The
Opalios holotype specimen,
AM F132596–AM F132599, was discovered in 2001 in sediments of the
Griman Creek Formation (Wallangulla Sandstone Member) near
Lightning Ridge,
New South Wales, Australia. The specimen consists of four broken pieces of the left
dentary. In 2024, Flannery et al.
described Opalios splendens as a new genus and species of early monotreme based on these fossil remains. They erected the new
monotypic family
Opalionidae within the
Ornithorhynchoidea—the
superfamily also containing modern
echidnas and
platypus—to house
Opalios due to its similarity to both of these extant lineages. The
generic name,
Opalios, is a
Greek word meaning "
opal", referencing the holotype's opalized nature. The
specific name,
splendens, is a
Latin word meaning "shining" or " distinguished", in reference to the holotype's striking translucency and size. Several other monotremes are known from the Griman Creek Formation, including
Dharragarra and
Parvopalus—which were described in the same
publication as
Opalios—as well as
Kollikodon,
Steropodon, and
Stirtodon. == References ==