Middle Silurian
Gower Formation
Clinton County, IA, USA
22mm arthropod on 71mm slab
Arthropod,

Here is an early myriapod, a member of a subphylum of arthropods, Myriapoda, which contains millipedes and centipedes.  The oldest-known millipede comes from from a younger formation in the Late Silurian but this specimen falls in line with studies predicting earlier myriapods may be found even into the Late Cambrian.

The Silurian was so long ago, the earth’s rotation was noticeably faster than it is today, closer to its Precambrian speeds.  One complete rotation, one day, took only about 21 hours.