An early Miocene extinction in pelagic sharks Sibert, Elizabeth C.; Rubin, Leah D.
Science (American Association for the Advancement of Science),
06/2021, Letnik:
372, Številka:
6546
Journal Article
Recenzirano
Mysterious mass extinction
The term “shark” inspires predictable images of stealthy and streamlined marine predators that are key components of modern ecosystems. Studying shark teeth buried in deep ...sea sediment, Sibert and Rubin reveal that current shark diversity is a small remnant of a much larger array of forms that were decimated by a previously unidentified major ocean extinction event (see the Perspective by Pimiento and Pyenson). The extinction led to a reduction in shark diversity by more than 70% and an almost complete loss in total abundance. There is no known climatic and/or environmental driver of this extinction, and its cause remains a mystery. Modern shark forms began to diversify within 2 to 5 million years after the extinction, but they represent only a minor sliver of what sharks once were.
Science
, aaz3549, this issue p.
1105
; see also abj2088, p.
1036
A shark extinction event 19 million years ago led to the loss of the majority of shark species and reduced their diversity.
Shark populations have been decimated in recent decades because of overfishing and other anthropogenic stressors; however, the long-term impacts of such changes in marine predator abundance and diversity are poorly constrained. We present evidence for a previously unknown major extinction event in sharks that occurred in the early Miocene, ~19 million years ago. During this interval, sharks virtually disappeared from open-ocean sediments, declining in abundance by >90% and morphological diversity by >70%, an event from which they never recovered. This abrupt extinction occurred independently from any known global climate event and ~2 million to 5 million years before diversifications in the highly migratory, large-bodied predators that dominate pelagic ecosystems today, indicating that the early Miocene was a period of rapid, transformative change for open-ocean ecosystems.
Ray-finned fishes (Actinopterygii) comprise nearly half of allmodern vertebrate diversity, and are an ecologically and numerically dominant megafauna in most aquatic environments. Crown teleost ...fishes diversified relatively recently, during the Late Cretaceous and early Paleogene, although the exact timing and cause of their radiation and rise to ecological dominance is poorly constrained. Here we use microfossil teeth and shark dermal scales (ichthyoliths) preserved in deep-sea sediments to study the changes in the pelagic fish community in the latest Cretaceous and early Paleogene. We find that the Cretaceous–Paleogene (K/Pg) extinction event marked a profound change in the structure of ichthyolith communities around the globe: Whereas shark denticles outnumber ray-finned fish teeth in Cretaceous deep-sea sediments around the world, there is a dramatic increase in the proportion of ray-finned fish teeth to shark denticles in the Paleocene. There is also an increase in size and numerical abundance of ray-finned fish teeth at the boundary. These changes are sustained through at least the first 24 million years of the Cenozoic. This new fish community structure began at the K/Pg mass extinction, suggesting the extinction event played an important role in initiating the modern “age of fishes.”
Marine ecosystem models predict a decline in fish production with anthropogenic ocean warming, but how fish production equilibrates to warming on longer timescales is unclear. We report a positive ...nonlinear correlation between ocean temperature and pelagic fish production during the extreme global warmth of the Early Paleogene Period (62-46 million years ago Ma). Using data-constrained modeling, we find that temperature-driven increases in trophic transfer efficiency (the fraction of production passed up trophic levels) and primary production can account for the observed increase in fish production, while changes in predator-prey interactions cannot. These data provide new insight into upper-trophic-level processes constrained from the geological record, suggesting that long-term warming may support more productive food webs in subtropical pelagic ecosystems.
Oxygen minimum zones (OMZs) play a critical role in global biogeochemical cycling and act as barriers to dispersal for marine organisms. OMZs are currently expanding and intensifying with climate ...change, however past distributions of OMZs are relatively unknown. Here we present evidence for widespread pelagic OMZs during the Pliocene (5.3-2.6 Ma), the most recent epoch with atmospheric CO
analogous to modern (~400-450 ppm). The global distribution of OMZ-affiliated planktic foraminifer, Globorotaloides hexagonus, and Earth System and Species Distribution Models show that the Indian Ocean, Eastern Equatorial Pacific, eastern South Pacific, and eastern North Atlantic all supported OMZs in the Pliocene, as today. By contrast, low-oxygen waters were reduced in the North Pacific and expanded in the North Atlantic in the Pliocene. This spatially explicit perspective reveals that a warmer world can support both regionally expanded and contracted OMZs, with intermediate water circulation as a key driver.
Feichtinger
. assert that the reduction in denticle abundance and diversity we found are incorrect, claiming that we failed to consider changes in sedimentation rate. However, we used standard ...methods that explicitly account for changes in sedimentation rate and density. We maintain that our initial dataset and conclusions are robust and provide evidence for a major early Miocene extinction in pelagic sharks.
Naylor
. argue that the existence of multiple denticle types within a single species precludes the use of this metric as a measure of the decline of multiple shark species. We show that species-level ...shark diversity would have to decrease by >90% to account for the observed >70% denticle extinction, implying that the early Miocene shark extinction was larger than previously recognized.
Increasingly more students with disabilities, including autistic or otherwise neurodiverse students, are studying for degrees in STEM field subjects. In recent years, there has been an increased ...effort from the geoscience education community to make teaching more accessible and inclusive to these students. However, much of the literature on this topic lacks the voice of the individuals these practices aim to serve. This, combined with the medical, deficit-based understanding of autism typically presented in the literature, has resulted in the perpetuation of harmful stereotypes, along with recommendations that may not actually serve as best practice. Here we present a more accurate and holistic explanation of what autism actually is, using our lived experiences as autistic geoscientists. We then outline a comprehensive framework for best supporting autistic and neurodiverse geoscience students, with a focus on field-based learning. This framework includes three pillars: (a) develop effective communication pathways with autistic students, (b) presume competence and include autistic students in the planning of their own accommodations, and (c) employ strategies for expectation management. We also touch on the importance of recognizing the sensory processing aspects of autism spectrum conditions and suggest strategies for minimizing these difficulties in a field environment. By centering autistic voices in the discussion of how to support autistic geoscience students, we hope to change the narrative of inclusion for this diverse, but significant population.
Sedimentary ancient DNA (sedaDNA) analyses are increasingly used to reconstruct marine ecosystems. The majority of marine sedaDNA studies use a metabarcoding approach (extraction and analysis of ...specific DNA fragments of a defined length), targeting short taxonomic marker genes. Promising examples are 18S-V9 rRNA (~121-130 base pairs, bp) and diat-rbcL (76 bp), targeting eukaryotes and diatoms, respectively. However, it remains unknown how 18S-V9 and diat-rbcL derived compositional profiles compare to metagenomic shotgun data, the preferred method for ancient DNA analyses as amplification biases are minimised. We extracted DNA from five Santa Barbara Basin sediment samples (up to ~11 000 years old) and applied both a metabarcoding (18S-V9 rRNA, diat-rbcL) and a metagenomic shotgun approach to (i) compare eukaryote, especially diatom, composition, and (ii) assess sequence length and database related biases. Eukaryote composition differed considerably between shotgun and metabarcoding data, which was related to differences in read lengths (~112 and ~161 bp, respectively), and overamplification of short reads in metabarcoding data. Diatom composition was influenced by reference bias that was exacerbated in metabarcoding data and characterised by increased representation of Chaetoceros, Thalassiosira and Pseudo-nitzschia. Our results are relevant to sedaDNA studies aiming to accurately characterise paleo-ecosystems from either metabarcoding or metagenomic data.
Large‐scale, comparative studies of morphological variation are rare due to the time‐intensive nature of shape quantification. This data gap is important to address, as intraspecific and ...interspecific morphological variation underpins and reflects ecological and evolutionary processes.
Here, we detail a novel software package, AutoMorph, for high‐throughput object and shape extraction. AutoMorph can batch image many types of organisms (e.g. foraminifera, molluscs and fish teeth), allowing for rapid generation of assemblage‐scale morphological data.
We used AutoMorph to image and generate 2D and 3D morphological data for >100,000 marine microfossils in about a year. Our collaborators have used AutoMorph to process >12,000 patellogastropod shells and >50,000 fish teeth.
AutoMorph allows users to rapidly produce large amounts of morphological data, facilitating community‐scale evolutionary and ecological studies. To hasten the adoption of automated approaches, we have made AutoMorph freely available and open source. AutoMorph runs on all UNIX‐like systems; future versions will run across all platforms.
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