Research Suggests Early Cynodont Ancestor Gave Birth to Live Young 236 Million Years Ago
New findings in Frontiers in Mammal Science propose that some cynodonts may have been viviparous, giving birth to live young much earlier in evolutionary history than previously thought. This study presents compelling evidence of live birth in Chiniquodon theotonicus, a mammalian ancestor that lived approximately 236 million years ago, as claimed by lead author Dr. Leandro Gaetano from Argentina’s National Scientific and Technical Research Council (CONICET).
Dr. Gaetano stated, “We show for the first time that live birth was present in at least one mammalian ancestor. This implies that viviparity among early cynodonts originated in the mammalian lineage at least 95 to 90 million years earlier than previously thought.”
Insights into Cynodont Reproduction
Cynodonts thrived during the Triassic period, a crucial time for ecosystem recovery following one of the largest mass extinctions in history. Senior author Adriana Mancuso, a researcher at CONICET, explained that the environmental conditions led to high competition for resources and strong predatory pressures, suggesting that viviparous species would provide better protection for embryos compared to egg-layers. “Combined with a trend toward aridity and strong seasonality, embryos of viviparous species would be better protected than those of egg-laying species,” she noted.
The investigation began when researchers noticed a distinct growth mark in the fossilized remains of C. theotonicus found in northwestern Argentina, identified as a neonatal line that indicates a growth spurt shortly after birth. To verify this feature, researchers estimated its body mass at birth and compared it to its adult mass using data from thousands of living mammals, reptiles, and birds.
“If mammalian ancestors were egg-laying or viviparous has been considered an inscrutable mystery,” Gaetano remarked. The study employed innovative methods to tackle this difficult question within the fossil record.
Body Mass Findings
The researchers calculated that C. theotonicus weighed approximately 1.7 kg at birth and reached about 12 kg at maturity, signifying that the newborn was about 14% of its adult body mass. This ratio contrasts sharply with those of modern reptiles; for instance, some turtles and crocodilians weighing between 8 kg and 14.5 kg produce hatchlings weighing only 9 to 53 g, resulting in neonate-adult ratios of roughly 0.1% to 0.6%. Birds, too, exhibit similar trends with hatchling weights significantly lower than that of the adult birds.
Conversely, mammals of comparable adult size (8 kg to 15 kg) typically give birth to significantly heavier young. A notable example is the bay duiker antelope, which produces offspring weighing roughly the same as C. theotonicus.
Implications for Evolutionary Biology
This research prompts a reevaluation of the timeline concerning the evolution of live birth. As co-author María Miceli Baro from the University of Buenos Aires highlighted, features like the neonatal line were previously not associated with cynodonts. “Through its analysis, we found that a trait that is generally linked to evolutionary success was present in animals long before true mammals originated,” she remarked.
The study suggests that live birth might have emerged much earlier in the mammalian lineage than has been traditionally accepted. Gaetano noted, “It is very well possible that C. theotonicus does not represent an isolated case of viviparity among cynodonts. It could be evidence of the general switch from laying eggs to giving birth to live young early on in the mammalian lineage.” However, he emphasized the need for further evidence to substantiate these claims, concluding that some cynodonts appear strikingly similar to present-day mammals.
