Restricted morphospace occupancy of early Cambrian reef-building archaeocyaths

Abstract

AbstractThe 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.