The taphonomic clock in fish otoliths

Paleobiological and paleoecological interpretations rely on constraining the temporal resolution of the fossil record. The taphonomic clock, that is, a correlation between the alteration of skeletal material and its age, is an approach for quantifying time-averaging scales. We test the taphonomic clock hypothesis for marine demersal and pelagic fish otoliths from a 10–40 m depth transect on the Mediterranean siliciclastic Israeli shelf by radiocarbon dating and taphonomic scoring. Otolith ages span the last ~8000 yr, with considerable variation in median and range along the transect. Severely altered otoliths, contrary to pristine otoliths, are likely to be older than 1000 yr. For pelagic fish otoliths, at 30 m depth, taphonomic degradation correlates positively with postmortem age. In contrast, no correlation occurs for demersal fishes at 10 and 30 m depth, mostly because of the paucity of very young pristine (<150 yr) otoliths, possibly due to a drop in production over the last few centuries. Contrary to molluscan and brachiopod shells, young otoliths at these depths are little affected and do not show a broad spectrum of taphonomic damage, because those that derive from predation are excreted in calcium- and phosphate-rich feces forming an insoluble crystallic matrix that increases their preservation potential. At 40 m depth, all dated otoliths are very young but rather damaged because of locally chemically aggressive sediments, thus showing no correlation between taphonomic grade and postmortem age. Our results show that local conditions and the target species population dynamics must be considered when testing the taphonomic clock hypothesis.

Cybernetic times: Norbert Wiener, John Stroud, and the ‘brain clock’ hypothesis

In 1955, Norbert Wiener suggested a sociological model according to which all forms of culture ultimately depended on the temporal coordination of human activities, in particular their synchronization. The basis for Wiener’s model was provided by his insights into the temporal structures of cerebral processes. This article reconstructs the historical context of Wiener’s ‘brain clock’ hypothesis, largely via his dialogues with John W. Stroud and other scholars working at the intersection of neurophysiology, experimental psychology, and electrical engineering. Since the 19th century, physiologists and psychologists have been conducting experimental investigations into the relation between time and the brain. Using innovative instruments and technologies, Stroud rehearsed these experiments, in part without paying any attention at all to the experimental traditions involved. Against this background, this article argues that the novelty of Wiener’s model relies largely on his productive rephrasing of physiological and psychological findings that had been established long before the Second World War.

Einstein’s “Clock Hypothesis” and Mössbauer Experiments in a Rotating System

An extra energy shift between emitted and received radiation on a rotating disc – next to the conventionally recognised second-order Doppler shift – has been revealed in a series of recent Mössbauer experiments, where a radioactive source is fixed at the centre and an absorber is attached to the rim of the rotating disc. This disclosure gives indication to a possible violation of the “clock hypothesis” by Einstein: i.e. the independence of the rate of a clock on its acceleration. At the moment, there seem to be two plausible interpretations of this result: (i) the deviation of the geometry of the rotating disc from that predicted by the general theory of relativity (GTR), or (ii) the existence of a specific maximal acceleration in nature, when transformation between two accelerated frames differs from the corresponding transformation of the relativity theory. We take a closer look at both ways leading to the violation of the clock hypothesis; particularly, by analysing the outcomes of recent experiments in rotating systems and by suggesting a new Mössbauer rotor experiment to determine the most feasible mechanism for testing the dependence of the rate of a clock on its acceleration.

What do light clocks say to us regarding the so-called clock hypothesis?

The clock hypothesis is taken to be an assumption independent of special relativity necessary to describe accelerated clocks. This enables to equate the time read off by a clock to the proper time. Here, it is considered a physical system–the light clock–proposed by Marzke and Wheeler. Recently, Fletcher proved a theorem that shows that a sufficiently small light clock has a time reading that approximates to an arbitrary degree the proper time. The clock hypothesis is not necessary to arrive at this result. Here, one explores the consequences of this regarding the status of the clock hypothesis.

Genetic equidistance at the nucleotide level

The genetic equidistance phenomenon was first discovered in 1963 by Margoliash and shows complex taxa to be all approximately equidistant to a less complex species in amino acid percentage identity. The result has been mis-interpretated by the ad hoc universal molecular clock hypothesis, and the much overlooked mystery was finally solved by the maximum genetic diversity hypothesis (MGD). Here, we studied 15 proteomes and their coding DNA sequences (CDS) to see if the equidistance phenomenon also holds at the CDS level. We performed DNA alignments for a total of 5 groups with 3 proteomes per group and found that in all cases the outgroup taxon was equidistant to the two more complex taxa species at the DNA level. Also, when two sister taxa (snake and bird) were compared to human as the outgroup, the more complex taxon bird was closer to human, confirming species complexity rather than time to be the primary determinant of MGD. Finally, we found the fraction of overlap sites where coincident substitutions occur to be inversely correlated with CDS conservation, indicating saturation to be more common in less conserved DNAs. These results establish the genetic equidistance phenomenon to be universal at the DNA level and provide additional evidence for the MGD theory.

The Genetic Equidistance Phenomenon at the Proteomic Level

The field of molecular evolution started with the alignment of a few protein sequences in the early 1960s. Among the first results found, the genetic equidistance result has turned out to be the most unexpected. It directly inspired the ad hoc universal molecular clock hypothesis that in turn inspired the neutral theory. Unfortunately, however, what is only a maximum distance phenomenon was mistakenly transformed into a mutation rate phenomenon and became known as such. Previous work studied a small set of selected proteins. We have performed proteome wide studies of 7 different sets of proteomes involving a total of 15 species. All 7 sets showed that within each set of 3 species the least complex species is approximately equidistant in average proteome wide identity to the two more complex ones. Thus, the genetic equidistance result is a universal phenomenon of maximum distance. There is a reality of constant albeit stepwise or discontinuous increase in complexity during evolution, the rate of which is what the original molecular clock hypothesis is really about. These results provide additional lines of evidence for the recently proposed maximum genetic diversity (MGD) hypothesis.