THE phrase “prehistoric monster” might have been coined with sea spiders in mind. Though neither large (the biggest are hand-sized) nor threatening to people, their quintessential creepy-crawliness presses many of the buttons marked “horror” in the human psyche. And prehistoric they certainly are. Fossils show that they date from at least 500m years ago, during the Cambrian period, the dawn of the animals. True spiders, to which sea spiders (some of which have more than eight legs) are but distantly related, are known for certain only from as far back as the Carboniferous period, about 300m years ago.
One of the crucial evolutionary developments that permitted multicellular animals to come into being during, or shortly before, the Cambrian period was a circulatory system. Small creatures, consisting of one or a few cells, can absorb enough oxygen for their respiratory requirements directly from the water they inhabit. It simply diffuses into them. Larger ones, though, need a way of...Continue reading
Source: Science and technology http://ift.tt/2vhdAmN
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