Conodonts were the first vertebrates to develop mineralized dental tools, known as elements. Recent research suggests that conodonts were macrophagous predators and/or scavengers but we do not know how this feeding habit emerged in the earliest coniform conodonts, since most studies focus on the derived, ‘complex’ conodonts. Previous modelling of element position and mechanical properties indicate they were capable of food processing. A direct test would be provided through evidence of in vivo element crown tissue damage or through in vivo incorporated chemical proxies for a shift in their trophic position during ontogeny. Here we focus on coniform elements from two conodont taxa, the phylogenetically primitive Proconodontus muelleri Miller, 1969 from the late Cambrian and the more derived Panderodus equicostatus Rhodes, 1954 from the Silurian. Proposing that this extremely small sample is, however, representative for these taxa, we aim to describe in detail the growth of an element from each of these taxa in order to the test the following hypotheses: (1) Panderodus and Proconodontus processed hard food, which led to damage of their elements consistent with prey capture function; and (2) both genera shifted towards higher trophic levels during ontogeny. We employed backscatter electron (BSE) imaging, energy-dispersive X-ray spectroscopy (EDX) and synchrotron radiation X-ray tomographic microscopy (SRXTM) to identify growth increments, wear and damage surfaces, and the Sr/Ca ratio in bioapatite as a proxy for the trophic position. Using these data, we can identify whether they exhibit determinate or indeterminate growth and whether both species followed linear or allometric growth dynamics. Growth increments (27 in Pa. equicostatus and 58 in Pr. muelleri) were formed in bundles of 4–7 increments in Pa. equicostatus and 7–9 in Pr. muelleri. We interpret the bundles as analogous to Retzius periodicity in vertebrate teeth. Based on applied optimal resource allocation models, internal periodicity might explain indeterminate growth in both species. They also allow us to interpret the almost linear growth of both individuals as an indicator that there was no size-dependent increase in mortality in the ecosystems where they lived e.g., as would be the case in the presence of larger predators. Our findings show that periodic growth was present in early conodonts and preceded tissue repair in response to wear and damage. We found no microwear and the Sr/Ca ratio, and therefore the trophic position, did not change substantially during the lifetimes of either individual. Trophic ecology of coniform conodonts differed from the predatory and/or scavenger lifestyle documented for “complex” conodonts. We propose that conodonts adapted their life histories to top-down controlled ecosystems during the Nekton Revolution.

<p>Climate warming is expected to lead to a reduction in the body size of marine organisms, a trend already observed in commercial fishes, but the effects of temperature rise on size distribution in exploited populations are difficult to separate from the impact of overfishing and other anthropogenic stressors. We aim to test the hypothesis that fish body sizes, as well as growth rates changed during the late Holocene and Anthropocene in the northern Adriatic Sea due to environmental perturbations caused by climate warming. We perform sclerochronological analysis on modern otoliths from fish sampled alive, as well as radiocarbon-dated fossil otoliths of non-commercial, demersal gobies (<em>Gobius niger </em>Linnaeus, 1758) sampled from a sediment core taken off Piran (Slovenia) to quantify changes in body size and growth parameters throughout the Holocene. Otoliths are the aragonitic structures of the fish’ inner ear with species-specific morphology, and thanks to their incremental growth, they serve as unique environmental and life-history archive. Moreover, otolith size correlates with fish size. We use otoliths cut in half to perform both sclerochronology and radiocarbon dating, obtaining a high-resolution time series of changes in fish body size, growth dynamics and life history parameters. We employ backscatter electron (BSE) imaging and electron probe microanalysis (EPMA) to identify body sizes and growth dynamics, as well as to correlate their growth increments with climatic and other environmental parameters. The reconstructed changes in body size and growth rates of very common, non-commercial fish species over the last 7.000 years, can serve as an ecological baseline for evaluating the magnitude of ongoing temperature rise and future shifts in fish populations in response to global warming.</p>

Otolith death assemblages provide a valuable source of biological and ecological information that can help address three main problems in marine conservation: a) the lack of pre-industrial, pre-human-impact baselines for evaluating change; b) the inefficiency of survey methods for recording small and cryptic fish species; and c) the absence of long-term data on environmental change impacts on marine ecosystems and fishes. We review here the current knowledge on the formation and preservation of otoliths and their death assemblages, and the methods to obtain, date and analyse them in order to detect changes in the species traits and ecology, the fish population structure and the palaeoceanographic shifts that drove them. Supplementary material at https://doi.org/10.6084/m9.figshare.c.6428742