Photograph of La Brea Tarpits showing a pool of natural asphalt with a model of a Columbian mammoth in distress trapped within it. On the shore of the pit, models of an adult Columbian mammoth and a distressed baby mammoth look on. Palm trees and grass surround the pit.

Fossils of the Pacific Border

Simple map identifying the Pacific Border physiographic region of the western United States.
Page snapshot: Introduction to the fossils of the Pacific Border region of the western United States.

Topics covered on this page: Mesozoic fossils; Mesozoic marine fossils; Dinosaurs; Cenozoic marine invertebrate fossils; Paleogene marine invertebrate fossils; Neogene marine invertebrate fossils; Quaternary marine invertebrate fossils; Cenozoic marine vertebrate fossils; Cenozoic sharks and rays; Cenozoic marine mammals; Cenozoic aquatic and terrestrial fossils; Eocene Chuckanut Formation; Desmostylians; Quaternary terrestrial fossils; The La Brea Tarpits; The Pacific mastodon; The Channel Islands pygmy mammoth; Resources.

Credits: Some of the text on this page comes from "Fossils of the Western US" by Brendan M. Anderson, Alexandra Moore, Gary Lewis, and Warren D. Allmon, chapter 3 in The Teacher-Friendly Guide to the Geology of the Western US, edited by Mark D. Lucas, Robert M. Ross, and Andrielle N. Swaby (published in 2014 by the Paleontological Research Institution; currently out of print). The book was adapted for the web by Elizabeth J. Hermsen and Jonathan R. Hendricks in 2022. Changes include formatting and revisions and additions to the text and images. Credits for individual images are given in figure captions.

Updates: Page last updated May 3, 2022.

Image above: Recreation of a Columbian mammoth trapped in a tarpit, La Brea Tarpits, Los Angeles, California, 2012. Photo by Carol M. Highsmith (Reproduction number LC-DIG-highsm-21992, Carol M. Highsmith Archive, Library of Congress Prints and Photographs Division, no known restrictions on publication).

Mesozoic fossils

The Pacific Border includes terranes and former island arcs that accreted onto the West Coast, along with sediments deposited after this merger. Nearly every newly-exposed hillside or roadcut in this region exposes fossiliferous sediment, even in developed areas. Fossil-bearing rocks of Triassic, Jurassic, and Cretaceous age can be found in this region in north-central California to southwestern California and in southwestern Oregon.

Mesozoic marine fossils

Triassic to Jurassic

In north-central California, Triassic rocks yield ammonoids and marine reptiles, including the icthyosaur Shastasaurus and the thalattosaur Thalattosaurus. Jurassic marine fossils include abundant clams such as Buchia and ammonoids.


2-panel figure of photographs showing two views of a shell of the Triassic ammonoid Tropites from California. The shell coils in one plane and has faint ridges. A convoluted suture can be seen in an image of the narrow part of the shell.

Two views of the Triassic ammonoid Tropites sububullatus pacifica from Brock Mountain, California. Photos of UC 32543 by Alex P. Layng, Field Museum of Natural History (copyright Field Museum of Natural History, Creative Commons Attribution NonCommercial license, accessed via GBIF.org, images cropped and resized).


2-panel image showing Thalattosaurus alexandrae, a marine reptile from the Triassic of California. Panel 1: Drawings showing two views of the skull, from above and from the side. The skull is narrow and elongated with short, pointed teeth. Panel 2: Illustration of the live animal. The animal appear lizard-like with four limbs and a long tail flattened vertically. It is shown swimming in the water.

Thalattosaurus alexandrae, an extinct reptile named Annie Alexander, an heiress who financed the creation of the University of California Museum of Paleontology. Left. Drawings of the skull. Source: Merriam (1905) Memoirs of the California Academy of Sciences, vol. 5 (Wikimedia Commons). Right: Reconstruction of the living animal by Nobu Tamura (Wikimedia Commons, Creative Commons Attribution-ShareAlike 4.0 International license, image cropped).


4-panel image showing 3 photographs and a drawing of the skull of the ichthysaur Shastasaurus pacificus. The skull has a large eye socket; most of the snout is missing. It is thought that this animal may have had no teeth and a very short snout. 

Different views of the skull of Shastasaurus pacificus, an ichthyosaur from the Triassic of California. Scientists think that Shastasaurus may have had a short snout and no teeth. A, B. Lateral views. C. View from above. D. Oblique view showing the front of the skull. Source: Figure 5 from Sander et al. (2011) PLoS ONE e19480 (Creative Commons Attribution license, image resized).


Cretaceous

During the Cretaceous, sea levels were higher, and the Pacific shoreline was much further inland. The shore was lined with palms, and the waters were filled with bivalves such as Inoceramus and Trigonia. Recognizable relatives of many extant bivalves, such as oysters, also became common during the Cretaceous. Ammonoid cephalopods were extremely diverse and can be found in many Cretaceous rocks. Marine reptiles such as ichthyosaurs, plesiosaurs, and mosasaurs are also found through much of coastal California.

In addition to marine fossils, Cretaceous plants have been found on Sucia Island off the coast of Washington state, including fruits of plants in the dogwood family (Cornaceae) as well as winged fruits of a member of Cunoniaceae, a family of plants found in the southern hemisphere today.


Photograph of a shell of a heteromorph ammonoid from the Cretaceous of California. The shell is tightly U-shaped and has prominent ridges encircling it at regular intervals.

A heteromorph ammonoid (Helicancylus pilsbryi) from the Early Cretaceous Budden Canyon Formation, Shasta County, California. Photo of LACMIP 9951.3 by Natural History Museum of Los Angeles County (Creative Commons Attribution 4.0 International license, image cropped).


Photograph of a shell of an ammonoid from the Early Cretaceous of California. The shell is tan and coils in a single plane, with regular ridges encircling it. The height of the shell is about 3.5 centimeters.

An ammonoid (Acanthoplites subbigoti) from the Early Cretaceous Budden Canyon Formation, Shasta County, California. Photo of LACMIP 9951.2 by Natural History Museum of Los Angeles County (Creative Commons Attribution 4.0 International license, image cropped).


Photograph of a shell of a cephalopod from the Late Cretaceous of California. The shell is white and coils in a single plane, with regular ridges encircling it. The height of the shell is about 4 centimeters.

An acanthoceratoid cephalopod (Romaniceras deverioide), Late Cretaceous Redding Formation, Shasta County, California. Photo of LACMIP 10741.4 by Natural History Museum of Los Angeles County (Creative Commons Attribution 4.0 International license, image cropped).


Photograph of a large fossil bivalve, Inoceramus, from Little Sucia Island, Washington. The bilvalve has both valves preserved and the hinge can be seen. The shell is off-shite and shiny.

The giant bivalve Inoceramus from the Cretaceous of Little Sucia Island, San Juan Islands, Washington. Photo by Kevmin (Wikimedia Commons, Creative Commons Attribution-ShareAlike 3.0 Unported license, image cropped).



Dinosaurs

Although there were presumably many dinosaur species on land, only a few dinosaur fossils have been found in this region. These include the bones of hadrosaurs and the Late Cretaceous armor-plated ankylosaur Aletopelta. One specimen of Aletopelta found in California evidently floated out to sea, where its armor plates and spines were encrusted by bivalves!


Photograph of a display showing a specimen of Aletopleta, an armored dinosaur from the Cretaceous of California. Teh specimen has two legs and other bones heavily encrusted with bivalves.

Specimen of Aletopelta with encrusted bivalves on display in San Diego, California. Photo by ケラトプスユウタ(Wikimedia Commons, Creative Commons Attribution-ShareAlike 4.0 International license, image cropped and resized).


Illustration showing a life reconstruction of Aletopleta, an armored dinosaur from the Cretaceous of California. The dinosaur walks on four short legs, has a longish neck ending in a small head with a break-like mount, and a long tail. The back of the animal has rows of large spikes and the tail ends in a club. The dinosaur was about half as tall as a grown man, as indicated by small silhouettes in the corner of the illustration.

Life reconstruction of Aletopelta by Karkemish (Wikimedia Commons, Creative Commons Attribution 3.0 Unported license, image cropped and resized).


Map showing the distribution of dinosaurs on the Pacific Coast of Late Cretaceous North America (Larimidia). Dinosaurs are known from Alaska in the north to Baja California in the south.

Distribution of dinosaurs from the Late Cretaceous Pacific Coast known as of 2015. Source: Figure 1 from B. R. Peecock and C.A. Sidor (2015) PLoS ONE 10(5): e0127792 (Creative Commons Attribution 4.0 International license, image resized).

Cenozic marine invertebrate fossils

Paleogene marine invertebrate fossils

The Paleogene marine fossils of the Pacific Border strongly resemble the hard-shelled organisms living in the Pacific today, although some may seem geographically out of place, which is an important piece of evidence for environmental change through geological time. Gastropods and bivalves are the most common marine fossils of the Paleogene and include clams, oysters, whelks (family Buccinidae), moon snails (family Naticidae), and tower snails (family Turritellidae).


Photographs showing two views of a singe valve of a clam shell from the Eocene of Washington. The photos show the outer and inner surfaces. The shell is brown and is impacted with sediment on its inner surface. The shell is about 4 centimeters wide.

Single valve of a clam shell (Pitaria californiana) shell from the Eocene Cowlitz Formation, Cowlitz County, Washington. Photo of PRI 76124 by the Paleontological Research Institution/PRI (CC0 1.0/Public Domain Dedication, accessed via GBIF.org).


Photographs showing two views of an Eocene marine snail shell, spire pointed upward. The shell is a little over 3 centimeters wide.

Two views of a snail shell (Amaurellina clarki clarki) shell from the Eocene Cowlitz Formation, Lewis County, Washington. Photo of PRI 76176 by the Paleontological Research Institution/PRI (CC0 1.0/Public Domain Dedication, accessed via GBIF.org).


Photograph of a fossil nautiloid cephalopod shell from the Oligocene of Washington. The shell is orange-brown in color and shiny. Sinuous sutures can be faintly seen on part of the shell.

A nautiloid (Aturia angustata) from the Oligocene Lincoln Creek Formation, Grays Harbor County, Washington. Photo by Kevmin (Wikimedia Commons, Creative Commons Attribution-ShareAlike 3.0 Unported license, image cropped).


Photograph of the fossil crinoid Isocrinus from the Oligocene of Oregon. The fossil shows the stalk, calyx, and many arms splayed out radially.

A crinoid (Isocrinus oregonensis) from the Oligocene Keasey Formation, Columbia County, Oregon. Photo of USNM MO 560790 by the Crinoid Type Image Project (Smithsonian National Museum of Natural History, public domain).


Crabs are sometimes common, although it is rare to find fossils of whole animals, as these organisms typically break apart after death. However, some locations preserve crabs (and other fossils) within concretions. Concretions are hard, layered nodules, often with a different chemical makeup from the surrounding rock. They form when minerals precipitate (crystalize) around a nucleus within the sediment. While concretions are not fossils themselves, they may contain fossils. Even trace fossils may be preserved in concretions, as many organisms line their burrows with mucus, and the decay of that mucus may begin the formation of a concretion.


Photograph of a concretion containing a fossil crab from the Oligocene of Oregon. The concretion is a round, gray rock that has been spilt open to show a light brown crab. The body, part of the pinchers, and part of the legs of the crab on the right side of its body can be seen.

A crab (Branchioplax washingtoniana) from the Oligocene Callam Formation, Kitsap County, Washington. Photo of YPM IP 000195 by Jessica Utrup, 2017 (Yale Peabody Museum of Natural History/YPM, CC0 1.0/Public Domain Dedication, accessed via GBIF.org).


Photograph of concretion containing a fossil crab from the Oligocene of Oregon. The concretion has mostly been removed leaving a shiny brown, articulated crab partially embedded in gray rock. The body, pinchers, and legs of the crab are preserved.

A crab (Pulalius vulgaris) from the Oligocene Lincoln Creek Formation, Wahkiakum County, Washington. Photo by Michael Brett-Surman (Smithsonian National Museum of Natural History, public domain).


Neogene marine invertebrate fossils

The general character of fossils from the Neogene is similar to that of the Paleogene; Neogene fossils can be found near the coast as well as in the Central Valley area of California. Marine invertebrates include bryozoans, corals, mollusks, and echinoderms. Gastropods such as moon snails, whelks, and tower snails are important in the marine fauna. Mussels, clams, scallops, and oysters are all common and can often be found in the rocks exposed at beach cliffs along much of the Oregon and California coasts. 


Close-up of a photo of a fossil coral from the Pliocene of California. The photo shows a number of corallites, each circular with radiating septa (partitions) within them. The specimen is probably 3.5 to 4 centimeters across based on a ruler at the bottom of the image.

A scleractinian coral (Astrangia coalingensis) shell from the Pliocene San Joaquin Formation, Kings County, California. Photo of PRI 71881 by the Paleontological Research Institution/PRI (CC0 1.0/Public Domain Dedication, accessed via GBIF.org).


Photograph of a fossil bryozoan from the Miocene of Santa Barbara Island, California. The specimen is pinkish in color and has a series of regular depressions and smaller pits on the surface. The scale bar is half a millimeter, indicating the specimen is very small.

A bryozoan (Schizoporella cornuta) from the Miocene of Santa Barbara Island, a small island off the coast of Southern California. Photo of YPM IP 591115 by Daniel J. Drew, 2017 (Yale Peabody Museum of Natural History/YPM, CC0 1.0/Public Domain Dedication, accessed via GBIF.org).


Photos showing two views of a fossil top snail shell. The shell is show with its apex pointed upward and is wider than it is long. The shell is estimated at 2 centimeters wide based on a ruler provided for scale.

Two views of a Calliostoma top snail (Calliostoma coalingense) from the Pliocene Etchegoin Formation, California. Photo of PRI 71586 by the Paleontological Research Institution/PRI (CC0 1.0/Public Domain Dedication, accessed via GBIF.org).


Two views of a fossil moon snail shell from the Pliocene of California. The left view shows the apex of the shell from an oblique angle. The right view shows the aperture (opening) of the shell. The shell is smooth and has a low spire, being perhaps slightly wider than long. Overall, the shell is nearly spherical in shape.

Two views of a moon snail (Polinices galianor), Pliocene, San Diego County, California. Left photo and right photo by Mark A. Wilson (Wikimedia Commons, CC0 1.0/Public Domain Dedication).


Photograph of a group of clam shell valves (one part of a two part shell) from the Miocene of Washington. The valves are beige to white and show a series of concentric grown lines. The photo shows about seven valves and some broken valve fragments.

Clam (Pitar dalli) shells from the Miocene of Pacific County, Washington. Photo of PRI 76135 by the Paleontological Research Institution/PRI (CC0 1.0/Public Domain Dedication, accessed via GBIF.org).


Photograph showing two scallop shell valves from the Miocene of California. The photo shows a large valve on the left and a smaller valve on the right. Each valve has a series of ridges that begin at the hinge and radiate fan-like to the edge of the valve. 

Scallops (Patinopecten coosensis) from the Miocene Empire Formation, Coos County, Oregon. Photo of PRI 76312 by the Paleontological Research Institution/PRI (CC0 1.0/Public Domain Dedication, accessed via GBIF.org).


Photograph of a group of oyster shell vales from the Miocene of California. The shells are slightly to much longer than wide. The outer surface has a series of ridges with slightly irregular edges. The photo shows 7 valves of various sizes.

Oysters (Crassostrea titan) from the Miocene Santa Margarita Formation, Fresno County, California. Photo of PRI 72940 by the Paleontological Research Institution/PRI (CC0 1.0/Public Domain Dedication, accessed via GBIF.org).


Photograph of sand dollars from the Pliocene of La Jolla, California. The specimen is a chunk of sandy rock matrix with fragments of bivalve shell embedded in it. Multiple large, relatively complete sand dollars are exposed on the surface. 

Sand dollars (Dendraster excentricus) from the Pliocene of La Jolla, California. Photo of YPM IP 227814 by Susan H. Butts, 2021 (Yale Peabody Museum of Natural History/YPM, CC0 1.0/Public Domain Dedication, accessed via GBIF.org).


Photograph of a group of fossil barnacles from the Pliocene of California. The barnacles look like short tubes that are clustered together. The specimen is about 10 centimeters across based on a ruler at the bottom of the photograph.

Barnacles (Tamiosoma gregaria) from the Pliocene San Joaquin Formation, California. Photo of PRI 71257 by the Paleontological Research Institution/PRI (CC0 1.0/Public Domain Dedication, accessed via GBIF.org).


Quaternary marine invertebrate fossils

Prior to 100,000 years ago, much more of the coast was submerged, and the region was ultimately exposed when the expansion of glaciers caused a regression in sea level. The coasts of California, Oregon, and Washington and the central valley of California contain numerous Pleistocene-age fossil deposits. Bivalves and gastropods are the most common marine invertebrate fossils from this time, particularly scallops (Pecten) and oysters (Ostrea), but also clams such as Saxidomus, Mya, and Clinocardium and snails such as Polinices, Neptunea, Turricula, and Cancellaria.


Photographs showing two views of an oyster shell, one of the outer surface and one of the inner surface. The valve is longer than it is wide and other outer surface is not smooth, but has irregular ridges. The outline of the shell is also slightly irregular. The valve is about 3.5 centimeters long, as indicated by a ruler at the bottom of each photo.

Outer and inner surface of a single valve of an oyster shell (Ostrea lurida) from the Pleistocene Palos Verde Sand, Los Angeles, California. Photo of PRI 75917 by the Paleontological Research Institution/PRI (CC0 1.0/Public Domain Dedication, accessed via GBIF.org).


Photographs showing two views of a single valve of a clam shell, one of the outer surface and one of the inner surface. The shell is wider than tall and white in color. A ruler at the bottom each image indicates that the valve is about 3.5 centimeters across.

Outer and inner surface of a single valve of a clam shell (Simomactra planulata) from the Pleistocene Palos Verde Sand, Los Angeles, California. Photo of PRI 75950 by the Paleontological Research Institution/PRI (CC0 1.0/Public Domain Dedication, accessed via GBIF.org).


Photograph of fossil worm snails from the Pleistocene of California. The shells look like a series of narrow tubes, some in groups.

Worm snails (family Vermetidae) from the Pleistocene of Humbolt County, California. Photo of PRI 76553 by the Paleontological Research Institution/PRI (CC0 1.0/Public Domain Dedication, accessed via GBIF.org).


Photograph of a group of horn snail shells from the Pleistocene of California. The shells are elongated and narrow with a series of longitudinal ridges running around each turn of the shell. The photo shows about 20 shells that are randomly arranged. 

Horn snails (Cerithideopsis californica) from the Pleistocene of San Diego County, California. Photo of PRI 76599 by the Paleontological Research Institution/PRI (CC0 1.0/Public Domain Dedication, accessed via GBIF.org).

Cenozic marine vertebrate fossils

Marine vertebrates include sharks, rays, bony fish, and marine mammals such as seals, walruses, sea cows, toothed whales (including dolphins), and baleen whales. Birds, including auks (Mancalla vegrandis), are also known.

Cenozoic sharks and rays

Sharks and rays have skeletons made of cartilage instead of bone, which means that their skeletons are rarely found as fossils. The teeth of sharks and rays, however, are much more resistant. They are common fossils in Cenozoic sediments on the Pacific Border.

Sharks shed their teeth constantly, and a single individual can produce as many as 35,000 teeth during its lifetime. You can tell the difference between fossil and modern shark teeth by their color. Fossil teeth are often brown, gray, or black, whereas modern teeth are tan to white. Shark teeth come in many forms and sizes, and not all teeth in the mouth of a single individual look alike.

The largest shark that ever lived was a species related to the modern great white shark, but much bigger. Otodus megalodon—frequently called just "megalodon"—lived in the eastern Pacific Ocean from the Miocene to the Pliocene. The fossil record of megalodon consists almost entirely of teeth. While megalodon is regarded as one of the largest and most powerful ocean predators that ever lived, size estimates for this shark vary widely because of the lack of body fossils. Nevertheless, a reasonable estimate is that megalodon reached lengths of up to 18 meters (59 feet). It likely fed on large whales.


Photographs showing both sides of a megalodon tooth from the Pliocene of San Diego County, California. The tooth is tan in color and roughly triangular in shape. It is missing part of the tip and also a portion of the roots.

Two views of a megalodon (Otodus megalodon) tooth, Pliocene San Mateo Formation, San Diego County, California. Source: Figure 7 from Boessenecker et al. (2019) PeerJ 7: e6088 (Creative Commons Attribution 4.0 International license, image cropped).


Cenozoic marine mammals

Cenozoic whales

Whales are most closely related to artiodactyls, or the even-toed hoofed mammals (for example, bison, deer, and hippopotamuses). Whales evolved from four-legged mammals that lived on land during the Eocene epoch, beginning around 55 million years ago. The early ancestors of whales were animals that resembled wolves. Fossils—especially from Egypt and Pakistan—have revealed many extinct forms that show the evolutionary steps in the remarkable transition from land-dwelling to truly aquatic animals. Some transitional Eocene whales are known from the southeastern and south-central United States.

There are two major groups of modern whales. The toothed whales or odontocetes (meaning "toothed whales") include dolphins, porpoises, and the sperm whale. The toothless baleen whales or mysticetes include the large, plankton-feeding whales such as humpback whales, blue whales, and right whales.

Fossils of whales and dolphins can be found in Oligocene and younger sediments along the West Coast of North America. Transitional aetiocetid, or toothed baleen whales, are found on the West Coast in the Oligocene. Although "toothed baleen" may appear to be a contradiction in terms, aetiocetids are toothed ancestors of the modern baleen whales.


Photographs of fossil teeth of an aetiocetid whale from the Oligocene of Washington. The teeth range from single-cusped to multi-cusped and partial to complete with roots. All images are grayscale.

Teethed of an aetiocetid or toothed baleen whale (Fucaia buelli) from the early Oligocene Makah Formation, northwestern Washington. Source: Figure 10 from Marx et al. (2015) Royal Society Open Science 2: 150476 (Creative Commons Attribution 4.0 International license, image cropped).


Photograph of whale bones from the Miocene to Pliocene Purisima Formation. Bones include vertebrate and other bones of a baleen whale, bones of the front flipper of a baleen whale, ear bones of a toothed whale, and vertebrae of a toothed whale.

Whale bones from the Miocene to Pliocene Purisima Formation, central California. A. Vertebrae and other bones of a baleen whale. B. Bones of the front flipper of a baleen whale. C. Ear bones of a toothed whale. D. Vertebrae of a toothed whale. Source: Figure 20 from Boessenecker et al. (2014) PLoS ONE 9(3): e91419 (Creative Commons Attribution 4.0 International license, image resized).


Cenozoic sirenians

Manatees, dugongs, and sea cows belong to a group of aquatic mammals called sirenians. Sirenians are not closely related to whales. Among living animals, they are most closely related to elephants. Fossil dugongs or sea cows (both common names for members of the sirenian family Dugongidae) are found on the Pacific border. One of the oldest examples on the West Coast is from the Channel Islands off southern California and may date back as far as the late Oligocene.

Sirenians are no longer found in the eastern Pacific. The last West Coast sirenian, Steller's sea cow (Hydrodamalis gigas), had a Pleistocene range that extended from northern Alaska to Baja California in the east and from northeastern Russia to Japan in the west. By the time Europeans began exploring the North Atlantic, the range of the Steller's sea cow was restricted to a region around the Commander Islands east of the Kamchatka Peninsula (eastern Russia). The remaining small population was hunted to extinction sometime during the 1700s. During the Miocene and Pliocene, ancient relatives of the Steller's sea cow, Hydrodamalis cuestae and Dusisiren, lived in California.


Photograph of the ribs of a sea cow from the Oligocene to Miocene of Santa Rosa Island, California. The bones are white and are arranged in parallel. They are embedded in brown rock matrix.

Ribs of a sea cow, late Oligocene to Miocene, Santa Rosa Island, California. Photo by National Park Service/NPS (public domain).


Illustration reconstructing the sea cow Dusisiren from the Miocene to Pliocene of California. The animal is gray in color with a barrel-like body, two front flippers, and no hindlimbs. The tail has horizontally oriented flukes. The head has small eyes near the top, nostrils in the front, and a mouth on the underside. The mouth appears to be flanked by short whiskers.

An artist's interpretation of Dusisiren jordani from the Miocene to Pliocene of California as it may have appeared in life. Illustration by Nobu Tamura (Wikimedia Commons, Creative Commons Attribution-Share Alike 4.0 International license).


Map showing the range of the Steller's sea cow. The map shows the northern Pacific Ocean with eastern Asia on the left and western North America on the right. A region extending from Hokkaido Island (Japan) to northeastern Russia to northwestern Alaska, the Aleutian Islands, and south to Baja California is shaded yellow, indicating the Pleistocene range of Steller's sea cow. Red dots in the eastern Aleutian Islands, St. Lawrence Island, and west-central Alaska indicate archeological sites with sea cow remains. Blue dots on the Commander Islands show the range of the Steller's sea cow as of the 1700s, prior to extinction.

Ancient and historic ranges of Stellar's sea cow (Hydrodamalis gigas). Yellow shading = Pleistocene range; red dots = evidence from archaeological sites; blue dots = range at time of discovery by Europeans. Map from Figure 1 in Sharko et al. (2021) Nature Communcations 12: 2215 (Creative Commons Attribution 4.0 International license, image accessed via Wikimedia Commons).


Photograph of a Steller's sea cow skeleton against a black background. The animal has well-developed forelimbs and poorly developed hindlimbs.

Skeleton of the extinct Steller's sea cow (Hydrodamalis gigas). Specimen collection location unknown (presumably the Commander Islands region), on display in the Musée des Confluences à Lyon. Photo by Vassil (Wikimedia Commons, CC0 1.0 Universal/Public Domain Dedication).


Cenozoic pinnipeds

Pinnipeds are a group of marine mammals with four flippers that are capable of coming out of the water and resting or moving around on land, at least for limited distances. Pinnipeds include three families: earless seals or true seals (Phocidae), eared seals (Otariidae, including fur seals and sea lions), and walruses (Odobenidae). Among living organisms, pinnipeds may be most closely related to bears; they are not closely related to whales or sirenians.

Some of the oldest known pinnipeds are from the West Coast. The oldest from this region is Enaliarctos, which comes from the late Oligocene to early Miocene of California and Oregon. Enaliarctos already looked recognizably seal-like. A similar early pinniped is also known from Washington.

While eared seals and walruses date to the Miocene on the West Coast, earless seals did not appear until the Pliocene. Today, earless and eared seals still occur on the California, Oregon, and Washington coasts. Only one species of walrus is living today, and it occurs in the Arctic region; in North America, it is only found around the coastline of Alaska.


2-Panel image of the early pinniped Enaliarctos. Panel 1: Photograph of a partial fossil jaw iwth 3 molars and a canine tooth. Panel 2: Reconstruction of Enaliarctos mealsi, an early pinniped. The animal looks like a small eared seal.

The early pinniped Enaliarctos. Left: Jaw from the Oligocene Yaquina formation of Oregon. Photo of UCMP V 253400 by Patricia Holroyd (University of California Museum of Paleontology/UCMP, Creative Commons Attribution 3.0 Unported license, image cropped and scale added). Right: Reconstruction of Enaliarctos mealsi by Nobu Tamura (Wikimedia Commons, Creative Commons Attribution-ShareAlike 4.0 International license).


Photograph of the skeleton of an early pinniped from the Miocene of California. The skeleton is partially prepared out of a piece of beige stone. It has no skull, but most of the rest of the body is there, including the spine and ribs, a forelimb, and two hind-limbs. A meter stick is at the bottom of the image; the specimen is probably one and one-third meters, estimated. 

Skeleton of the early pinniped Enaliarctos mealsi, Miocene Jewett Sand Formation, Kern County, California. The ruler at the bottom of the image is 1 meter long (39.4 inches long). Photo of USNM PAL 374272 by Michael Brett-Surman (Smithsonian National Museum of Natural History, public domain).


Photograph of the skull of a tuskless walrus from the Miocene of California. Only the upper part of the skull (not the jaw) is shown from the side. Four pointed teeth can be seen at the front end of the skull.

Skull of a tuskless walrus (Titanotaria orangesis), Miocene Capistrano Formation, Orange County, California. Source: Figure 5 from Magallanes et al. (2018) PeerJ 6: e5708 (Creative Commons Attribution 4.0 International license, image cropped and resized).


Photograph of the skull of a four-tusked walrus in a display case. The walrus has robust jaws with two large tusks protruding from the top and bottom. The crown of the skull has a central ridge.

Cast (reproduction) of a skull of a four-tusked walrus (Gomphotaria pugnax), Miocene, California. Specimen on temporary display at the Alaska State Museum, Juneau, 2019. Photo by Jonathan R. Hendricks ([email protected]).


Multipart image showing the jawbones of four species of Miocene walruses from the U.S. West Coast in side and top views. 

Lower jawbones of four species of Miocene walruses from the West Coast. A. Pelagiarctos, "Topanga" Formation, Orange County, California. BImagotaria downsi, Santa Margarita Sandstone, Santa Cruz County, California. C. Proneotherium repenningi, Astoria Formation, Lincoln County, Oregon. D. Pontolis magnus, Empire Formation, Coos County, Oregon. Source (figure and adapted caption): Figure 7 from R. W. Boessenecker and M. Churchill (2013) PLoS ONE 8(1): e54311 (Creative Commons Attribution license, image resized).

Cenozoic aquatic and terrestrial fossils

Eocene Chuckanut Formation

The Eocene Chuckanut Formation outcrops on Sucia and Lummi islands off the coast of Washington. It also occurs in northwestern Washington near Bellingham and extends to the northeast, to near the boundary between the Pacific Border and the Cascade-Sierra Mountains regions.

Aquatic animals of the Chuckanut Formation include freshwater gastropods and mussels as well as turtles. Mammal and bird tracks are also preserved. Plant fossils include horsetails (Equisetum) associated with animal trackways, spectacular impressions of fan-like palm leaves, and other angiosperms, ferns, and conifers.


2-panel image. Panel 1: Black and white photograph of a turtle trackway from the Eocene of Washington. The tracks go from the upper left corner to the lower right corner of a rock slab. Panel 2: Drawing of the same slab shown in panel 1 indicating the positions of the tracks and the distances between them. Distances between left and right tracks are 7.5 to 16 centimeters. Distances between tracks in same line are 15 to 17 centimeters.

A turtle trackway Eocene Chuckanut Formation, Whatcom County, Washington. Left is a photo of the trackway, right is an illustration interpreting the trackway (R = right legs, L = left legs). Source: From Figure 7 in Mustoe (2019) Geosciences 9(7): 321 (Creative Commons Attribution 4.0 International license, image cropped).


4-part image. Part 1: Black and white photograph of two crocodile footprints on a rock slab, Eocene, Washington. Parts 2 and 3: Drawings of modern crocodile feet. Part 4: Drawing of a single fossil track with 5 toes showing the angles between the toes ranging from 24 to 38 degrees.

Crocodilian footprints (left), Eocene Chuckanut Formation, Whatcom County, Washington. The drawings in the center show modern crocodile feet; the drawing at the right is an outline of one of the fossil tracks with measurements. Source: From Figure 9 in Mustoe (2019) Geosciences 9(7): 321 (Creative Commons Attribution 4.0 International license, image cropped).


Photograph of a rock slab with two palm leaf impressions on it. One impression is a nearly complete leaf with a thick stalk and a fan-shaped blade. The second is a partial leaf blade. A mason-type rock hammer has been placed on the rock slab for scale.

Impressions of fossil palm leaves (Sabalites), Eocene Chuckanut Formation, Whatcom County, Washington. Source: From Figure 4 in Mustoe (2019) Geosciences 9(7): 321 (Creative Commons Attribution 4.0 International license, image cropped).


Photograph of a fossil angiosperm (flowering plant) leaf from the Eocene of Washington. The leaf is dark brown in color and oriented horizontally. It is elliptical in shape with a large midvein running down its center. Lateral veins are relatively straight and end at the edge of the leaf. The margin is toothed.

Unidentified angiosperm leaf, Eocene Chuckanut Formation, Whatcom Creek, Washington. Photo of YPM PB 064582 by Linda S. Klise (Yale Peabody Museum of Natural History/YPM, CC0 1.0/Public Domain Dedication, accessed via GBIF.org).


Desmostylians

Desmostylians are a group of enigmatic Oligocene to Miocene mammals that looked like and probably lived in similar habitats to modern hippos. Desmostylians are found only around the northern edges of the Pacific Ocean. In the eastern Pacific, they are found on the West Coast of North America from the Aleutian Islands in Alaska to Baja California Sur in Mexico. The modern relatives of these animals are unknown. The first desmostylian fossils were found in the 1800s in Alameda County, California. 


3-Part figure of photos showing a desmostylian tooth in three views. From the top, the cusps looks like a series of disks packed together. Each cusp has an outer dark ring surrounding a central lighter area. From the side, the tooth is highest at the front and back, dipping in the middle. Otherwise, the cusps look flat.

Three views of a desmostylian tooth (Desmostylus hesperus) from the Miocene Temblor Formation, Fresno County, California. Photos of specimen UCMP 136511 by Sarah Rieboldt (University of California Museum of Paleontology/UMCP, Creative Commons Attribution 3.0 Unported license, images cropped).


Drawing reconstracting a desmostylian sitting on a beach. The desmostylian is hippo-like, with four short legs, a barrel-like body, a large lead, and a short tail. A small tusk can be seen protruding from the upper jaw.

Reconstruction of a desmostylian (Desmostylus hesperus) on the shore. Illustration by Dmitry Bogdanov (Wikimedia Commons, Creative Commons Attribution 3.0 Unported license). 


Quaternary fossils

La Brea Tar Pits

During the Pleistocene, large terrestrial mammals were common, including mammoths, woolly rhinos, horses, camels, bison, saber-toothed cats, and dire wolves. The famous La Brea Tar Pits in downtown Los Angeles provide a spectacular window into the region’s Pleistocene mammal communities. The tar pits formed around 40,000 years ago, when natural asphalt deposits began to seep up from cracks in the ground to form pools. As this asphalt seeped from the ground, it became covered with leaves and dust. Animals that wandered in became trapped, as did predators that arrived to eat the mired animals.

The oldest remains from La Brea have been dated to 38,000 years old, and the tar pits still continue to trap unsuspecting animals today. Bones sink into the asphalt are stained dark brown or black, leading to the unique appearance of the fossils found here. Fossils of more than 600 species of animals and plants have been excavated from the asphalt. In addition to large mammals and birds, the tar pits have preserved a remarkable array of microfossils ranging from insects and leaves to pollen grains, seeds, and ancient dust.


Photograph of the underside of a Columbian mammoth skull with two molars exposed at La Brea Tar Pits, California.

Underside of the skull of a Columbian mammoth (Mammuthus columbi) called "Zed" at the George C. Page Museum, Los Angeles, California, 2014. This mammoth was discovered near the La Brea Tarpits during a construction project in 2006. Photo by Ian Abbott (flickr, Creative Commons Attribution-NonCommercial-ShareAlike 2.0 generic license).


Photograph of the skull of a dire wolf. The skull varies from brown to black in color. It is dog-like with sharp teeth.

The skull of a dire wolf (Canis dirus), La Brea Tarpits, Pleistocene, Los Angeles, California. Photo by James St. John (flickr, Creative Commons Attribution 2.0 Generic license, image resized).


Photograph of a chunk of tarry sediment with four beetles on its surface, three large and one much smaller.

Water scavenger beetles (Hydrophilius), Pleistocene, La Brea Tarpits, Los Angeles, California. Photo by Didier Descouens (Wikimedia Commons, Creative Commons Attribution-ShareAlike 4.0 international license, image cropped).


The saber-toothed cat

Smilodon fatalis, the saber-toothed cat, is among the most famous species represented in the La Brea Tar Pits. This extinct big cat is well known for its prominently elongated canine teeth. While these animals are sometimes referred to as “saber-toothed tigers,” they are not actually close relatives of tigers or any other living feline group. Smilodon belongs to the group known as “dirk-toothed” cats, which possess teeth with fine serrations.

The elongated canines of Smilodon were fairly thin and would have broken if they bit into bone, so these teeth would likely have been used to kill prey that was already subdued. The cats’ most common prey were likely bison and camels; dire wolves were probably their direct competitors.

Smilodon went extinct around 10,000 years ago, along with many of the large mammals it utilized for food. These extinctions have been related to both climate change and hunting by humans, although the relative importance of each of these factors is still debated.


Photograph of a mounted skeleton of a saber-toothed cat on display in a museum. The skull of the cat has two very long, slightly curved canine teeth in its upper jaw.

Skeleton of a saber-tooth cat (Smilodon fatalis), La Brea Tarpits, Pleistocene, Los Angeles, California. Photo by James St. John (flickr, Creative Commons Attribution 2.0 Generic license, image resized).


Illustration reconstructing a saber-toothed cat as it may have looked in like. The image shows a stout-bodied cat with a spotted tan coat, short tail, and long canine teeth.

Illustration of a saber-toothed cat (Smilodon fatalis) by Dantheman9758 (Wikimedia Commons, Creative Commons Attribution 3.0 Unported license).


The Pacific mastodon

In 2019, scientists named a new species of mastodon: the Pacific mastodon (Mammut pacificus). This mastodon species is similar to the more widespread American mastodon (Mammut americanum). The differences between the two species are subtle, such as the shape of the molars and the number of vertebrae making up the end of the backbone. The Pacific mastodon has been found only in California (including the La Brea Tarpits and many other sites), southeastern Idaho, and Montana.


Skull of a Pacific mastodon from Riverside, California, shown in side view. 

Skull of a Pacific mastodon (Mammut pacificus) from the Pleistocene of Riverside, California. Source: Figure 7 from Dooley et al. (2019) Peer J 7:e6614(Creative Commons Attribution 4.0 International license, label added and image resized).


Map of North America showing the distribution of the Pacific mastodon and selected American mastodons. The Pacific mastodon is mostly found near the coast of California, although it has been found at several inland sites in northern California, southeastern Idaho, and Montana. The American mastodon has been found in Alaska, northwest Canada, Washington, and Oregon, as well as  the Southwest, and much of eastern North America.

Map showing the distribution of the Pacific mastodon (red dots) and the locations of selected American mastodon specimens (blue dots) that were used for comparison in a study. Source: Figure 4 from Dooley et al. (2020) Peer J 8: e10030 (Creative Commons Attribution 4.0 International license, image resized).


The Channel Islands pygmy mammoth

During the Pleistocene, the large Columbian mammoth (Mammuthus columbi) ranged throughout most of the contiguous 48 U.S. states and into Mexico. A much smaller species of mammoth, the pygmy mammoth (Mammuthus exilis), was found only on the Channel Islands of California.

The Channel Islands are a group of islands off the coast of southern California near Los Angeles and Ventura. When sea levels were lower during the Pleistocene, the Channel Islands were colonized by Columbian mammoths, which are thought to have swum from mainland California over 80,000 years ago. At their largest, Columbian mammoths were more than 4 meters tall (up to 14 feet, measured at the shoulder). Over generations, the mammoths on the Channel Islands became smaller, eventually evolving into pygmy mammoths. At their largest, pygmy mammoths stood perhaps 2.4 meters (8 feet) tall.

The Channel Islands pygmy mammoths went extinct near the end of the Pleistocene epoch. Humans had reached the islands by that time. Bones of a person known as Arlington Springs Man were discovered on the Channel Islands in the 1950s and later dated to about 13,000 years old. The remains of Arlington Springs Man are among the oldest known human bones discovered in North America.

Diagram showing the relative sizes of the three species of mammoth that are found in the Pleistocene of North America. From largest to smallest they are the Columbian mammoth, the woolly mammoth, and the pygmy mammoth.

Size comparison of the three types of mammoths found in the Pleistocene of North America. Diagram modified by a diagram by FunkMunk (Wikimedia Commons, public domain).


Photograph of a pygmy mammoth skeleton preserved in a sand dune. The backbone, the skull with one tusk, and a front and a rear leg can be seen. Bones appear to be articulated (in life position).

Pygmy mammoth (Mammoth exilis) from the Pleistocene, Channel Islands National Park, California. Photo by National Park Service/NPS (public domain).


Photograph of a pygmy mammoth skeleton being excavated. The mammoth is preserved in a sand dune. Its skull with one tusk is on the right. Also visible are a front leg, shoulder bland, ribcage, pelvis, and femur. A grid on four poles has been placed over the mammoth skull. A man bends down over the skeleton and another man is kneeling behind him, partially obscured. In the foreground, a little of a third man can be seen.

Pygmy mammoth (Mammoth exilis) being excavated. Pleistocene, Channel Islands National Park, California. Photo by National Park Service/NPS (public domain).

Resources

Resources from the Paleontological Research Institution

Daring to Dig: Annie Alexander: https://www.museumoftheearth.org/daring-to-dig/bio/alexander (More about Annie Alexander, who Thalattosaurus alexandrae was named after)

Digital Atlas of Ancient Life Virtual Collection: https://www.digitalatlasofancientlife.org/vc/ (Virtual fossil collection featuring 3D models of fossil specimens sorted by group)

Digital Encyclopedia of Ancient Life: https://www.digitalatlasofancientlife.org/learn/

[email protected]: Quick guide to common fossils: https://earthathome.org/quick-faqs/quick-guide-common-fossils/


Go to the full list of resources about fossils in the western U.S.

Go to the full list of general resources about fossils