The extinct marine megafauna of the Phanerozoic

Abstract

The modern marine megafauna is known to play important ecological roles and includes many charismatic species that have drawn the attention of both the scientific community and the public. However, the extinct marine megafauna has never been assessed as a whole, nor has it been defined in deep time. Here, we review the literature to define and list the species that constitute the extinct marine megafauna, and to explore biological and ecological patterns throughout the Phanerozoic. We propose a size cut-off of 1 m of length to define the extinct marine megafauna. Based on this definition, we list 706 taxa belonging to eight main groups. We found that the extinct marine megafauna was conspicuous over the Phanerozoic and ubiquitous across all geological eras and periods, with the Mesozoic, especially the Cretaceous, having the greatest number of taxa. Marine reptiles include the largest size recorded (21 m; Shonisaurus sikanniensis) and contain the highest number of extinct marine megafaunal taxa. This contrasts with today's assemblage, where marine animals achieve sizes of >30 m. The extinct marine megafaunal taxa were found to be well-represented in the Paleobiology Database, but not better sampled than their smaller counterparts. Among the extinct marine megafauna, there appears to be an overall increase in body size through time. Most extinct megafaunal taxa were inferred to be macropredators preferentially living in coastal environments. Across the Phanerozoic, megafaunal species had similar extinction risks as smaller species, in stark contrast to modern oceans where the large species are most affected by human perturbations. Our work represents a first step towards a better understanding of the marine megafauna that lived in the geological past. However, more work is required to expand our list of taxa and their traits so that we can obtain a more complete picture of their ecology and evolution.

Keywords: body size; ecology; ecosystem function; megafauna; species extinction.

Graphical abstract

Figure 1.

A graphical representation of the earliest and largest extinct marine megafaunal taxa. Colours denote the taxonomic group to which each taxon belongs to, which is also used in the geological timescale on the right to denote stratigraphic range. Animal shapes were downloaded from www.phylopic.org. Credits are as follows: Shonisaurus sikanniensis and Leedsichthys problematicus: Gareth Monger; Otodus megalodon: T. Michael Keesey; Perucetus colossus: Michael Tripoli. Remaining animal shapes have a Public Domain licence without copyright (http://creativecommons.org/licenses/by/3.0).

Figure 2.

Representation of extinct marine megafauna in the Paleobiology Database (PBDB) to capture their current state of knowledge. (A) Number of occurrences of each taxon. Each horizontal line (n = 523) represents a taxon (see text). X-axis is log-transformed. (B) Representation of taxonomic groups in PBDB showed as percentages relative to total number of megafaunal taxa in each group. Colours denote the taxonomic group to which each taxon belongs to in A and B. Animal shapes in B are those from Figure 1. (C) Sampling completeness rates for the extinct marine megafauna and the baseline dataset (extinct species with a body length < 1 m) as estimated using a capture–mark–recapture approach. Thick lines indicate the 55% credible interval for the sampling rate, whereas thin lines indicate the 95% interval.

Figure 3.

Extinct marine megafauna over time. (A) Number of taxa per taxonomic group and across geological eras. (B) Stratigraphic ranges of the different taxonomic groups (horizontal lines) and percentage of First Appearance Datums (FADs; green), Last Appearance Datums (LADs; grey) in each geological period shown in vertical bars. See Table 3 for details. (C) Stratigraphic ranges of individual taxa. Grey dashed lines delimit the geological eras. See Supplementary Material Data S2 for details.

Figure 4.

Body size patterns among the extinct marine megafauna. (A) Distribution of maximum body sizes per taxonomic group based on density estimates. Taxonomic groups are ordered by mean maximum body size, with the largest estimate at the top. Sample size (number of extinct megafaunal taxa per group) is shown at the right of each density curve. (B) Maximum body size of each taxon over time, whereby the mid-point of the stratigraphic range was used. The black line shows the average linear trend in maximum body size over time considering all taxonomic groups. (C) Average linear trends in body size per taxonomic group. In A and B, the asterisks indicate statistical significance; the numbers show the average increase in body size per every million-year; maximum body size is log-transformed and grey dashed lines delimit the geological eras.

Figure 5.

Ecological traits across geological eras. The number of taxa per taxonomic group and ecological trait, including counts where the ecological data is missing. (A) Guild, or most common feeding mechanism. (B) Vertical position, or distribution in the water column where animals feed. (C) Habitat, or lateral position where animals live.

Figure 6.

Major clades within the extinct marine megafauna taxonomic groups. (A) The number of taxa per clade within taxonomic groups, whereby the maximum body size of each clade is depicted by the point size. (B) Presence of each megafaunal clade across geological eras where the size of the points depicts the maximum body size, and the coloured surrounding ring represents the corresponding era. No point means that the clade is not occurring in that geological era. *Here, the clade Archosauromorpha only refers to early branching taxa and excludes Paracrocodylomorpha.

Figure 7.

Distribution of ecological traits (guild, position in the water column, and habitat) for the extinct marine megafauna assemblage. (A) The relative frequency of each ecological trait per taxonomic group as percentage. (B) The log-transformed maximum body size in metre per taxon over time and per ecological trait. The mid-point of the stratigraphic range for each taxon was used to plot the maximum body size. Grey dashed lines depict boundaries between eras.

Figure 8.

Extinction selectivity of marine megafauna compared with non-megafauna species (i.e., baseline, taxa that belong to the same genus as the extinct megafauna, but that are < 1 m). (A) The extinction risk for fossil taxa as estimated by a Bayesian generalised linear mixed-effects model. Points show the average extinction risk for each taxonomic group, and lines the 95% Credible Interval. (B) Extinction selectivity over time on a logit scale for each megafauna group as estimated by the Bayesian model. Positive values indicate an extinction selectivity towards baseline taxa and negative values preferential extinction of megafauna taxa. Thick coloured lines depict the average trend per taxonomic group and the shaded area the corresponding 95% Credible Interval. Logit values are defined as the logarithm of the extinction probability for megafaunal taxa divided by the extinction probability for baseline taxa.