the horseshoe crabs, are related to the extinct Eurypterids, and
more distantly to spiders and scorpions. Their lineage traces back
to the Cambrian, and extends to modern times as the genus Limulus.
This fossil is of a similar genus known as Mesolimulus from the
Solnhofen lithographic limestone deposits of Eichstatt, Germany.
The 150 million year old lagerstätte deposits of Solnhofen
are famous for their exceptionally well-preserved organisms, the
most famous of which are the handful of specimens of the ancient
there are only three extant genera and five extant species of Class
Xiphosura, they were quite diverse during the Palaeozoic
Era. Because they have apparently undergone little change, the
extant horseshoe crabs are often considered to be living fossils.
Horse shoe crabs have a large shield that covers the cephalothorax,
and the carapace is hinged between the cephalothorax and abdomen.
The sturdy exoskeleton comprises three parts, the large semicircular
cephalothorax, the opisthosoma which is the posterior portion of
body behind the cephalothorax, and a long tail spine or telson.
The resemblance to trilobites
is apparent, and, in fact, the Xiphosura are considered by many
to be the closest living relatives of the long-extinct trilobites.
see: Solnhofen Fossils