The vast ice sheet that covered Canada during the last Ice Age began to melt rapidly about 15,000 years ago, creating enormous glacial lakes. The largest glacial lake, known as Lake Aggasiz, was bigger than all of the present day Great Lakes combined. The ice dam impounding this incredible volume of water collapsed 12,900 years ago, and a massive flood of cold freshwater, icebergs, and debris gushed into the North Atlantic via the St. Lawrence River. This event caused a sudden drop in global temperatures and a reversal back to Ice Age conditions at northern latitudes because the influx of cold fresh water shut down ocean currents that brought tropically-heated salt water north. The cold climate phase lasted for about 1500 years, and climate scientists refer to it as the Younger Dryas.
The colder ocean of the Younger Dryas should have spawned fewer hurricanes than the warmer oceans of today. Hurricanes are a product of energy released from warm ocean water. However, scientists discovered evidence hurricane activity increased off the coast of Florida during the Younger Dryas. They discovered deposits of turbidite near the Dry Tortugas Islands, dating to the Younger Dryas. Turbidite is sediment and rock resulting from underwater perturbations. Earthquakes can cause turbidite formation, but this region is not prone to seismic activity. Instead, hurricanes produced underwater currents that formed turbidite here.
Map of the Dry Tortugas–site of the study referenced in this blog entry.
Image showing how turbidite deposits are formed.
Scientists aren’t sure why hurricane activity increased during the Younger Dryas at this locality. Some of their climate models suggest the oceans were much colder to the north and west of the Florida coast but only slightly colder than present day ocean temperatures off the modern Florida coast. Perhaps the tropically-heated water that pooled near the equator spawned hurricanes that reached the Florida and south Atlantic coasts.
Increased hurricane activity contributed to the expansion of longleaf pine savannahs. The wind felled forests, and the accompanying lightning-sparked fires maintained longleaf pine savannah ecosystems while repressing closed canopy hardwood forests. Pleistocene megafauna became extinct during the Younger Dryas, even though longleaf pine savannahs are ideal habitat for grazers such as mammoths, bison, horses, giant tortoises, and many other species.
Reference:
Toomey, M. ; et. al.
“Increased Hurricane Frequency Near Florida during Younger Dryas Atlantic Meridional Overturning Circulation Slow Down”
The authors of a new statistical analysis of megafauna extinctions boast their method is superior to previous efforts, and they can’t reject human interactions as a cause of the extinctions. However, they also can’t rule out climate change as a factor, so despite their supposed superior method, their new study (referenced below) resolves nothing. The statistical method they used is known as kriging, a kind of interpolated algorithm. The data included 95 of the last radiocarbon dates of megafauna species and 75 of the earliest dated archaeological sites in North America. They then mapped the last appearance dates of the megafauna with the earliest archaeological evidence of humans. They concluded climate change caused the extinction of mastodons in Alaska because this species disappeared there long before humans colonized the region, but humans may have been a factor causing extinctions at lower latitudes. Megafauna persisted until the very end of the Pleistocene in some regions including Mexico, Texas, Tennessee, and the Great Lakes region. Humans may have overlapped in these regions for as long as 3000 years. The study doesn’t find much evidence for the blitzkrieg (rapid overhunting) model of extinction, though there may have been “localized” examples of this in western North America. But it is possible humans gradually disrupted ecosystems in a way that was detrimental to megafauna populations. This is known as the sitzkrieg model of extinction, and it is the scenario that makes the most sense to me.
There is a major flaw in the reasoning behind the conclusions of this study. The authors of this study equate the regional disappearance of a species with its extinction. For example mastodons became extirpated in Alaska about 30,000 years ago, but they did not become extinct. Populations remained robust south of the ice sheet until about 13,000 years ago. If man never colonized North America, it seems likely mastodons would have re-colonized Alaska and Canada during the present day interglacial. Habitat in present day Canada and Alaska is very much like that of some Ice Age regions mastodons formerly inhabited before their extinction. Mastodons were a wetland species, and aquatic habitats have greatly expanded in Canada and Alaska since the end of the Ice Age. (During the Last Glacial Maximum Canada was covered by ice sheets and Alaska was an arid grassland–both unsuitable environments for mastodons.) Climate change may have driven the redistribution of megafauna geographic ranges, but that is not the same as extinction. During climate phases that favored the expansion of grassland, woodland species were forced to migrate farther to find suitable habitat, and vice versa. Again, this is not extinction…it is a species adjusting to a new range map.
I’m convinced man, and man alone, is responsible for the extinction of the Pleistocene megafauna. These wide-ranging species (some occurred all across the continent) were adaptable species that survived dozens, if not hundreds, of dramatic climatic swings over hundreds of thousands of years. But when man begins to appear in the archeological record, they disappear permanently. There is no way that can be coincidental. I think man disrupted the ecological balance through a combination of overhunting, increased fire frequency, and interdiction of migratory corridors. It took a few thousand years, but when human populations reached a certain level, most species of megafauna could not adapt. They required an ecosystem with low levels of people and did even better with no humans on the landscape at all.
Graph of average annual temperatures over the past 150,000 years based on data from Greenland ice cores. Note all of the dramatic climate fluctuations. Megafauna species survived these climate fluctuations but became extinct in North and South America within a few thousand years of when man entered the continents.
Reference:
Weatherall, M.; Brianna McHorse, and E. Davis
“Spatially Explicit Analysis Sheds New Light on the Pleistocene Megafaunal Extinctions in North America”
Rob Nelson stood next to a wall of fossils on 1 episode of Secrets of the Underground, a Science channel tv series. He was visiting Friesenhahn Cave in Bexar County, Texas about 20 miles north of San Antonio during the taping of the series he hosts. The tusk of a mammoth or mastodon, a baby mammoth tooth, and many small fossils were visible; and they were cemented together. It’s remarkable that such an undisturbed matrix could still exist here because people have been excavating fossils from this site off and on for about 100 years. Specimens collected by local amateurs were first described from this site in a paper published during 1920. For awhile the landowner stopped permitting people to collect fossils in the cave, but then in 1949 Mr. Friesenhahn himself invited some professors to excavate fossils in the cave. They found the complete skeletons of scimitar-toothed cats and a long-nosed peccary plus the bones of 30 other species of mammals and the remains of reptiles and amphibians. The discovery of the complete scimitar-toothed cat skeletons was important because before this the species was known from an incomplete skull, a few teeth, and some isolated bones. Large numbers of juvenile mammoth and mastodon bones were found associated with the scimitar-toothed cat skeletons, and the paleontologists came to the conclusion the big cats used the cave as a den and dragged their prey inside.
A flurry of papers about the cave were published, but access was again restricted until Concordia University purchased the property in 1998. Apparently, since the purchase, some scientists have been working with the disturbed sediments, but they are waiting for a private or government grant before tackling the remaining undisturbed strata. I suppose they want to use the most modern techniques when going through this material. During the original dig 68 years ago, scientists mention fossils that were in such poor condition “they weren’t worth preserving.” (I was appalled when I read this.) There are modern methods that can preserve fossils that are in poor condition, but they can be costly. Scientists have also developed better ways of excavating fossils. Nevertheless, nothing has been published in the scientific literature about this cave since Concordia University purchased the property. It has been nearly 20 years, and they still haven’t been able to obtain funding for new excavations, though they do have a corporate grant to study the disturbed sediments. Still, it seems as if someone currently studying the cave would have at least published a paper by now entitled “Additional fossils recovered from Friesenhahn Cave.” To be honest, I am not impressed with their academic efforts here.
Brief excerpt of an episode of Secrets of the Underground, featuring Friesenhahn Cave.
A grate protects the cave from looters and keeps trespassers from falling inside.
The paleontologist, Grayson Mead, with the complete skeleton of a scimitar-toothed cat discovered in Friesenhahn Cave during 1949.
So far, 13 adult and 5 juvenile scimitar-toothed cat remains have been found in the cave. It’s unclear which of these were recovered in 1949 and which were discovered more recently. The cave has also yielded 1 bone of a saber-toothed cat, hundreds of baby mammoth and mastodon teeth, the bone of 1 ground sloth; and the remains of bison, deer, camel, tapir, long-nosed peccary, black bear, dire wolf, and coyote. The latter was especially abundant. Smaller animals that inhabited the area during the late Pleistocene, based on the bone accumulation in the cave, were jack rabbit, cottontail, desert cottontail, pocket mouse, and 4 species of mice in the Peromyscus genus. Some of these species are listed in the paleobiology database, and others are mentioned in the below referenced bulletin or on the Texas University website. The lists don’t match up. Someone needs to do a more thorough review of the specimens to determine exactly which species were found by whom and during which excavation.
Evidence suggests a pond periodically existed in the cave, depending upon rain and drought cycles. The basin filled during rainy years but dried out during droughts. No fossil evidence of pond turtles exists here. Instead paleontologists report remains of 2 terrestrial species–a large extinct subspecies of box turtle and an extinct tortoise (Geochelonewilsoni) related to the extinct giant tortoises that ranged throughout the south during the Pleistocene. G. wilsoni is known from just a few sites in North America but was first discovered in Frisenhahn Cave. Pond turtles never found the ephemeral water hole in the cave, but northern leopard and barking frogs did. Diamondback rattlesnakes used the cave as a den as well.
The species composition suggests the region around the cave was an arid grassland with some scrub. Woodlands existed alongside local rivers. The mammoth, bison, camel, coyote, and jackrabbit indicate dry grassland environments. However, the presence of deer, tapir, long-nosed peccary, and black bear suggest some woodlands or forest edge habitat existed nearby.
The cave formed when rainwater dissolved limestone rock underground. The initial entrance was small, and the oldest levels contain small vertebrates deposited in the form of owl pellets. Gradually, the entrance enlarged so that larger vertebrates began to use it as a den. Some of the fossil remains are from animals that died in the cave, but others were brought in by predators. Periodic flash floods may have added small bones to the collection. Eventually, the cave entrance collapsed, and the chamber was sealed for thousands of years until recent times when a sinkhole formed on top of the cave, allowing modern day access. The remains are estimated to be 19,000 years old, but it’s unclear where this estimate originated. I’m unaware of any carbon-dating of the objects in this cave. It was originally discovered before carbon-dating was invented. The site is badly in need of a more modern review, and I’m not sure Concordia University is up to the task.
References:
Evans, Glen; and Grayson Meade
“The Friesenhahn Cave” and “The Saber-toothed Cat, Dinobastis serus”
Bulletin of the Texas Memorial Museum September 1961
I enjoy deciphering articles published in scientific journals and translating them into language a layman can understand. I learned how to do this because of my long fascination with Pleistocene ecology. Information about Pleistocene ecology almost entirely comes from scientific journal articles, and I found the language in these publications can be unnecessarily complex and oftentimes the writing is just bad. I had to learn how to interpret them. Some scientists are good writers, but others are not. R.D. Lawrence (1921-2003) was a writer who felt the same way I do about language in scientific journals. At 1 point in his life he was studying to be a biologist. He wrote a thesis about stickleback fish, and his professor told him it was good, but he wanted him to rewrite it in the language used by scientific journals instead of the easy to understand language Mr. Lawrence had used. He rejected this “babblespeak” and dropped out of school. He later wrote 36 books about Canadian wildlife and won 7 awards.
The late R.D. Lawrence relaxing at home with his pet raccoon.
R.D. Lawrence was born in Spain to a Spanish mother and an English journalist who worked for Reuters. At the age of 14 he was separated from his family during the Spanish Civil War, and he joined the side fighting against the fascists. (Ironically, his brother joined the fascists.) Though just a teenager, he led 1 military attack in the sewers against the fascists. Eventually, he escaped to southeastern France and was later reunited with his family in England. He fought for Great Britain during World War II. He was at Dunkirk, rode a tank in North Africa, and was severely wounded during the D-Day invasion. His injuries ended his military career. He moved to Canada and worked as a journalist, while studying nature in his spare time. He gathered enough material so that he was able to start getting his books about Canadian wildlife published. Recently, I’ve read 3 of his books.
Mr. Lawrence and his wife bought some land in the wilderness of Ontario during 1962. Here, they built a cabin where they spent weekends. (He still worked as a journalist during the week.) He bought the land before most of Ontario was logged over and converted into suburbs, so much of the wildlife was naïve and not particularly afraid of people. He wrote about his experiences at this cabin in his book, The Place in the Forest. The semi-tame animals frequenting his cabin yard included red squirrels, fox squirrels, flying squirrels, white-footed mice, snowshoe hares, and birds. Bird seed and table scraps encouraged the creatures of the forest to hang around the cabin, and if the door was left open, they would enter the cabin and help themselves. Mr. Lawrence and his wife adopted 2 orphaned raccoon kits and after they were grown and freed, they often returned and joined the feast. Mr. Lawrence also wrote about some of the less tame inhabitants in the local wilderness–beavers, deer, wolves, black bears, and birds of prey. He didn’t let worms and insects go unnoticed either. My favorite chapter relates his encounter with a bald-faced hornet’s nest when he was climbing a tree to photograph a hawk’s nest on another nearby tree.
Mr. Lawrence’s wife died of a brain aneurism at a quite young age, prompting him to move to British Columbia where he decided to buy a boat and travel up the Pacific coast from Vancouver to southern Alaska by himself. He wrote about this experience in his book, Voyage of the Stella. When he fished for salmon to eat, he often caught weird species of fish–wolf fish, Pacific lancet fish, barrel eyes, and dogfish. He fed these to the killer whales that occasionally swam near his boat, and he even dove in the water with them while wearing his scuba gear. The killer whales never bothered him, but he once had to fend off a blue shark. On his journey he also encountered pods of Dall’s porpoises and a pair of whale sharks. He refueled his boat at the Indian villages that dotted the coast. Most of the Indians were friendly, but 1 drunk tried to hit him over the head with a ketchup bottle while he was trying to eat supper at a restaurant. The brave war veteran floored the Indian with an open palm blow to the forehead.
Mr. Lawrence demonstrated even more courage in his next book, The Ghost Walker. He spent 8 months in a wilderness cabin located in a remote area of British Columbia that was 60 miles from the nearest town, and the only feasible connection to civilization was an hazardous canoe ride down a river. He used this makeshift cabin as an home base for tracking a large male cougar. He gained the cougar’s trust, and the big cat let the man follow him around. Mr. Lawrence experienced several dangerous situations, aside from trusting the cougar not to turn around and eat him. During a blizzard, Mr. Lawrence sought shelter in a creekside cave but found himself staring eye-to-eye with an hibernating grizzly bear. Mr. Lawrence popped out of the cave like a “champagne cork” and fled, dropping his backpack which the angry bear tore to shreds. On another occasion he slipped down an icy slope, hit a tree, and sustained a concussion. He administered his own first aid. He often tracked the cougar after sunset, walking in the dark woods by himself for hours. He was 1 brave soul.
I have 1 criticism of Mr. Lawrence. He imagined he had established ESP connections with a cougar and a killer whale. There is no rational scientific basis for his belief. I’m sure it was his imagination, not an ESP connection. He was just lucky the animals he “communed” with didn’t decide to attack and eat him.
The scimitar cat roamed Eurasia and North America for millions of years until its extinction about 11,000 years ago. This apex predator sported fangs and was about the size of a lion. Evidence from Friesenhahn Cave in Texas suggests it preyed upon juvenile mammoths and mastodons as well as peccaries. A study of bone chemistry in Europe indicates 1 specimen ate yaks. The scimitar cat is not as well known as the more robust saber-toothed cat (Smilodon fatalis) that apparently was restricted to North and South America. Another species of saber-toothed cat (S. populator) occurred in eastern South America. Remains of scimitar-toothed cats are uncommon in the fossil record. Entire skeletons were found in Friesenhahn Cave, but elsewhere, if evidence is excavated of scimitar cat, it is usually just a single bone or tooth. This reflects low populations compared to the prey species it fed upon. Generally, large carnivores are less common at fossil sites than the species they depended upon for food because predators are necessarily less abundant than their prey. This low population caused an huge gap in the fossil record of Eurasian scimitar-toothed cats. There was no evidence of scimitar-toothed cats in Eurasia after 300,000 years BP until the jaw bone of 1 individual was netted by a fishing boat in the North Sea a few years ago. (Much of the North Sea was above sea level during Ice Ages.) This new specimen dates to 28,000 years BP. There is also 1 stone statuette that may or may not be of a scimitar-toothed cat–the identification is unclear. Thousands of generations of this species lived on the continents of Europe and Asia, but this is all the evidence we have of scimitar-toothed cats living there during the late Pleistocene.
Illustration of scimitar cat.
This Pleistocene-aged stone statuette may be a representation of a lion or a scimitar-toothed cat. The deep chin, evolved to protect the fangs, and the bob-tail are evidence it represents Homotherium. However, the absence of fangs and straight back (Homotherium had a sloping back) indicate it represents a lion. Unfortunately, this artifact was lost a century ago.
Scientists analyzed the DNA from the North Sea specimen and compared it with the DNA from specimens of scimitar-toothed cats from the Yukon, Canada and some specimens of South American Smilodon populator. They determined the lineage of scimitar-toothed cats (Homotheridae) diverged from the lineage of saber-toothed cats (Smilodontheridae) 18 million years ago. Despite their similar convergent appearance they were about as closely related to each other as house cats are to tigers. The genetic study also suggests the Eurasian scimitar-toothed cats (Homotherium latidens) and the North American scimitar-toothed cat (H. serum) should be considered the same species. The authors of the study found “low genetic diversity despite wide geographical occurrence.” However, the specimens the scientists used in the study were north and west of the ice sheet that cut most of North America off from Beringia and Eurasia. Scientists need to study DNA from specimens south of the ice sheet to conclusively show H. latidens = H. serum. A previous genetic study of lions (Panthera spelea, P. leo, and P. atrox) determined North American lions south of the ice sheet were a different species than those north of it.
Reference:
Anton, Mauricio; et. al.
“Soft Tissure Reconstruction of Homotherium latidens (Mammalia, Carnivora, Felidae): Implications for the Possibility of Representation in Paleolithic Art”
Geobios 42 (5) 2009
Paijmans, Johanna; et. al.
“Evolutionary History of Saber-toothed Cats Based on Ancient Metagenomes”
GeorgiaBeforePeople 