| rdfs:comment
| - Ranging in size from coin to VW Bug, ammonoids (also called ammonites), form a great part of the world's marine fauna from their inception in the Devonian all the way through the Mesozoic. In our home time-line, the story of the ammonoids ended there, at the end of the Cretaceous, but Spec, it seems, has more to tell.
|
| abstract
| - Ranging in size from coin to VW Bug, ammonoids (also called ammonites), form a great part of the world's marine fauna from their inception in the Devonian all the way through the Mesozoic. In our home time-line, the story of the ammonoids ended there, at the end of the Cretaceous, but Spec, it seems, has more to tell. Ammonoids, although they are outwardly similar to the coil-shelled nautiloids, and actually of a more advanced group, related to the squids and octopuses. Formally, ammonoids are distinguished from nautiloids by the construction of their septa, the barriers between different chambers of their shell, which in nautiloids are simple sheets of calcium carbonate, but in ammonoids are intricately wrinkled and folded. Ammonoids also distinguish themselves by possessing 12-8 sucker-laden tentacles, a squidlike beak composed of chitin (although in some species, the chitin is replaced by calcite later in life), and a toothed tongue or radula. An ammonoid's nervous system is similarly advanced, with central brain-like nerve ganglion and complex eyes that allow some ammonoid species to distinguish shapes and colors with the same facility as a human. All ammonoids begin life as a planktonic ammonitella, a tiny swimming creature protected by a simple protoconch, the precursor of its parents' shells. As the ammonitellae age, their protoconchs elongate, adding sections separated by increasing complex septa, producing a structure resembling a corckskrew. The shell, itself, is composed of aragonite, or mother-of-pearl, the mineral form of the calcium carbonate an ammonoid injests in seawater. The shell grows as the ammonoid lays down layer after layer of this material to a base of calcite (another compound of calcium carbonate) to form a hardened chamber, which it then walls off with a septum. The living ammonite dwells in only the foremost chamber, the one still composed of calcite, but retains control over the other chambers by means of the siphuncle, a tube of living tissue that extends through the shell. The ammonite uses the siphuncle to pump nitrogen gas and water around the chambers of its shell, altering its own buoyancy to ascend or sink through the water. As the larval ammonite ages, the shell's coils tighten into the familiar ramshorn. Depending upon the species, the ammonitellae then either descend to the bottom of the ocean, remain floating, or head for fresh water. Ammonoids have adhered to this life cycle for millions of years, and indeed, the fossil protoconchs of the tiny ammonitellae form a signifigant part of the marine fossil record.
|
![[RDF Data]](/fct/images/sw-rdf-blue.png)