Didymograptus artus/murchisoni
Didymograptus was a graptolite, an extinct colonial animal related to the primitive chordates.
It lived in Early and Middle Ordovician periods. The several described species of Didymograptus, with their wide geographic distribution and relatively narrow time ranges, are guide fossils for correlation of Early and Middle Ordovician rocks and time.
It is characterized by its two-branched form, frequently suspended from a circular disk-shaped structure. Didymograptus includes some of the largest known graptolites.