Detrital zircon spectra reflect the tectonic setting of the basin in which they are deposited. Convergent plate margins are characterized by a large proportion of zircon ages close to the depositional age of the sediment, whereas sediments in collisional, extensional, and intracratonic settings contain greater proportions with older ages that reflect the history of the underlying basement. These differences can be resolved by plotting the distribution of the difference between the measured crystallization ages (CA) of individual zircon grains present in the sediment and the depositional age (DA) of the sediment. Application of this approach to successions where the original nature of the basin and/or the link to source are no longer preserved constrains the tectonic setting in which the sediment was deposited.
Was autotomy a pervasive adaptation of the crinoid stalk during the Paleozoic?
Stephen K. Donovan, Netherlands Centre for Biodiversity-Naturalis, Postbus 9517, NL-2300 RA Leiden, Netherlands. Posted online ahead of print 22 Aug. 2012; doi: 10.1130/G33262.1.
The stalked crinoids, or sea lilies, that are alive at the present day but living in the deep sea are relations of seastars and sea urchins. They were diverse and numerically common in the Paleozoic from about 480 to 250 million years ago, as well as being one of the most important groups of sessile invertebrates. Autotomy (self-mutilation or self-amputation) in the stem -- the casting off of the more distal part -- is only known to occur in one extant group of crinoids. Unlike these crinoids, Paleozoic species lacked specialized articulations adapted to autotomy but had other adaptations that may have singly or collectively facilitated
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