Early Limbless Snake-like Amniotes Appeared in the Carboniferous

演化之聲
Mar 10
3 min read

Reconstruction of Nagini mazonense – with blue stripes（感謝Henry Sutherland Sharpe提供） — Reconstruction of *Nagini mazonense* – with blue stripes（感謝Henry Sutherland Sharpe提供）

The evolution of four limbs was one of the defining innovations that enabled vertebrates to colonize land. Yet in some lineages this key adaptation was later reduced or completely lost, accompanied by the evolution of elongated bodies adapted for crawling rather than walking. Among living squamate reptiles, more than sixty lineages have independently evolved reduced or absent limbs, including snakes, pygopodids, amphisbaenians, dibamids, and several groups of skinks and other lizards. Similar trends also occur in certain amphibians, such as caecilians and sirenids. These evolutionary transformations are generally interpreted as adaptations to particular ecological conditions, including burrowing lifestyles, aquatic habitats, or other environmental pressures. Despite its repeated occurrence, limb reduction or complete limb loss has so far been confined to specific groups within the tetrapod evolutionary tree, and it has never arisen in mammals.

The origin of limbless body plans in early reptiles and amphibians has long been a subject of interest for paleontologists. Unfortunately, the fossil record provides only limited information about the transitional stages through which limbs were gradually lost. One difficulty lies in preservation: the delicate bones of shortened limbs and their surrounding vertebrae are rarely preserved together in a clear and complete state. As a result, the process by which the earliest tetrapods first lost their limbs entirely has remained poorly understood.

A recent discovery from the Carboniferous Mazon Creek fossil deposits in Illinois, United States, has shed new light on this question. The fossils belong to a tetrapod that already possessed characteristics of early amniotes and has been named Nagini mazonense. Two specimens are known, both preserving traces of soft tissues. The fossils reveal that the forelimbs and pectoral girdle had disappeared completely, while small and reduced hindlimbs were still present. This pattern closely resembles the inferred evolutionary sequence in snakes, in which the forelimbs and shoulder girdle were lost first, followed later by the hindlimbs.

The new species is interpreted as a basal member of a group of early reptiles known as recumbirostrans. It appears to be most closely related to genera such as Infernovenator and Brachydectes. Members of this clade lived primarily during the Carboniferous and Permian periods and were typically characterized by elongated bodies and shortened limbs, although completely limbless forms had not previously been documented within the group.

Brachydectes newberryi（圖片來源：Nobu Tamura，採用 CC BY-SA 4.0 授權） — *Brachydectes newberryi*（圖片來源：Nobu Tamura，採用 CC BY-SA 4.0 授權）

During the embryonic development of tetrapods, the lateral plate mesoderm gradually produces limb buds. These buds then grow outward in a coordinated pattern, forming cartilage elements that are eventually replaced by bone to produce fully developed limbs. In tetrapods with reduced or absent limbs, mutations affecting the genes that regulate limb development disrupt one or more stages of this process, preventing normal limb formation.

Different evolutionary lineages have achieved limb reduction through different genetic mechanisms. In skinks, for instance, limb reduction often results from the loss of normal growth activity in the epiphyseal growth plates of cartilage during later stages of embryonic development, producing animals with very short limbs. This condition is also commonly observed among recumbirostrans. In contrast, modern snakes and caecilians typically exhibit disruptions in the earliest stages of limb bud formation during embryogenesis, leading to the complete loss of limbs and the shoulder girdle. This appears to be the condition affecting the forelimbs of Nagini mazonense.

Comparisons between Nagini mazonense and other recumbirostrans suggest that the disappearance of limbs occurred through a gradual sequence of evolutionary modifications. In lineages such as snakes and caecilians, the fully limbless condition is believed to have been preceded by stages in which limbs were reduced but still present. Even today, tiny vestigial hindlimbs retained by pythons and boas provide living evidence of this transitional history. The discovery of Nagini mazonense therefore offers important insight into how limb loss may have unfolded in early tetrapods and suggests that the mechanisms underlying limb reduction in ancient species were broadly similar to those observed in modern animals.