid and can be split if bent too far. This leaves the defeated Anomalocaris a small bit vulnerable to pesky opportunists that might nibbles at his new injury, as well as the other older injuries he already possesses. Already, he is being followed by a large swarm, somewhat larger than the Anomalocaris, of little soft-bodied brown Creatures. These tiny narrow swimmers, about 0.98 inches in length, are Haikouichthys. These very early forms of Agnathan Vertebrate. The Animal has a distinct head and tail. The head has six gills. There are a number of segments (myomeres) with rear directed chevrons in the tail. This little Creature has a notochord within the body, instead of possessing an external exoskeleton, thus being tough on the inside. There is a prominent dorsal fin with fin radials. The fin radials angle “forward” toward the end. Haikouichthys, along with its close coexisting relatives (the 30 to 40 millimeter long Haikouella, the 28-millimeter-long Myllokunmingia, and the elongated Zhongjianichthys), are among the first Animals to possess an internal structure, a notochord, which is on the path towards the spines of the future.
The Haikouichthys begin to nibble at the big Anomalocaris’s injuries and dart frequently when the invertebrate twitches from feeling them. Although they lack pectoral fins, their flexible notochords make them darters, and certainly more maneuverable than a spineless Dinocaridid.

To Be Continued