The oldest animal-built reef was discovered in southern Namibia, in a region known for its ancient sediments.
The first known coral reef off the coast of Iraq has just been discovered, according to a paper in the journal Scientific Reports. The conditions at the site -- the mouth of the Shatt al-Arab River in southeastern Iraq -- are surprisingly turbulent and chilly for a reef, with seawater temperatures often in the 50s (Fahrenheit). "We were entirely surprised to find a living coral reef under such harsh conditions," lead author Thomas Pohl of Germany's Institute for Geology Scientific Diving Center and his colleagues wrote. They added that the waters are often polluted with oil and are sediment loaded, making the discovery all the more surprising.Best Ocean Animal Photos of 2013
From a distance, this brown coral might look like a crater, but it's very much alive. Coral reefs are thought to be environmentally sensitive marine ecosystems. They tend to develop in sites with clear water, ambient temperature, moderate salinity and strong wave action. This reef, named the Palinurus Rock Reef, clearly falls at the edge of that spectrum, given the harsher conditions.PHOTOS: New Japanese Island Forming In Pacific Ocean
Political instability, affecting scientific exploration, likely hindered discovery of the reef. The water's low visibility also didn't help. Pohl and his team struggled to study the reef -- measuring 2.5 by 4.4 miles -- and capture images of it, given the challenges.PHOTOS: Steampunk Artist Transforms Ocean Trash
The authors identified a number of living stony corals and octocorals (which lack a stony skeleton), as well as sponges and aquatic mollusks that may compete with the corals for space on the reef -- or that may cause the coral structure to erode.PHOTOS: Oceans Could Power Our Homes
Despite the intense competition between species at the reef, the closeness between some organisms is very evident. Here, colorful marine animals known as brittle stars intertwine like snakes around octocorals.PHOTOS: Life on the Ocean Floor Garbage Patch
Sediment, light levels, sponges and the coral itself give much of the scene a greenish hue. Four of the coral groups identified at the reef are slow-growing, massive species that are robust enough to develop under the harsh environmental conditions.PHOTOS: Going Blue: Ocean Landscapes Need Protection
Brittle stars typically have five thin, segmented arms extending from a central disc. They are called "brittle" because, when disturbed, they can cast off parts of their arms, leaving predators hunting them completely befuddled. Related animals called serpent stars and basket stars are also found at the Iraqi reef.PHOTOS: James Cameron Explores the Mariana Trench
Brittle stars have a mouth, but no anus. They feed, usually at night, on small organic particles. Sea stars at reefs may exhibit an array of different colors and body patterns.BLOG: World's Most Dangerous Oceans Identified
Although underwater visibility is often limited to 3 feet or less at the reef, you can find fish in the Shatt al-Arab river. The river is the only continuous freshwater source in the region.PHOTOS: Small World Under the Sea
Green-colored sponges are well distributed within the Palinurus Rock coral reef system.PHOTOS: Tiny People Swim With the Fishes
Mounds of coral within the reef sometimes take on mammal-like shapes. Like gazing at clouds, interpreting what those shapes may resemble is up to the viewer.PHOTOS: Life in a Drop of Water
The discovery of this unique coral reef could provoke the interest of the international scientific community working to understand coral ecosystems and climate. These habitats urgently need protection, conservation and research, especially given their location in areas of oil and gas exploration, the authors conclude.VIDEO: Why We Sent Jellyfish to Space
An ancient reef that once teemed with primitive sea life has been unearthed in Africa.
The reef, which dates to 548 million years ago, is the oldest animal-built reef ever found.
The coral-like creatures, dubbed Cloudina, may have built the superstructures to protect themselves from predators or to soak up the nutrients from ocean currents, said study co-author Rachel Wood, a geologist at the University of Edinburgh in Scotland. (See Images of the Ancient Reef Fossils)
During the Ediacaran Period, which lasted from about 635 million to 542 million years ago, all life lived in the sea, and most creatures were immobile and soft-bodied, with mysterious wavy, frondlike shapes.
But in the 1970s, scientists discovered evidence of Cloudina, the earliest fossil animals to have skeletons. The pencil-shaped sea creature could grow to about 5.9 inches (15 centimeters) long. A cross-section of the tubular shape shows that it would've been about 0.3 inches (8 millimeters) in diameter, Wood said.
"It's like a series of hollow ice-cream cones all stacked up," Wood told Live Science, referring to the appearance of the Cloudina skeleton. "It might have been related to corals and anemones and jellyfish."
Like modern-day corals, the youngest cone in the stack would have been alive, while the rest would be dead, Wood said.
But scientists knew little about how these enigmatic creatures lived.
Late last year, while excavating in Namibia in a region known for Ediacaran fossils, Wood and her colleagues found evidence for a vast network of reefs built by Cloudina about 548 million years ago. Like modern-day corals, the primeval creatures excreted calcium carbonate, which cemented them to each other and helped grow the reef.
The new finds are the oldest animal-built reefs ever discovered. Previously, the oldest animal-built reefs dated to around 530 million years ago, during the Cambrian Period, when the complexity and diversity of life on Earth exploded. (Slimy bacterial communities known as stromatolites have built vast, limestone reefs for almost 3 billion years, Wood said.)
The ancient Cloudina reefs the team discovered grew in patches atop a massive stromatolite-formed reef complex that spans nearly 4.3 miles (7 kilometers), Wood said.
"If you were snorkeling over it nearly 550 million years ago, you'd see areas of green surface, from stromatolites, and then you'd see these little patches of tubes all growing together, forming a little thicket, or mound, on the seafloor," Wood said.
Cloudina likely formed reefs to protect itself from predators. In China, for instance, scientists have unearthed Cloudina fossils with holes drilled in them, likely from acid secreted by a predator animal, Wood said.
Clumping together on reefs would have also brought nutrient-rich currents close to the filter feeders at a time when more life forms were competing for space and food, the authors said.
"All together it paints a picture of quite significant ecological complexity," Wood said.
The fossils were described today (June 26) in the journal Science.
More from LiveScience:
Colorful Creations: Incredible Coral
Cambrian Creatures Gallery: Photos of Primitive Sea Life
Extreme Life on Earth: 8 Bizarre Creatures