'Carreg Coetan Arthur on the Nyfer estuary, north Pembrokeshire, is the most coastal of the group of Nevern valley chambered tombs, often categorised as belonging to the Irish Sea portal dolmen tradition... In 1979-80 excavations were undertaken over three seasons to ensure that the full extent of the site was protected from an adjacent housing development... Excavations tested the extent of the preservation of the cairn and chamber interior. The site had been substantially disturbed by cultivation and human and rodent interference, but remnants of an old ground surface protected by a covering of redeposited subsoil survived outside the south and east side of the chamber, below which lay one complete quartz-tempered round-bottomed Developed Bowl inverted on a prepared 'paved' surface and associated with cremated bone and charcoal which gave a date of c. 3650-3020 cal. BC.'
Edited from Rees, S. 2012. page 51.

Royal Commission photo survey, 6th February 2020 by Toby Driver in winter light during which a deep cupmark was noted low down on the capstone close to its edge.

T. Driver, RCAHMW, Feb 2020.

References:

Barker, C.T., 1992, The Chambered Tombs of South-West Wales, Oxbow Monograph 14, pages 19-21.

Rees, S. 2012. Excavations at Carreg Coetan Arthur chambered tomb, Pembrokeshire. Archaeologia Cambrensis 161, 51-163.