The sponge pump as a morphological character in the fossil record

Sally P. Leys,Pablo Aragonés Suarez

doi:10.1017/pab.2021.43

Abstract

AbstractThe timing of early animal evolution remains one of the biggest conundrums in biology. Molecular data suggest Porifera diverged from the metazoan lineage some 800 Ma to 650 Ma, which contrasts with the earliest widely accepted fossils of sponges at 535 Ma. However, the lack of criteria by which to recognize the earliest animals in the fossil record presents a challenge. The sponge body plan is unchanged since the early Cambrian, which makes a sponge-type animal a good candidate for the earliest fossils. Here we propose a method for identifying an organism as sponge grade by translating the sponge pump character into a quantifiable morphological trait. We show that the ratio between the two major components of the aquiferous system, the cross-sectional area of the osculum (OSA) and the surface area of the whole sponge (SA), is an effective metric of the pump character of extant sponges and that the slope of this ratio is distinct for three classes of Porifera: Demospongiae, Calcarea, and Hexactinellida. Furthermore, this metric is effective at distinguishing as sponges both extant taxa and fossils from two extremes of the Phanerozoic, the Cambrian and Paleogene. We tested this metric on the putative Ediacaran spongeThectardis avalonensisfrom Mistaken Point, Newfoundland, and foundThectardisfits both with Cambrian sponges and with modern demosponges. These analyses show that the OSA/SA ratio is a reliable character by which to identify fossils as sponge grade, opening up exciting possibilities for classifying new fossils as sponges.

Highlights

The tempo and mode of early animal evolution remains one of the biggest conundrums in biology
We found that for a range of individuals of different sizes, the osculum cross-sectional area (OSA)/surface area (SA) metric is characteristic for each sponge class, which suggests that each class has distinct pumping capabilities constrained by the structure of the aquiferous system
The correlations between morphology and pumping rates shown here confirm that the character that best represents a sponge is its total osculum cross-sectional area (OSA), because it is proportional to the total surface area of a sponge (Bidder 1923; Goldstein et al 2019; Kealy et al 2019; Morganti et al 2021)

Summary

Introduction

The tempo and mode of early animal evolution remains one of the biggest conundrums in biology. It is known that phylum-level lineages were long established by the early Cambrian, ca. The apparently sudden emergence of multicellular animals at the base of the Cambrian greatly puzzled Darwin (1859), who argued that fossil discoveries would eventually solve this conundrum. Many body fossils from the Ediacaran are known, interpretations of them as independent lineages or stem groups of metazoans is still debated (Cunningham et al 2017), with only Kimberella (Fedonkin and Waggoner 1997) argued to be “widely accepted” as a metazoan (Wray 2015). Aside from the varied interpretations of Ediacaran soft-bodied biota, there is a large gap in time between fossils and the molecular clock estimates that place the divergence of major animal clades at 800 Ma or earlier (Peterson and Butterfield 2005; see Cunningham et al 2017). The lack of consensus on the timing of the appearance of the first multicellular animals is exacerbated by, or maybe due to, the fact that we lack criteria by which to recognize early animals definitively in the fossil record (Antcliffe et al 2014; Turner 2021)

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Discussion

Conclusion