Very weird: 236-million-year-old fossil proves mammals stopped laying eggs earlier than we thought
Our understanding of mammalian reproduction is being reborn.Analysis of a 236-million-year old fossil could completely rewrite our understanding of how mammals gave birth, per a study in the journal Frontiers in Mammal Science.The groundbreaking remains belong to Chiniquodon theotonicus, a species of cynodont — prehistoric reptiles that were likely the ancestor of modern mammals, Phys.org reported.For years, scientists believed that our evolutionary forebear laid eggs much like other reptiles, but direct evidence of how they reproduced remained inconclusive — until now.However, by analyzing the bones of a Chiniquodon theotonicus individual — which lived during the late Triassic period — scientists determined that they likely birthed live young, winding the clocks back on this reproductive adaptation by a staggering 95 million years.“It looks like some cynodonts were in fact very similar to present-day mammals,” exclaimed lead author Dr.Leandro Gaetano, a paleontologist at the National Scientific and Technical Research Council (CONICET) in Argentina.The researchers came upon this revelation by accident while examining the bones of a fully grown C.
theotonicus, and finding that they had “embryonic tissue,” Gaetano told Gizmodo.“This is very weird, I have never seen this,” he said.By analyzing data from living species, the scientists deduced that the mark was a neonatal line, a distinct growth ring found in bones or teeth that forms following the post-birth growth spurt.This post-natal stretch mark gave researchers a novel idea on how to test whether cynodonts laid eggs — by comparing their body mass at birth to their adult size.They then compared this ratio to that of several thousand species of modern birds, mammals and reptiles.“If mammalian ancestors were egg-laying or viviparous (live birth-giving) has been considered an inscrutable mystery,” said Gaetano.
“We came up with a somewhat ingenious set of methods to get at somethi...