Two pulses of morphological diversification in Pacific pelagic fishes following the Cretaceous–Palaeogene mass extinction

Abstract

Molecular phylogenies suggest some major radiations of open-ocean fish clades occurred roughly coincident with the Cretaceous–Palaeogene (K/Pg) boundary, however the timing and nature of this diversification is poorly constrained. Here, we investigate evolutionary patterns in ray-finned fishes across the K/Pg mass extinction 66 million years ago (Ma), using microfossils (isolated teeth) preserved in a South Pacific sediment core spanning 72–43 Ma. Our record does not show significant turnover of fish tooth morphotypes at the K/Pg boundary: only two of 48 Cretaceous tooth morphotypes disappear at the event in the South Pacific, a rate no different from background extinction. Capture–mark–recapture analysis finds two pulses of origination in fish tooth morphotypes following the mass extinction. The first pulse, at approximately 64 Ma, included short-lived teeth, as well as forms that contribute to an expansion into novel morphospace. A second pulse, centred at approximately 58 Ma, produced morphotype novelty in a different region of morphospace from the first pulse, and contributed significantly to Eocene tooth morphospace occupation. There was no significant increase in origination rates or expansion into novel morphospace during the early or middle Eocene, despite a near 10-fold increase in tooth abundance during that interval. Our results suggest that while the K/Pg event had a minor impact on fish diversity in terms of extinction, the removal of the few dominant Cretaceous morphotypes triggered a sequence of origination events allowing fishes to rapidly diversify morphologically, setting the stage for exceptional levels of ray-finned fish diversity in the Cenozoic.

Enhanced fish production during a period of extreme global warmth

“…We note the only major diversification of fish tooth morphotypes occurred at the beginning of the record (62-58 Ma; ref. 44 ), a time with relatively little change in temperature. There is also no structural or evolutionary change in fish diversity at DSDP Site 596 during the remainder of the study interval (58-46 Ma) 44 , suggesting that the observed variation in the ichthyolith record is most likely due to changes in fish production, rather than changes in fish community composition or diversity.…”

“…The final analyzed dataset consisted of a per-Myr time-varying tooth-size distribution where the area under the curve (the total IAR) is a proxy for total fish community production. We adopt this simple measure of total relative fish community production following the previous work 38,43,44 .…”

“…To account for this, we scaled the productivity size distribution such that the reference solution has total productivity of one unit per Myr and changes in productivity over time can be modeled relative to the reference solution in %. This procedure assumes that the unknown tooth number per biomass conversion remains constant over time, a reasonable assumption in this case as there are no major changes in fish morphotype community composition that would indicate a major shift in the fish community or biology 44 . We note that a power-law distribution was tested for the primary productivity distribution but we could not find a satisfactory model solution.…”

The extinction and survival of sharks across the end-Cretaceous mass extinction

“…Despite the proliferation of certain tooth types in the aftermath of the extinction event, static disparity does not support a Paleocene diversification of sharks, which is in contrast to the substantial ecological and taxic diversification of coeval actinopterygian fishes (e.g., acanthomorphs and carangarians) [16,19,79–82]. Nevertheless, Sibert et al [83] recently found that post-Mesozoic actinopterygian disparity underwent alteration in only a few dominant tooth morphotypes. Likewise, we suggest that while, in general, sharks do not seem to have experienced much change in disparity across the K/Pg boundary, they did suffer sufficient ecological disturbance to trigger a compositional transformation in the diversity of certain constituent clades [20,83], such as lamniform and carcharhiniforms.…”

Timing the extant avian radiation: The rise of modern birds, and the importance of modeling molecular rate variation

“…The fossil record is rich in examples of rapid shifts in organismal size and form (Finarelli and Flynn 2006;Evans et al 2012;Bellwood et al 2014;Huttenlocker 2014;Near et al 2014;Berv and Field 2018). For major groups these shifts are particularly pervasive in the aftermath of mass extinction events (Twitchett 2007;Friedman 2010;Sibert et al 2018). In these cases, the most recent common ancestors of survivors are often hypothesized to be relatively small bodied (Cardillo et al 2005;He et al 2010;Huttenlocker 2014).…”