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80 Million Year-Old Fossils at Granite Bay

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by Dick Hilton and Pat Antuzzi

At Granite Bay northeast of Sacramento in Placer County, outcrops of Chico Formation provide evidence toward a greater understanding of environments that existed here approximately 80 million years ago. These fossils come from rocks laid down originally as ocean sediment after the Jurassic Period during the Cretaceous Period. Exposed during grading and ditching for house construction, and now under homes and pavement, the area contains offshore marine fossils, fossils from a nearby cliff-lined coast, plus fossils from the land environment as well.

The outcrop area at Granite Bay: The authors had collected in the area for many years prior to the time when the site was to be developed. Placer County officials were aware that the area was a sensitive paleontological site and in order to satisfy the environmental impact to paleontological resources the contractor was asked to hire a consultant to monitor excavation and collect fossils on site. Richard Hilton, a paleontologist and Professor of Geology at Sierra College, was hired as consultant and Pat Antuzzi, a local fireman and amateur fossil collector, volunteered to help. Pat made most of the important fossil finds.

The first fossils collected here were collected by Dr. John B. Trask, the first State Geologist of California, and described by paleontologist William M. Gabb in 1864. In 1911 Waldemar Lindgren described these fossils as coming from the Upper Cretaceous Chico Formation. In Granite Bay the Chico Formation lies on the granitic rocks of the Sierra Nevada where it was originally deposited as mud and sand on the sea bottom near the edge of the ancient Sierra. During the deposition of this sediment the site was probably well offshore from a steep cliff-lined shore resembling the Oregon coast of today.

The three ancient environments: This unique locality of Chico Formation contains evidence of three destinct ancient environments: offshore, nearshore and land. Fossils collected here by Pat Antuzzi and Dick Hilton paint a vivid picture of those three ancient environments.

1. The fossils from the offshore environment show it was a muddy bottom with many species of clams and snails, as well as other mollusks, some living upon and some making their living in the sediment. There were possibly shrimp behaving much like the ghost shrimp today, but their burrows were lined with woody flotsam collected from the bottom. There was a crinoid (sea lily), an animal related to sea stars but looking much like a plant with a five-sided stem and a bulbous "head". Swimming in the water above were numerous Nautilus as well as many other chambered relatives including the coiled ammonites and the straight but also chambered baculites. Sharks and other fish, such as Enchodus, abounded as did at least three types of large marine turtle, one with a shell about 10 feet long. A mosasaur (a huge, 20 foot-long sea-going reptile with sharp-edged, pointed teeth) most likely fed on a diet of fish and other swimming creatures.

Fossil mix2. An undersea debris flow of rocks, sand and shells was deposited on the bottom here. The material in this deposit gives us a glimpse as to what the shoreline environment was like. This nearshore environment would have included a rocky coast with an active beach receiving the waves of the much larger ancestral Pacific Ocean. The Atlantic Ocean was only in its beginnings as North America and the Old World were only starting to drift apart. The waves were probably breaking against a cliff-lined coastline where just offshore lived an abundance of clams, snails, oysters, limpets and other mollusks. Some of these animals lived on rocks while others coursed across and then burrowed into the sand and mud. Grazing over this undersea landscape were numerous urchins, one type with spines resembling tiny bowling pins. Swimming overhead were sharks and other fish. The fossils of the hard parts of animals only give us a glimpse into a much more diverse environment that must have included soft-bodied plants and animals as well.

3. Fossils from the land were brought into the site in two ways: first as flotsam that was carried down rivers and eventually became waterlogged and settled to the bottom: second as material brought in by the undersea debris flows.These fossils provide a sketch of the ancient Sierra Nevada. Picture a landscape not unlike the rugged Oregon coast, with an actively eroding coastline in front of a lush forest. The forest would have been very different, however, as there would have been many primitive leafy, flowering trees (some much like present day magnolias) plus coniferous trees resembling the Norfolk Island pine and monkey puzzle tree. Many of these trees are today found only in the southern hemisphere. The understory would have included tree ferns, seed ferns, horsetails and cycads (Sago palms). Even land snails were found including a new species, Polygyroidea hiltoni. Among the trees, dinosaurs would have roamed about like deer and forst elephants. Lurking in their midst was a killer, a theropod dinosaur, that like a forest tiger needed flesh to survive.

bone fragmentDr. Greg Erickson of University of California-Berkeley identified one of the bone fragments found at the site as being consistent with a theropod dinosaur. It is most likely the mid-section of a leg bone and the lack of distinct annual growth rings in the bone shows that it is from a young animal. This is the first evidence of a carnivorous dinosaur from California.

This small outcrop of Chico Formation probably contains the most diverse array of Late Cretaceous fossils found in California. This site is private property and is now developed with homes and streets covering the site.

Richard Hilton is the author of Dinosaurs and Other Mesozoic Reptiles of California, Univ. of California Press, 2003. Chapter 3 is available as a downloadable .pdf.

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