I’ve had this burning question inside me since taking a Zoology class in college… it’s really rather simple. If 65 million years ago something happened (and it doesn’t really matter what)that wiped out all the known families of dinosaurs (including those in the oceans) how come co-resident reptiles like turtles and crocs didn’t die off also? Were they really around back then? Does the fact that they hide their eggs in sand etc. have anything to do with it? Does anyone really know?
No dinosaurs lived in the oceans. The large marine reptiles were different types of reptiles, more closely related to lizards than to the dinos and crocodilians. I don’t believe that any ichthyosaurs or mosasaurs made it to the end of the Cretaceous, and the plesiosaurs were extremely reduced in variety before the end.
So far as we can tell, it was only the large dino species that died off at tha K/T boundary. The smaller ones had grown feathers and learned to live in trees before then. The species of turtles and crocs which squeaked through into the Eocene were also much smaller than the average dinosaur.
So size matters? But surely not all the small dinosaurs ultimately became birds? Is that the current theory?
Sorry dude, it’s too late in the evening for me to dig up any cites, so I’m working from memory here.
IIRC, no land animals which massed more than about ten kilos survived the K/T event. As the recent finds from China show, it appears that any dinosaur in that size range had feathers (or feather-like structures) presumably for insulation. Therefore all small dinos were birds, and had been for a long time before the end of the Mezozoic. Cladistically it makes sense to consider modern birds to be just the types of dinos adapted for flight, just as bats are merely mammals adapted for flight.
Okay… I believe you. Just one more question. You mentioned that the marine reptiles, presumably both large and small (were their small ones?) also died out about this same time. Is it presumed that whatever killed the dinosaurs also killed the large marine reptiles too? And are we still hanging onto to the “meteor hit the earth and caused a climatic catastrophy that the smaller animals (like amphibians) somehow managed to survive” theory?
Yes, the comet/asteroid hit is still the leading explanation for the K/T mass extinction. Any large comet/asteroid hit (IIRC, it is thought that it was a 6-mile wide object that hit Earth…or was it 10-mile? anyway…) will send up huge amounts of dust into the atmosphere. So, for several months, there is very little sunlight reaching the Earth’s surface. Thus, lots of plants die. Then large herbivores starve. Then the carnivores that eat the herbivores starve. Large animals require lots of food…and there wouldn’t be enough to go around after a hit like that. The smaller animals managed to struggle through those months and, when the dust settled, they found they had reign of the planet. Since plants are the base of the food chain for both land and water animals, both land & water animals were affected.
But Phobos, what about the effect on late-Cretaceous flora and fauna of the Deccan Traps eruptions? I understand that increased volcanic activity in that period started to reduce the diversity of large animals through climate change and acid pollution, to name but two factors.
I’d like to point out a distinction that seems to have been overlooked: Dinosaurs were not reptiles. Chelonians (turtles) and crocodilians, true reptiles, had split evolutionarily from dinosaurs many millions of years prior to the K/T event.
This is not meant to detract from the otherwise solid answers that have been posted in this thread; only to clear up a common misconception.
How did being small improve chances for survival? If food became scarce, small creatures generally have a higher metabolism realtive to larger animals so they’d starve first. I guess you could argue that they require less food mass, but still…
If it was cold, smaller creatures have more difficulty retaining body heat than larger ones so they’d freeze first. Perhaps the feathers or fur of the survivors was a major determinant? If the small creatures were (coincidentally) furry or feathery, then perhaps smallness is just an association?
I believe the cladistic definition of reptilia is all anapsids and all diapsids. Anapsids include turtles. Diapsids include crocodiles, dinosaurs, and birds. Because turtles are anapsids, and crocs and dinos are both synapsids, turtles likely split off before the crocodile/dinosaur split. Thus if turtles are reptiles and crocs are reptiles, it makes no sense for dinosaurs not to be reptiles.
BTW, before anyone says “What? Birds aren’t reptiles!” keep in mind that there’s a recent shift in classification schemes. Linnaeus created the Linnean system of classification, which grouped creatures pretty much by how much they looked alike. Birds were given their own group, Aves. Creatures were either Avians, or reptiles, or some other creature, but never both. But paleontologists began to have terrible troubles: If birds evolved from dinosaurs, each time they found a bird-like dinosaur fossil they had to decide, should this be stuck into the Aves group, or should it be in Reptilia? It became more and more evident that it was an artificial and arbitrary system.
The newer system groups organisms by their evolutionary lineage, and is known as phylogenetic taxonomy. If birds are evolved from reptiles, then birds are reptiles, and Aves becomes a subset of dinosauria, which is a subset of reptilia. It’s a much more natural, sensible system.
Random note: There’s no taxonomic definition for “bird.”
Well, if you’re going to get cladistic, there’s no such thing as a reptile at all. Crocodiles are much more closely related to birds (and to dinosaurs) than crocodiles are to turtles. The former Class “Reptilia” has pretty much been discarded in recent classifications for exactly that reason. You can’t include crocodiles with turtles without also including birds in the same taxonomic group.
Ice Wolf, the Deccan Traps basalt flows have been cited as an alternative explanation for the K/T extinction. However, I saw a very interesting lecture a few months ago that mentioned speculation that such flows could be triggered by a celestial impact on the other side of the planet - something about focussing shockwaves. Supposedly the Indian plate was exactly opposite the Yucatan impact site at the time.
A minor nitpick with Dr Fidelius: there is increasing evidence that many of the small meat-eating dinosaurs were feathered (not necessarly including ones in the herbivore lineages). However, not all of these were on the lineage that led to birds. “Birds” may be dinos, but not even all small feathered carnivorous dinos were “birds.”
Otherwise Phobos and DrF have summed up the answer pretty well.
choosybeggar, just because it was dark doesn’t necessarily mean it was cold. Most of the animals that survived the K/T crisis were evidently either (1) small and needing little food (2) scavengers, or able to feed on seeds, etc. (3) able to go dormant (this could include both turtles and crocs).
And doesn’t already know, the place generally regarded as the likely impact is the Chicxulub Crater. You can find tons of info on it by plugging it into any search engine.
Imagine you have two types of animals, say a Brachiosaur that eats 1 ton of plants per day, and a smaller dino that eats a tenth that, 200 pounds of plants per day.
If their available food is cut by a factor of ten, a tenth of the population of small creatures may survive. But you can’t have a tenth of a brachiosaur wandering around.
This is of course a simplification, but you get the idea.
(And I’m not saying this is the only factor, it’s just my favorite)
True. In the middle of winter, when I turn off my lights to go to bed, the temp in my room doesn’t change. Is there any evidence that temps remained relatively stable during the K/T crisis?
Colibri, re the suggestion that the Chicxulub impact caused the Deccan Traps. Ya gotta be kidding me! It was a magma hot spot, which punched a hole through the mantle where India was at the time. Apparently, Reunion Island is over it right now. Are hot spots the product of dramatic space events now?
Just seen another theory concerning radiation levels and the K/T extinctions. Also see that in December 2000 it was found that Chicxulub was too small to be a planet-killer (I’ve seen a documentary in the last two weeks putting the theory that the asteriod/comet/whatever impact caused underground sulphur deposits to rise up into the atmosphere. Which means we’re back to the acid-rain theory).
If the radiation theory has some credence (switching of the poles, increased solar flare activity etc.), this could explain why the larger dinosaurs, more radiation sensitive than others, died out. Brachiosaurs, BTW, died out before the Cretaceous. Also, why was it that the majority of extinctions were among animals, not plant-life?
I think there were a range of factors that changed Earth’s fauna at K/T.
Has anyone on these boards seen the theory that dinosaurs actually descended from bird-like reptiles? They came from the trees …
He’s not kidding. This is a pet theory of Michael Rampino’s. A couple guys at Caltech also suggested it. It even made Time magazine a few years back. They mapped the hotspots versus the impact craters–unfortunately, that Yucatan impact site was not exactly opposite. The idea is that the impact creates the hotspot.
Who was giving the lecture, Colibri?
Also, the Siberian traps seem to be connected with that other big extinction event–but I don’t believe they’ve found much iridium around it. So, maybe…
Great discussion and I appreciate all of the information but I still don’t understand how some climatic factor that caused a massive die off of plants didn’t impact the poor defensless amphibians. Based on how they react today, very little change in the environment can cause massive frog die offs… yet they survived this catastropic event unharmed? They must be a lot tougher than we thought…
Short answer, dolphinboy – frogs and their kin of today are relatively small. They survived partly due to body size (see earlier posts on this topic). Their bigger relatives, after years of being kicked out of niches by reptilian cousins, died out by the K/T boundary.
Thank you everyone for a really interesting topic. I’ve learned heaps.
The guy I heard lecture was Gregory Retallack, an ancient-soils expert from the University of Oregon. He’s been working on the extinction event at the end of the Permian, which was The Mother of All Global Catastrophes - much bigger than the K/T event. About 90% of all species in the ocean and 70% of all species on land apparently bought it then. There previously didn’t seem to be much evidence of an impact for that one, so it was often attributed to all the continents coming together to form Pangaea, with the loss of continental shelf habitats and greater aridity on land. But now there is evidence of an impact, including shocked quartz and IIRC some iridium, but not nearly as much as at the K/T boundary. The supposed impact crater in Australia is opposite the paleo-position of the Siberian basalt flows. Retallack and others have produced evidence that the extinction took place extremely rapidly in geological terms.
There are five great mass extinction events in earth history: end Ordovician, late Devonian, Permian-Triassic (P/Tr), end Triassic, and Cretaceous-Tertiary (K/T). As the first two mainly involve clams and such they rarely make the cover of Time.
About a month after I heard Retallack I heard a seminar by another scientist, whose name escapes me at the moment, who claimed that the “Big Five” mass extinctions are tied to extremely long-term fluctuations in Earth’s climate between hot and cold periods. (Others have recognized this periodicity in extinctions, but attribute to some kind of periodicity in impacts - the “Nemesis” hypothesis, for instance.) He made a fairly convincing case, but the P/Tr event didn’t really fit in the scheme. He did some handwaving about it being a time of rapid climate transition, but if it was actually due to an impact this objection to his ideas disappears.
The interesting thing is that the K/T impact event apparently took place when one of his “hot climate” crises was already underway. Many scientists have objected to the K/T impact being the cause of the extinction, since some dinosaurs, sea reptiles and other groups were already in decline or extinct before the impact. However, it appears to me that both explanations could be correct - that the asteroid just happened to hit during a period when a mass extinction was due anyway due to climate cycles. And if the impact triggered massive volcanic eruptions too, then poor old T. rex didn’t stand a chance.
Here’s a couple of links for additional reading:
Regarding amphibians, I think that their survival tends to argue against the acid-rain idea. In fact, fresh waters were one of the few habitats on Earth where there was not much extinction at the K/T boundary. Many freshwater fish, turtles, crocs, frogs and amphibians pulled through. This says to me that there probably couldn’t have been a massive influx of acid rain, especially as far as the amphibians were concerned. As to other reasons for their survival, besides being small, as Ice Wolf said, they also are capable of going dormant. Anyway, since decay-feeding insects may have had a field day after the K/T event, they would not have lacked for food.
Wow, Colibri, good s–t! Any chance this is close to your field? I thoroughly enjoyed being de-ignoranced by you. Let’s do it again sometime.