Post by inyo on Jan 3, 2016 0:48:32 GMT -5
Traditional turkey-eating season is over, of course, but here's another way to observe a "wishbone"--it's the roughly furcula-shaped graptolite specimen, genus Didymograptus, I collected a number of years ago from the lower Ordovician Al Rose Formation, California. That golden glow of preservation is due to replacement by the mineral limonite, a hydrated iron oxide (FeO(OH)·nH2O). The Al Rose Formation is California's premiere graptolite-producer; it also yields excellently preserved brachiopods and trilobites. Actual length of graptolite specimen in photographs, below, is 12 millimeters (0.47 inch).
Graptolites first appear in the geologic record during the middle stages of the Cambrian Period, some 505 million years ago. Even though they persisted all the way up to the late Mississippian age, or roughly 325 million years ago, most species of graptolites had already become extinct by the latest Devonian Period 35 million years earlier. Graptolites achieved their highest degree of success during the Ordovician Period, when they attained worldwide distribution by adapting with ingenuity to three distinct modes of life. One order of graptolite, for example--the fan to leaf-shaped dendroids--led a sessile life attached to the sea floor, apparently straining the marine waters for microscopic organisms. Another type developed a special flotation device which allowed the graptolite colony, termed a rhabdosome, to drift in the open ocean; and a third kind solved its own planktonic challenge by attaching itself to floating strands of seaweed to hitch a free ride through the open ocean in search of better feeding grounds; presumably it too strained the sea waters for microscopic particles of food.
In all three examples of graptolitic adaption, the actual colonial animal lived inside the minute rows of cups called thecae that developed along each individual segment of the rhabdosome; technically, these segments are called a stipe. The tiny saw-tooth compartments that housed the graptolite animals along the stipe show to best advantage under magnifications of ten or more power. Thus, a good-quality hand lens is indispensable in order to gain a detailed and aesthetic appreciation of graptolite specimens.
The exact zoological classification of graptolites has presented a serious challenge to paleontologists. Early investigators referred graptolites to such disparate groups as coelenterates or bryozoans; yet, there certainly was no unanimity of opinion among fossil specialists throughout the 19th century. The breakthrough came when some perfect, three dimensional specimens were etched out of cherts using powerful brews of acids around 1948. Paleontologists then realized that the graptolite colony most closely resembled the modern pterobranch, a tiny marine hemichordate, which by definition is a primitive chordate whose notochord (a spine-like notch) is restricted to the basal part of the head.
