The common assumption that the evolution of the first reptiles, birds, and mammals was centered around hard-shelled eggs is being challenged. A recent study, examining both fossilized and current species, indicates that it was extended embryo retention (EER) within the mothers, rather than egg-laying, that provided these primeval creatures with an evolutionary edge.
Scientists from Nanjing University and the University of Bristol have discovered that the ancestors of the earliest mammals, reptiles, and birds may have actually given birth to live offspring.
The conventional wisdom attributed the evolutionary success of amniotes – vertebrates that develop within an amnion, a protective membrane inside the egg – to the existence of the hard-shelled egg.
This view is contested by a new investigation of 51 fossil species and 29 extant species, categorized as either oviparous (egg-laying) or viviparous (live-birth giving), which suggests a different conclusion.
The study’s findings, now published in Nature Ecology & Evolution, reveal that the primary evolutionary branches of Amniota, including Mammalia, Lepidosauria (lizards and their kin), and Archosauria (dinosaurs, crocodilians, birds), exhibit viviparity and EER in their ancestral forms.
The image of the amniotic egg with its semi-permeable shell and extraembryonic membranes has long been hailed as a major evolutionary innovation. However, this research posits that it was EER that bestowed the definitive safeguard to this specific group of animals.
Extended embryo retention (EER) refers to the variable period during which mothers hold the young, likely dependent on optimal survival conditions.
Professor Michael Benton of Bristol’s School of Earth Sciences described the significance of the amniotic egg in the evolution of the first tetrapods from amphibious habits, likening it to a “private pond” that protected against dehydration and enabled terrestrial dominance.
Project Leader Professor Baoyu Jiang emphasized that this traditional perspective has now been questioned. The study noted the flexibility in reproductive strategies among lizards and snakes, as well as the discovery of live-bearers among various fossil species, including Mesozoic marine reptiles.
A Cretaceous choristodere skeleton, found inside a parchment-shelled egg in China, also indicates a transitional pattern between egg-laying and live-bearing, not exclusive to lizards.
Dr. Joseph Keating detailed that EER is a common occurrence in today’s vertebrates, with variable timing in lizards and snakes, presenting potential ecological benefits such as optimal temperature and food availability.
Professor Benton’s conclusion emphasized the outdated nature of the classic ‘reptile egg’ model, replaced by the understanding that the earliest amniotes had evolved EER as a means of adaptive parental protection. The exact nature of the birth, whether in parchment eggs or as live offspring, remains unclear, but this adaptation offered them an edge over preceding tetrapods.
Reference: “Extended embryo retention and viviparity in the first amniotes” by Baoyu Jiang et al., 12 June 2023, Nature Ecology & Evolution. DOI: 10.1038/s41559-023-02074-0
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Frequently Asked Questions (FAQs) about fokus keyword: evolution
What does the new study about early reptiles, birds, and mammals reveal about their reproduction?
The new study reveals that the first reptiles, birds, and mammals might not have relied on hard-shelled eggs as previously believed. Instead, they may have given birth to live young, with extended embryo retention (EER) in the mothers providing an evolutionary advantage. The research examined both fossil and living species and found evidence of viviparity and EER in their ancestors, challenging traditional views on early animal reproduction.
What is Extended Embryo Retention (EER)?
Extended Embryo Retention (EER) is when the young are retained by the mother for a varying amount of time, depending on when conditions are best for survival. The research suggests that EER, rather than egg-laying, gave certain ancient creatures a significant evolutionary advantage.
How does this research challenge the traditional view of the amniotic egg in evolution?
Traditionally, the success of amniotes was attributed to the hard-shelled egg, considered as one of the greatest innovations in evolution. This new study, however, implies that it was EER that provided the ultimate protection for these animals, rather than the hard-shelled egg. This finding contradicts the conventional understanding and signifies a shift in how we perceive early animal reproduction.
What species were examined in this study?
The study involved an analysis of 51 fossil species and 29 living species, categorized as either oviparous (laying hard or soft-shelled eggs) or viviparous (giving birth to live young).
Were there any examples of transitional patterns between egg-laying and live-bearing found?
Yes, fossils like the Mesozoic marine reptiles and a choristodere from the Cretaceous of China showed evidence of live-bearing and a transition between egg-laying and viviparity, adding weight to the findings of the study.
Who conducted this research?
The research was conducted by a team from Nanjing University and the University of Bristol, and it was published in Nature Ecology & Evolution.
What are the implications of this research for understanding early animal evolution?
This research provides a new perspective on the reproduction of early reptiles, birds, and mammals, emphasizing the role of live birth and EER rather than hard-shelled eggs. It has the potential to reshape our understanding of early animal evolution, challenging existing models and opening new avenues for exploration and study.
