Late Ediacaran and Early Cambrian (560-520 Ma)

Based on their behavior, the animals of phase 1 had an internal cavity and a hydrostatic skeleton, which means that their body was supported by internal pressure. [5] The animals of phase 1 probably slid across the sea floor, possibly with push-pull or peristalsis, which is a wave-like movement due to constricting and relaxing muscles. Feeding may have occurred by directly absorbing nutrients from the sea floor or by scratching and moving food into the body. DNA analysis indicates that Xenoturbella (a simple acoelomate worm) may be ancestral to the triploblastic bilaterians (protostomes and deuterostomes). It has no internal organs and no nerve cord. It has no anus. There is just a cavity in which food is processed. Organisms like this simple worm were probably the inhabitants of this period.

At the beginning of the Cambrian Period (Mangano’s Phase 2, Figure 8‑13, 540-538 Ma), there were branching burrow structures, more complex trails, and more overall disturbance of the sea floor, which indicate an increase in animal complexity and behavior. A specific type of burrowing wormhole marks the beginning of the Cambrian Period in geologic formations. It is similar to holes made by primitive priapulid worms (Figure 8‑15). Although the priapulids do not appear to be carnivorous, they wait in their burrows for slow-moving animals to walk over their holes and nab them with their teeth as the animals pass by. They are soft-bodied, which is why just the holes (trace fossils) but not the worms were preserved as fossils. These worms have a body cavity, but it is not a coelem. Scientists classify it as a hemocoel, which means a blood space.

There were many “small shelly fauna” on both sides of the Ediacaran – Cambrian boundary (542 Ma). They were not necessarily small or shelly. They were preserved when covered with phosphate. Many of them are just parts of organisms and their identification is uncertain. These were also part of the trace fossil assemblage.

During the Fortunian period of the early Cambrian (538-530 Ma, Mangano’s phase 3), there was an explosion in undermat miners.[6] Novel burrows indicate new body architectures and the evolution of many new body plans. In this case, the trace fossils indicate this divergence of body plans prior to their preservation in the fossil record. There is also evidence of predation in the form of holes drilled in organisms. Cnidarians and sponges were abundant in this phase. There is evidence of colonization of shallower and deeper waters than in the previous phases. Trace fossils indicate the presence of navigation devices (sensory systems) in animals that were tuned to find food. There are many scratch imprints, which indicate the presence of animal mechanisms that were able to disturb the surface or subsurface. There are possible indications of protostome annelid worms, but the deuterostome enteropneust (acorn worm) might also produce the same trace fossils.[7] Thus, there appear to be stem or crown group fossils of animal phyla in the Fortunian. According to Budd, the first crown group bilaterians appear in 535 Ma, protoconodonts (chaetognatha) and lophotrochozoan conchs (Halkeria), and arthropods);[8] The Halkeria appear in the Lower (older) Cambrian (535-530 Ma) fossil record (Figure 8‑16). They were small, 2 mm, “slugs in chain mail” (Figure 8‑16). They grew a shell on top (like a conch), and the lower part resembled a slug that inches along the ground.

In Cambrian Stage 2, (528 – 521 Ma, Mangano Phase 4A), there were many animal suspension feeders, which are organisms that extend a lophophore or other arms into the water and collect passing particles. The phoronids, brachiopods, are representative of this type of feeding, in which the animal buries itself in the seafloor and extends the lophophore into the water column. When necessary, the animal retracts the lophophore for protection. Animal suspension feeders, such as protostome bryozoans and brachiopods, and deuterostome pterobranches, have similar morphology.

The Archaeocyatha were reef building organisms in warm tropical waters during Cambrian Stage 2 (Figure 8‑17). They had a cavity between inner and outer shells. They might have been similar to sponges by allowing water to pass through or pushing water through pores and removing nutrients from the water. The pores are large enough to allow small plankton to pass through.

The first definitive arthropod fossil, a trilobite appears in the fossil record at 521 Ma. Primicaris larvaformis was similar to a trilobite and appeared 525 Ma. This means that the sensory, motor, and organ systems of the invertebrate animal phyla were in place by the end of Cambrian Stage 2 (521 Ma). One early trilobite group was the Redlichiidae (Figure 8-18).

An increase in oxygen might have set the stage for Cambrian Stage 3 (521 – 514 Ma, Mangano’s phase 4B) and the appearance of nearly all remaining animal phyla. A higher oxygen content was probably critical for the Cambrian explosion and the functionality of animals. Think about breathing at Mt. Everest basecamp with 50% of the oxygen at sea level and on top of Mt. Everest with 33% (supplemental oxygen required), and you can imagine the effect of increased oxygen on animal function.

There are two great fossil deposits in this period that reveal an amazing diversity and complexity of animals, the Chenjiang biota (517 Ma) and the Burgess Shale (505 Ma). The cephalochordates and the vertebrates appear in the Chengjiang biota (517 Ma). Almost all of the 35 animal phyla appeared in the sea during this period. These body plans have continued until the present time, and almost no new body plans have arisen since then, just variations on the Cambrian body plans.