Nevertheless, as indicated by O’Dor (In Darmaillacq et al. The aphorism “live fast, die young” describes quite well the life history strategy of these coleoid cephalopods, which have epipelagic and benthic habitats. The broad category of “coastal and shelf species” (neritic species) covers the relatively well-known cephalopod groups. 3.4), which is not a true shell but a brood chamber. This last group includes Argonauta species whose females produce an external calcareous structure (Fig. 3.3), vampire squids, Dumbo octopuses and octopods. 3.2), long-fin and short-fin squids (Fig. 3.1), which are the only living cephalopods with outer shells, and the Coleoidea, which is represented by about 800 species, containing the cuttlefishes and bobtail squids (Fig. There are two major divisions within present-day cephalopods: the Nautiloidea with six species of the pearly nautilus (Fig. These traits make cephalopods the most active and intriguing of the molluscs (Nixon and Young 2003). Increase in brain size and complexity, development of effective sense organs, and changes in the skin concurred to the development of sophisticated behaviours. Some of the shelled ammonites that were the dominant elements of the marine fauna during the Mesozoic were of 3 m in diameter. The most well known of these fossil records are the nautiloids, ammonoids, and belemnites. The shells of some of these species reached nearly 10 m in length. They flourished in Palaeozoic oceans between the Ordovician (488 mya) and Triassic periods (200 mya). All members of these clades were squid-like, but had straight external shells. Palaeontologists have identified three distinct fossil clades that are entirely extinct. Clearly, the lineages of extinct taxa were prolific and diverse. Some 17,000 fossil species are known, most of them provided with an outer calcareous shell, whose abundance and distribution have experience important fluctuations throughout the different geologic eras. They have acquired the ability to regulate buoyancy, followed by reduction and internalization of the shell and the development of the mantle musculature. Its evolution is related directly to the development of low-pressure buoyancy mechanisms. This group has been among the dominant large predators in the ocean at various times in geological history. Cephalopods constitute one of the most complex groups of invertebrates and the most evolved of molluscs. The Cephalopoda is an ancient class of the Phylum Mollusca dating from the Upper Cambrian Period (around 500 million years ago mya).
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