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[Infinity Blade]

So if I'm understanding the rope analogy and what the square cube law says correctly, the total volume of a rope is not what determines its tensile strength, it is its cross section. However, the volume increases by the cube while the cross sectional area increases by the square. The same holds true for muscles; as an animal gets bigger (read: more massive), its muscles consequently get bigger (read: more voluminous). But just as with rope, the volume of the muscle increases faster than the cross sectional area does. This is why larger animals will seem "pound for pound" weaker than smaller ones, requiring the need to account for the square cube law to truly find out how strong certain muscles are in one animal compared to another animal that in real life differs from the first in size.

I just wrote down a derivative of the mathematical expression of the square cube law. I think I have it right.



EDIT: after TinyPic crapped itself and made this image invisible, I took it upon myself to see if I could obtain this derivative from the square cube law over a year later and reupload it to Imgbb, until I found the original picture-> in my emails. I remembered the first half, but not the second. Luckily when I saw the original I quickly remembered the purpose of the second half.

[creature386]

Yeah, I got the same result.

[theropod]

Yep, correct.

I think you wouldn’t call it pressure, but tension, or stress (as it’s not compressive) but the principle is the same. Basically, the "pressure" the muscles can take, or pull against, is the muscle’s tension, and that tension divided by their cross-sectional area is the specific tension, which is relatively constant across animals (not knowing soft tissue details for extinct animals we usually take it as constant, though in some cases there might be reason to assume higher pennation angles, which would effectively increase the number of muscle fibres relative to the cross-section, thereby increasing the force).

In addition to the ones creature listed, other good questions you could throw up would be "why can ants lift objects much heavier than themselves, whereas we cannot?" or "why do insects get by with such thin legs?" or "Why don’t eel larvae or lungless salamanders need respiratory organs". The latter bases on the same principle, but here it’s the skin area for gas exchange that is proportionately larger in smaller animals.

[Infinity Blade]

@theropod

One other tangentially-related question: if a shorter muscle isn't any weaker than a longer one, is a longer one weaker than a shorter one? This book (Dogs: Their Fossil Relatives and Evolutionary History) claims that the properties of muscle tissue make a shorter muscle contract more efficiently than a longer one, and so the former can exert more power with less effort all else being equal. Is that true?

[theropod]

Feb 10, 2019 1:31:46 GMT 5 Infinity Blade said:

@theropod

One other tangentially-related question: if a shorter muscle isn't any weaker than a longer one, is a longer one weaker than a shorter one? This book (Dogs: Their Fossil Relatives and Evolutionary History) claims that the properties of muscle tissue make a shorter muscle contract more efficiently than a longer one, and so the former can exert more power with less effort all else being equal. Is that true?

The length of a muscle doesn’t affect it’s tension no. But a shorter muscle needs less energy to fully contract (because the number of sarcomeres, which are the functional units that require energy), at the payoff of only moving a shorter distance. What they mean by "more efficient" is referring to work, the energy required for the contraction, so force (assumed to stay equal) times distance (smaller in shorter muscle). But the maximum tension they can generate is only dependant on diameter and fiber type.
What the book says is that a "shorter neck can excert more power with less effort", which is true, but due to a completely different issue; because a longer neck means a longer out-lever for the muscles, with the same force of muscle pull the force output of the neck (e.g. in moving the head) will be lower. That’s why it is easier to hold a heavy weight close to your body than with an outstretched arm.
Hence also why animals with very heavy heads to support tend to have short necks, because that requires less musculature to just support the weight (e.g. T. rex, or an elephant). Whereas animals with small heads can affort having longer necks because they need to produce less force at the end (e.g. Sauropods).
Of course a shorter neck also has shorter muscles, but that’s not the reason it can produce more force, just a by-product that will further improve its efficiency but reduce its range of motion.

Power in the physical sense btw is work divided by time, so the amount of energy used in a give time. That word doesn’t really make sense, I strongly presume the authors really mean force, as excerting more power with less effort is sort of an oxymoron.

An important thing to keep in mind is the difference between the pure pulling force of a muscle, and the force output at whatever structure it is moving. If your jaw muscles can pull a total of 10kN of force, that doesn’t mean your bite force if also 10kN, because these muscles don’t just pull the jaw straight up, they insert in different places that are a given distance from the point of rotation, and thus have different mechanical advantage with respect to the different tooth positions. When talking about length being irrelevant, this only refers to the pulling force. With different mechanical advantage of course, the length can become relevant if it allows a muscle to move a longer lever.

[creature386]

Much like Infinity Blade with his blog post, I wanted to review the script for a future video here:

„Carbonates are born, not made“ (Noel James, 1979)
If this pseudo-philosophical sentence confuses you, you’re in good company. I had to think about it myself. Rocks don’t give birth to other rocks, neither is there a rock god.
Apparently, it is supposed to be a metaphor for how sedimentary rocks form. The „made“ symbolizes the formation of siliciclastic rocks. Siliciclastic rocks form when a parent rock is disintegrated and transported away.
Carbonates meanwhile form through skeletal grains or precipitation in the depositional environment. Perhaps that’s what „born“ means. They are often of organic origin.
Anyway, carbonates are our topic. They are the rocks where most of the oil and gas is to be found, but forget about this, my channel runs on renewable energies. They have something else: Fossils!
Limestones typically form in tropical, oceanic shelves, but not always. Evaporites form when lakes or certain marine environments, well, evaporate. Travertine are deposited my mineral springs, such as those you find in caves. As they typically form in water, you should not expect much terrestrial life in limestones. Unless you live in Solnhofen, of course.
A carbonate rock or limestone is per definition mainly composed of the mineral calcium carbonate, CaCO3. Dolomite, or CaMgCO3 is also a thing, but less important. You can recognize them because they’re the only rocks that react with hydrochloric acid. They can form through chemical reactions in the water or when water organisms have to put something in their shells.
There are two main types of CaCO3, calcite and aragonite. Aragonite is predominant today, but it is more unstable and tends to be replaced by calcite over time. If it preserves at all. Aragonite mostly forms/deposits in shallow marine water while calcite (especially with little magnesium) also occurs in freshwater and deep marine water.
Carbonates will not deposit below a so-called carbonate compensation depth which is the depth at which all CaCO3 dissolves. It lies at an ocean depth of about 4-6 km, though calcite can deposit deeper than aragonite.
As you might guess, the grain size classification you learned in my sediment video is irrelevant to limestones. In siliciclastic sediments, we bother about grain sizes because the environment’s energy determines which grains get transported. If only the big grains are left, you’re dealing with a strong current. But limestones form where they are deposited, so why bother?
Well, even a limestone’s depositional environment has energy. The energy tends to decrease the further you go away from the coast. Exception are lagoons (low energy) and the outer edges of a carbonate platform (energy). limestones are often composed of fine-grained mud and bigger skeletal fragments of living organisms, such as shells. Let’s say the water flows really fast, like at a beach or the edge of a carbonate platform. All the mud gets blown away. You only have the grains and cement between them. Let’s say the water stands still, like in a lagoon. Well, a lot of mud is deposited. So much that the grains are only a far and few.
We quantify the percentage of mud and grains in the Dunham classification scale.
If a limestone is almost all mud, it’s called, you guessed it, a mudstone. More than 10% skeletal grains and it’s a wackestone. More than 50% and it’s a packstone. If there’s no mud, only cement, it’s a grainstone.
When I told you carbonates don’t get transported, I was messing with you. I lied to you for the same reason we tell kids too young to spell „oblate spheroid“ that the Earth is a ball.
Those types of carbonates with the mud and grains are the allochthonous carbonates. They get transported a bit. Carbonates who don’t get transported at all are autochthonous carbonates.
A good example are reefs. Reefs win the „most significant biogenic structures“ award, the silver medal goes to human buildings. Their paleontological importance cannot be overstated. There are three examples or autochthonous limestones you should remember, all classified by the organisms who built them. One are framestones. They are built by constructors, like corals, who are really massive and upwards growing. Another are bafflestones. They are built by bafflers, like green algae, who built upwards growing, but much finer structures by catching sediment. The last are bindstones. They are built by binders, like many sponges, who bind the sediments. As if the other classification, this kinda correlates with water energy, e.g. framestones occur in higher energy than bafflestones.
Now, I will tell you about some of the most significant grains.
First, we have ooids which are basically little eggs, about 2 mm in diameter. They often have these concentric layers around a nucleus and form in high-energy marine depositional environments. Also, rocks with ooids often contain oil.
Next, we have oncoids. They are like ooids, but larger and with more overlapping layers. They can form in high energy, but also further off the coast and even in fresh water.
Peloids are little balls even smaller than ooids with no clear internal structure.
Apart from these tree, you can have many different organisms. The mineralogy of carbonates is pretty boring, you only have CaCO3, but the biology is very interesting. Carbonates often contain brachiopods, bivalves, trilobites, echinoderms, foraminifera, corals, algae and sponges.
All this information is important for knowing the world in which these organisms lived in. Mostly, you need a combination of many different data points to make conclusion. Example: If you have a mudstone, you have low energy, could be deep sea or a lagoon. However, if you have photosynthetic organisms in it, it's more likely the lagoon.
I have talked a lot about energy, but you can tell so much more, especially from what we already know about the organisms. Green algae tell you the water was not very deep, they need red light. Corals tell you that the salinity conditions were stable.
Shells are useful for two things. If you have a bunch of shells whose dorsal (back) sides point in one direction, they were likely all arranged by a storm. Even better, they tend to be arranged in a way that offers the least resistance to the water current which means with the ventral, wide end at the ground. So, by looking at how the shell is arranged, you can tell apart up and down. This is important to determine if a rock is a bind-, baffle- or framestone. If you have both shells, it means the shell fossilized in situ in its burial position.
Bioturbation (e.g. burrowing by organisms) is also very useful. Its density can tell you about the past biodiversity which is itself a good proxy for oxygen levels.

Besides oil and intellectual curiosity, why does it matter what environments prehistoric organisms lived in? There is a whole field about conservation paleobiology. It is difficult to predict how habitat changes influence biodiversity, evolution and migration on long scales. We can extrapolate from current trends, but only the geologic record can actually test such hypotheses. I will go more in depth on the significance and methods of paleoecological studies in a future video.

I admit I did not do much research. I types this off the top of my head from what I remember from college.
A few particular questions:
To those members well-versed in geology: Is the information factually accurate? Is important information missing (remember that I don't want to go far above 10 minutes though)? Did I set the right priorities (it's sad how the actual fossils get so little attention, but I felt like the stuff above that was important)?
To those members not well-versed in geology, but interested in paleontology (e.g., most of my target audience): Can you follow this? Does the relevance to paleontology become clear?

[theropod]

I’d add that there are sometimes dinosaur fossils in pure carbonatic rocks, look no further than Solnhofen. Probably more important though is that lots of important dinosaur sites are marls, so they do contain a lot of carbonate as well.

In terms of the "boring" mineralogy, perhaps add something about dolomite? Much rarer, but still relevant.

Tying in with that, maybe also mention deposition outside of the classical carbonate shelf settings, such as evaporites or travertine?

[creature386]

Good points, I think mentioning Solnhofen will make my target audience interested.
Dolomite and non-traditional depositional environments are surely noteworthy, though I need a way to implement them that does not take too much screen time.

[theropod]

Maybe just mention in the introduction that other carbonates exist, but you want to focus on CaCO3, and specifically on biogenic deposition.

[Infinity Blade]

Do we have any preserved skin from Megalonyx? Apparently one of the characteristics Kurtén and Anderson (1980) used to distinguish Paramylodon from Nothrotheriops and Megalonyx was the presence of dermal ossicles, which implies Nothrotheriops and Megalonyx lacked these. That sounds about right for Nothrotheriops (we have skin from it too, but no osteoderms to my knowledge). I don't know about the latter, though.

[creature386]

This quote implies there is:

As is well known, Megatherium and Megalonyx (as well as Nothrotherium) are without dermal osicles, but the skin of Mylodon is thickly studded with these.

pdfs.semanticscholar.org/ca12/6f237143098a6d4542e41b693e3169a8b9fe.pdf?_ga=2.85676112.857177708.1552396311-852069318.1552396311

[Infinity Blade]

Hmm, that would imply that we also have skin from Megatherium as well. But this book seems to suggest not (link).

Reconstructions of Megatherium americanum usually portray this giant ground sloth with a thick furry coat (for example, as in Figs. 6.22 and 7.10). This appearance is based largely on the fact that most mammals possess a good amount of hair covering their bodies and the demonstrated presence of hair in the sloths Mylodon and Nothrotheriops. However, the possibility that Megatherium, given the implications of its large body size and geographic distribution, may have been largely hairless is considered in Chapter 7.

Could it be they're just referring to the fact that while we have skeletons from Megatherium and Megalonyx, we haven't found evidence of dermal ossicles for them yet?

[creature386]

Maybe that's because hair preserves less well than dermal ossicles and its presence is thus harder to rule out.

[Infinity Blade]

Okay, yet again I need two questions answered this time around.

1.) Can anyone identify any instances where flightless, ground-dwelling birds have been hunted into extinction by mammalian carnivores? I know humans have done this before (e.g. to moa), but what about non-human mammalian predators? Does the fossil record ever document such an instance? Are there comparatively recent historical examples I'm forgetting about?

2.) Can anyone point me to information about the leg lengths of mammalian predators and their prey increasing throughout the Cenozoic?

[Infinity Blade]

Mar 16, 2019 9:51:45 GMT 5 Infinity Blade said:

Okay, yet again I need two questions answered this time around.

1.) Can anyone identify any instances where flightless, ground-dwelling birds have been hunted into extinction by mammalian carnivores? I know humans have done this before (e.g. to moa), but what about non-human mammalian predators? Does the fossil record ever document such an instance? Are there comparatively recent historical examples I'm forgetting about?

2.) Can anyone point me to information about the leg lengths of mammalian predators and their prey increasing throughout the Cenozoic?

And a third.

How/what would you describe the original ecological niche of humans as before we left Africa (or at least before we started using agriculture)?