Post by elosha11 on Apr 21, 2014 4:22:09 GMT 5
In researching the weight of the largest estimated Megalodons, (approx. 18-20 meters), we have seen many predictions of maximum bulk. From Gottfried's 1996 estimate approaching and slightly exceeding 100 tons for a shark of 20.3 meters, to Mike Sivverson's estimate of 50-60 tons (he thinks sharks of 100 tons would have 30-33 cm vertebral centra, which hasn't shown up yet in the fossil record), to the recent Pimiento and company's estimate of 70-75 tons based on the largest teeth found in the Megalodon fossil nursery, we have a wide range of opinions.
What bearing does the shark's lighter cartilage skeleton have on these estimates? In the typical shark skeleton, the cartilage has about half the density of bone, making it significantly lighter. On the other hand, larger sharks like the great white, and to an even greater degree, Megalodon, have heavily calcified cartilage structure, which adds to its density and therefore its weight. But I still can't imagine an 18 meter Megalodon vertebral column weighing as much an 18 meter sperm whale vertebral column. Moreover, comparatively sized cetacean have a much more developed rib structure than sharks, so overall their entire skeleton should be much heavier than the shark's.
However, I note that a robust shark like the great white usually compares favorably in weight to a cetacean of similar length such as a false killer whale. Is this because the shark's liver is much heavier than the whale's and because it has a higher percentage of heavier muscle as opposed to the cetacean's blubber, which is presumably lighter than muscle, like other types of fat.
What weight (pun intended) should we give to these factors when comparing mega-toothed sharks to their cetacean or pliosaur/mosasaur equivalents?
Here's an informative bit of relevant commentary from www.elasmo-research.org/education/topics/p_cartilage.htm
Unlike most vertebrates, sharks do not rely on their internal skeleton to provide them with firm sites for muscle attachment. Instead, underneath their sandpapery outer covering (studded with countless thousands of tiny, tooth-like scales called 'dermal denticles' ), sharks have a thick skin composed of a complexly criss-crossed meshwork of tough-but-springy fibres made of a protein called collagen. This meshwork of collagen fibers forms a kind of 'corset' to which the swimming muscles attach. By having their swimming muscles attached directly to the tough, armor-like skin, the skin is able to act as a kind of external skeleton. From a mechanical perspective, having the muscles attached directly to an external skeleton is a very efficient arrangement, resulting in very little waste of muscular energy — which explains why even the tiniest crab can give such a disproportionately painful nip!
Blacknose Shark (Carcharhinus acronotus) © Jeremy Stafford-DeitschAll sharks, like this Blacknose Shark (Carcharhinus acronotus), have a an internal skeleton composed of cartilage, which is tough, flexible, and light-weight. This gives sharks many of the structural and 'flight' benefits of model aircraft constructed of balsa wood.
Photo © Jeremy Stafford-Deitsch; used with the gracious permission of the photographer, who asks that you support the Shark Trust.
Another benefit of having an internal skeleton made mostly of cartilage is weight reduction. Cartilage is tough but supple and only about half as dense as bone. Because of its relatively low density, a shark' body mass is significantly reduced and therefore requires substantially less energy to propel through the water. Since sharks lack any trace of a swim bladder — an internal organ that many bony fishes use to achieve neutral buoyancy by the controlled secretion and absorption of the gases within it — they are largely dependent upon dynamic lift, much like a glider. The less muscular energy a shark has to commit to avoid sinking, the more is available for forward propulsion. Thus, by having a light-weight internal skeleton, sharks reduce the amount of energy needed to swim. And by relying on a tough, 'external' skeleton formed by the skin, sharks increase the efficiency of swimming muscle contraction without incurring any loss of bodily support.
What bearing does the shark's lighter cartilage skeleton have on these estimates? In the typical shark skeleton, the cartilage has about half the density of bone, making it significantly lighter. On the other hand, larger sharks like the great white, and to an even greater degree, Megalodon, have heavily calcified cartilage structure, which adds to its density and therefore its weight. But I still can't imagine an 18 meter Megalodon vertebral column weighing as much an 18 meter sperm whale vertebral column. Moreover, comparatively sized cetacean have a much more developed rib structure than sharks, so overall their entire skeleton should be much heavier than the shark's.
However, I note that a robust shark like the great white usually compares favorably in weight to a cetacean of similar length such as a false killer whale. Is this because the shark's liver is much heavier than the whale's and because it has a higher percentage of heavier muscle as opposed to the cetacean's blubber, which is presumably lighter than muscle, like other types of fat.
What weight (pun intended) should we give to these factors when comparing mega-toothed sharks to their cetacean or pliosaur/mosasaur equivalents?
Here's an informative bit of relevant commentary from www.elasmo-research.org/education/topics/p_cartilage.htm
Unlike most vertebrates, sharks do not rely on their internal skeleton to provide them with firm sites for muscle attachment. Instead, underneath their sandpapery outer covering (studded with countless thousands of tiny, tooth-like scales called 'dermal denticles' ), sharks have a thick skin composed of a complexly criss-crossed meshwork of tough-but-springy fibres made of a protein called collagen. This meshwork of collagen fibers forms a kind of 'corset' to which the swimming muscles attach. By having their swimming muscles attached directly to the tough, armor-like skin, the skin is able to act as a kind of external skeleton. From a mechanical perspective, having the muscles attached directly to an external skeleton is a very efficient arrangement, resulting in very little waste of muscular energy — which explains why even the tiniest crab can give such a disproportionately painful nip!
Blacknose Shark (Carcharhinus acronotus) © Jeremy Stafford-DeitschAll sharks, like this Blacknose Shark (Carcharhinus acronotus), have a an internal skeleton composed of cartilage, which is tough, flexible, and light-weight. This gives sharks many of the structural and 'flight' benefits of model aircraft constructed of balsa wood.
Photo © Jeremy Stafford-Deitsch; used with the gracious permission of the photographer, who asks that you support the Shark Trust.
Another benefit of having an internal skeleton made mostly of cartilage is weight reduction. Cartilage is tough but supple and only about half as dense as bone. Because of its relatively low density, a shark' body mass is significantly reduced and therefore requires substantially less energy to propel through the water. Since sharks lack any trace of a swim bladder — an internal organ that many bony fishes use to achieve neutral buoyancy by the controlled secretion and absorption of the gases within it — they are largely dependent upon dynamic lift, much like a glider. The less muscular energy a shark has to commit to avoid sinking, the more is available for forward propulsion. Thus, by having a light-weight internal skeleton, sharks reduce the amount of energy needed to swim. And by relying on a tough, 'external' skeleton formed by the skin, sharks increase the efficiency of swimming muscle contraction without incurring any loss of bodily support.
