Restricted morphospace occupancy of early Cambrian reef-building archaeocyaths

Abstract

The evolution of novel morphologies can signify expansion of a clade into new niches. This can be studied in the fossil record by investigating the morphospace occupancy of organisms, with small morphospaces signifying low morphological disparity and more diffuse morphospaces suggesting a broader range of morphologies adapted to different environments. Morphological disparity of many taxa (arthropods, crinoids, etc.) from the Cambrian to modern intervals have been studied in this manner. However, no study has investigated this in archaeocyaths, which, as reef builders, can have a disproportionate effect on early Cambrian biodiversity relative to their frequency. Here, we collect morphological data on archaeocyathan sponges, mostly from Laurentia. More than 600 museum specimens and 400 field samples were measured for traditional morphometric characters and discrete gross morphological characteristics. We find that archaeocyaths have an average cup/individual (body) diameter of 10.6 mm. This is significantly smaller than a selected group of modern demosponges and lithistid sponges that measure 94.1 mm and 66.8 mm in diameter, respectively, and each has a larger size variance. Archaeocyathan gross morphologies are also highly constrained to a few simple morphologies (three to six categories), while modern demosponges and lithistids are more diverse (nine categories each). These data indicate that Laurentian archaeocyaths were restricted in their morphological disparity, potentially due to limitations imposed by having a robust calcareous skeleton while still maintaining a large intervallum cavity space to facilitate passive entrainment. The fact that these Cambrian reef builders were restricted in their morphological complexity may have had a strong influence on the biodiversity of early Phanerozoic ecosystems. Furthermore, a clade limited to only a few specific morphologies is at an increased risk of extinction.

Reconstruction of a Ross lost CambrianSeries 2mixed siliciclastic–carbonate platform from carbonate clasts of the Shackleton Range, Antarctica

“…They have reported an average diameter of 10.6 mm and a median value of 8.67 mm (0.78 mm minimum to 74 mm maximum). Cordie & Dornbos (2019) reported an average diameter of 10.6 mm from more than 1000 measured archaeocyaths. Therefore, the archaeocyaths from the Shackleton Range record are small compared to published data.…”

“…Furthermore, typical conical cups of Ajacicyathidae are around 5-15 mm (Debrenne et al 2012). Cordie & Dornbos (2019) have measured the traditional morphometric characters of more than 1000 archaeocyaths, mainly from the orders Ajacicyathida and Archaeocyathida. They have reported an average diameter of 10.6 mm and a median value of 8.67 mm (0.78 mm minimum to 74 mm maximum).…”

Reconstructing the feeding ecology of Cambrian sponge reefs: the case for active suspension feeding in Archaeocyatha

“…530-509 Ma), forming metre-scale bioherms akin to modern patch reefs [3,4] to much larger constructions tens of metres in scale [5,6]. Most species are short (millimetre-to centimetre-scale), conical and characterized by porous skeletal walls [7][8][9][10]. Their calcite skeletons typically consist of inner and outer walls ('cup in cup' structure) connected by porous tabular (horizontal) and septal/taenial (vertical) partitions forming an intervening space termed the intervallum, which surrounds a larger central cavity that was probably open to the ambient ocean environment ( [8,11]; figure 1).…”

Prey fractionation in the Archaeocyatha and its implication for the ecology of the first animal reef systems

“…Given their abundance and global distribution, the archaeocyaths have become important index fossils for correlating rocks on a global scale, meaning that we have detailed knowledge of their taxonomy and systematics (Debrenne et al 2012; Gradstein et al 2012). The Archaeocyatha are the first undoubted metazoans to form extensive reef-like bioconstructions in association with calcimicrobes (Brasier 1976; Rowland and Gangloff 1988; Gandin and Debrenne 2010), and they therefore play a pivotal role in early Cambrian ecology (Wood et al 1992; Wood 1999; Zhuravlev et al 2015; Cordie and Dornbos 2019). Studies on their functional morphology have been extensive and show the Archaeocyatha were suspension feeders (Wood et al 1992; Debrenne et al 2012).…”