During the Ediacaran period (635–542 MYA), life evolved more diverse and sophisticated cell chemistry, including genetic mechanisms which would be conserved through time. At the cellular level, some organisms had novelties that straddled classification. Exotic single-celled microorganisms emerged – neither bacterial nor animal – that had features like fungi and multicellular algae. The Ediacaran was a period of fundamental experimentation in life form.
Complex animals got their start millions of years before the Cambrian, during the Ediacaran. ~ American paleontologist Rachel Wood
Life forms were microscopic prior to the Ediacaran. That changed as size became a way to spread.
Rangeomorphs were a frondose animal, standing tall while attached to the seafloor by a holdfast, though some lay flat. Rangeomorphs were among the earliest large organisms: some grew to 2 meters tall, though others were only a few centimeters. Their soft bodies were fractal branches unlike any other complex macrobe.
Rangeomorphs could rapidly shape-shift: growing into a long, tapered column if the seawater above them had an elevated oxygen level or food flow. As filter feeders, rangeomorph frond size and shape were responsive to nutrient availability and uptake.
Rangeomorphs sent out waterborne propagules to reproduce in distant areas. Plants later imitated this technology with stolons.
Rangeomorphs were adaptively optimal for the low-competition, high-nutrient conditions of Ediacaran oceans. With the Cambrian explosion in animal diversity (from 541 MYA), fundamental changes in ecological and geochemical conditions led to their extinction. ~ English paleobiologists Jennifer Hoyal Cuthill & Simon Conway Morris
Dickinsonia were another Ediacaran marine oddity. Ranging from a few millimeters to 1.4 meters, Dickinsonia were a bilaterally symmetrical, ribbed, oval pancake. They grew by adding new rib-like segments.
Classifying Dickinsonia has been problematic. Proposals include that they were fungi, or some extinct kingdom. In 2018 paleontologists discovered that Dickinsonia produced cholesterol, “a hallmark of animals.”
Most the marine life during the Ediacaran were sessile: living quiet lives attached to the seabed. Mats of cyanobacteria covered the sea floor along continental shelves during the Ediacaran.
Then soft-bodied sea worms arose that were the start of big trouble. The oceans became starved of oxygen (anoxia). Animals began burrowing in search of O2. They tore into the microbial mats which were relatively oxygen rich. The metazoan rampage altered the seabeds, killing off organisms dependent upon the mats. This bioturbation by early burrowers had an outsized outcome in initiating a negative feedback loop, beginning with increasing oceanic phosphorus burial from critters in the benthic sediment.
The decrease in marine phosphate level lessened organic carbon burial and lowered oceanic oxygen. This only spurred the burrowers on, exacerbating the situation and leading to the extinction event that ended the Ediacaran.
Consensus opinion has long been that all Ediacaran life was oceanic. Based upon analyses of soil crusts (paleosols) and fossil finds, American geologist Gregory Retallack discovered in 2013 that fungi, lichen, and microbial colonies lived on land by the end of the Ediacaran.
Tiny bilaterian animals evolved 650 MYA. (The earliest multicellular animals were as long as a human hair is wide.) Bilateria have a longitudinal plane of symmetry, and specialized internal organ systems. The bilaterian gut can digest more food. This architecture was ideal for adaptively scaling-up body size.
The fossil record of the Ediacaran is scant, as it was during this time that animals were developing the hard shells which would begat fossils. Exoskeletons became possible because an increasing abundance of calcium became available in seawater: an output from oceanic volcanic activity. Animals with complex skeletons evolved by 550 MYA.
Oxygen was like a slow fuse to the explosion of animal life. ~ American Earth scientist David Catling
Increased atmospheric oxygen afforded the diversification of animal life that characterized the Cambrian. The oxygen increase came by burying decaying plant matter in sediment. Burial removes the carbon from Earth’s surface, preventing it from bonding with molecular oxygen in the atmosphere.
The evolution of metazoans, and their subsequent diversification during the Cambrian explosion, was caused by the ability of the continents to accumulate and store sedimentary organic carbon. ~ American geologists Shanan Peters & Jon Husson
Organic sedimentation accelerated as the short-lived supercontinent Pannotia splintered ~550 MYA. This triggered a spreading of shallow ocean water across the continents, releasing calcium which allowed animals to build hard shells; whence the Cambrian explosion.