Our grasp on how mammals reproduce is undergoing a dramatic transformation.
A 236-million-year-old fossil is causing experts to rethink the reproductive evolution of mammals, as detailed in a recent study published in the journal Frontiers in Mammal Science.
These extraordinary fossils belong to Chiniquodon theotonicus, a species within the cynodont group — prehistoric creatures considered the precursors to modern mammals, according to findings reported by Phys.org.
For many years, the prevailing scientific belief held that these ancient relatives reproduced by laying eggs, similar to other reptiles. Yet, definitive proof of their reproduction methods remained elusive — until recent discoveries provided new insights.
Upon thorough examination of the skeletal remains of a Chiniquodon theotonicus, which roamed the Earth in the late Triassic period, researchers concluded these creatures likely gave birth to live offspring. This revelation shifts our understanding of mammalian evolution, pushing back the timeline of live birth by an astonishing 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 something very difficult to analyze in the fossil record.”
They found it weighed 3.7 pounds at birth and 26.5 pounds when full-grown — a 14% increase — which was most similar to the newborn-adult body mass ratio of mammals that give live birth.
These placental critters, which comprise 95% of mammals, can weigh between 17.6 pounds and 33.1 pounds as adults but give birth to babies tipping the scales at 35.5 grams and 4.1 pounds (1.87 kg), which translates to a ratio of 18.77%.
Meanwhile, bay duiker antelope of Africa weighs approximately the same as a newborn C. theotonicus. Coincidentally, relatives of this primeval species were also displaying lactation, body hair, and brain size similar to modern-day placental mammals, leading researchers to believe that this live birth adaptation could’ve emerged around the same time.
What was the advantage of giving live birth, which was previously seen as a fairly modern acquisition in the mammalian kingdom?
Senior author Adriana Mancuso, a researcher at CONICET who specializes in terrestrial ecosystem evolution, believes that the adaptation helped them adapt to a harsh environment.
“Cynodonts thrived in the Triassic, a period of recovery and restructuring of ecosystems after one of the most devastating mass extinctions in life history,” she said. “This meant high competition for resources and strong predatory pressures.”
Due to the increasingly dry conditions and seasonality, embryos of live birth-giving animals “would be better protected than those of egg-laying species,” she explained.
Researchers hope more research to shed light on whether C. theotonicus’ reproductive mechanism was an anomaly or representative of a major trend toward giving live birth among mammal ancestors.