According to Official Guinness Records,
The earliest-known example of live birth is a fossilised 380-million-year-old mother and embryo placoderm (prehistoric armoured fish). The extremely well-preserved fossil clearly reveals the presence of an embryo attached to its mother via an umbilical cord. Measuring 25 cm long, this remarkable specimen was uncovered in the Gogo area of Western Australia in 2005 by a team from Victoria, but its existence was not made public until 2008. Representing a new species, it was named Materpiscis attenboroughi, after British television wildlife presenter Sir David Attenborough.
For a complete list of Unknown date records, please visit Unknown date Guinness Records in Australia.