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80

Elephant, rhinoceros, &c all have much thicker legs in proportion. The answer, I think, lies in the fact that the animals you mention all evolved as cursorial animals (that is, they run to escape predators). Less mass in the lower leg means it swings easier, so the animal can run faster. There are two things you're apparently not noticing in that ...


51

This is a mistake in comparative anatomy which is somewhat common. When looking at four-legged animals, people often mistakenly map the parts of the hind legs. Here is an image that shows the different morphology of the same bones in horses and humans: What people often think of as the thigh of a four legged creature is really our calf and shin bone! And ...


20

How come large herbivores have such thin legs? They don't. The following book does an extensive comparison of the bones of humans with other animals: Adams BJ, Crabtree PJ. 2008. Human vs Horse. In: Comparative Skeletal Anatomy. Humana Press. Here are some images of human bones (left) next to horse bones (right): Radius/Ulna (fused in horses) Humerus ...


7

No, fish scales are dermal (= formed in derma) bones like skull roof bones. Scales in reptiles are formed by epidermis and are made primarily of protein (from keratinocytes), being similar in derivation to hair, feather and nails. On the other hand, in reptiles one must differentiate between scales and osteoderms (= scutes). Scutes are widespread among ...


7

Pangolin scales are certainly not retained from fish. Pangolins are within the clade Pholidota (Tree of Life page for Eutheria) and their nearest relatives are anteaters, sloths, and armadillos. According to the University of California Museum of Paleontology, similarities between pangolin and armadillos are the results of convergent evolution. So the ...


6

From a little research I have been able to find some of the hypotheses and speculations but no papers expressly confirming or denying the matter. One theory displayed in "The evolution of mammalian keratinized structures" (abstract only) is that mammalian hair developed along the following pathway: Hair follicles develop between reptilian like scales. A ...


5

As to the early evolution of mammalian hair, Rowe et al. (2011) hypothesized that the primitive function for hair was not thermoregulatory, but rather for tactile sensation (contra the hypotheses of Spearman and Maderson). Rowe et al. say: Body hair develops as migrating neural crest cells induce patterns of tiny placodes that mature into hair follicles ...


2

I certainly agree with your general observation, but not being an ichthyologist, I can only offer a partial answer, based on a particular family of fish. Within the family Tripterygiidae, most species lack scales on their head, or have highly modified types of scales. A comparative study of 48 species showed that only 2 have "normal" scales of their head (...


1

According to these Wikipedia pages - https://en.wikipedia.org/wiki/Geologic_time_scale#Table_of_geologic_time, https://en.wikipedia.org/wiki/Precambrian - the Precambrian is a "supereon" not an "eon". The ICS International Chronostratigraphic Chart, ambiguously, puts "Eonothem/Eon" over both the "Precambrian" column and the "Hadean - Archean - Proterozoic" ...


1

The pangolin scale is a horny derivative of the epidermis. It is complex in structure and is divisible into three distinct regions. The dorsal plate forms approximately one-sixth of the scale thickness. It is composed of flattened solid keratinized cells without basophilic nuclear remnants. This region tends to fray easily. The dorsal plate ...


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