126 research outputs found
Forced Moves or Good Tricks in Design Space? Landmarks in the Evolution of Neural Mechanisms for Action Selection
This review considers some important landmarks in animal evolution, asking to what extent specialized action-selection mechanisms play a role in the functional architecture of different nervous system plans, and looking for âforced movesâ or âgood tricksâ (see Dennett, D., 1995, Darwinâs Dangerous Idea, Penguin Books, London) that could possibly transfer to the design of robot control systems. A key conclusion is that while cnidarians (e.g. jellyfish) appear to have discovered some good tricks for the design of behavior-based control systemsâlargely lacking specialized selection mechanismsâthe emergence of bilaterians may have forced the evolution of a central ganglion, or âarchaic brainâ, whose main function is to resolve conflicts between peripheral systems. Whilst vertebrates have many interesting selection substrates it is likely that here too the evolution of centralized structures such as the medial reticular formation and the basal ganglia may have been a forced move because of the need to limit connection costs as brains increased in size
Cryptic Disc Structures Resembling Ediacaran Discoidal Fossils from the Lower Silurian Hellefjord Schist, Arctic Norway
The Hellefjord Schist, a volcaniclastic psammite-pelite formation in the Caledonides of Arctic Norway contains discoidal impressions and apparent tube casts that share morphological and taphonomic similarities to Neoproterozoic stem-holdfast forms. U-Pb zircon geochronology on the host metasediment indicates it was deposited between 437 ± 2 and 439 ± 3 Ma, but also indicates that an inferred basal conglomerate to this formation must be part of an older stratigraphic element, as it is cross-cut by a 546 ± 4 Ma pegmatite. These results confirm that the Hellefjord Schist is separated from underlying older Proterozoic rocks by a thrust. It has previously been argued that the Cambrian Substrate Revolution destroyed the ecological niches that the Neoproterozoic frond-holdfasts organisms occupied. However, the discovery of these fossils in Silurian rocks demonstrates that the environment and substrate must have been similar enough to Neoproterozoic settings that frond-holdfast bodyplans were still ecologically viable some hundred million years later
The two phases of the Cambrian Explosion
Abstract The dynamics of how metazoan phyla appeared and evolved â known as the Cambrian Explosion â remains elusive. We present a quantitative analysis of the temporal distribution (based on occurrence data of fossil species sampled in each time interval) of lophotrochozoan skeletal species (nâ=â430) from the terminal Ediacaran to Cambrian Stage 5 (~545 â ~505 Million years ago (Ma)) of the Siberian Platform, Russia. We use morphological traits to distinguish between stem and crown groups. Possible skeletal stem group lophophorates, brachiopods, and molluscs (nâ=â354) appear in the terminal Ediacaran (~542âMa) and diversify during the early Cambrian Terreneuvian and again in Stage 2, but were devastated during the early Cambrian Stage 4 Sinsk extinction event (~513âMa) never to recover previous diversity. Inferred crown group brachiopod and mollusc species (nâ=â76) do not appear until the Fortunian, ~537âMa, radiate in the early Cambrian Stage 3 (~522âMa), and with minimal loss of diversity at the Sinsk Event, continued to diversify into the Ordovician. The Sinsk Event also removed other probable stem groups, such as archaeocyath sponges. Notably, this diversification starts before, and extends across the Ediacaran/Cambrian boundary and the Basal Cambrian Carbon Isotope Excursion (BACE) interval (~541 to ~540âMa), ascribed to a possible global perturbation of the carbon cycle. We therefore propose two phases of the Cambrian Explosion separated by the Sinsk extinction event, the first dominated by stem groups of phyla from the late Ediacaran, ~542âMa, to early Cambrian stage 4, ~513âMa, and the second marked by radiating bilaterian crown group species of phyla from ~513âMa and extending to the Ordovician Radiation
Reverse breakdown in long wavelength lateral collection CdxHg1âxTe diodes
Long wavelength diodes in CdxHg1âxTe show large deviations from ideality in their reverse characteristics. The excess currents are attributed in many published papers on band to band tunneling at high reverse bias and to trap assisted tunneling at low reverse bias. Measurements of photocurrent multiplication, currentâvoltage characteristics, and noise have been made on long wavelength loophole diodes to determine the breakdown mechanism. This has produced strong evidence that the reverse characteristics of good quality diodes of this type are limited by impact ionization. At higher biases, there is evidence of an additional breakdown mechanism, probably tunneling
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