Dartmouth professor Justin Strauss (right) and University of Washington professor Akshay Mehra examine an exposed ancient reef high in Canada’s Mackenzie Mountains for fossil samples. The now-fossilized reef formed about 850 million years ago and was once under a vast sea that covered western North America. (Photo by Robert Gill)

The organic residue of ancient organisms such as the fossil Tawuia dalensis can leave impressions in fine-grained rocks like shale or mudstone that are known as carbonaceous compressions. Though researchers are not certain what T. dalensis was, it has been suggested that they were a large species of algae or a primitive organism with an advanced cell structure. (Photo by Robert Gill)

Earth sciences professor Justin Strauss loads the helicopter that will fly the researchers and their equipment to a remote field site in the Mackenzie Mountains near the border of the Northwest Territories and Yukon in western Canada, where they spent two weeks in the wilderness. (Photo by Robert Gill)

The research team camps in the valley between the soaring jagged peaks of the surrounding Mackenzie Mountains. Once the bottom of an ancient sea, the mountains were raised by the collision of Earth’s plates over hundreds of millions of years and are part of the Canadian Rocky Mountain range.  (Photo by Robert Gill)

Professors Justin Strauss (right) and Akshay Mehra, who was a Neukom Postdoctoral Fellow in Strauss’ research group at Dartmouth, plan their daily hunt for potential sponge fossils.  (Photo by Robert Gill)

Phoebe Cohen (right), professor of geosciences at Williams College, plans a safe route over the rugged terrain to the reefs with Dartmouth professor Justin Strauss and the team. University of Washington professor Akshay Mehra, Strauss, and Cohen co-led the project, which was funded by a grant from the National Science Foundation. (Photo by Robert Gill)

Professor Justin Strauss treks with Meghan King (left), a postdoctoral scholar at the University of Washington, and Caroline Nadalin (foreground), a sophomore at Williams College, to a fossilized reef. The researchers climbed 1,000 to 3,000 feet up steep slopes to access the reefs from their base camp. (Photo by Robert Gill)

Williams College professor Phoebe Cohen (front) bushwhacks a path to a reef with (left to right) Ross Anderson, a senior researcher of natural history at the University of Oxford;  Oxford graduate student George Wedlake; and University of Washington professor Akshay Mehra. (Photo by Robert Gill)

Professor Justin Strauss and Williams student Caroline Nadalin collect and label the exact location of rock samples from below the reefs. Unlike today’s reefs that are built by invertebrate animals such as corals, the ancient reefs in the Mackenzie Mountains were constructed by single-celled microbes before abundant multicellular life existed. (Photo by Robert Gill)

From left, George Wedlake, Phoebe Cohen, and Caroline Nadalin examine a block of layered rock known as a stromatolite that was created by communities of microbes. The existence of sponge fossils inside these reefs would indicate the existence of animals about 100 million years earlier than the current oldest confirmed record. (Photo by Robert Gill)

Justin Strauss and Akshay Mehra take notes on the area around the field site. Strauss is leading the geochronology of the rock and sediment that constitute the fossilized reef to establish their age, while Mehra’s group is creating 3D reconstructions of the reef structures and of individual fossils to confirm the existence of sponges. (Photo by Robert Gill)

Justin Strauss, Phoebe Cohen, and Caroline Nadalin trek across a steep hillside of fragmented rock. Loose stones made navigating many of the slopes of the 7,000-foot peaks to access the researchers’ sampling and mapping locations difficult.  (Photo by Robert Gill)

Ross Anderson (left) and George Wedlake collect fossil samples from hillside shale deposits. In the months after the trip, they have been working with Williams College professor Phoebe Cohen to prepare the samples for micropaleontology, in which host rock minerals are dissolved to look for organic residues and microscopic fossils.  (Photo by Robert Gill)

Fossils of primitive sponges have previously been identified in complex formations of microbes and inorganic materials like those found in the Mackenzie Mountains reef. The sample above contains the remnant of open pores in the reef system (white areas) that may have existed between compressed layers of reef-building microbes. (Photo by Justin Strauss)

Rocks known as flat-pebble conglomerate are evidence of the vast ocean that covered the terrain that rose to form the Mackenzie Mountains. These deposits usually result from high-energy storm waves ripping up pieces of partially cemented seafloor no more than 200 feet below the ocean’s surface to create a coarse-grained sedimentary rock. They can also form when underwater rock- and mudslides carry large sediment particles down steep slopes into deeper-water basins.  (Photo by Robert Gill)

Evidence of an ancient shoreline is seen in casts of rock salt, or halite, that form when seawater evaporates. The distinct cubes result when salt crystals growing in shallow water are quickly buried in mud or sand. Although the salt ultimately dissolves, the sediment encasing the crystals hardens into a stone mold of the underlying mineral. (Photo by Justin Strauss)

From right, Phoebe Cohen, Ross Anderson, Caroline Nadalin,  George Wedlake, and Justin Strauss hike down from the reef with backpacks full of rock and fossil samples. The researchers plan to return to the Mackenzie Mountains next summer. (Photo by Robert Gill)