# Unraveling the Role of Spicules in Shaping Sponge Body Structure: Evidence from the Early Cambrian Shuijingtuo Formation

## TL;DR

This study examines ancient sponge fossils and spicules to understand how spicule types and arrangements contribute to sponge body structure.

## Contribution

The paper provides the first systematic analysis linking isolated spicules to the sponge body plan in early Cambrian fossils.

## Key findings

- Pentactine spicules form the framework of parietal gaps in sponge fossils.
- Monaxon spicules support the overall sponge body structure.
- Smaller spicules stabilize the sponge framework.

## Abstract

Complete fossils of sponges are typically only found in Konservat-Lagerstätten, with most fossil sponge remains instead consisting of discrete spicules. This study focuses on grid-like skeletal sponge fossils and associated discrete spicules from the black shales of the Shuijingtuo Formation (South China Yangtze Platform) to reconstruct the function and spatial arrangement of the different spicule types that make up the sponge skeleton. Morphological and functional analysis confirms the examined sponge material belongs to the Hexactinellids, with pentacines and stauractine spicules the dominant spicule types. The taxonomic affinity between macroscopic sponge specimens and isolated spicules is demonstrated in this paper. The spicule system exhibits clear functional zonation: monaxon spicules support the sponge body, pentactines construct the framework of the parietal gaps, and smaller spicules provide structural reinforcement. These results provide a new understanding of how spicule morphology contributes to the overall sponge body plan.

In most cases, sponge fossils are preserved as isolated spicules, with complete sponge body fossils largely confined to Konservat-Lagerstätten. Although the classification and diversity of sponges and their isolated spicules have been extensively studied, no systematic attempts have been made to define the relationship between fossil spicules and the sponge body plan. By utilizing relatively well-preserved sponge fossils from the black shales of the Shuijingtuo Formation (South China) in conjunction with isolated spicules from the same locality, we assess spicule morphology to identify the potential functional roles of spicules and chart their arrangement within the sponge body. The elemental distribution and three-dimensional morphology of the examined sponge body fossil (likely a hexactinelid) are assessed using both micro-XRF and micro-CT. Tetractine, stauractine and pentactine spicules are the most abundant spicule types, both in the body fossil and in acid residues, with an additional spicule type (monaxons) also present. The larger pentactine spicules (five-ray spicules) frame the structure, whereas the smaller tetractines and stauractines (four-ray spicules), along with smaller pentactines, are arranged along the branches of the larger spicules. Based on the arrangement of the different spicules, it is proposed that each of the spicule types represents a discrete functional form: monaxons support the overall sponge body plan, pentactines construct the framework of the parietal gaps, and the smaller pentactines or tetractines stabilize the framework of the parietal gaps. These results provide a new understanding of sponge morphology, spicule function and the relationship between isolated fossil spicules and associated sponge body fossils.