Twenty years and 7,500 fossils reveal what blurs Earth's marine fossil record
In productive marine environments, a square meter (10.8 square feet) of seafloor can be perforated by hundreds to thousands of isolated and interconnected tunnels through which crawl and writhe a cornucopia of clams, shrimp, ...
All that excavation mixes up the sediment, along with any shells and other skeletal remains that happen to be there. This temporal smearing is a problem for paleontologists because when that piece of seafloor is buried and becomes part of the fossil record, it's difficult and expensive to determine how much mixing took place.
"What continually amazes me is just how much time a bunch of fossils collected from a single sediment layer can represent. In some cases, well-preserved fossil organisms that are found next to each other might have lived hundreds or thousands of years apart," said Rafal Nawrot, a paleontologist at the University of Vienna.
The work is published in the journal Proceedings of the National Academy of Sciences.
The mixing of fossils that lived at different times but are preserved together is called time averaging.
According to Daniele Scarponi, a colleague of Nawrot's and an associate professor at the University of Bologna, time averaging dictates the types of questions paleontologists can ask.
"Before interpreting a fossil assemblage, we need to know the interval it represents. Some fossil assemblages are like the ruins of Pompeii—buried rapidly and thus providing a snapshot of past communities frozen in time. Others are more akin to a prehistoric graveyard used continuously over centuries, in which human remains from different generations are slowly accumulating over time. Both types can provide valuable insights into the past, but the kinds of data we can extract from them will be different in each case," Scarponi wrote.
Scientists carbon-dated more than 7,500 marine fossils from ocean beds around the world and used that data to determine which environmental factors are most responsible for mixing together fossils from different time periods in the same stratum. Credit: Kristen Grace, Florida Museum
Limestone beds like this one in Cuba demonstrate just how rich a fossil bed can be when conditions influencing their burial and preservation are just right. Credit: Florida Museum Photo, Roger Portell
Sea urchins, like this Brissus latecarinatus, roam the seafloor and comb through sediment searching for food. Credit: Florida Museum Invertebrate Zoology Photo
Chaetopterus djiboutiensis is a type of worm that lives within the sediment in U-shaped tunnels that it strengthens with a layer of mucus. Credit: Florida Museum Invertebrate Zoology Photo