A nearly complete respiratory, circulatory, and excretory system preserved in small Late Cretaceous octopods (Cephalopoda) from Lebanon

Although they are rare, fossilized gills are well known in Mesozoic coleoid cephalopods. In the Late Jurassic Solnhofen (South Germany) and Late Cretaceous Hâkel and Hâdjoula (Lebanon) plattenkalks, the feather-like gill remains are usually preserved as a yellowish staining. Small coleoids from Hâkel—tentatively determined as octopods—attracted our attention because these stains occur throughout the entire mantle sac in an unusual symmetrical pattern. Actualistic comparisons point to a compound of diverse vascular structures that most likely reflect central parts of the venous blood system (afferent branchial vessels, branchial hearts, vena pallialis, blood sinus) as well as the nephridial sacs. The nephridial sacs are clearly separated, which confirms the octopod nature of the fossils. A reticulated staining pattern in the rear of the mantle, which may reflect the gonad capillary system, suggests the presence of mature small-sized octopods. Based on its colour, its amorphous habit, and energy-dispersive X-ray spectroscopy (EDX) elemental analyses, the major components of the coelomic cavities have been replicated by an iron-rich phase (presumably goethite; copper was not detected). The goethite does not replace the tissues; rather, it traces their gross form as a well-defined “stain”. It is assumed the goethite is secondary after pyrite, which precipitated as a consequence of the oxygen-binding capacity of the copper-bearing haemocyanin and its ability to locally regulate redox potentials immediately postmortem.