New insights on the systematics, palaeoecology and palaeobiology of a plesiosaurian with soft tissue preservation from the Toarcian of Holzmaden, Germany.

The Posidonienschiefer Formation (Toarcian) of Holzmaden, Baden-Württemberg in southwestern Germany has yielded several excellently preserved plesiosaurian specimens and received considerable research attention. The plesiosaurians found within these deposits are always significantly outnumbered by ichthyosaurs, and close examination of these rare specimens is crucial to a better understanding of the diversity and palaeoecology of Plesiosauria in this very peculiar ecosystem. The plesiosaurian specimen SMNS 51945 found in this area is a juvenile individual consisting of a partial, crushed skull and an exquisitely preserved post-cranial skeleton. Its anatomical characters seem to differ from the long-necked plesiosauroids Microcleidus brachypterygius and Seeleyosaurus guilelmiimperatoris that are the most abundant taxa within the plesiosaurian assemblage. The post-cranial skeleton preserves very likely soft tissues composed of buff-coloured and dark-coloured structures around the vertebral column and hindlimb of the animal. A network of buff-coloured fibres located posterior to the hindlimb most likely represents phosphatised collagen fibres as already found in some ichthyosaur specimens, confirming that wing area in plesiosaurians was much larger than that suggested by skeletal remains alone. The specimen also contains gastroliths (sand-sized grains mainly composed of quartz) in the stomach cavity suggesting the animal spent at least some of its time in shallow coastal waters, tens or hundreds of kilometres from the final place of burial.

Reconstructing the ecology of a Jurassic pseudoplanktonic raft colony.

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.