Early Cambrian Pentamerous Cubozoan Embryos from South China

Abstract

BackgroundExtant cubozoans are voracious predators characterized by their square shape, four evenly spaced outstretched tentacles and well-developed eyes. A few cubozoan fossils are known from the Middle Cambrian Marjum Formation of Utah and the well-known Carboniferous Mazon Creek Formation of Illinois. Undisputed cubozoan fossils were previously unknown from the early Cambrian; by that time probably all representatives of the living marine phyla, especially those of basal animals, should have evolved.MethodsMicroscopic fossils were recovered from a phosphatic limestone in the Lower Cambrian Kuanchuanpu Formation of South China using traditional acetic-acid maceration. Seven of the pre-hatched pentamerous cubozoan embryos, each of which bears five pairs of subumbrellar tentacle buds, were analyzed in detail through computed microtomography (Micro-CT) and scanning electron microscopy (SEM) without coating.ResultsThe figured microscopic fossils are unequivocal pre-hatching embryos based on their spherical fertilization envelope and the enclosed soft-tissue that has preserved key anatomical features arranged in perfect pentaradial symmetry, allowing detailed comparison with modern cnidarians, especially medusozoans. A combination of features, such as the claustrum, gonad-lamella, suspensorium and velarium suspended by the frenula, occur exclusively in the gastrovascular system of extant cubozoans, indicating a cubozoan affinity for these fossils. Additionally, the interior anatomy of these embryonic cubozoan fossils unprecedentedly exhibits the development of many new septum-derived lamellae and well-partitioned gastric pockets unknown in living cubozoans, implying that ancestral cubozoans had already evolved highly specialized structures displaying unexpected complexity at the dawn of the Cambrian. The well-developed endodermic lamellae and gastric pockets developed in the late embryonic stages of these cubozoan fossils are comparable with extant pelagic juvenile cubomedusae rather than sessile cubopolyps, whcih indicates a direct development in these fossil taxa, lacking characteristic stages of a typical cnidarian metagenesis such as planktonic planula and sessile polyps.

Developmental biology of the early Cambrian cnidarian Olivooides

“…Han et al . () did not share the concerns of Steiner et al . () in fitting a cnidarian bodyplan to the internal structures described by Dong et al .…”

Embryology in Deep Time

“…A small number of specimens have shown that the external pentaradial symmetry is imposed more fundamentally upon the internal anatomy which has been preserved in only the most exceptional of circumstances, revealing a series of circumference parallel walls with pentaradial divisions and diverticulations, leading to an open adapertural space otherwise occupied by an axial pentaradial process (Dong et al 2013 ;Han et al 2013 ). Additionally, a set of two juxtaposed pentaradial objects has been described in association with Olivooides and interpreted as ephyrae (juvenile medusae) in the process of strobilation (budding in scyphozoan cnidarians; see Chapter 6 ) (Dong et al 2013 ).…”

“…Debate over the affi nity of the olivooids has been wide-ranging, including echinoderms , scalidophorans (Steiner et al 2014 ), cnidarians (Bengtson and Yue 1997 ;Yue and Bengtson 1999 ;Han et al 2013 ), and diploblastic stem-eumetazoans (Yasui et al 2013 ). The hypothesis of echinoderm affi nity is based on little more than pentaradial symmetry and can be rejected since the olivooids lack key echinoderm apomorphies, not least a mineralised skeleton comprised of calcite stereom (Dong et al 2013 ;Han et al 2013 ;Steiner et al 2014 ).…”

“…The hypothesis of echinoderm affi nity is based on little more than pentaradial symmetry and can be rejected since the olivooids lack key echinoderm apomorphies, not least a mineralised skeleton comprised of calcite stereom (Dong et al 2013 ;Han et al 2013 ;Steiner et al 2014 ). The scalidophoran interpretation is based principally on the general similarity between the theca of olivooids and the lorica of loriciferans and larval priapulids, together with similarities in general symmetry and the requirement of the aperture to open and close akin to a scalidophoran introvert (Steiner et al 2014 ).…”

“…The pattern of pentaradial symmetry seen in Olivooides is also unusual for cnidarians, as is the aperture which is diffi cult to rationalise with the presence of a polyp. Many of these concerns are diminished if not dismissed based on recent description of internal anatomy, which Han et al ( 2013 ) interpret (and likely grossly over-interpret) in the image of a cubozoan polyp. Details of this comparison are problematic, not least the evident fact that the embryo of Olivooides develops into a sessile polyp (if it is indeed a cnidarian), not a medusa, and what is more, the polyp stage of living cubozoans is grossly reduced.…”