Animals living more than 550 million years ago could have survived inhospitable oceans by associating with dense mounds of cyanobacteria called microbial mats, an international team of researchers argues in a new study. Such clumps of oxygen-producing gunk could have supplied the first mobile animals with food to eat and air to breathe, the group reports online May 15 in Nature Geoscience.
The animal kingdom’s vanguard would have needed all the help it could get. Recent fossil finds show that wriggling animals first emerged at least 555 million years ago, when atmospheric oxygen concentrations may have been about one-tenth what they are today. Yet as creatures moved around more, they needed more oxygen. So how early mobile critters, which probably resembled worms or slugs, eked out a living in these choked environments has been a big puzzle for paleontologists, says study coauthor Murray Gingras. “Biomats provided the oxygen that ironically enabled the animals to better exploit biomats as food,” he says.
He got his first clue after drilling into a frozen pond in Alberta, Canada. The pond, almost entirely deprived of oxygen, hosted a small number of insect larvae surrounding a layer of photosynthesizing algae. “They were eating the biological material, and they were using it as a scuba tank at the same time,” says Gingras, a paleontologist at the University of Alberta in Edmonton.
Alberta’s frozen lakes don’t look much like ancient oceans, however, so Gingras and his colleagues turned to supersalty lagoons in Venezuela. Here, gelatinous masses of cyanobacteria, a type of oxygen-producing microbes with ancient origins, clog the waters. Animals first evolved in a similar “world ruled by microorganisms,” Gingras says, in which microbial mats like these may have dominated shallow oceans across the globe. In fact, he adds, the oldest discovered fossil trace of animal life depicts the tooth marks of a long-dead creature biting into such a mound of bacteria.
The modern lagoons, like their ancient counterparts, carry few traces of oxygen. But the gas does run high — reaching near or above typical water levels — right above and below the mats, the team discovered. These lagoons usually host scant animal life but, as in Alberta, clusters of insect larvae gather around the mats, taking bites and maybe breaths. The first mobile animals easily could have done the same, Gingras says.
“In a way, it’s dead obvious,” says Mary Droser, a paleontologist at the University of California, Riverside.
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