The transition occurred partly because amniotes evolved to reproduce with hard-shelled, rather than soft-shelled, eggs.
“The vertebrates’ move onto land was an important part, and within that a key step was the evolution of the amniotic egg in the immediate common ancestors of reptiles and mammals,” Ahlberg said. “So these events form a key episode in our own ancestry as well as the history of the planet.”
The new study pushes the origin of amniotes much deeper into the Carboniferous Period, 299 million to 359 million years ago, which allows a much greater length of time for the diversification of early reptiles, said Stuart Sumida, president of the Society of Vertebrate Paleontology and professor of biology at California State University, San Bernardino. Sumida, who wrote an accompanying article to release with the study, did not participate in the new research.
The search for amniote origins
Long has been studying ancient fish fossils from the Mansfield district, where the slab was found, since 1980.
“The Mansfield area has produced many famous fossils, beginning with spectacular fossil fishes found 120 years ago, and ancient sharks. But the holy grail that we were always looking for was evidence of land animals, or tetrapods, like early amphibians. Many had searched for such trackways but never found them — until this slab arrived in our laboratory to be studied,” he said.
Fossils from the Mansfield district have shed light on how sexual organs might have first evolved in ancient armored fish.
Now, the researchers want to know what else lived in Gondwana alongside the ancient reptile they found.
The findings have inspired researchers to broaden the search for fossils of the earliest amniotes, and their close relatives, to the southern continents, Sumida said.
“Most of the skeletal fossil discoveries of the earliest amniotes are known from continents derived from the northern components of Pangea,” Sumida said in an email. “Discoveries there suggested that amniote origins might be in those regions. It seems clear to me now that we must now expand our search for Early Carboniferous localities in Australia, South America, and Africa.”