(CNN) -- Finding fossils can be a fact of life for construction crews excavating in California. That's what happened when crews broke ground to begin the new Bay Area Calaveras Dam in 2013. They just didn't expect to find so many.
The existing 93-year-old Calaveras Dam stands only about a thousand feet from the Calaveras Fault, a proximity that prompted earthquake safety concerns.
The dam impounds the Calaveras Reservoir, which holds 40% of the area's water supply capacity. It's the largest Bay Area reservoir, said Betsy Lauppe Rhodes, regional communications manager for the Hetch Hetchy Regional Water System.
With 2.7 million Bay Area customers relying on its water, the stability of the dam is critical. After careful study, a decision was made to rebuild the dam completely next to its existing location, Rhodes said in an email.
The dam's excavation required moving 10 million cubic yards of rock and soil. During initial project planning, shell fossils were noticed at the site, she said.
"Because of this, the project team included a paleontologist who would monitor excavations and document and preserve anything we found," Rhodes said.
"What we were not expecting was this many fossils, of this variety. That was a complete surprise."
The construction workers were trained about what to look out for and instructed to cease work and alert the site paleontologist if they saw anything out of the ordinary. The paleontologist would then mark the fossil's location using GPS and remove it in a block of rock and dirt, sometimes with a plaster jacket around it to protect it during transportation.
The paleontologist for this site was probably busier than expected: It proved to be home to a treasure trove of fossils revealing what life was like in the area 15 million to 20 million years ago, and the most complete collection of fossils found in the Bay Area for more than 50 years. A combination of plant and animal fossils gives scientists a very clear picture of what conditions in an area were once like.
To ensure that the collection remained as intact as possible, the team reached out to regional institutions to see who could take on such a vast collection. Rhodes said that fossils found during construction on public or government land must by law be preserved and cared for by an official repository.
The University of California Museum of Paleontology, at the University of California, Berkeley campus, stepped up to the challenge. The school spent $500,000 to reopen a fossil prep lab. "UCMP has been a great partner in that endeavor," Rhodes said.
"They have assembled a tremendous team and lab to prepare and categorize the fossils and make them available for future generations."
Among the finds were numerous fossilized palm trees and pine cones, hundreds of invertebrates including snails and crabs, shark teeth and whale skulls. There was also evidence of a previously unknown species of fossilized baleen whale. As the researchers continue their work, they expect to find more new species.
They have upwards of 20 whale skulls, each about 3 feet long. This is highly unusual for a "salvage" project, in which scientists try to excavate fossils from an active construction site that may be damaged.
"Thus far we have made significant progress on five complete skulls, and quite a few individual bones," Cristina Robins, senior museum scientist at the museum and head of the the project, wrote in an email.
"There are individual teeth from Desmostylus [a hippo-like creature] and seal. We have evidence for 4 different baleen whale species, and at least 2 toothed whale [dolphin or orca-like] species. Our largest whale is actually the most complete -- we have a 5-foot skull and 17 vertebra, plus some ribs," she wrote.
Robins said she was surprised by the quality, as well as the quantity, of the fossils.
Although the small invertebrates may seem less exciting, they help complete a time capsule of what life was like millions of years ago in what is now the Bay Area, especially the paleo-environment and climate, according to Robins.
"These are the first fossils ever collected from this particular part of the East Bay, and it has turned out to be one of the richest sites for marine mammals in northern California," she said.
"It is the first time that we have so many individuals of the same species of fossil whales from the same site. It is rare to find vertebrate, invertebrate and plant fossils that have been scientifically collected from the same rock units, so it will allow us to reconstruct the past environments with a level of detail that is very unusual for any site, and especially for ones on a construction site. Additionally, the plant fossils are terrestrial -- palm trees and pine trees -- preserved with the marine fossils. This shows us that the coastline was not far away."
Water covered much of the area millions of years ago. Where people live and work now, whales roamed over modern Berkeley and Oakland, and giant megalodon sharks were chasing prey in San Jose.
Desmostylus would have waded along a coastline that was decorated with palm and pine trees, while giant seals were splashing in the water.
Cleaning, preparing and studying this number of fossils takes time. The project will end in July 2019, so the researchers are documenting what they can find. They will do all they can until then, and that's when whatever is left will be open and available for others to study.
"We are really just beginning to understand the scientific significance of the finds," Robins said.
"This collection adds significantly to our knowledge of the paleontology of California from the Miocene -- about 15-20 million years ago. The quantity and quality of the fossils is extremely impressive, and that comes down to both luck and the skill and care of the mitigation paleontologists and the construction workers who often found the fossils."
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