Earliest vertebrate embryos

The earliest vertebrate embryos currently known to science are a series of tiny unborn embryos discovered inside the 385-million-year-old fossil of an adult female specimen of Watsonosteus fletti. This was a live-bearing species of prehistoric armoured fish or placoderm, and could measure up to 1 m (3 ft 3 in) long. The specimen, dating from the mid-Devonian Period, was discovered in a loch on the island of South Ronaldsay in Orkney, a collection of islands off the northern Scottish mainland, UK. This very significant fossil is now housed within the collections of the National Museums of Scotland, and is approximately 3 million years older than the previous earliest-known fossilized vertebrate embryos, found in Australia. The discovery was first described in a paper published in the journal Palaeontology on 24 September 2020.

Placoderms or armoured fishes constitute a very distinctive taxonomic class of fishes that are now entirely extinct, the youngest species currently known from the fossil record having died out around 358 million years ago during the Late Devonian. As their common name suggests, they were characterized by the articulated armoured plates that covered their head and thorax, and were among the first jawed fishes to have evolved. During their existence, the placoderms were very successful, diverse, mostly predatory and inhabited both freshwater and marine waters.

The oldest fossilized animal embryos overall are even older, dating back around 500 million years. Around 100 embryos of Markuelia hunanensis - an extinct species of worm, and so an invertebrate species - were discovered in Hunan, China, by a team from Bristol University, UK, and Beijing University, China. Their results were announced in the journal Nature in January 2004.