I'm going by what my Grandmother wrote on the box she sent the two items in. What she called a platystrophia looks something like a clam with three rising and lowering main lobes to the shell which have sub-lobes in them. It sort of looks like a fat Shell oil sign only with three folds folds across the shell from side to side. I did take them both to University of California at Berkeley when I was about 10 years old and the guy there confirmed what they were, but did not, as I recall, confirm the age, if we talked about it at all. Then again, that was 56 years ago. He did say the Trilobite was a particularly fine specimen. It's curled up, sort of biting its tail, not spread out flat.
With my Grandmother's handwriting the 585,000,000 could just as easily have been 535,000,000 years. LOL! She used a fountain pen and her hands were pretty shaky. . . but what was 50 million years give or take to an 8 year old boy? I'm pretty sure she wrote the other fossil was a hundred million years older than the Trilobite.
Thanks for the information. . .
"The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period (521 million years ago), and they flourished throughout the lower Paleozoic era before beginning a drawn-out decline to extinction when, during the Devonian, all trilobite orders except the Proetids died out.
Trilobites finally disappeared in the mass extinction at the end of the Permian about 250 million years ago. The trilobites were among the most successful of all early animals, roaming the oceans for over 270 million years.[4]"
https://en.wikipedia.org/wiki/Trilobite
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"Platystrophia is an extinct genus of brachiopod that lived from the Ordovician to the Silurian in Asia, Europe, North America, and South America. It has a prominent sulcus and fold. It usually lived in marine lime mud and sands."
https://en.wikipedia.org/wiki/Platystrophia
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Devonian 419-359 million years ago
Silurian 443-419 million years ago
Ordovician 485-443 million years ago
Cambrian 541-485 million years ago