4.2: California’s Fossil Record

The fossil record is the set of all fossils - and it just happens to chart the course of evolution through time. It is one of the most significant pieces of supporting evidence for the idea that evolution is a rational and real explanation for the origin of the diversity of species. In general, the term "fossil record" refers to all fossils that have been excavated, examined, and characterized so far by thousands of hardworking scientists and paleontologists over the last few hundred years. The fossil record is used as evidence for many of things, including for common descendants, mass extinctions, the Cambrian explosion and for the fact that Earth is considerably older than 6000 years (in fact, its 4.6 billion years old).

Paleobiogeography

Paleobiogeography goes one step further to include paleogeographic data and considerations of plate tectonics. Using molecular analyses and corroborated by fossils, it has been possible to demonstrate that perching birds evolved first in the region of Australia or the adjacent Antarctic (which at that time lay somewhat further north and had a temperate climate). From there, they spread to the other Gondwanan continents and Southeast Asia – the part of Laurasia then closest to their origin of dispersal – in the late Paleogene, before achieving a global distribution in the early Neogene (figure 4.2). Not knowing that at the time of dispersal, the Indian Ocean was much narrower than it is today, and that South America was closer to the Antarctic, one would be hard pressed to explain the presence of many "ancient" lineages of perching birds in Africa, as well as the mainly South American distribution of the suboscines.

Figure 4.2: Distribution of Four Fossil Groups During the Super Continent Gondwanaland. Image by Wikimedia is licensed under the CC-BY-SA 4.0 license.

The science of paleobiogeography helps us understand the history of where different species lived and how they got there. This knowledge is particularly useful for figuring out when major events like vicariance (splitting populations) or geodispersal (spread of organisms) happened in the past. It also sheds light on how regional groups of plants and animals formed.

For instance, by looking at the genes and geographic distribution of different fish species in the Amazon, scientists can tell that these fish didn't all appear suddenly in a burst of evolution. Instead, the vast diversity of Amazonian fish built up gradually over tens of millions of years. This process, called allopatric speciation, occurs when populations become isolated from each other (often due to geographic changes) and eventually evolve into separate species. This contrasts with some island ecosystems, where new species can quickly arise from a single ancestor due to the unique pressures of their isolated environment (like the famous finches of the Galapagos Islands).

For animals that live in freshwater like fish, their world is naturally divided into separate watersheds like individual "bowls." These "bowls" can be temporarily separated by land and then reconnected by erosion over long periods. However, in places like the Amazon basin, where the land is very flat, the many rivers have a complex history of connections. Here, a process called stream capture becomes important for how fish evolve and where they live. Stream capture happens when part of one river system gets "stolen" by another, often due to land movements or erosion. This can significantly change the distribution of fish species in the area.

The California State Fossil - The Smilodon

The Smilodon, often referred to as the Saber-Toothed Cat, is one of the best-known saber-toothed predators and most famous prehistoric mammals. The Smilodon lived in the Americas during the Pleistocene epoch (2.5 mya – 10,000 years ago). The genus was named in 1842 based on fossils from Brazil; the generic name means "scalpel”, or "two-edged knife" combined with "tooth". Three species are recognized today: S. gracilis, S. fatalis, and S. populator. The two latter species were probably descended from S. gracilis, which itself probably evolved from Megantereon. The hundreds of individuals obtained from the La Brea Tar Pits in Los Angeles constitute the largest collection of Smilodon fossils.

Figure 4.3: Size Comparison of All Members of the Smilodon Genus. Image by Alhadis is licensed under CC BY-SA 4.0.

Overall, Smilodon was more robustly built than any current species (extant) of cat, with particularly well-developed forelimbs and exceptionally long upper canine teeth. Its jaw had a bigger gape than that of modern cats, and its upper canines were slender and fragile, being adapted for precision killing. S. gracilis was the smallest species at 55 to 100 kg (120 to 220 lb.) in weight. S. fatalis had a weight of 160 to 280 kg (350 to 620 lb.) and height of 100 cm (39 in). Both species are mainly known from North America but remains from South America have also been attributed to them. The S. populator from South America was the largest species, at 220 to 436 kg (485 to 961 lb.) in weight and 47 in (120 cm) in height and was among the largest known felids. The coat pattern of Smilodon is unknown, but it has been artistically restored with plain or spotted patterns.

In North America, Smilodon hunted large herbivores such as bison and camels, and it remained successful even when encountering new prey species in South America. Smilodon is thought to have killed its prey by holding it still with its forelimbs and biting it, but it is unclear in what manner the bite itself was delivered. Scientists debate whether Smilodon had a social or a solitary lifestyle; analysis of modern predator behavior as well as of Smilodon's fossil remains could be construed to lend support to either view. Smilodon probably lived in closed habitats such as forests and bushes, which would have provided cover for ambushing prey. Smilodon died out while most North and South American megafauna disappeared, about 10,000 years ago. Its reliance on large animals has been proposed as the cause of its extinction, along with climate change and competition with other species.

Out of the Collection: Smilodon & Skull

Want to learn more about the Smilodon, the California State Fossil? Either scan the QR code or visit this link to see Professor Patrich share his museum replica of an actual Sabretooth Cat skull (Video length: 5min).

Case Study - The Pygmy Mammoth

The pygmy mammoth or Channel Islands mammoth (Mammuthus exilis) is an extinct species of dwarf elephant descended from the Columbian mammoth (M. columbi) of mainland North America. During glacial periods, the mammoths swam from the mainland to Pleistocene Santa Rosae Islands. During interglacial sea level rise isolated those as the original Sant Rosae island became 4-5 islands, later to know as the Channel Islands. Natural selection favored smaller mammoths- restricted from food sources, ultimately becoming pygmies.

This species became extinct during the Quaternary in which many megafauna species became extinct due to changing conditions to which the species could not adapt. A case of island or insular dwarfism, from a recent analysis in 2010 it was determined that Mammuths exilis was on average, 1.72 m (5.6 ft) tall at the shoulders and 760 kg (1,680 lb.) in weight, in stark contrast to its 4.3 m (14 ft) tall, 9,070 kg (20,000 lb.) ancestor.

Discovery

Mammoth remains have been known on the northern Channel Islands of California since 1856. They were first reported in scientific literature in 1873. In 1994 the National Park Service called in scientists to inspect an uncovered, unidentified skeleton found on the northeast coast of Santa Rosa Island. They found bones of the axial skeleton of a large land vertebrate and decided to excavate and dig up the skeleton. They recovered 90% of a mature male pygmy mammoth's skeleton. The mammoth was about 50 years old when it died. The small bones were preserved in life position, which represented that it had died where it was found rather than being scattered around the island. The bones were returned to the Santa Barbara Museum of Natural History. After the discovery of the skeleton, a pedestrian survey of the island began. This resulted in the discovery of 160 new locations of mammoth remains, the vast majority being found on Santa Rosa Island. This was the first discovery of a nearly complete specimen of the pygmy mammoth. Fortunately, the skeleton was only missing a foot, a tusk, and a couple of vertebrae. The remains were covered by a sand dune, which prevented the bones from scattering and kept them intact.

Figure 4.4: Excavation of a Pygmy Mammoth Skeleton on Santa Rosa Island, California. Image by Wikimedia is licensed under CC BY-SA 4.0

Habitat

Remains of M. exilis have been discovered on three of the northern Channel Islands of California since 1856: Santa Cruz, Santa Rosa, and San Miguel, which together with Anacapa were the highest portions of the now mostly submerged super island of Santarosae. The late Pleistocene elephant appears to have survived on the islands until the arrival of the humans associated with Arlington Springs Man around 13,000 years ago and the last known mammoth occurrence was 13–12.9 thousand years ago which predates the later Chumash people's arrival during the early Holocene, between 10,800 and 11,300 years ago. Radiocarbon dating indicates M. exilis existed on the island for at least 47,000 years prior.

Figure 4.5: A Map of Santarosae Island, around 15,000 years ago, which today makes up the Channel Islands, California. Image by University of Nebraska, Lincoln Digital Commons is in the public domain.

Modern elephants are excellent swimmers, and the ancestors of M. exilis swam the 4 mi (6.4 km) to Santa Rosae. As the population of mammoths increased, the lack of large predators such as the dire wolf, Smilodon and the American lion and the loss of habitat caused by the rise of sea levels at the end of the ice age as Santa Rosae split into four islands favored smaller animals. Because of this, the pygmy mammoths began evolving through generations as a survival mechanism to stay alive on the ever-shrinking Santa Rosa Island; their body size became smaller because it required less food and resources to remain energized and alive. After this evolutionary period, the mammoth had become a distinct species, the pygmy mammoth.

The pygmy mammoth was able to thrive in all the many different ecosystems found on Santa Rosae, such as high elevation plateaus, to dune, grassland, riparian, and steppe-tundra ecosystems. Their fossils are found in the Channel Islands and in the California Channel. The evidence of their habituation in all those diverse dwellings is revealed by the pollen and plant pieces found in sediments and in dung. In addition, each habitat has a specific isotope mark from the unique types of soil, plants, and water. These mammoths sometimes modified their habitats, specifically in Channel Islands, where they created more spacious grassland as a result of their roaming.

Evolution

Land bridges were once theorized to have connected the northern Channel Islands to the mainland, because it was assumed the mammoths could not swim. A land bridge between the mainland and Santa Rosae did not exist during the Quaternary; however, the distance to the mainland was reduced to 7 kilometers (4.3 mi). When the Ice Age caused the sea levels to lower, the four northern Channel Islands formed a single island that was closer to the mainland and larger in size. Also, their mainland predators such as the dire wolves, the Smilodon and the American lion were not present. After this evolutionary period, the mammoth had become a distinct species, the pygmy mammoth. The pygmy mammoth's evolution on Santa Rosae took over 30,000 years.

Extinction

A single cause of extinction of the pygmy mammoth is unknown, as it could have been caused by over-hunting by humans, wildfires, climate change, or some combination thereof.

· Human interference often has a greater effect on island species than on continental species, and there is evidence that Native Americans hunted the pygmy mammoth on Santa Rosa. Mammoths were still extant on the islands when humans arrived, and mammoth remains were associated with charcoal of the same radiocarbon date. Two mammoth skulls with the brain removed were found adjacent to a fire pit, of the 100 fire pits at least a third contained mammoth bones.

· A shift in sea level driven by climate change likely played a part in the extinction: as the sea level rose, about 61% of the island landmass was submerged. Some 4,000 years prior to extinction the island had an area of roughly 1,900 square kilometers (730 sq mi). Further increase in sea levels left four smaller islands with nearly eighty percent less total landmass.ut of the Collection: The Mammothote

Come check out these incredible pieces of California history! Either scan the QR code or visit this link to see Professor Patrich share a real piece of a California mammoth bone, & real woolly mammoth hair! (Video length: 6min).