Yilingia spiciformis
- 演化之聲
- Mar 19
- 3 min read
Updated: 1 day ago
Age
Ediacaran
551-539 Ma
Taxonomy
Kingdom: Animalia
Clade: Bilateria
Genus: Yilingia
Species: Yilingia spiciformis
Morphological description
Yilingia spiciformis is an elongate, strap-like bilaterian organism characterized by clear segmentation and a trilobate body organization. The entire body is composed of repeated trilobate segments, each consisting of a rhomboidal central lobe and two posteriorly directed, subtriangular lateral lobes that articulate with the lateral faces of the central lobe. The arrangement of segments exhibits polarity: the lateral lobes are consistently inclined at approximately 30–60° and oriented toward the posterior end. This directional consistency allows inference of anterior–posterior orientation even in incomplete specimens.
Some specimens show podomere-like subdivisions within the lateral lobes; however, this feature is not consistently observed and may represent taphonomic artefacts rather than true anatomical structures .
The anterior region gradually tapers, whereas the posterior end narrows more distinctly. Evidence for body regionalization or cephalization is weak or absent. Known specimens range from approximately 5 to 26 mm in width and up to 27 cm in length, with around 50 segments .
In dorsal view, the central lobe is fully exposed, while in ventral view it is partially covered by the lateral lobes. Transverse sections reveal a biconvex profile, with lateral lobes tucked ventrally beneath the central lobe, indicating a degree of dorsoventral differentiation. The size of the lateral lobes increases progressively along the body axis toward the posterior, corresponding with an overall increase in body width, although the terminal posterior region shows slight narrowing.
Segment morphology remains largely uniform along the body, indicating homonomous segmentation and a lack of tagmosis.
The trace fossil associated with locomotion, a mortichnium, has a width of approximately 2.5 cm, comparable to the maximum body width. It is preserved as a shallow positive hyporelief with parallel lateral grooves. Additional trace fossils range from 7 to 25 mm in width and 7.6 to 60.8 cm in length; they may be straight or curved, with some preserving chevron-like structures reminiscent of trilobate segments, while others are smooth and featureless .
The holotype specimen is catalogued as NIGP-166252.
Etymology
Yilingia is named after the Yiling District of Yichang City, Hubei Province, China, where the fossils were discovered.
The species name spiciformis is derived from Latin, meaning "spike-shaped," referring to the fossil's wheat-ear-like appearance .
Biological description
Yilingia spiciformis lived during the terminal Ediacaran period, approximately 551 to 539 million years ago, and is known from the Shibantan Member of the Dengying Formation in the Yangtze Gorges region of South China .
This organism represents a segmented bilaterian with clear segment polarity and directional locomotion. It is likely related to either panarthropods or annelids. Its segmentation reflects true metamerism rather than superficial annulations. The lateral lobes may be homologous to appendages in arthropods or parapodia in annelids. Each segment bears only a single pair of lateral lobes, a condition that differs from Cambrian lobopodians but resembles that of euarthropods. If the podomere-like features are artefacts, then an annelid affinity becomes more plausible .
Yilingia spiciformis was capable of active locomotion and sediment disturbance, producing long and continuous trails. It is among the few known Ediacaran organisms demonstrated to generate such sustained trace fossils. This ability provides direct evidence linking body fossils with trace fossils, bridging a long-standing gap in understanding the identity of Ediacaran trace makers .
The combination of segmentation and motility in Yilingia spiciformis suggests that these features may have already evolved in the common ancestor of bilaterians. These biological innovations likely contributed to the ecological transformations known as the Cambrian substrate revolution and agronomic revolution, during which widespread bioturbation reshaped seafloor environments. The discovery of Yilingia spiciformis therefore offers crucial insight into the early evolution of segmentation and the emergence of active movement in animal history.
（Author: Shui-Ye You）
Reference
Chen Z et al. (2019). Death march of a segmented and trilobate bilaterian elucidates early animal evolution. Nature.
30.78, 111.05
Comments