Shell decollation is a growth strategy that has been adopted by a number of invertebrate taxa to offset the metabolic and ecological disadvantages of shell growth. However, little is known about the origin and evolution of this process. We here describe well-preserved specimens of the hyolith, Cupitheca decollata sp. nov., preserving the decollation process from the early Cambrian Yu’anshan Formation (ca. 518 million years ago) of South China. Based on a large number of specimens collectively representing different developmental stages, we use high-resolution X-ray microtomography and scanning electronic microscopy to reconstruct the process of decollation in this taxon. Cupitheca is among the earliest known small shelly fossils, and thus our discovery confirmed that periodic decollation had evolved by the onset of the Cambrian explosion, reflecting the high intensity of the predator-prey arms race in early Cambrian ecosystems. A comparison between the decollation processes of Cupitheca and other shelly invertebrates suggests that periodic decollation and the associated molecular mechanisms of calcium dissolution, uptake, allocation, and deposition may have had multiple independent origins.Shell decollation is a growth strategy that has been adopted by a number of invertebrate taxa to offset the metabolic and ecological disadvantages of shell growth. However, little is known about the origin and evolution of this process. We here describe well-preserved specimens of the hyolith, Cupitheca decollata sp. nov., preserving the decollation process from the early Cambrian Yu’anshan Formation (ca. 518 million years ago) of South China. Based on a large number of specimens collectively representing different developmental stages, we use high-resolution X-ray microtomography and scanning electronic microscopy to reconstruct the process of decollation in this taxon. Cupitheca is among the earliest known small shelly fossils, and thus our discovery confirmed that periodic decollation had evolved by the onset of the Cambrian explosion, reflecting the high intensity of the predator-prey arms race in early Cambrian ecosystems. A comparison between the decollation processes of Cupitheca and other shelly invertebrates suggests that periodic decollation and the associated molecular mechanisms of calcium dissolution, uptake, allocation, and deposition may have had multiple independent origins.
FIG. S1. Locality and stratigraphy of Changqing section in Huize County, Yunnan Province, China.
A, location of Yunnan Province in China. B, position of Changqing section in Huize County. C, stratigraphic column in Changqing section. Abbreviations: SYT, Shiyantou Formation; HJS, Hongjingshao Formation.
Figure S1-166.tif
FIG. S2. Morphology and decollation structures of Cupitheca decollata sp. nov.
A, NIGPAS-HC-018, larval shell with a circle of penetrating channels, detail of box shown in G. G, enlargement of the penetrating channels, secondarily eroded as a groove. B, NIGPAS-HC-019, tubular shell with a belt-like structure, box indicates the position of H. H, detail of the belt-like structure along the circle of penetrating channels. C, NIGPAS-HC-020, aperturally broken shell with a ring, detail of box shown in I. I, enlargement of the ring. D, E, NIGPAS-HC-021, 022, tubular shells with rings and micropillar structures, boxes indicate the positions of J and L, respectively. J, enlargement of the ring and poorly developed micropillars, detail of box shown in K. L, enlargement of the ring and the micropillar structure, detail of the micropillar structure shown in M. F, NIGPAS-HC-023, apically broken shell with a ring and micropillar structure, detail of box shown in N. N, enlargement of the deformed ring and micropillar structure, detail of box shown in O. K, M, O, vertical micropillars. Scale bars represent: 100 μm (A); 200 μm (B-F); 10 μm (G, K, M); 60 μm (H, L); 50 μm (I); 20 μm (J); 40 μm (N); 12 μm (O).
figure S2-166.tif
FIG. S3. The ring and the micropillar structure of Cupitheca decollata sp. nov.
A, B, NIGPAS-HC-006, 007, incomplete tubular shells with rings and micropillar structures. C, NIGPAS-HC-008, broken tubular shell with a ring. Scale bars represent 200 μm.
figure S3-166.tif
Table S1. Original measurement data.
DF-DA, measurements of diameter of flange and diameter of aperture. DF-HN, data of diameter of flange and height of neck. DR-LDS, data of diameter of ring and length of decollated shell. DR-WR, measurements of diameter of ring and width of ring.
Table S1.xls
image stacks-NIGPAS-HC-006
The specimen with a ring and micropillars on the shell surface was scanned using a three-dimensional X-ray microscope, Zeiss Xradia 520 Versa at the Micro-CT Lab of Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences (NIGPAS). The operating voltage of the X-ray source was 60 kV. To obtain high resolution, a CCD (charge-coupled device)–based optical objectives with 4× was applied in the scan, and voxel size is 1.3455 μm. Thin filter LE1 was used to avoid beam hardening artefact. For this scan, 3201 projections in total were obtained over 360 degrees, and the exposure time for each projection was 1.8 s. The volume data were processed using VGSTUDIO MAX (3.0 version) software.
image stacks-NIGPAS-HC-007
The specimen with a ring and micropillars on the shell surface was scanned using a three-dimensional X-ray microscope, Zeiss Xradia 520 Versa at the Micro-CT Lab of Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences (NIGPAS). The operating voltage of the X-ray source was 60 kV. To obtain high resolution, a CCD (charge-coupled device)–based optical objectives with 4× was applied in the scan, and voxel size is 0.9086 μm. Thin filter LE1 was used to avoid beam hardening artefact. For this scan, 3601 projections in total were obtained over 360 degrees, and the exposure time for each projection was 5.5 s. The volume data were processed using VGSTUDIO MAX (3.0 version) software.
image stacks-NIGPAS-HC-008
The specimen with a ring on the shell surface was scanned using a three-dimensional X-ray microscope, Zeiss Xradia 520 Versa at the Micro-CT Lab of Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences (NIGPAS). The operating voltage of the X-ray source was 65 kV. To obtain high resolution, a CCD (charge-coupled device)–based optical objectives with 4× was applied in the scan, and voxel size is 1.1076 μm. Thin filter LE2 was used to avoid beam hardening artefact. For this scan, 3201 projections in total were obtained over 360 degrees, and the exposure time for each projection was 3 s. The volume data were processed using VGSTUDIO MAX (3.0 version) software.
3D reconstruction model
3D reconstruction models of specimens NIGPAS-HC-006, NIGPAS-HC-007 and NIGPAS-HC-008. The volume data were processed using VGSTUDIO MAX (3.0 version) software.
