A Silurian Graptolite Window: Guluoping Section Fills a Gap in South China’s Fossil Record

Deep in Hubei Province, China, a modest outcrop of black shale is rewriting what scientists know about life on Earth roughly 440 million years ago. The Guluoping section, exposed south of the Yangtze River in Zigui County, has yielded a rich assemblage of graptolites—extinct colonial animals that drifted through ancient oceans—that firmly establishes the presence of the Stimulograptus sedgwickii Biozone on the South China Platform for the first time. This interval, corresponding to the uppermost Aeronian Stage of the Llandovery Epoch in the Lower Silurian, has long been a puzzle for palaeontologists working in China, where it was rarely recognized and often misidentified.

The research, published in the Swiss Journal of Palaeontology, centers on approximately 25 meters of dark gray to black, often silty shale that rests above a massive sandstone and is capped by another sandstone unit of unknown thickness. These carbonaceous mudstones belong to the Longmaxi Formation, a widely distributed rock unit across the Yangtze Platform that has been studied extensively in recent decades. Yet despite numerous investigations of Lower Silurian graptolite faunas in the region, the uppermost Aeronian interval remained elusive, with previous reports either based on unconvincing specimens or complicated by tectonic deformation that obscured diagnostic features.

Graptolites are among the most valuable index fossils in lower Paleozoic stratigraphy. These planktic colonial organisms, whose skeletal remains resemble delicate pencil strokes on shale, evolved rapidly enough that their successive species define narrow biozones used to correlate rock layers across continents. The Stimulograptus sedgwickii Biozone, named after its characteristic species first described in 1843, marks a specific slice of time near the end of the Aeronian Stage. Pinpointing this interval matters because it sits close to the base of the overlying Telychian Stage, now formally defined by the first appearance of the graptolite Spirograptus guerichi in the El Pintado section of Spain.

What makes the Guluoping discovery compelling is the sheer clarity of the faunal signal. Stimulograptus sedgwickii appears in nearly every sample collected from the section, alongside other diagnostic species such as Lituigraptus urceolus and Parapetalolithus praecedens. Together with Pseudoretiolites perlatus and Pristiograptus xiushanensis, these taxa form a moderately diverse community of both biserial and uniserial graptolites. The fauna belongs to the higher part of the biozone, though the exact boundaries of the interval remain unexposed at the section due to faulting and erosion in this tectonically complex region.

The section itself is divided into eight distinct lithological units, each characterized by subtle differences in sediment composition. Bentonite layers—volcanic ash deposits that can be dated precisely—appear in some units, while others show spherical weathering patterns and secondary pyrite enrichment along fractures. The base of the overlying sandstone is uneven, indicating a distinct erosional surface that may represent a gap in the geological record. Such sandstone intervals hint at local erosion of mudstones and possible biostratigraphic breaks that researchers must account for when reconstructing the sequence of ancient environments.

Beyond graptolites, the Guluoping section preserves an unexpected cast of supporting characters. Small oval sponges, ranging from 2 to 5 millimeters in length, are associated with the graptolite fauna. Experts have suggested these are lyssacinosidan hexactinellid sponges, probably belonging to the family Rossellidae, and almost certainly representing a new species. Remarkably, they appear to be the youngest known sponges of this type from the post-LOME interval in China—LOME referring to the Late Ordovician Mass Extinction, one of the largest biodiversity crises in Earth’s history. Flattened imprints of phosphatic brachiopods, possibly lingulids, round out the assemblage, though their original shells have not survived.

Preservation quality varies dramatically across the outcrop depending on weathering conditions. In fresh, unweathered material, the dark gray to black shales reveal graptolites as silvery, shining films of organic material that contrast sharply with the surrounding rock. In weathered samples from the surface, the shales turn yellowish-brown and the graptolites appear as dark brown to black remnants. Weathering begins around the specimens themselves, forming light-colored halos before consuming the entire sediment. This variation in preservation, combined with tectonic deformation, makes some identifications genuinely difficult.

Deformation is a recurring theme in the Guluoping material. Specimens of Pseudoretiolites decurtatus from the same slab show dramatically different measurements depending on their orientation—some compressed, others stretched by tectonic strain. The two-thecal repeat distance in Parapetalolithus praecedens varies between 1.0 and 1.8 millimeters, a range too wide to be explained by natural intraspecific variation alone but easily accounted for by directional deformation. The silvery color of the fresher graptolite material also indicates heating during tectonic deformation, with thermal maturity comparable to what geologists measure using vitrinite reflectance in coal studies. Green to whitish pressure-shadow minerals surrounding many specimens provide further evidence of this tectonic modification.

This deformation carries real taxonomic consequences. Historical examples abound of species erected on the basis of deformed specimens that were misinterpreted as genuine morphological features. The cautionary tale of Diplograptus antithecaeoides, described in 1939 with supposedly unusual thecal development, was later shown to be based on deformation similar to what is seen in the Guluoping Pseudoretiolites specimens. The authors of the new study warn that failure to detect tectonic deformation may lead to the erection of unwarranted new taxa, a problem that has often been overlooked in graptolite descriptions.

Internationally, the picture is equally complex. The Stimulograptus sedgwickii Biozone is widely recognized, but the overlying Stimulograptus halli Biozone has been confirmed only in Britain, Avalonia, and Baltica. In many other regions, including Laurentia and Gondwana, the two are treated as a single combined interval. In South China, differentiation between the two biozones is not currently possible, as Stimulograptus halli has not been found at Guluoping. The presence of Lituigraptus urceolus, however, suggests correlation with equivalent faunas in the Czech Republic and the United Kingdom. Notably, the Guluoping record clarifies the age of Pristiograptus xiushanensis, a species previously known only from monospecific assemblages whose biostratigraphic position had been uncertain—its association with Stimulograptus sedgwickii now indicates it belongs to the upper Aeronian rather than the Telychian as previously suggested.

The Guluoping section thus fills a genuine gap in the global graptolite record. In Baltica, the equivalent interval is missing entirely in several key successions, including cores from Bornholm, Latvia, and Estonia, where unconformities or gaps span much of the upper Aeronian. In the Argentinian Precordillera, the interval has only recently been documented. By providing a well-documented fauna from the South China Platform, the Guluoping section offers palaeontologists a new reference point for correlating Silurian strata worldwide and understanding how marine ecosystems recovered and reorganized in the millions of years following the Late Ordovician Mass Extinction.

Subject of Research: Silurian graptolite biostratigraphy of the Stimulograptus sedgwickii Biozone at the Guluoping section, Yangtze Platform, South China

Article Title: The Guluoping section and the understanding of the Stimulograptus sedgwickii graptolite Biozone on the South China Platform (Yangtze Platform)

Article References: Wang, C., Maletz, J., & Wang, X. (2026). The Guluoping section and the understanding of the Stimulograptus sedgwickii graptolite Biozone on the South China Platform (Yangtze Platform). Swiss Journal of Palaeontology, 145, 1041-1059. https://doi.org/10.3897/sjp.145.211690

Image Credits: AI Generated

DOI: 10.3897/sjp.145.211690

Keywords: graptolites, Silurian, Aeronian, Llandovery, biostratigraphy, South China, Yangtze Platform, Longmaxi Formation, Stimulograptus sedgwickii, tectonic deformation, palaeontology, Hubei Province