According to paleontologists, the oldest hard bodied life forms only date back to the beginning of the Cambrian, roughly 550 or so million years ago. Or did you mean the "Platystrophia" was about 100 million years more recent than the 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 419.2-358.9 million years ago
Silurian 443.4-419.2 million years ago
Ordovician 485.4-443.4 million years ago
Cambrian 541.0-485.4 million years ago
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. . .