An international research team led by Prof. ZHANG Huaqiao from the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, has reported the discovery of extraordinary fossil embryos of Ecdysozoa, a group of animals that include roundworms, horsehair worms, mud dragons, penis worms, velvet worms, water bears, insects, shrimps, and crabs, from the early Cambrian Kuanchuanpu biota (ca. 535 million years ago, or Ma) in southern Shaanxi Province, China.
The discovery expands our understanding of the morphological disparity and diversity of the early Fortunian fossil embryos and ecdysozoans.
The study was published in Palaeogeography, Palaeoclimatology, Palaeoecology on Dec. 11.
Invertebrate embryos have relatively low fossilization potential; however, when preserved, they can offer crucial insights into the evolutionary developmental biology of extinct animals. Invertebrate embryos have been reported from the early Cambrian through the Early Ordovician, but are represented only by cnidarians and a single scalidophoran taxon Markuelia. In the early Cambrian Kuanchuanpu biota, cnidarian embryos and their hatched stages are abundant and diverse. In contrast, fossil embryos belonging to the Ecdysozoa are still unknown in the Kuanchuanpu biota, although their hatched stages have been reported since ten years ago.
In this study, the researchers described exceptionally preserved fossil embryos from the early Fortunian (early Cambrian) Kuanchuanpu Formation at Zhangjiagou section, Xixiang County, Hanzhong City, Shaanxi Province, China. In total, seven specimens are illustrated, and they are all three-dimensionally phosphatized. Micro-CT analysis shows that they are internally hollow, thus leaving no trace of internal soft anatomies. Based on the differences of the number and arrangement of the sclerites at their anterior and posterior ends, these embryos are assigned to two new taxa, Saccus xixiangensis gen. et sp. nov. (Fig. 1) and Saccus necopinus gen. et sp. nov. (Fig. 2).
The study shows that the embryos are enclosed within a thin and smooth envelope, ranging in diameter from 730 μm to 1 mm. The embryos are relatively large, indicating that their eggs are yolk-rich and thus can provide enough energy for development (lecithotrophy). The embryos have a bag-shaped body with no introvert or paired limbs. High-resolution scanning electron microscopic images show that the integument is non-ciliated. The sclerites at the anterior end are radially arranged, whereas those at the posterior end are bilaterally arranged. The integument shows soft deformation, while the sclerites remain largely undeformed, suggesting that the sclerites are stiffer and likely cuticularized. The embryos lack any orifice, thus representing a developmental stage prior to the formation of a mouth or anus.
The bilaterally arranged sclerites at the posterior end imparts bilaterality, classifying these new embryos as bilaterians. Furthermore, the absence of cilia or cilium insertion sites and the presence of cuticularized sclerites suggest an ecdysozoan affinity. The presence of cuticle implies that the embryos are in later embryonic stages, possibly close to hatching.
In the absence of hatched specimens, the developmental mode of Saccus is unknown. It is inferred that it underwent indirect development, hatching as lecithotrophic (yolk-feeding) larvae. In this case, the morphology of the juvenile and adult forms is uncertain due to metamorphosis during later growth. Alternatively, it could undergo direct development, hatching as lecithotrophic juveniles, with both juveniles and adults potentially having a similar bag-shaped body without an introvert or paired limbs, closely reminiscent of Saccorhytus (Fig. 3). In either case, the embryos reply on yolk as the only source of energy until they develop a functional mouth and begin to take food.
The direct development hypothesis has implications for the body shape evolution of early ecdysozoans. If Saccus and Saccorhytus are constrained to be total- or stem-group ecdysozoans (Fig. 3), it indicates that a bag-shaped body may be primitive to the ecdysozoans, whereas a vermiform one, which characterizes the crown-group ecdysozoans, evolved later.
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