The ground in and around the city of Holzmaden contains rich layers of well preserved fossils (or Lagerstätten) of the Jurassic period. The fossils are found in the 180 million year old Posidonia Shale. The Posidonia Shale is a Lower Jurassic geological formation famous for its exceptionally well-preserved complete skeletons of fossil marine fish and reptiles. The Posidonienschiefer, as German paleontologists call it, takes its name from the ubiquitous fossils of Posidonia bronni that characterize its fauna. The formation comprises finely laminated layers of bituminous shales formed of fine-grained sediments intercalated with bituminous limestones and crops out in a number of locations in southwestern Germany, although most famously near the village of Holzmaden. The European bituminous shales deposited on a sea floor during the Early Toarcian in the ancient Tethys Ocean are described as being deposited in an anoxic, or oxygen depleted, deep water environment, although the details of the depositional environment are the subject of debate by researchers of the formation. In addition to Posidonia bronni, the shales contain some spectacularly detailed fossils of other Jurassic sea creatures: ichthyosaurs and plesiosaurs, spiral-shelled ammonites and crinoids. Ichthyosaurs – the name means ‘fish lizards’ – were reptile inhabitants of the sea, living at the same time as dinosaurs prowled the land and pterosaurs glided in the air. They looked rather like modern dolphins. The smallest was around 1 metre, the biggest, Shonisaurus, an amazing 23 metres, both extremes arising early in their evolution. Ichthyosaurs had eyes as big as those of any animal known. Despite their fish-like appearance, their anatomy shows they once lived on land, as demonstrated by their two pairs of limbs, with digit-like bones rather than rays or spines in their flippers, and a shoulder girdle connected to the skull. The roof of the skull had a pair of openings called fenestra: a hallmark of reptiles. Another indication that they had a terrestrial origin is their lack of gills. Like marine mammals, they had to draw oxygen from the atmosphere. This was why, when an ichthyosaur embryo in the womb uncurled itself in preparation for birth, it instinctively oriented itself so that it passed out of the womb tail first. Only when its head emerged did it need to breathe, at which point it could take its first gulp of oxygen by immediately swimming to the surface.
Holzmaden, Germany; discovered c. 1932-4 whilst digging the autobahn from Stuttgart to Munich