500 million years of errors: Brachiopod shells record shadow of arms race in ancient oceans
Blacksburg, Va. - A study of fossils from the Paleozoic Era, collected across the world, reveals that ancient brachiopods were little bothered by predators. However, the rare predation traces left on brachiopod shells by unknown assailants coupled with a subtle increase in their frequency through time may be the shadows on the wall that show killers were in the room and their numbers increased with time.
From 550 million years ago until 250 million years ago, brachiopods, or "lampshells," were plentiful in the earth's oceans. Today these shelly creatures that superficially resemble clams are rare, mostly lingering in cryptic habitats and subpolar regions of the oceans. But the fossil record, generously endowed with lampshells of diverse shapes, offers an impressive testament to the bygone glory of brachiopods.
A report in the June 17, 2005, issue of Science ("Secondary Evolutionary Escalation between Brachiopods and Enemies of Other Prey") by Michal Kowalewski and Alan Hoffmeister of the Virginia Tech Department of Geosciences, Tomasz Baumiller of the University of Michigan Museum of Paleontology, and Richard Bambach, Virginia Tech professor emeritus of paleontology now at the Harvard University Botanical Museum, indicates that attacks on Paleozoic brachiopods, as recorded by round drill holes bored in prey shells by some unknown drilling attackers, were rare but widespread and continuously present through the entire Paleozoic Era.
The researchers report that the frequency of drilled victims went up slightly in the mid to late Paleozoic, but these late Paleozoic attacks were still less frequent when compared to drill holes found in shells of brachiopods dredged from modern ocean floors. "These subtle increases in drill holes on brachiopods, from less than 1 percent in the early Paleozoic era to several percent today suggest that predators have become increasingly abundant and active through history of life," said Kowalewski, associate professor of geobiology. "This is consistent with previous studies of other fossil organisms that suggest that marine ecosystems escalated and have become an increasingly hostile theatre of survival."
However, even modern brachiopods are drilled at very low frequencies when compared to most common shellfish of today such as clams and snails (mollusks). "It is not uncommon in modern seas that 25 percent or more of mollusks are done in by voracious drilling killers such as whelks and moon snails," Kowalewski said.
With funding from the National Science Foundation, Kowalewski and his colleagues looked at the evidence of predation on brachiopods through the entire Paleozoic Era - the period from 543 million to 248 million years ago. Brachiopods were a common shellfish up until the mass extinction at the end of the Paleozoic that may have killed as much as 95 percent of all marine life. The rich Paleozoic fossil record of brachiopods yielded ancient shells that retained traces of predation, in particular, drillings by predators and parasites, offering researchers quantifiable record of ecological interactions between brachiopod victims and their attacker. While many previous researchers studied drilled brachiopods at numerous sites, and there have also been extensive studies on mollusks in the more recent fossil record, this is the first comprehensive study of predation on brachiopods across the entire Paleozoic Era.
"The rarity of drill holes in brachiopods shows they were, most likely, the victims of mistaken identity or opportunistic attack when preferred menu items were absent," said Kowalewski. "There was a modest increase in frequency of drill holes in middle-to-late Paleozoic times and then another increase sometime after the end of the Paleozoic Era," he said.
"These increases are consistent with macroevolutionary models postulating two major intervals of increased ecospace utilization and escalating predation pressures: the mid-Paleozoic Marine Revolution followed by the Mesozoic Marine Revolution," the researchers write in Science.
"During this marine revolution, the basic food supply of the oceans - phytoplankton - increased significantly, organisms up the food chain became more meaty, and predators - bony fish, snails, and crustaceans - increased in frequency and diversified," Kowalewski said. This evolutionary "escalation" started in the Late Mesozoic led to the predator heyday we observe in modern oceans, with drilling predators being just one of many carnivorous guilds that feed on shellfish. "As predators became specialized in attacking their shell-protected victims, prey groups also diversified and became better armored. Some invertebrates, like sand dollars and many types of clams, invaded soft substrates by burrowing underneath the ocean floor," Kowalewski said. "However, they may have not only been seeking refuge from predators, but also pursuing their own dietary needs by exploring the deeper layers of substrate that became enriched with nutrients as phytoplankton productivity increased in the oceans."
The fact that instances of drill holes creep up only slightly in brachiopods shows that drilling predators were never interested in eating brachiopods. But, as ecosystems became increasingly competitive, the predators were forced to go after less desirable prey more often, expending energy to drill the less nutritious, metabolically slower, and perhaps less palatable brachiopods, according to the Science article.
"Elevated competition, higher food demands of metabolically more active faunas, higher failure rates in attacks on more active or better defended prey, and increased predation pressures on drillers themselves could all have contributed to more frequent incidences of opportunistic and mistaken attacks on brachiopods," the researchers write.
Being unattractive to predators did not help brachiopods to maintain their ecological dominance in the marine biosphere, however. Following the Permo-Triassic mass extinction, the mollusks took over the ocean floors. This status quo has persisted through today, despite the fact that mollusks continue to be the meal of choice among many drilling predators and other types of carnivorous seafood lovers.
"Most interesting, perhaps, is this evolutionary persistence of incidental interactions between brachiopods and enemies of other prey," Kowalewski said. "We tend to view long-term predator-prey interactions in terms of evolutionary arms races that can lead to escalation or coevolution between assaulters and their victims. Yet, the rare opportunistic or mistaken attacks on brachiopods are not likely to have been evolutionarily important - they may reflect more than 500 million years of menu selection errors," he said.
"Finally, the subtle increase in errors and chance attacks has an interesting corollary for the famous Red Queen Hypothesis," Kowalewski said. Over 20 years ago, Leigh Van Valen of the University of Chicago postulated that organisms must keep evolving all the time to survive in constantly changing environments. The name refers to the Red Queen in Lewis Carroll's Through the Looking Glass, who tells Alice that she must keep running simply in order to stay in the same place on a chessboard that keeps shifting beneath her feet. "It is intriguing to consider that the increase in error frequency through time may be a Red Queen phenomenon," Kowalewski said.
Virginia Tech. June 2005.
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