December 23. Primates

The end of the Paleocene and start of the Eocene was apparently a stimulating time, in terms of evolutionary diversity for the mammals. The oldest known primate-like animal dates to this time, from lake beds in France and from North America, about 56 million years ago. Plesiadapis was a small lemur-like animal adapted to climbing trees and eating plants, although it may have been an opportunistic omnivore. Its ancestors are uncertain, but it probably evolved in North America and migrated to Europe.

Mesopithecus (Gaudry, 1867)

By Oligocene time, the lineages of both the Old World and New World monkeys had been established. Many fossils have come from the East African rift valleys, also the source of many hominid fossils. The lake deposits there preserve these rare remains relatively well. 

The Miocene, around 12 to 18 million years ago, saw the divergence of groups such as the great apes, orangutans, and gibbons. By very late Miocene, 5 to 7 million years ago, many monkeys were probably quite recognizably modern in appearance. There’s an excellently preserved example called Mesopithecus, found in Greece, that lived about that time and likely lived on fruits and leaves. It was about 16 inches long.

The primitive primates of Madagascar, lemurs and their kin, present a challenge to plate tectonics. Madagascar was already separated from the other continental masses by Paleocene, when these animals developed. So how did they get there? In fact there are only five orders of land mammal on Madagascar today. The favored idea has usually been rafting – drifting of land animals on floating mats of water-borne debris, though that idea relies on a lot of luck. The luck depended on a relatively short window of geologic time, between 60 and 20 million years ago, when the distances and currents would make the rafting idea plausible. The best estimate for the colonization of Madagascar by primates is probably early to middle Eocene time, 40 to 52 million years ago. 

Alternatives to the rafting idea include the presence of an ancestral primate on Madagascar, but none have been found, and “island hopping,” crossing the distance in short spurts to intervening islands – but the likelihood of such islands is low. I don’t think there is unanimous agreement as to how primates got to Madagascar, but for now I think the rafting idea is most likely, even if it was actually pretty unlikely. Just more likely than the other possibilities. Once they got there, the lemurs and other primates probably survived because they lacked the competition from the more advanced monkeys that developed in Africa by Oligocene time.

—Richard I. Gibson

Lemur origins 

Reconstruction of Mesopithecus from A. Gaudry, Animaux Fossiles et Géologie de l’Attique. Recherches faites en 1855–1856, 1860 (1867), public domain.

December 22. Sierra Nevada and Wasatch Front

Today’s episode is a follow-up to the Basin and Range discussion the other day, and a follow-up to the Sierra Nevada Batholith which we talked about back in the Jurassic, in October. The Sierra Nevada Mountain Range in California coincides pretty well with the Jurassic-Cretaceous Sierra Nevada Batholith, the deep roots of the subduction and magmatic arc system that was established out there by at least 150 million years ago. 

But the modern range is vastly younger than that. The Sierra Nevada is basically one huge fault block, tilted to the west, with the fault marking its eastern front. It’s a big normal fault, with the mountains up to the west and the region to the east dropped down. That fault is essentially the western margin of the Basin and Range Province, the extended, faulted suite of uplifts and basins that continues all the way east to the Wasatch Mountains in central Utah. 

The action on the Sierra Nevada Fault started about the same time as the Basin and Range became active, in the Miocene Epoch, 15 to 20 million years ago. And most of the uplift is in the past 5 million years. There’s easily 10,000 feet of displacement on the fault and probably quite a bit more. 

Intermountain Seismic Belt
marked by dashed black line.

It may be that the size and strength of the huge granitic batholith provided a mechanical boundary to the part of the crust that was able to break into the Basin and Range Province. The other side of the Basin and Range, the Wasatch Mountains, mark the western edge of the Colorado Plateau, which is also a region that is fundamentally different in terms of the nature of the crust from the Basin and Range. The Colorado Plateau’s crust is thicker, cooler, and mostly older than the Basin and Range. So, thinking back to the episode a couple days ago, this helps us look at the unique broken nature of the Basin and Range as likely connected to its mechanical properties, such as thickness, composition, and age.

Today, there is some seismic activity within the Basin and Range, but it’s pretty spotty. In California, the main tectonic focus is along the San Andreas Fault Zone. But the Sierra Nevada Fault is almost certainly still active. The uplift of the Sierra Nevada isn’t simply on the one fault, and in 1872, a magnitude-7.8 quake hit the Lone Pine area, in the Owens Valley east of the Sierra. The movement wasn’t on the Sierra Nevada Fault, but on the nearby and related Owens Valley Fault, which is part of the system of faults continuing to uplift the Sierra Nevada. Some segments had vertical offset of as much as 20 feet.

On the other side, the Wasatch Mountain Front, which forms the backdrop of Salt Lake City and all of central Utah, is part of an earthquake belt called the Intermountain Seismic Zone. Its heritage is also about the same timing as the Basin and Range, the Miocene, but there has been some level of discontinuity there, at the western edge of the Colorado Plateau, for many tens of millions of years.

The activity along this belt is usually nothing like that on, say, the San Andreas Fault, but great earthquakes do occur, including the Hebgen Lake Quake in 1959, which had a magnitude of about 7.4. The Intermountain Seismic Belt goes from southern California, through southern Nevada and central Utah, eastern Idaho and western Wyoming, through western Montana and into Canada. If you drive Interstate 15, you’re pretty much following this zone. There hasn’t been a big earthquake on the Salt Lake City segment of the Wasatch Fault for about 1,300 years. That doesn’t mean one is due; typically, the historical record of earthquakes is far too short to infer any kind of regular periodicity, and it can be challenging to date prehistoric quakes with accuracy. Nonetheless, seismologists always try to figure this out for general hazard preparedness, and there do appear to have been sizeable earthquakes on some part of the long Wasatch Fault zone about every 350 years. A big one will happen there, sometime.

* * *

Our anniversary today is an earthquake, on December 22, 856 A.D., at Damghan, Iran. It had an estimated magnitude of 7.9 and killed about 200,000 people, one of the most deadly earthquakes ever. It occurred in the Alborz Mountains, the range in northern Iran along the shores of the Caspian Sea. The South Caspian Basin is a small bit of oceanic crust that is trapped in the ongoing Cimmeride and Alpine-Himalayan collisions that we’ve talked about previously. Here, it seems likely that the dense bit of oceanic crust is subducting, at least somewhat, beneath the Iranian continental block. Volcanoes in the Alborz Range, including Damavand, support that idea. And so do the earthquakes.

—Richard I. Gibson

Sierra Nevada
Sierra Nevada Fault Scarp (1898)
Intermountain Seismic Zone

December 21. Saber-toothed cat

Smilodon, the famous saber-toothed cat, was not a tiger nor even closely related to tigers. They were felids, cats in general, but they branched off from the families that include all modern cats pretty early in the development of the feline families. They probably diverged sometime during the Miocene, more than 10 million years ago. 

Smilodon and dire wolves (drawing by Robert Horsfall, 1913)

Smilodon’s scientific name doesn’t mean “smiling tooth,” but rather comes from the Greek meaning “carving-knife tooth,” and they did certainly have huge sharp canine teeth up to 11 inches long. The animal was about the size of a lion, and was probably the last in a lineage of similar carnivorous animals. It lived during the glacial period, the Pleistocene Epoch, from about 2.5 million years ago until 10,000 years ago. We’ll talk a bit more about the Pleistocene extinction of the saber-toothed cat and other large animals in a few days. Smilodon is the state fossil of California.

Lots of Smilodon fossils have been found in the famous La Brea Tar Pits in Los Angeles, along with a hugely diverse fauna. The presumption is that animals were attracted to the tar mistaking it for water or simply crossed it inadvertently, and became trapped. The asphalt, the tar, in the tar pits, is basically a heavy oil, a natural seep that has been active for many thousands of years. Oil, migrating from depth, has reached the surface. The material is less fluid than most oil, but more fluid than, say, the tar sands of Alberta. It’s not really a surprise that we’d find oil through the spectrum of possibilities, from highly liquid to completely solid, as in oil shale. There is a real oil field at depth at the La Brea site, the Salt Lake Oil Field, where oil is trapped in Miocene and Pliocene sediments in anticlines and fault blocks. The buoyant oil continued to migrate upward along one fault to reach the surface and form the tar pits. The material is heavy and viscous because the volatile portions have evaporated at the surface, leaving the tar behind. Brea is Spanish for tar.

The oldest fossils in the La Brea Tar Pits date to about 38,000 years ago – and many of them are extinct. The assemblage is huge, and includes extinct bison species, llamas, cheetahs, bats, sloths, mastodons, more than 100 bird species, snakes, insects, and plants. It is truly a lagerstätte, one of those amazing places where fossils are preserved in extraordinary detail.

—Richard I. Gibson

Drawing of tar pits by Robert Bruce Horsfall, 1913 (public domain)