Most authors agree that the oldest paraconodonts like Protohertzina derive from the "Anabarites trisulcatus - Protohertzina anabarica Zone". According to the geological time-scale 2016 (Ogg et al. 2016), this would yield a minimum age of about 532.7 Ma. Of course, Protohertzina already occurs at the base of this zone, so my question would be are there any earlier "First Appearance Data of Protohertzina" or other early protoconodonts which are bounded below radiometric age or can be reasonable correlate to an interval below 532.7Ma. Your help would be greatly appreciated.
First Appearance Data of Protohertzina might be constrained to 545–540 Ma according to SSFs (including Protoherzina and other protocodonts) reported from Ust’-Yudoma Formation, Russia. see the ref below:
Zhu M , Zhuravlev A Y , Wood R A , et al. A deep root for the Cambrian explosion: Implications of new bio- and chemostratigraphy from the Siberian Platform. Geology, 2017, 45(5):459-462.
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