Just How Old Are Deep-Sea Creatures?
Little has been known about the evolution and overall age of deep-sea creatures due to the absence of sediments that could establish a long history of fossilized remains. There have been a few cores of deep-sea sediments that contain some benthic life, but they have been assumed to almost never contain larger fauna. Recent findings in sediment pulled from the Ocean Drilling Program in the western tropical Atlantic show that deep-sea ocean life is probably older than previously known.
This team found echinoderm fossils, which are marine life that lack backbones. Some examples that you may be familiar with are sea cucumbers and starfish. Samples of deep-sea sediment cores were chosen based on the depth and preservation of the microfossils. Once the 74 cores were chosen, they were treated and washed over a screen so as to liberate the fossils. The measure of abundance was then determined by comparison to various references from different locations and depths of the ocean. This allowed for a holistic view of previous and current echinoderm abundance in the ocean, further allowing for a determination of whether or not the abundance has changed.
Previous to these findings, it was believed that the deep-sea was repeatedly re-colonized by shallow-water life. However, the species found by the study are drastically different from the shallow-water communities that we would expect to see similarities with today. Instead of aligning with this re-colonization theory, this new evidence suggests that the composition of deep-sea fauna has mostly remained the same since the Early Cretaceous time period. This was about 145-100 million years ago.
This discovery has many implications, one being that many historical oceanographic events didn’t have as much of an impact on deep-sea life as paleontologists have inferred. This then points to a resiliency of this deep-sea life, which is stronger than that of shallow-water life. The deep-sea environment most likely gives evolutionary stability to its inhabitants, hence why there are so many similarities between the fossilized remains found and the fauna that we find today. Specifically, the echinothurioids and histocidarids that were found are older than the oldest shallow-water forms. This implies that their evolution largely took place in the deep-sea environment. The resiliency of these echinoderms that were discovered was most likely due to the potential and occurrence of their wide dispersal throughout the deep-sea. This is a quality that is observed in deep-sea fauna today, and may have been what allowed these organisms to prevail despite large disturbances that occurred in the ocean.
While this discovery tells much about the deep-sea environment that was previously unknown or believed to be different, it must be stated that these findings may only apply to the Northern Atlantic where they were found. However, the notion that deep-sea and shallow-water have very different resiliencies to ocean disturbances is one that may cause the research of oceanic communities to have a different approach.
Thuy, B., Gale, A. S., Kroh, A., Kucera, M., Numberger-Thuy, L. D., Reich, M., & Stöhr, S. (2012). Ancient origin of the modern deep-sea fauna. PLoS ONE, 7(10). https://doi.org/10.1371/journal.pone.0046913