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Where were the earliest Heterostraci fossils discovered?
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agnathan
Lamprey (Lampetra) on rainbow trout.
The earliest known vertebrate fossils belonging to the order Heterostraci were discovered in North America. These remains, which consist of fragments of dermal armour, date back to the Upper Ordovician Period, approximately 450 million years ago.1
It remains uncertain whether these early jawless fishes inhabited shallow coastal marine waters or were freshwater vertebrates that were transported into coastal deposits by stream action.1