Biologists attempting to conserve and restore eroded underwater environments are limited by their scant knowledge of what those habitats looked like before humans arrived. For decades, scientists have had to rely on the fossils of one particular group of animals — mollusks, which are found in plentiful abundance — for clues.
But a new study published today in the journal PeerJ has confirmed a reliable analogue of modern marine ecosystems that lie just beneath the surface. The results suggest that fossils from a wide range of marine groups — including worms, mollusks, crabs, and sea urchins — are preserved at nearly the same proportions of abundance and diversity as their living counterparts. This provides scientists with a more complete catalog of fossilized remains to work with when researching the oceans’ depths.
The findings are significant, researchers said, because the ability to compare ancient and modern ecosystems gives scientists greater insight into the specific ways that a marine habitat's history might influence future conservation strategies. Diversity in the fossil record also helps pinpoint the specific sustainability needs unique to different regions.
“Most of what we know, in terms of biases in the fossil record, is based on mollusks,” said lead author Carrie Tyler, a marine conservation paleobiologist and professor of geoscience at UNLV. “We designed our study to determine whether those biases are consistent when you include many types of organisms, not just mollusks. Is the fossil record still reliable when you have worms and sea urchins and all other groups in a marine ecosystem?”
Marine Sea, Marine Do
Fossils are a partial and imperfect record of Earth’s past. Organisms made primarily of soft tissue are less likely to be preserved than those with hard, durable parts that are resistant to decay, such as bones and shells. These parts have varying degrees of thickness and strength depending on the organism and stage of development, which also affects the likelihood that they’ll be preserved.
To get around this problem, researchers have used mollusks as a proxy for the overall health of ecosystems. Mollusks are particularly well-represented in the fossil record, and previous research shows they are faithful indicators of past diversity. Using fossils and historical records, researchers in Europe recently demonstrated the native molluscan biodiversity of the eastern Mediterranean Sea has almost entirely collapsed due to global warming. This finding likely means that other marine groups in the region are nearing similar thresholds.
Like a doctor taking a patient’s vital signs, scientists can use fossil mollusks to broadly infer the health and stability of an environment. But to distinguish patterns within population declines, shifting ranges and the introduction of invasive species, a full checkup is required.
The Fossil Findings
To jumpstart their examination, Tyler and study co-author Michal Kowalewski, the Florida Museum Thompson chair of invertebrate paleontology, first had to find a suitable marine ecosystem in which to compare living and fossil organisms and study the discrepancies between past and present communities.
The authors settled on a comparatively unaltered environment off the coast of North Carolina that contained both living animals and dead skeletal remains. There, they collected samples from 52 localities along transects that extended from near-shore coastal waters out to sea.
“We chose this system because it included a spectrum of habitats along an onshore-offshore gradient, from estuary to open shelf,” Kowalewski said. Each of the habitats supports specialized communities, which allowed Tyler and Kowalewski to test the preservation potential for a host of disparate organisms and environmental conditions.
Over the course of two years, they counted more than 60,000 living and dead specimens representing hundreds of marine invertebrates. As expected, the thick shells of mollusks resulted in an overabundance of their remains in the fossil record compared with other groups. However, the fragments of dead corals, sand dollars, tube-forming worms, and other non-mollusks were broadly represented at the same level of abundance and diversity as their living counterparts.
Groups with lower existing diversity in the region, such as sea stars and brachiopods, weren’t recovered from the fossil record due, in part, to their low numbers. In many cases, past and present habitats were also dominated by different species — a type of hermit crab common today didn’t show up in the fossil record, for example. But the overall number of species in different groups remained consistent.
Most marine ecosystems lack anything near a complete inventory of the species that inhabit them, and the existing roster is dwindling as some species decline in abundance and others succumb to extinction. But if other marine ecosystems are archived with the same fidelity as those in North Carolina, researchers will have a new baseline with which to evaluate the long-term viability of the communities they support.
“We can use the whole fossil assemblage as a picture into the past for a particular place despite differences in preservation among animals. By comparing it to the living community, we can see how much an ecosystem has changed and decide on the best conservation strategies based on those changes,” Tyler said.