Abstract
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.
Highlights
Transoceanic rafting is a fundamental feature of marine evolutionary biogeography and ecology, which is used to explain the origins of (a) royalsocietypublishing.org/journal/rsos R
The diffusion analysis is represented by two models, which show that the largest of the log systems (Holzmaden) could have survived for a minimum of 2 years and a maximum of 14 years (20 years if we account for the maximum values of the longitudinal diffusion coefficient) with an atmospheric relative humidity of 100% allowing the crinoid colony to grow to maturity and confirming the viability of the pseudoplanktonic hypothesis
When the diffusion model was applied to the small logs (S1 and S2) without the crinoid population, it showed that the log could have stayed afloat for 800 months; when the colony was added it sank after one time increment
Summary
Transoceanic rafting is a fundamental feature of marine evolutionary biogeography and ecology, which is used to explain the origins of (a) royalsocietypublishing.org/journal/rsos R. The deep time ecology of these communities has never been investigated using the latest quantitative methods to test these different hypotheses [6] empirically. In recent communities, such rafts have included highly adapted bivalves, barnacles, limpets, bryozoans, sea anemones, amphipods and isopods [5]. In the Jurassic, these communities consisted of specially adapted crinoids, whose apparent maturity suggests that these pseudoplanktonic communities had to have lasted longer than modern examples (more than 6 years) [6]. This study uses the latest quantitative ecological techniques used in palaeobiology to reconstruct the ecology and duration of these rafts
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