In the aquatic environment of the Ordovician and the Silurian, the dominant and diversed fish group was the agnathans which had no jaws like Arandaspis, Sacabambaspis or Cephalaspis. By the Early Devonian, the succeeded group in the rivers, lakes, and seas was the Placoderms, especially, the Arthrodires radiated in both marine and non-marine environments during the Middle and Late Devonian 5).
- Earthwise - British Geological Survey -
- Miles R, Westoll T (1968). IX.—The Placoderm Fish Coccosteus cuspidatus Miller ex Agassiz from the Middle Old Red Sandstone of Scotland. Part I. Descriptive Morphology. [abstract] Transactions of the Royal Society of Edinburgh, 67(9): 373-476.(DOI:10.1017/S0080456800024078).
- Orkney Landscapes - Life in a Middle Devonian Lake -
- Prehistoric Wildlife
- Young GC (2010) Placoderms (Armored Fish): Dominant Vertebrates of the Devonian Period. Annu. Rev. Earth Planet. Sci. 38:523-550. (DOI: 10.1146/annurev-earth-040809-152507).