Early Cambrian pentamerous cubozoan embryos from South China.
BACKGROUND
Extant 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.


METHODS
Microscopic 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.


RESULTS
The 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.
Early Life Institute and Department of Geology and State Key Laboratory of Continental Dynamics, Northwest University, Xi'an, PR China. elihanj@nwu.edu.cn
Jian
J
Han
Shin
S
Kubota
Guoxiang
G
Li
Xiaoyong
X
Yao
Xiaoguang
X
Yang
Degan
D
Shu
Yong
Y
Li
Shunichi
S
Kinoshita
Osamu
O
Sasaki
Tsuyoshi
T
Komiya
Gang
G
Yan
D000818:Animals; D005075:Biological Evolution; D002681:China; D003063:Cnidaria / Q000033:anatomy & histology / Q000145:classification / Q000648:ultrastructure; D004625:Embryo, Nonmammalian; D005260:Female; D005580:Fossils