Fossil Mix-Up Could Rewrite the History of Beetles, the Largest Group of Animals on Earth

Science
دوشنبه 18 شهریور 1398 ساعت 21:41
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Smith sonian mag
The most diverse family of animals on the planet just got a little bit younger due to a possible mix-up in the fossil record.
Rove beetles have more than 60,000 living species. The beetles are spread all over the world and have adapted to a great number of ecological niches. The only problem is that Leehermania prorova —the presumed oldest known member of the species—may not be a rove beetle at all.
“It’s not related to that group of beetles at all but related to a smaller group of beetles,” says Martin Fikáček , a collection curator and researcher in the insect department at the Czech National Museum in Prague and a co-author of a study about the beetle published today in Systematic Entomology .
If Leehermania prorova is not a rove beetle, but rather a beetle related to the Myxophaga suborder as Fikáček and his group believe, it could mean the four beetle suborders we know today are much older than previously believed. Since insects represent the largest animal class (72 percent of all known animal species, according to the Catalogue of Life ), and beetles represent about 35 percent of all known insects (and 25 percent of all known animals, with about 400,000 named species), a better knowledge of beetles can help us understand some of the fundamental processes of evolution.
“It’s an important question to get this right, because this is in fact not just the largest beetle family but the largest family in the animal kingdom,” says Joe Parker, an assistant professor in biology at the California Institute of Technology. “So understanding how old it is and how it’s diversified into so many species is really a key piece of information for understanding animal evolution in general.”
Leehermania prorova is about 226 million years old, but the species first came to the attention of scientists after fossilized specimens about as big as the width of a nickel were found along the Virginia-North Carolina border in the 1990s. Photos circulated for a couple decades, exciting beetle biologists, but it wasn’t until 2012 that a study described the species as the oldest known rove beetle.
This claim raised eyebrows among some beetle researchers around the world, and a collective dialogue opened up about the proper taxonomy of Leehermania prorova , lasting for several years between Fikáček and beetle researchers in China, Japan, Europe, Australia and the Field Museum of Natural History in Chicago. The new study is based on this collective effort, making the case that Leehermania prorova was not a rove beetle at all, but something more closely related to a skiff beetle.
A Modern Myxophaga beetle that the researchers compared to the fossil beetle to in order to figure out what family the ancient beetle belonged to. (Martin Fikáček et al.)
Skiff beetles belong to Myxophaga, one of the other four extant suborders of beetles along with Polyphaga, the suborder which includes rove beetles. In contrast with the latter order, Myxophaga is a relatively small suborder with only a few dozen species. The insects of this suborder are also tiny—most skiff beetles are about 1 millimeter in size.
Leehermania prorova is only about 2 millimeters in size as well. Other characteristics are difficult to tell from the fossil, but Fikáček says that one clue is what the 2012 authors believed were visible mandibles, or lower jaws, characteristic of rove beetles. The recent study, however, identifies these features as appendages called maxillary palpi.
“This is basically a small segmented leg-like thing next to the mouth which helps the insect manipulate food,” Fikáček says. Skiff beetles and others from their order have hidden mandibles, since the mostly aquatic beetles feed on things like algae. If the “mandibles” of Leehermania prorova were really more leg-like, it could mean the species’ actual mandibles were hidden as well.
Furthermore, the antenna on Leehermania prorova didn’t quite look like those of a rove beetle, and it lacked certain features on its abdomen usually present in rove beetles.
“You have to be careful not to misinterpret things,” Fikáček says, adding that another group of Myxophaga also looks a lot like rove beetles. “People make mistakes about fossils quite frequently because it’s not easy.”
The authors of the 2012 did not respond to a request for comment on the new research, but Parker, who was not involved in either study, believes the evidence more strongly favors the recent paper’s placement of Leehermania prorova in the Myxophaga group.
“This fossil has been contentious since its description in 2012,” he says.
Adam Brunke, an entomologist who studies beetles at the Canadian National Collection of Insects, Arachnids and Nematodes in Ottawa who was not involved in either study, says that this research represents the “gold standard in modern paleontology” because it’s a synthesis of all available evidence from both living and extinct species. He says that Fikáček and his co-authors’ conclusion is “robustly supported,” and that their arguments are more compelling than those of the 2012 study.
“This is not to say that the story is finished, new fossils that are better preserved or show different features could reveal that [the beetle species] belongs somewhere else in the Myxophaga tree,” Brunke says via email. The specimen of Leehermania prorova is an important fossil due to the paucity of such samples that have survived the eons.
“Fossils of proper beetles (there are many fossils of ‘proto-beetles’ in Permian and Triassic deposits) from the Triassic are very rare, and Leehermania , regardless of its systematic placement, is one of our only glimpses of what beetles were around right after the end Permian mass extinction,” Brunke says.
Any fossil only gives researchers the minimum age rather than the maximum age of a species. But the new classification could mean that the age of all beetles is pushed back. When researchers thought the species was a unique ancestor of the rove beetle, it hinted at the time period when these beetles evolved. But if the species is from the skiff beetles’ suborder, it would push back the origin of that group by about 100 million years. Rove beetles, on the other hand, could be about 50 million years younger than previously estimated.
Leehermania prorova doesn’t look as different from modern skiff beetles as researchers expect an ancient ancestor to look, Fikáček says, which “means that the real ancestors of the whole Myxophaga had to be older, quite likely much older actually.” If Leehermania prorova is a skiff beetle, it may provide evidence against a previous theory that the modern beetle orders evolved after the mass extinction at the end of the Permian about 250 million years ago.
“ Leehermania is the first opening of the black box,” Fikáček says in an email.
As far as explaining the evolutionary success of rove beetles, Fikáček and Parker say that the evidence that they are younger—about 176 million years old—means that the most successful family of animals would have diversified rather quickly. Their soft abdomens, which contrast with other beetle species, may have allowed them to move through leaf litter and get into niche habitats such as the burrows of other animals, tree bark and a number of other places to find food and hide from predators.
“One of the ideas that people have is that this actually helped the rove beetles to inhabit and use space and niches which few other insects used at that moment,” Fikáček says.
Rove beetles may still be as old as thought, even if Leehermania isn’t one. New fossils will have to fill in the incomplete picture of entomological evolution. But regardless of their age, rove beetles are certainly one of the most successful groups of animals to crawl the Earth.
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