Modern cephalopods are unique among molluscs for their relatively high intelligence. Most extant cephalopods (Coleoidea, represented by squids and octopuses) lack a mineralized external shell; instead, they bear an internal hard or soft shell, or even lack a shell entirely. However, ancestral cephalopods (Nautiloidea) are characterized by heavily mineralized shells with internal structures—such as the siphuncle, chambers, septa, and septal necks—that regulate buoyancy. The siphuncle represents one of the critical innovations in cephalopods evolving from the stem mollusk group. Unfortunately, owing to the lack of a continuous fossil record, the detailed evolutionary pathway of the siphuncle remains unclear. The earliest accepted cephalopod, Plectronoceras cambria, dates from the late Cambrian, which is several tens of millions of years later than the divergence time of cephalopods estimated by molecular clocks during Cambrian Explosion (early Cambrian). However, the abundant, multi-phylum microfossils known as 'small shelly fossils' that occur worldwide in the early Cambrian may offer great potential for discovering the earliest cephalopods.
Recently, Dr. PAN Bing, an assistant researcher in Professor ZHU Maoyan’s group at the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences (NIGPAS), in collaboration with a team of researchers from China, the UK, and the USA led by Prof. Guo Junfeng (Chang'an University, China), conducted research on small shelly fossils from South China. Fortunately, among the numerous small shelly fossils from the Shuijingtuo Formation (c. 520 Ma), they discovered the earliest siphuncle-bearing cephalopod, Eoceras shaanxiense, and systematically analyzed 32 specimens of this new species. The scientific results were recently published in Nature.
Based on observations of specimens preserved through different types of phosphatization, using scanning electron microscopy (SEM) and micro-computed tomography (micro-CT) (Fig. 1), the researchers revealed the overall external morphology and internal anatomy of Eoceras. Eoceras is millimeter-sized, with an orthoconic shell, an oblique apertural margin, multiple septa, and a ventrally situated segmented tube that appears to bridge the septa via minute canals (Fig. 2). Through comprehensive comparison with the internal structures of typical siphuncle-bearing cephalopods, the segmented tube of Eoceras is interpreted as a siphuncle. Therefore, they argue that Eoceras is likely a very early cephalopod—more specifically, an intermediate evolutionary stage between earlier chambered shells and later species with more developed buoyancy systems.
They also outline a general early evolutionary pathway for cephalopods: firstly, early cephalopods originated from a taxon with orthoconic shells and multiple septa; secondly, they evolved a sealed, segmented siphuncle within the orthoconic shell; and thirdly, they formed a true siphuncle with septal necks and connecting rings. Considering its primitive and simple system for regulating buoyancy, Eoceras may have had a mostly benthic mode of life. However, the fossil lacks several features seen in later cephalopods, and its soft-body anatomy remains uncertain. Future discoveries and further studies of similar Cambrian fossils may help clarify the earliest evolutionary history of cephalopods and the origins of their distinctive shell architecture.
This research was supported by the National Natural Science Foundation of China (NSFC), the National Key Research and Development Program of China, and the Opening Foundation of the State Key Laboratory of Continental Evolution and Early Life.
References: Song, Z.C., Pan B., Guo, J.F.*, Vinther, J.*, Li, G.X., Han, J., Van Iten, H., Zhao, X.F., Pei, X.Z., Peng, J.X., Qiang, Y.Q., Zhang, B.Y., Wen, H.J.*, 2026. Earliest siphuncle-bearing cephalopod from the early Cambrian. Nature. https://doi.org/10.1038/s41586-026-10868-y.
Fig. 1 Preservation types of Eoceras gen. nov. from the Cambrian Shuijingtuo Formation, South China
Fig. 2 Overall out morphology and inner anatomy of the shell of Eoceras
Fig. 3 Hypothesized phylogenetic position and life reconstruction of Eoceras gen. nov.
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