The abrupt appearance of complex animal fossils in the Cambrian period, known as the Cambrian Explosion, has long puzzled paleontologists. Some scholars have proposed the 'artifact hypothesis,' suggesting that complex animals existed well before the Cambrian but simply failed to fossilize because they lacked hard parts (such as shells or exoskeletons) or because prevailing taphonomic conditions—the environmental factors conducive to fossilization—were unfavorable during the preceding Ediacaran period.
However, recent discoveries challenge this view. The Burgess Shale-type deposits from the Cambrian preserve both hard-bodied and soft-bodied organisms in exquisite detail. Crucially, similar taphonomic windows—environments featuring rapid burial in fine-grained, anoxic sediments—have now been identified in Ediacaran strata. Despite these favorable conditions, these older deposits yield an abundance of entirely different, largely immobile, quilted organisms (the Ediacaran biota) and microbial mats, but no unequivocally complex bilaterian animals.
Furthermore, the trace fossil record—the preserved evidence of animal activity, such as burrows and tracks—undergoes a dramatic shift at the Precambrian-Cambrian boundary. Ediacaran trace fossils are almost exclusively superficial, consisting of simple horizontal trails on the surface of microbial mats. In contrast, early Cambrian strata record the sudden advent of vertical burrowing, a behavior that fundamentally altered the structure of the seafloor by oxygenating the sediments and breaking up the microbial mats. This 'agronomic revolution' indicates not just a change in what organisms were preserved, but a fundamental ecological shift in how organisms interacted with their environment. If complex, mobile bilaterians had existed during the Ediacaran, we would expect to find their distinct three-dimensional burrows in those sediments, especially given that the physical conditions necessary to preserve such trace fossils were demonstrably present. The absence of these traces, combined with the presence of appropriate taphonomic windows, strongly suggests that the Cambrian Explosion represents a genuine evolutionary radiation rather than merely a change in the fossil record's fidelity.
Based on the passage, which of the following can be most reasonably inferred regarding the 'agronomic revolution'?
- AProponents of the artifact hypothesis argue that it was responsible for destroying the fossilized remains of soft-bodied animals.
- BIt was made possible by a sudden influx of atmospheric oxygen, which allowed complex bilaterians to survive and burrow deeply.
- It involved behaviors that physically dismantled the biological ground cover that had characterized the seafloor during the preceding period.Answer
- DIt completely eliminated microbial mats from the marine ecosystem, thereby permanently altering the types of fossils that could be preserved.
- EIt introduced the physical taphonomic conditions necessary to preserve three-dimensional trace fossils in fine-grained sediments.