And if not, why not? As someone who is vaguely familiar with evolution, I will gladly accept the answer “just cuz.”
I remember reading somewhere about turn of the century German biologists writing about Bauplan,* on the general morphology of animals, and a beautiful book by a Brit called Young or Thomas* on similar themes (even suggesting, in retrospect, some themes of fractal growth. But I can’t remember anything on pentapods.
*Cite?
Seastars often have five limbs, is that what you mean?
Most living creatures have symmetry. Either bilateral or radial. Almost all animals are bilateral. Even the echinoderms start out as bilateral, even though adult echinoderms possess pentaradial, or five-sided, symmetry. So, I guess that’s what you are looking for?
Some monkeys possess prehensile tails, giving them a “5th limb” of sorts.
Starfish?
Thank you for your quick reply.
This is fascinating. Mammal and all that.
Why do you think pentapods are so rare on dry land compared to the sea?
(Thanks Tyger Girl, also)
Biology likes even numbers
Not only starfish – echinoderms in general* have five-fold symmetry, including crinoids and sea urchins:
http://www.newworldencyclopedia.org/entry/Symmetry_(biology)
*although not invariably. Some are bilateral
The closest land animals are the macropods (kangaroos and wallabies). Their tail acts as a third leg when they are standing, or travelling fast, but they can use their arms as legs, as in this picture. The description in the Wiki article on Macropods says:
Because there aren’t any radially symettrical land animals. Most of the radial animals are sessile, which means they are filter feeders of some kind or another. But that doesn’t work on land, you can’t just pump air through your feeding apparatus and expect to collect anything.
Because there aren’t enough bands like Dave Brubeck to provide dance music for them?
Seriously, animals using a tail as a fifth limb aside, it’s hard to imagine how a land animal with five limbs arranged in radial symmetry would get about in an efficient manner. I can imagine fanciful gaits - picture the thing spinning through the landscape like a whirling dervish. Since wild predators seem to generally lack a sense of humor, it couldn’t escape while they were laughing …
The Buer?
Nearly all Metazoans except the Cnidarians (jellyfish, corals, sea anenomes, and their allies) and the Echinoderms are bilaterally symmetrical, meaning they’ll have an even number of limbs if they have limbs at all. (Even earthworms have setae, bristles used in locomotion, and they’re two to a segment.)
Almost all the echinoderms are pentaradially symmetrical, with five or a multiple of five limbs. (The former exceptions, homalozoans, are now tentatively considered basal chordates in some classifications.) In the case of the holothurians and edhinoics (sea cucumbers and sea urchins/sand dollars) the pentaradial symmetry is obscured, in the latter case by an apparent spherical symmetry, but is still present in underlyng structures. In the crinoids and their extinct relatives the blastoids and cystoids, the asteroidea and the ophiuroidea, the five-armed structure is obvious.
Of interest is a group of degenerate parasitic forms related to the arthropods, the Pentastomida. Here the mouth and four limbs mimic a pentagonal attach-to-host structure.
New World monkeys, with prehensile tails, could sort of be said to have five limbs.
