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.
Highlights
The phylum Cnidaria is a diverse group of relatively simple diploblastic animals with highly complex and typical venomous cell, ‘cnida’
The specimen ELISN108-343 is apparently different from ELISN31-5 in many aspects, such as external morphology and internal anatomy, especially the organization of gastric pockets and the derivation of subumbrellar endoderm
In ELISN108-343, the endoderm of the subumbrella is continuous with the upper part of accessory septa, whereas in ELISN31-5, it is mainly derived from the upper part of gonad-lamellae
Summary
The phylum Cnidaria is a diverse group of relatively simple diploblastic animals with highly complex and typical venomous cell, ‘cnida’. Medusozoans comprise four classes: Staurozoa, Scyophozoa, Hydrozoa and Cubozoa [1] The latter, popularly known as ‘box jellyfish’ or ‘sea wasps’, is a monophyletic group characterized by a four-sided box-shaped appearance, four bunches of interradial tentacles each with a wing-like pedalium at the proximal end, and four well-developed complex eyes as well as a circular velarium suspended perpendicularly by four bracket-like perradial frenula [1,2] (see Figure 1A–B). The fossil record of cubozoans is quite sparse [7,8]; a few probable cubozoan fossils with simple and unbranched pedalia are known from the Pennsylvanian Mazon Creek Formation of central USA and the Middle Cambrian Marjum Formation of Utah, western USA [8]. 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
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