Hunter AW et al. (2020), Reconstructing the ecology of a Jurassic pseudo...

ECB-ART-49643

R Soc Open Sci 2020 Jul 22;77:200142. doi: 10.1098/rsos.200142.

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Reconstructing the ecology of a Jurassic pseudoplanktonic raft colony.

Hunter AW , Casenove D , Mayers C , Mitchell EG .

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Pseudoplanktonic crinoid raft colonies are an enigma of the Jurassic. These raft colonies are thought to have developed as floating filter-feeding communities due to an exceptionally rich oceanic niche, high in the water column enabling them to reach large densities on these log rafts. However, this pseudoplanktonic hypothesis has not been quantitatively tested, and there remains some doubt that this mode of life was possible. The ecological structure of the crinoid colony is resolved using spatial point process analyses and the duration estimates of the floating system until sinking using moisture diffusion models. Using spatial analysis, we found that the crinoids would have trailed preferentially positioned at the back of the floating log in the regions of least resistance, consistent with a floating, not benthic ecology. Additionally, we found using a series of moisture diffusion models at different log densities and sizes that ecosystem collapse did not take place solely due to colonies becoming overladen as previously assumed. Our analyses have found that these crinoid colonies studied could have existed for more than 10 years, even up to 20 years, exceeding the life expectancy of modern documented raft systems with possible implications for the role of modern raft communities in the biotic colonization of oceanic islands and intercontinental dispersal of marine and terrestrial species.

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	Figure 1. Crinoid fossil raft, the ‘Hauff specimen’ from Holzmaden (G1). (a) Log with spatial analysis data points; key: blue = crinoid crowns, black = attachment discs. (b) Spatial analysis plot. (c) Close up view of the crinoid crown and stem sections. (d) PCF distance plot.
	Figure 2. Estimates for the soaking duration (in years) of cylindrical log due to (a) longitudinal and (b) radial diffusion of water. Wood logs can stay afloat several years before absorbing enough water to reach a critical density allowing them to sink. Modelled values on the vertical axis indicate the number of years required for three cylindrical logs to sink and are presented in relation to the length and density of the material. For each geometry, durations were calculated for three distinct values of radial diffusion coefficient (Dlong/Drad = 10; Dlong/Drad = 100 and Dlong/Drad = 1000). In all cases, the wood log was assumed to have reached an equilibrium moisture content (EMC) with an atmosphere at 20°C and 100% humidity. In such conditions, wood densities above 0.75 × 10³ kg m−3 will sink instantly.
	Figure 3. Duration of the floating phase (in months) for varying density (horizontal axis) and varying equilibrium moisture content (EMC). Calculations are run for three specimens: Gottingen (S1), Frankfurt (M1) and Holtzmaden (G1). (a) S1 log sample with no crinoid population (unloaded). (b) S1 log sample with crinoid community (loaded). (c) M1 unloaded. (d) M1 log sample loaded. (e) G1 log sample unloaded. (f) G1 log sample loaded.
	Figure 4. Reconstruction of the Holzmaden ‘Hauff specimen’ crinoid raft colony (G1).

References [+] :

Carlton, Tsunami-driven rafting: Transoceanic species dispersal and implications for marine biogeography. 2017, Pubmed