T. Rex, one of the largest, clocked in at about 9 tons. Not much less than the Giganotosaurus and the record holding for weight carnivorous dinosaur, the Spinosaurus. Other theropods not so huge.
Those biggest were a bit bigger than the the current largest land animal, the African elephant (about 6 tons with the record known to be 11 tons) but “significantly” is a stretch.
Cites given earlier in this thread though are of mammals that hit over 15 tons and maybe even one over 20 tons historically. So “somewhat larger than the largest land mammals ever” seems to be a clear false statement.
That level of gigantism land mammals are documented to have done as well and is in the rough range of the largest extant ones.
Why so many more large dinosaur predators? Could their size have been driven by the size of the prey? Even adolescent sauropods were big.
Not so much since there is no longer any such coincidence in dates.
Of course, that doesnt mean humans didnt eat their share. Not impossible at all that hunting drove a species or two to extinction- altho it hardly every happens. Otoh humans did bring along other species and diseases and also started modifying the environment with fire. So, sure, humans are no doubt a culprit.
There’s nothing that says that the extinction of any species had to have a single cause. Maybe the mammoths could have survived climate change, and maybe they could have survived humans, but they couldn’t survive both happening nearly at once.
A carbon-rich black layer, dating to ≈12.9 ka, has been previously identified at ≈50 Clovis-age sites across North America and appears contemporaneous with the abrupt onset of Younger Dryas (YD) cooling. The in situ bones of extinct Pleistocene megafauna, along with Clovis tool assemblages, occur below this black layer but not within or above it. Causes for the extinctions, YD cooling, and termination of Clovis culture have long been controversial. In this paper, we provide evidence for an extraterrestrial (ET) impact event at ≅12.9 ka, which we hypothesize caused abrupt environmental changes that contributed to YD cooling, major ecological reorganization, broad-scale extinctions, and rapid human behavioral shifts at the end of the Clovis Period. Clovis-age sites in North American are overlain by a thin, discrete layer with varying peak abundances of (i) magnetic grains with iridium, (ii) magnetic microspherules, (iii) charcoal, (iv) soot, (v) carbon spherules, (vi) glass-like carbon containing nanodiamonds, and (vii) fullerenes with ET helium, all of which are evidence for an ET impact and associated biomass burning at ≈12.9 ka. This layer also extends throughout at least 15 Carolina Bays, which are unique, elliptical depressions, oriented to the northwest across the Atlantic Coastal Plain. We propose that one or more large, low-density ET objects exploded over northern North America, partially destabilizing the Laurentide Ice Sheet and triggering YD cooling. The shock wave, thermal pulse, and event-related environmental effects (e.g., extensive biomass burning and food limitations) contributed to end-Pleistocene megafaunal extinctions and adaptive shifts among PaleoAmericans in North America.
The radiocarbon dates also suggest that northeastern megafauna underwent two major declines before finally going extinct. The first was 14,100 years ago, before any humans were in the region, but the number of animals then recovered after about 500 years; the second and final population crash began 12,700 years ago, when Paleoindians had just arrived in the region, according to the archaeological record. Moreover, the team reports in the 1 February issue of Quaternary Science Reviews, even though humans and megafauna continued to coexist for about 1000 years before the animals finally went extinct, the animals were already on their way out: Between 75% and 90% of the northeastern megafauna were gone before humans ever came on the scene. Yet even during the millennium of human and animal overlap, the team argues, there is no evidence for hunting: Neither megafaunal nor Paleoindian sites in the northeast contained animal bones that were butchered or otherwise modified.*
I’m not sure it is a question gross content, more that it is a question of ratios.
You’d need one of the more scientific guys to give you any factual specific answers
But one reason the percentage of oxygen fluctuates is that the earth sequesters and releases various other gases on an ever changing basis.
Carbon is a real common one that most people understand.
Trees and plants take in carbon over their life times, they die.
If they become buried away so they dont normally just decompose, their carbon content is locked away in the ground and not returned to the atmosphere.
Have lightning make a huge forest fire, all those trees burning are releasing their carbon, it is no longer sequestered
Oceans are also big carbon sinks, and various things can induce the ocean to give up its carbon to the atmosphere.
Release a large amount of carbon, and the oxygen content per volume goes down.
The raw amount of oxygen did not go away, there is just now more of other things.
Like if you take a cup of bleach, you cant drink that, poison…
Now if you pour 100 gallons of water in with it, you can drink it.
Yet its still the same exact 1 cup of bleach, except to get that 1 cup in your tummy
you have to drink the 100 gallons of water it is diluted into.
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There is no one reason, many different things and different things in different places.
Some places that were hospitable are now frozen and buried under ice, some are now desert, many are no longer tropical nor sub tropical.
Some could have possibly been wiped out by the results of a large impact, volcanic event, tectonic movements etc.
Environment and climate is a big deal for plants, they can not simply migrate or make rapid adaptations.
Once antarctica and alaska were more like florida is today, imagine what would happen to plants native to florida if it became like alaska?
How about if it became like antarctica?
Some were simply beaten out by emerging modern plant forms.
Pretty sure we have some people here that can give you a better more specific answer.
Quote function not working for me for the post from DrDeth a couple of posts above, but the Firestone/West hypothesis has been an insane bag of ignorant crazypants from the very beginning, and I have followed it from the very beginning, including reading their moronic book. I could go into it at length when I’m not tapping it out on a phone–for now, I’ll leave just this tidbit.
Whether is it right or wrong it was published in a peer reviewed scientific journal, so calling them “insane bag of ignorant crazypants” is pretty much the opposite of how science works.
Okay, more of Firestone and West. They published their “theory” in the form of a mass-market book with the title The Cycle of Cosmic Catastrophes. Here is just a small sampling of their ideas, quoting from the book. (Spoilered for length.)
41,000 years ago, the initial burst from a nearby supernova reached Earth.:
The explosion happened while above the horizon in Southeast Asia and Australia, so it caused a megafaunal die-out there, but Europe and the Americas were fine. Then 34,000 years ago, the shock wave from the supernova hit the Earth:
Then there was the third wave of extinctions. I won’t quote the more lengthy descriptions of this, but at the moment of the supernova, giant chunks of silicates formed in its atmosphere and were tossed out in all directions. The chunks were miles across but less dense than a snowflake. Many of those chunks reached the Earth between 16,000 and 13,000 years ago. One of them, 300 miles across but less dense than styrofoam, exploded over the Canadian ice sheet at 70,000 miles an hour. That explosion is what wiped out the Clovis people and the lost megafauna in the Americas. It left no crater in Canada, but debris from it carved out all of the Carolina Bays and similar structures with different names. Also, all the meteorites recovered in the American Midwest through the efforts of H.H. Nininger (of various dense silicate, iron, and other types) are debris from this 300 mile wide snowflake. Also, all the gold and precious stone deposits in the middle of the US were debris thrown up from Canada by the atmospheric exploding snowflake, and the sky rained diamonds for months after the impact. (I’ve just barely skimmed over my ebook of this for these sample passages—some of those claims may not have appeared in the book but instead in reports in the credulous popular media, which published “news” articles on these ideas.)
Oh, heck—I said I wouldn’t quote from this, but I’m quoting from this:
So, that’s your scientific credibility of Firestone and West right there. These people were crackpots spewing insane horseshit. If you really want to side yourself with them, that’s your choice, but that isn’t glory that you are covering yourself with.
Because going bigger will let a predator eat elephants?
I’m not sure if it’s true that elephants have no predators. Crocodiles will attack elephantsin the water or at the water’s edge. My Google-fu has failed to detect if they are ever successful.
I quoted their article in a peer reviewed scientific journal. Nor was I siding with them. Their article was interesting, and relevant. The peer reviewed scientific journal is hardly a refuge for crackpots spewing insane horseshit.
Has a article in a peer reviewed scientific journal been posted that refutes them? I would like to read it.
That’s how science works , you know. Not by name calling.
Balance
Predator has to remain in balance with his environment at large, or he will cease to be a predator and become a fossil.
Elephant is not the most highly stocked shelf in the meat market and all the other meats come in smaller packages.
And getting those meats is expensive.
So you sit in the median range with the largest possibility of food.
Even the dinosaurs were like this, the largest predators were not the biggest boys on the block, can’t over grow the available groceries.
They are on the calves, not sure on a healthy adult, risky dinner to say the least
Especially since, in the rare event that a lioness manages to take down an elephant calf, the elephants will band together to seek out and kill the lioness and all of her cubs. I imagine that they do the same to crocodiles, too.
I would be very interested in seeing some reputable source for that behavior, a group of elephants hunting down a specific lioness to wreak revenge on both her and her cubs.
Darren Garrison, it certainly seems that the Younger Dryas impact hypothesis has been very throughly investigated and pretty much completely debunked.
Two questions for you.
Given the lack of any impact experts as part of the original group and the Pinter et al characterization of impact hypotheses as “novel, self-contradictory, rapidly changing, and sometimes defying the laws of physics”, how did they get published in PNAS in the first place? That’s not some vanity journal, it’s real peer review. You’d think the impact experts who reviewed it would have seen the problems. Were these people “big enough” for other reasons that giving their pet crackpot ideas a full airing was somehow required?
Your response to seeing the hypothesis brought up was quite strong - not just educating we TMs that the hypothesis has already been throughly investigated and has been tossed aside by now - do you have some additional personal connection to this story or is there some other reason that this “cautionary tale for researchers, the scientific community, the press, and the broader public” has particular meaning for you? Is it just frustration with pop media running with marginal speculative hypotheses as if they are serious science while relatively ignoring the major scientists in a field? Or more personal?
Firestone is a real scientist in the area of studying isotopes–he wrote a book on the subject that is apparently well regarded. But his understanding of science outside of atomic isotopes is insanely wrong, and when he got some clue about how wrong something was, he would adjust it to be equally wrong in the opposite direction. For instance, his original claims were that his supernova comets were traveling at around 10,000 miles per second when they hit Earth–when it was pointed out how wrong that was, he adjusted it to 1,000 miles per hour, which is impossibly low. And in the book, the comets (that formed inside a supernova explosion) were hundreds of miles across but with a density of 0.02 grams per cc. An utter impossibility-- those comets would collapse into a denser state even under their small gravity (of course, comets wouldn’t be forming in an exploding supernova in the first place.) I have no idea how he got his article published in a peer-reviewed journal other than the fact that reviewers can’t be well versed in all areas of science. But for someone who is even just a hobbyist in meteoritics and impacts such as myself, there were many glaringly obvious errors from the very beginning, and as he constantly adjusted his story, he constantly generated new errors.
West, on the other hand (as mentioned in the fist article I linked) was simply a con-man with a degree from a bible college (if any degree at all) who was hiding behind a pseudonym to cover his criminal history.